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63 on surface water quality and on recharge of groundwater Volume I--Technical Issues for Monitoring Highway Runoff aquifers. The types of barriers for containment and reten- and Urban Stormwater. FHWA-EP-03-054, Prepared by the tion of sediment and pollutants from runoff and the effec- USGA for the Federal Highway Administration (2003) 479 pp. tiveness of each device were discussed. The report also Granato, G. E., Dionne, S. G., Tana, C. K., and T. L. King. (2003). National Highway Runoff Water-Quality Data and Methodology addressed the quantity and quality of highway runoff from Synthesis, Volume II--Project Documentation. FHWA-EP-03- different types of road surfaces, drainage and conveyance 055, Prepared by the USGA for the Federal Highway Adminis- systems, and various types of highways. In addition, meth- tration (2003) 22 pp. ods and strategies for the handling and control of highway Granato, G. E. National Highway Runoff Water-Quality Data and runoff and effectiveness of pollution control devices were Methodology Synthesis, Volume III--Availability and Documen- reviewed. tation of Published Information for Synthesis of Regional or National Highway-Runoff Quality Data. FHWA-EP-03-056, Prepared by the USGA for the Federal Highway Administration 3.1.6. Identification of Research Needs (2003) 71 pp. Strecker, E. W., Quigley, M. M., and B. R. Urbonas. A Reassess- Clearly there is a need to compile major syntheses of ment of the Expanded EPA/ASCE National BMP Database. stormwater runoff research, such as the major examples pre- Proc., National Conference on Urban Stormwater--Enhancing sented above and this current research effort. The NDAMS Programs at the Local Level, Chicago, IL (February 1720, 2003) effort, with the generation of the bibliographic database is a pp. 555573. good starting point for such a compilation of research litera- Strecker, E. W., Urbonas, B. R., Quigley, M. M., Howell, J., and T. Hesse. Urban Stormwater BMP Performance Monitoring: A ture. However, since only 252 references out of the 1,300 Guidance Manual for Meeting the National Stormwater BMP abstracts in the database were reviewed and classified, a Database Requirements. Prepared for the U.S. EPA and ASCE, more extensive effort appears to be needed. Also, the classi- (February 2002) 216 pp. fication of documents could be extended and refined to Strecker, E. W., Quigley, M. M., Urbonas, B. R., Jones, J. E., and include subcategories within the major categories. J. K. Clary. Determining Urban Stormwater Best Management The International Stormwater BMP Database currently Practice Removal Efficiencies. Final Data Exploration and Eval- contains primarily BMP design and monitoring data with no uation Report, ASCE/EPA Assistance Agreement Number CX direct link to published literature. A research project that 824555-01 (2000). attempts to link an extensive bibliographic database (such as a refined NDAMS database) to a water quality and BMP per- formance database (such as the International Stormwater 3.2. EVALUATION OF STORMWATER CONTROL FACILITIES AND PROGRAMS BMP Database) would result in a very useful tool for storm- water practitioners. Such a project would likely require the Over the past 30-plus years, stormwater researchers have participation of several state and federal agencies, as well as evaluated the performance of stormwater BMPs. These eval- private organizations, in order to produce a user-friendly uations have come in many forms and permutations. For database. However, this type of project could be limited only instance, some evaluations simply may investigate the pollu- to highway runoff-related studies, substantially reducing the tant removal effectiveness of a BMP by monitoring the influ- size of the final database and the costs associated with its ent and effluent concentrations or loads, or both, and com- development. paring results at the storm event, seasonal, or annual scales. More advanced evaluations have included attempting to associate performance with specific site conditions or design 3.1.7. Primary References variables, evaluating methods to improve pollutant removal Barrett, M. E., Charbeneau, R. J., Collins, E. R. III, Malina, J. F. Jr., in existing drainage systems, and characterizing the water Ward, G. H., and R.D. Zuber. A Review and Evaluation of Liter- quality achieved rather than the removals. Furthermore, some ature Pertaining to the Quantity and Control of Pollution from BMP evaluations may have looked beyond the pollutant Highway Runoff and Construction. CRWR Online Report 95-5. removal effectiveness through the use of surrogate perfor- Center for Research in Water Resources (1995) 186 pp. mance measures, such as the hydraulic regimes (hydraulic Caltrans. Caltrans New Technology Report. Report CTSW-RT- residence, bypass volume, etc.), retention of previously cap- 03-010 (February 2003) 130 pp. tured pollutants, maintenance requirements, or even biolog- Clark, S., Pitt, R., Field, R., Fan, E., Heaney, J., Wright, L., and ical indicators. S. Burian. Annotated Bibliography of Urban Wet Weather Flow Because of the increasing use of stormwater BMPs by Literature from 1996 through 2002. Compilation of Annual Literature Review Issues of Water Environment Research, www. state DOTs, the evaluation of stormwater control facilities epa.gov/ednnrmrl/repository/wef_lit_rvws/index.htm (2003) and programs will be an area of increasing interest for DOT 380 pp. stormwater managers. The survey of state DOTs revealed Granato, G. E., Zenone, C., and P. A. Cazenas (eds.). National High- that a large portion of transportation agencies across the way Runoff Water-Quality Data and Methodology Synthesis, country implement stormwater control practices. Table 3-5

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64 TABLE 3-5 Categories of stormwater control practices nity. Many state DOTs have evaluated BMP performance, but in use at state DOTs without agreed-upon methods and terminology, it is difficult Stormwater Number of States Number of States to compare meaningfully the evaluations. Quality Practices Using Indicating that The survey of state DOTs revealed that 15 of the respond- Used They Do Not Use ing DOTs have conducted studies or have prepared reports Temporary erosion 50 0 that evaluate the effectiveness and efficiency or performance, soil control or both, of source control or treatment control stormwater Permanent 40 10 management measures at DOT facilities. As more DOTs stormwater facility begin monitoring the effectiveness of their BMPs, more sta- Stormwater retrofit 17 33 tistical summarizations and comparisons of performance Stormwater 26 24 data should be completed; however, this will not be easy if monitoring different methods are used to evaluate and report BMP per- Water quality 35 15 formance. There is a need for reaching a consensus on stan- BMPs in operations and maintenance dard BMP performance measures and terminology. As discussed in Guidance Manual for BMP Monitoring, efficiency is a measure of how well a BMP or BMP system removes or controls pollutants, performance is a measure of shows the number of states that currently implement standard how well a BMP meets its goals for the stormwater the BMP stormwater control practices, and Table 3-6 shows the num- is designed to treat, and effectiveness is a measure of how ber of states that currently implement specific controls. well a BMP system meets its goals in relation to all storm- Of the nearly 900 documents and abstracts reviewed, the water flows (ASCE/EPA, 2002). In other words, performance project team identified more than 400 studies that generally and effectiveness are project and site specific, in that they are evaluated stormwater control facilities and programs. These measures relative to specific goals, such as meeting a prede- evaluative studies were further categorized according to the termined effluent quality. Efficiency, on the other hand, is an primary subtopic area of the study. As studies that evaluate absolute measure, independent of effluent quality expecta- different BMP types often have similar objectives, the pri- tions. Cost efficiency is not included in any of these mea- mary subtopic areas were based on the primary objectives of sures. However, a systems analysis approach to stormwater the study rather than on specific BMP types to assist in iden- management should consider capital investments and opera- tifying research gaps and needs with regard to BMP evalua- tion and maintenance costs when evaluating the overall effi- tions. This section is organized according to the following ciency of various treatment alternatives. Efficiency as defined primary topic areas: above will be the only measure discussed herein. In addition to clarifying differences in terminology as dis- General Evaluation, cussed above, the primary research questions with regard to Gross Pollutant Removal, measures of BMP performance appear to be Hydraulic Assessment, Pollutant Retention, What are the various measures of BMP performance? Methods to Improve Pollutant Removal in Existing What are the positive and negative attributes of each? Stormwater Systems, Erosion and Sediment Control, Design Variables, and 3.2.1.1. Historical BMP Efficiency Unit Processes. Calculation Methods Many publications reporting efficiency values do not 3.2.1. General BMP Evaluation provide adequate information (such as the equation or even the parameter values) to determine the method used. When Although there are several different ways to evaluate BMPs, removal efficiency calculation methods are reported, values the most common methods monitor the effluent and influent can be based on a number of different methods. Historically, water quality. The literature shows significant variation among the methods used to calculate BMP efficiency include the the methods used to collect and analyze such water-quality efficiency ratio (ER), summation of loads (SOL), regression data and make inferences about BMP performance. ASCE, of loads (ROL), mean concentration (MC), efficiency of in cooperation with the EPA, attempted to develop a standard individual storm loads (ISL), and reference watersheds and BMP performance monitoring protocol through the publica- before-and-after studies, plus various alterations of the afore- tion of Urban Stormwater BMP Performance Monitoring: A mentioned methods. The efficiency method used most often Guidance Manual for Meeting the National Stormwater BMP is the ER method, which has serious shortcomings, as do all Database Requirements (GeoSyntec Consultants, 2002). How- of the others listed. Recent research indicates that BMPs are ever, the methods and terminology recommended in the man- effluent-limited, and the effluent concentration of some ual have yet to be adopted widely by the stormwater commu- BMPs, including extended detention basins, has little to no

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65 TABLE 3-6 Stormwater control technologies in use at state DOTs* Innovative Technique or Number of States Number of States Technology Using Practice Not Using Practice Water quality inlets 16 32 Constructed wetlands 32 16 Grassed/vegetated swales and buffer 43 5 strips Wet ponds 31 17 Dry ponds 39 9 Wet vaults/tanks 9 38 Dry vaults/tanks 6 41 Porous/permeable pavement designs 5 42 Oil and water separators 27 21 Silt fences 31 16 Infiltration basin/trench 32 15 Sand filter 15 32 Low impact design 11 36 Hydrodynamic ultra-urban BMPs 9 38 Filtration ultra-urban BMPs 14 33 Natural stream channel design and 24 22 stabilization Herbicide alternatives for roadside 23 23 vegetation maintenance Gross solid separators (trash) 16 30 Dry weather diversion 9 38 Flocculating agents 10 37 *Not all state DOTs responded to every question dependence on the influent concentration. Therefore, effi- concentrations (EMCs) of several storms. By definition, the ciency calculation methods using influent concentrations ER is the ratio of the difference between the average EMCs tend to overestimate the efficiency of the BMP when influ- of the inlet and outlet to the average inlet EMC. It can be ent concentrations are high and to underestimate the effi- expressed mathematically as ciency when the influent concentrations are low. The para- graphs below describe briefly the most common and currently average outlet EMC ER = 1 - accepted methods used to calculate efficiency. A more com- average inlet EMC plete description of historically used BMP efficiency calcu- lation methods can be found in Guidance Manual for BMP Summation of Loads. The SOL efficiency calculation is used Monitoring (ASCE/EPA, 2002). often to evaluate long-term performance of a BMP. Summing loads over a period greater than the residence time reduces Efficiency Ratio Method. As indicated above, the ER method the inherent outlet independence to inlet concentrations for is the most commonly used method and is accepted by vari- detentionstorage BMPs. This method, recommended by the ous organizations including the EPA (U.S. EPA, 2002). As Bay Area Stormwater Management Agencies Association opposed to a storm-by-storm estimate, this method tends to (BASMAA, 1996), has been used by Texas DOT to evaluate minimize the influence of clean influent concentrations at the treatment effectiveness of the DOT's highway runoff underestimating BMP efficiency by averaging the event mean controls (Keblin et al., 1997). By definition, the SOL is equal

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66 to the difference in the sums of the inlet and outlet loads assumes temporal transferability of drainage characteristics. divided by the sum of the inlet loads over a specified time Because of site constraints or poorly defined inlet or outlet period. Individual loads are calculated by multiplying the structures (such as infiltration facilities), these two methods EMC by the entire flow volume of the storm. The SOL can are often the only methods available for analysis. Fairly be calculated mathematically as recent examples of reference drainage studies include those of Legret and Colandini (1999), where the effectiveness of sum of outlet loads porous pavement at removing heavy metals is evaluated, and SOL = 1 - sum of inlet loads of Sansalone (1999), where the effectiveness of a partial exfiltration trench (PET) on a highway shoulder is evaluated. Regression of Loads. In this method a least squares linear The major difficulty of this method is the large number of regression of the influent and effluent loads is conducted, parameters that need to be consistent between the two with the regression line constrained at the origin. Percent drainages in the reference watershed method or constant in removal then is defined as the compliment of the slope () of time in the before-and-after studies. the regression line, or mathematically as ROL = 1 - 3.2.1.2. Innovative Approaches and Variations of the Historical Approaches Over a large range of loadings, there is sufficient evidence to demonstrate that outlet concentrations are not correlated In the past few years of stormwater BMP data collection linearly to inlet concentrations. Therefore, this method is not and assessment, the shortcomings of the historical BMP effi- a recommended method. Endorsement of the ROL method ciency calculation approaches have become more apparent. could not be found during a review of current literature (i.e., Also, inconsistent use of the several available methods has within the last 5 years). led to a wide range of efficiency values for BMPs, as well as inappropriate transfer of data. In an effort to overcome some Mean Concentration. The MC method generally is not a of these shortcomings, several stormwater quality profes- recommended method; however, when flow-weighted data sionals have proposed alternative methods for calculating or storm volumes are not available (such as from grab sam- efficiency. Most of the newer methods are variations on the ples, which are required when sampling for oil and grease), historical approaches; however, some innovative methods this method may be of some value. Data transfer is not advis- able when this method is employed because of the general have been proposed. All of the more recently proposed meth- lack of storm information. The MC equation is identical to ods attempt to address the shortcomings of the historical the ER equation except that average outlet concentrations are approaches and to increase the transferability of BMP data. used instead of average EMCs. Thus, an inherent assumption During a review of the most recent literature on BMP per- of this method is that the grab sample is representative of a formance monitoring protocols, four promising alternative flow-weighted composite sample. methods were found. Efficiency of Individual Storm Loads. The ISL method is Effluent Probability Method. The effluent probability a ratio of the loads removed to the loads entering a BMP dur- method was the recommended method in the ASCE/EPA ing a single storm event. The mean ISL efficiency of several National Stormwater BMP Database Guidance Manual. This individual events is then considered to be the average effi- method evaluates statistically the influent and effluent EMCs ciency of the BMP. This method weights all storms equally to determine if the differences in concentrations are statisti- and does not account for pollutant storage and release during cally significant and, subsequently, to discover trends or successive storm events. The most serious shortcoming of characteristics in the two data sets by analyzing cumulative this method is the assumption of effluent quality dependence distribution functions or standard parallel probability plots. on influent quality, particularly when applied to detention Useful information, such as ranges in influent values that storage BMPs that have residence times greater than the storm yield the greatest percent removal, is provided by this method. event duration. Because of the relatively large amount of analytical infor- mation generated, as well as the relatively more complicated Reference Drainage and Before-and-After Studies. These analysis as compared to the historical approaches, BMP effi- two methods differ slightly; however, the inherent assump- ciency estimates may be more difficult to include in a user- tions are essentially the same--characteristics of the refer- friendly database. Instead of a single efficiency value, a range ence drainage, or the study drainage, before BMP imple- of values at specific influent conditions or graphical plots mentation are the same as the study drainage with the BMP. would need to be reported. Nonetheless, this method pro- The reference drainage method assumes spatial transferabil- vides the most innovative assessment of BMP efficiency and ity of drainage characteristics, and the before-and-after method has the potential to generate widely transferable data.

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67 Flow-Dependent Removal Efficiency Method. This method, can be used adequately to estimate the efficiency of BMPs which is a variation of the ER method, uses partial EMCs. without well-defined inlets and outlets, such as infiltration- Storm hydrographs are bracketed into storm sampling peri- type facilities or even source controls. As mentioned above, ods, during which flow-weighted composite samples are col- reference watersheds and before-and-after studies have been lected. Partial EMCs are then averaged according to average used to estimate infiltration performance. In other perfor- inflow rates. Therefore, instead of reporting a single effi- mance estimates, infiltration is considered to be 100% effi- ciency value, as in the ER method, the removal efficiency is cient at removing pollutants and is, therefore, based solely on calculated at various inflow rates. This method has been pro- the amount of stormwater infiltrated. This type of efficiency posed by the Environmental Technology Evaluation Center measure is appropriate for some pollutants such as suspended and David Evans and Associates (EvTEC and DEA, 2000) solids; however, it is inappropriate for highly mobile pollu- and endorsed by the Washington State Department of Envi- tants, particularly when groundwater resources are threatened. ronmental Quality (WADOE, 2002). A serious assumption Alternative estimates could be based on changes to ground- of this method is that the flow rate is essentially steady during water quality or in soil concentrations. For large infiltration the storm sampling period. Also, as with the ER approach, this operations, efficiency could be based on changes to local approach neglects the fact that outflow concentrations are groundwater quality. For small infiltration operations, such often independent of influent concentrations over the course as roadside ditches, calculating the percent removal before of a storm event. the introduction to the groundwater would be necessary. Changes in soil concentrations would provide an idea of the Minimum Influent Concentration. The concept of this pollutant removal; however, the transport and chemical trans- method is similar to the previous method, in that it is an attempt to discretize the efficiency according to influent con- formations of the pollutant of concern would need to be ditions. In this method, influent concentration, instead of assessed adequately. flow rate, is used to provide an efficiency ratio with a lower Based on the review of the many methods for evaluating limit on the influent quality. Specific guidance on this method and reporting the efficiency of BMPs, consensus and clear has not been found. However, it was referred to in Storm- guidance are needed. water Best Management Practice Demonstration Tier II Pro- tocol for Interstate Reciprocity as an appropriate BMP per- 3.2.1.4. Primary References formance claim (Tier II Protocol, 2001). For example, a stormwater BMP performance claim could be "The Model X GeoSyntec Consultants. Urban Stormwater BMP Performance Mon- system can capture and treat the first half-inch, 24-hour storm itoring: A Guidance Manual for Meeting the National Storm- for a 10-acre runoff area. Under these conditions, a TSS water BMP Database Requirements. American Society of Civil removal rate of 85%, 5% (at a 95% confidence interval), Engineers and U.S. Environmental Protection Agency (Febru- can be achieved with inflow TSS concentrations greater than ary 2002). 100 mg/L." Placing a lower limit on influent quality addresses EOA, Inc. Guidance for Monitoring the Effectiveness of Stormwater the fact that the efficiency of BMPs tends to decrease as Treatment Best Management Practices. BASMAA Report (1996). influent concentrations are reduced. EvTEC and DEA. EvTEC Evaluation Plan for Ultra-Urban Stormwater Technologies. WA Department of Transportation Pollutant Flux Ratio. This method is a variation of the SOL (March 2000). method with average event flow rates being used instead of Keblin, M. V., Barrett, M. E., Malina Jr., J. F., and R. J. Charbeneau. flow volumes. This modification results in a pollutant flux The Effectiveness of Permanent Highway Runoff Controls: Sedi- ratio instead of a loads ratio. This method likely would result mentation/Filtration Systems. CRWR Online Report 97-4, http:// in similar values as the SOL method, depending on how event www.ce.utexas.edu/centers/crwr/reports/online.html (1997). Legret, M., and V. Colandini. Effects of Porous Pavement with flow volumes are calculated. The WADOE guidance docu- Reservoir Structure on Runoff Water: Water Quality and Fate of ment does not provide any preference to either method Heavy Metals. Water Science and Technology, Vol. 39, No. 2 (WADOE, 2002). However, the appropriateness of summing (1999) pp.111117. flux values from individual storms is questionable. Sansalone, J. J. In Situ Performance of a Passive Treatment System for Metal Source Control. Water Science and Technology, Vol. 3.2.1.3. Identification of Research Gaps 39, No. 2 (1999) pp. 193200. Tier II Protocol. Stormwater Best Management Practices Demon- and Needs stration Tier II Protocol for Interstate Reciprocity. Endorsed by California, Massachusetts, New Jersey, Pennsylvania, and Vir- Of all the BMP efficiency calculation methods analyzed, ginia (August 2001). the most promising methods are the effluent probability and U.S. Environmental Protection Agency. ETV Verification Protocol the minimum influent concentration removal efficiency meth- Stormwater Source Area Treatment Technologies. Draft Report ods. The former provides greater detail of the actual perfor- 4.1, Washington, DC (March 2002). mance of a BMP. The latter provides an easier-to-understand Washington State Department of Ecology. Stormwater Treatment and transferable measure of BMP efficiency. Neither method Facility Performance Evaluation Guidance Document. Draft

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68 Report, Washington State Department of Environmental Quality Street-sweeping efficacy studies have been conducted by (April 2002). several researchers with variable results. One of the conclu- sions of the EPA-sponsored Nationwide Urban Runoff Pro- gram (NURP)--in which more than $30 million was expended 3.2.2. Gross Pollutant Removal in an intensive 3-year investigation of urban runoff quality The subtopic of gross pollutant removal generally refers to at 28 locations throughout the United States (U.S. EPA, stormwater treatment facilities or programs designed to reduce 1983)--was that street sweeping was generally an ineffective the amount of trash, debris, or large sediments discharged technique for improving the quality of urban runoff. Similar from constructed road surfaces or storm drain systems. Ero- results were reached by Pitt (1979) and Pitt and Shawley (1982). Nevertheless, recent studies suggest that street sweep- sion controls, which are designed to hold soils in place either ing programs can be optimized to reduce significantly pollu- temporarily during construction or permanently on cutfill tant washoff from urban streets (Sutherland and Jelen, 1997) slopes, will be addressed in a separate subsection. Some com- Lippner and Moeller (2000) conducted a paired watershed mon research questions with regard to gross pollutant removal study to evaluate how end-of-pipe litter discharges were include affected by street sweeping frequency and the type of sweeper used. The study included field-tests of vacuum, How effective are source controls--such as street sweep- regenerative air, and high-efficiency and mechanical broom ing, catch basin cleaning, and public education--at sweepers to determine which sweeper type would be most reducing trash and debris in transportation facility runoff? appropriate for sweeping frequency analysis. Results of the How effective are trash racks, screens, or other GSRD tests found that while the high-efficiency and regenerative air at removing and retaining bulk pollutants? sweepers left the pavement cleaner than the broom sweepers, large material often was lodged in the air intake hoses of Several state DOTs have participated in recent studies that regenerative air sweepers or was pushed in front of the suc- try to answer these research questions. Caltrans has taken a tion head of the air machines rather than being sucked up. lead role with impetus from new TMDLs for trash; however, Also, Caltrans was concerned that the maximum operating survey results showed only 16 DOTs use gross solid separa- speed of the high-efficiency sweeper precluded it from being tor devices. The surprisingly low implementation of this sim- used in freeway applications; thus, the agency chose a broom ple treatment technology indicates a potential need for edu- sweeper (Mobil model M-8A) for the sweeping frequency cation and outreach in this area. study. The analysis indicated that litter reduction from sweep- To investigate the characteristics of litter in freeway storm- ing monthly as compared to weekly was not statistically sig- water and the effectiveness of various BMPs, Caltrans con- nificant at the 95% confidence level. Analysis of conven- ducted a 2-year litter management pilot study in the Los tional water quality constituents such as metals, nutrients, oil Angeles area (Lippner et al., 2001). New litter sampling and and grease, TSS, and coliform bacteria showed that increas- monitoring protocols were devised to characterize litter and ing sweeping from monthly to weekly actually may have to test BMP effectiveness. Twenty-four freeway catchments increased the concentrations of hardness, total and dissolved were monitored. Half of the catchments were treated with copper, dissolved nickel, and total petroleum hydrocarbons one of five BMPs; the others were controls. Tested BMPs (diesel). The cause of this is unknown, however, it could be included increased street sweeping frequency [the results of due to the abrasive action of the sweeper on the road surface, which were included in Lippner and Moeller (2000), dis- the pollutant sorption ability of street litter no longer available cussed at the end of this section], increased frequency of once removed, or simply the random variability of the data. manual litter pick up, a modified drain inlet, a bicycle grate In another study, Smith (2002) evaluated the street sweep- inlet, and a litter inlet deflector developed during the study. ing effectiveness of mechanized street sweepers for particu- Litter discharges were quantified by weight, volume, and late removal. The first mechanized street sweeping had no count and were further classified by composition. Roughly observable effect on subsequent storm loads of suspended half the freeway stormwater litter was found to consist of sediment. Following the second sweeping, a net increase of paper, plastic, and Styrofoam. With the exception of cigarette the suspended-sediment load was observed at one station, butts, the origins of most litter items could not be identified and a net decrease of the suspended-sediment load was because of their small size. Of the five BMPs tested, only observed at the second station; however, these effects were increased litter pick up and the modified drain inlet demon- only temporary. The highway was swept a third time after strated some reduction of litter in stormwater runoff, although continuous monitoring was terminated. The particle-size dis- the data are highly variable. Increased frequency of sweeping, tribution in sweeper samples for the size fraction <4 mm in the bicycle grate, and the litter inlet deflector did not effec- diameter was similar to the particle-size distribution in bot- tively reduce litter in stormwater discharges monitored dur- tom sediment in the catch basin. The concentration of parti- ing the study, although the trash bags placed on the outfall to cles >0.5 mm in diameter was higher in sweeper samples evaluate what trash was leaving the system were effective. than in samples from the oilgrit separators, allowing for the

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69 conclusion that the sweepers were successful in removing the tion, monitoring, and assessment of the performance of each larger particles. device. The peak runoff generated by a 25-year storm event The Wisconsin DOT Bureau of Highway Operations con- was set as the minimum hydraulic design criteria for the pilot ducted a research project to study the effectiveness of a high- GSRD. Design criteria to address operation and maintenance efficiency street sweeper used on an urban freeway section concerns included adequate parking and access for mainte- to control the quality of stormwater runoff from the pave- nance and monitoring vehicles; no lane closures for servic- ment surface (Martinelli et al., 2002). The research process ing or monitoring a device; minimized shoulder closures for used a paired basin approach on a test section that was swept major device maintenance activities; maintenance equipment, once per week and on a control section that was not swept limited to equipment commonly available in the Caltrans during the study period. The results of the study indicated maintenance fleet; and an annual maintenance cycle for with a 90% confidence interval that there was a difference of removal of accumulated gross solids. 1% and 280% in suspended-sediment concentration (SSC) For this study, device effectiveness was defined as the per- between the control and test sites. This upper limit indicates centage of total litter captured by the device. The pilot GSRD that the control site may have had higher average baseline removed a combination of gross solids, including solids, veg- conditions than the test site, which is one of the problems etation, and litter. Removal efficiencies for gross solids ranged with paired watershed studies. from approximately 82 to 100% on a wet mass (weight) To eliminate the spatial variability, a before-and-after study basis and from approximately 55 to 100% on a wet volume could have been conducted (though this approach introduces basis. Removal efficiencies for litter ranged from approxi- temporal variability). Alternatively, a calibrated simulation mately 66 to 100% on a dry mass (weight) basis and from model--such as the Simplified Particulate Transport Model approximately 66 to 100% on a dry volume basis. (SIMPTM) developed by Sutherland and Jelen (1993) or the Key findings from this pilot study include the follow- Source Loading and Management Model (SLAMM) devel- ing: (1) GSRD are sensitive to gross solids loading rates; oped by Pitt and Voorhees (2002)--could have been used. (2) design loading rates must consider total gross solids These models have been calibrated and applied successfully (solids, vegetation, and litter), because the simple screening by some researchers to estimate loads and concentrations, as technologies used in these devices do not automatically seg- well as to evaluate the BMP effectiveness, including street regate the litter component regulated under the TMDL from sweeping. Please refer to section 3.2.10, BMP Modeling, for overall gross solids; (3) litter is a relatively small component descriptions of studies that have calibrated and successfully of gross solids on both a total mass and total volume basis; applied these models. (4) gross solids loading rates require further study to define Catch-basin cleaning is considered a source control BMP the average and range of expected values; (5) screen blind- designed to reduce the potential for stormwater bypass and ing, and subsequent bypass, is the most common cause for a resuspension of previously captured pollutants and subsequent device to exhibit a low level of effectiveness for litter removal; discharge to receiving waters. Dammel et al. (2001) conducted and (6) gross solids storage and screen blinding prevention must be considered individually during design. the Drain Inlet Cleaning Efficacy (DICE) Study for Caltrans Virtually every municipality and several state DOTs have to evaluate whether catch-basin cleaning improves the water public education and outreach programs that discourage lit- quality of highway stormwater runoff. The runoff water qual- tering. However, the effectiveness of this type of source con- ity was monitored and analyzed to determine any difference trol is difficult to evaluate and therefore done rarely. Caltrans in water quality between stormwater discharge from a drain- has embarked on an extensive Public Education Litter Mon- age system with clean drain inlets versus discharge from itoring Study (PELMS) to implement and assess a public unclean systems. Results from 4 years of monitoring have education program targeted at reducing stormwater litter pol- not indicated a statistically significant difference between lutants (Caltrans, 2002). Public education media rollout for cleaned and uncleaned catchments for all 21 monitored cases. the Public Education Resource Study (PERS) started in mid- The DICE Study is continuing with additional sampling sites February 2002, so the effectiveness of the program has yet to and with the sampling of litter and other macro debris from the be determined. Stormwater litter monitoring is one of several flow stream added to the list of monitored constituents. As methods that will be used to gauge public education effec- additional data become available, efforts will be made to deter- tiveness. Other methods include public opinion surveys and mine if cleaning drain inlets has a measurable impact on the an assessment of roadside litter collection before-and-after water quality of effluent emanating from Caltrans freeways. program implementation. Another Caltrans study tested three nonproprietary in-line devices that could be incorporated into existing or future highway drainage systems to remove trash from stormwater 3.2.2.1. Identification of Research Needs discharges, subsequently meeting the waste load allocation of the trash TMDL (Endicott et al., 2002). The pilot study Based on the literature review pertaining to gross pollutant included conceptual design of trash removal devices, site stormwater control facilities and programs, litter removal selection, development of device design criteria, construc- using solids separation devices has been demonstrated by a

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70 number of researchers, and the effectiveness of these devices Smith, K. P. Effectiveness of Three Best Management Practices for at removing large particles (>5 mm) is well documented. A Highway-Runoff Quality along the Southeast Expressway, Boston, smaller amount of literature is available on the effectiveness Massachusetts. Water-Resources Investigations Report, U.S. Geo- of source controls such as public education and catch-basin logical Survey (2002) 170 pp. Sutherland, R. C., and S. L. Jelen. Simplified Particulate Transport cleaning, particularly with regard to the overall effects of Model--Users Manual, Version 3.1. (1993). catch-basin bypass. Sutherland, R. C., and S. L. Jelen. Contrary to Conventional Wis- The effectiveness of street sweeping technologies has not dom: Street Sweeping Can Be an Effective BMP. Advances in been demonstrated clearly, even with numerous studies, but it Modeling the Management of Stormwater Impact, Vol. 5 (1997). appears that mechanical sweepers may be better at removing U.S. Environmental Protection Agency. Final Report of the Nation- larger pollutants, and air machines may be better at removing wide Urban Runoff Program. Water Planning Division, Wash- fine particulates. The likely cause for street sweeping studies ington, DC (1983). being inconclusive is that the overall reductions in runoff loads and concentrations caused by street sweeping are rela- tively small in comparison to the high degree of noise or vari- 3.2.3. Hydraulic Assessment ability of the data. Such noisy data require many more sam- One method of evaluating the performance and applica- ples to detect differences than are collected typically. Most bility of a stormwater control facility is to analyze the flow studies do not have the resources to collect and analyze this rates and volumes of stormwater into, within, and out of a many samples. facility. Hydraulic residence times and the volumes of over- One potential research need is identification of a uniform definition of gross solids (and the related components) for the flow, or bypasses, often are used as sizing criteria before purpose of standardizing data reporting. There is an ASCE/ BMP construction and as performance measures after con- EWRI committee working on this issue, but protocols devel- struction. The degree of short-circuiting, which is a function oped for highway situations may be appropriate to help stan- of BMP design, has a direct effect on the hydraulic residence dardize BMP performance. time within a stormwater BMP. The hydraulic conductivity, or flow-rate capacity, for flow-based BMPs and the design volume for detention-based BMPs have a direct effect on the 3.2.2.2. Primary References bypass or overflow volume for a given rainfall-runoff event. Assessment of these hydraulic phenomena sometimes is con- Caltrans. Caltrans Public Education Litter Monitoring Study sidered when evaluating the performance of individual BMPs. 20012002. Preliminary Report CTSW-RT-02-021 (2002). Other times, researchers have reported only the treated efflu- Dammel, E. E., Berger, B. J., Regenmorter, L. C., and G. S. Lippner. ent, but not the amount that was bypassed. In evaluating the Evaluating Drain Inlet Cleaning as a Storm Water Best Manage- overall performance of a stormwater management program ment Practice. International Water Association 5th International Conference, Milwaukee, WI (June 1015, 2001). within a watershed, which may include a combination of Endicott, J. D., Berger, B. J., and S. S. Stone. Design and Perfor- nonstructural source control and structural treatment control mance of Non-Proprietary Devices for Highway Runoff Litter BMPs, it often is desirable to evaluate the amount of water Removal. Global Solutions for Urban Drainage, 9th International stored and released slowly, evapotranspired, or infiltrated Conference on Urban Drainage (September 813, 2002). throughout the watershed. Thus, this distributed BMP approach Lippner, G. S., and G. Moeller. Study Quantifies Broom Sweeper was coined LID (see section 3.2.9). Litter Pickup Ability. American Sweeper, Vol. 8 (2000). BMP design guidelines and criteria frequently include Lippner, G. S., Johnston, J., Combs, S., Walter, K., and D. Marx. recommended or required hydraulic residence times. For Results of California Department of Transportation Litter Man- instance, the City of Portland BMP design manual specifies agement Pilot Study. In Transportation Research Record 1743, TRB, National Research Council, Washington, DC (2001) that the outlet of stormwater quality ponds be designed such pp. 1015. that the pond drains to the permanent pool volume in 12 hours Martinelli, T. J., Waschbusch, R., Bannerman, R., and A. Wisner. (Woodward-Clyde, 1995). For extended detention, which is Pollutant Loadings to Stormwater Run-Off from Highways: The believed to provide a higher level of treatment, the hydraulic Impact of a Sweeping Program. Wisconsin Department of Trans- residence time may be increased by 24 to 48 hours for deten- portation, Division of Transportation Infrastructure Develop- tion facilities. Hydraulic residence time also is sometimes a ment, Bureau of Highway Operations (2002) 94 pp. design criterion for other BMP types, such as vegetated Pitt, R. E. A Demonstration of Nonpoint Pollution Abatement swales, where it is recommended that stormwater be in con- through Improved Street Cleaning Practice. Final Report, U.S. tact with biofiltration media 59 minutes (Water Environ- Environmental Protection Agency (1979). Pitt, R. E., and G. Shawley. A Demonstration of Nonpoint Pollu- ment Foundation and ASCE, 1998). Despite these recom- tion Management on Castro Valley Creek. Report, Alameda mended design parameters, the relationship between BMP County Flood Control and Water Conservation District, Hay- performance and hydraulic residence times, as well as other ward, CA (1982). hydraulic characteristics of BMPs, is not understood clearly. Pitt, R., and J. Voorhees. http://www.winslamm.com (2002). As such, some of the common research questions with regard

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71 to the hydraulic assessment of individual stormwater control efficiency was impossible to measure directly because of the facilities include water loss problems. However, it was estimated that the over- all percent removal of TSS including bypass was 78%, as How do hydraulic residence times and bypass volumes compared to 98% when excluding bypass. relate to BMP performance? Keblin et al. (1997) studied the effects of bypass on the What design variables influence hydraulic residence removal efficiency of the Texas DOT's Seton Pond facility times and short-circuiting? in Austin, which includes a sedimentation basin and a sand What methods are available to evaluate, improve, and filter. Of the 10 storms that were monitored, the authors maintain stormwater infiltration? observed that 20% of the total runoff volume bypassed the What is the potential for storage and reuse of urban facility, resulting in the total TSS load removal efficiency stormwater? being reduced from 89% excluding bypass to 71% including bypass. 3.2.3.1. Hydraulic Residence and Bypass 3.2.3.2. Hydraulics of Infiltration Facilities A couple of studies have shown that typically the longer the hydraulic residence time, the better the overall pollutant Rather than maximizing residence time, infiltration facili- removal performance (Kulzer, 1989; Driscoll, 1986). With ties usually have the goal of maximizing percolation rates. regard to particulate settling, Galli (1992) noted that several Common types of infiltration facilities include porous pave- researchers have found that a large portion of suspended par- ment, infiltration basins and trenches, sand filters without an ticulates (3070%) settle out within the first 612 hours of underdrain, and PETs. Sand filters or other media filters that detention. Fine silt and clay-sized particles settle out over a have an underdrain are not considered infiltration facilities. much longer period, on the order of days and weeks. Infiltration practices are one of the most valuable urban storm- With regard to increasing the hydraulic residence within water BMPs, because they help to reduce not only storm- stormwater BMPs, Newberry and Yonge (1996) studied the water pollutants but also stormwater volume, which increases factors that influence the hydraulic residence on highway groundwater recharges and reduces the potential for scour and grass strips. They found that a change in flow rate has a bank erosion in receiving waters. Livingston (2000) provides greater effect on hydraulic residence than an equivalent per- a comprehensive review of the successes and failures of cent change in slope and that a lower degree of soil com- stormwater infiltration, a summary of the lessons learned paction would allow for more subsurface flows and a longer about the use of infiltration practices, and a list of recommen- average hydraulic detention time. A tracer study by Price and dations of when and how they should and should not be used. Yonge (1995) found that the installation of a baffle at the inlet The review of highway stormwater literature revealed sev- of a detention basin would reduce short-circuiting and increase eral studies that evaluated the hydraulics of porous pavement hydraulic residence. Increased sediment and adsorbed metals removal also were observed. (Nawang et al., 1993; Goforth et al., 1984; Dempsey and As a flood-control precaution or to protect the BMP, water Swisher, 2003; Bond et al., 1999; Pratt et al., 1995; Wada quality BMPs often are designed to bypass stormwater runoff et al., 1997; and Backstrom and Bergstrom, 2000). The that exceeds their design capacity. Sometimes, bypass occurs model simulation study by Wada et al. (1997) found that the unintentionally when filter media or inlet structures become construction of permeable pavements with infiltration pipes clogged or blocked. Ultra-urban BMPs--such as storm drain (a perforated pipe within a gravel bed beneath the pavement) inlet filters, oilgrit separators, and infiltration facilities--are significantly increased the percolation rate of the pavement. the most susceptible to bypass. The overall effect of bypass The study by Backstrom and Bergstrom (2000) that evalu- on BMP performance, and ultimately receiving water qual- ated the hydraulics of porous pavement in cold climates ity, is relatively unknown. Consequently, several BMP per- found that when porous asphalt was exposed alternatingly to formance evaluation protocols require an evaluation of BMP melting and freezing over a 2-day span (conditions similar to performance with the inclusion of bypass volumes. Very few the snowmelt period), the infiltration capacity was reduced studies were found that evaluated the effects of bypass on by approximately 90%. Based on the results of this study and BMP performance. previous studies, the infiltration capacity of porous asphalt A study by Greb et al. (1998) evaluated the effects of was estimated to be 15 mm/min for snowmelt conditions. bypass in a Stormceptor and a multi-chamber treatment These results have serious implications with regard to the use train (MCTT) installed in public works maintenance yards. of porous pavements in cold climate areas. For the Stormceptor, 11 out of 45 storms bypassed the unit; An increasing concern, especially with the implementation the total water volume that bypassed equaled approximately of EPA's UIC regulations is that if infiltration rates are too 9%. The reported percent removal including the bypass vol- high, many pollutants could be introduced to groundwater. ume was 22% for TSS, as compared to 25% when excluding This issue is addressed in section 3.4, Highway Runoff Char- the bypass volume. For the MCTT, the overall TSS removal acterization and Assessment.

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72 3.2.3.3. Identification of Research Needs Kulzer, L. Considerations for the Use of Wet Ponds for Water Qual- ity Enhancement. Office of Water Quality, Municipality of Met- Based on the literature review addressing the hydraulic ropolitan Seattle (1989) 90 pp. assessment of stormwater control facilities in relation to BMP Livingston, E. H. Lessons Learned About Successfully Using Infil- performance, the most pressing gaps appear to be in the eval- tration Practices. Proc., of National Conference on Tools for Urban Water Resource Management and Protection, Chicago, uation of the characteristics and effects of short-circuiting and IL (February 710, 2000) pp. 8196. bypass or overflow (e.g., ponds or wetlands discharging over Nawang, W. M., and S. Saad. Stormwater Infiltration Investigation the low-flow outlet or bioswales when depths and velocities Using Porous Pavement. Proc., 6th International Conference on for good treatment are exceeded). The influence of hydraulic Urban Storm Drainage, Niagara Falls, Canada, Vol. I, pp. 405410. residence time on BMP performance has been studied well, Newberry, G. P., and D. R. Yonge. Retardation of Heavy Metals in and it has been confirmed that detention time is correlated Stormwater Runoff by Highway Grass Strips. Washington State positively with pollutant removal (at least for particulate- Department of Transportation, Olympia, WA (1996) 81 pp. bound pollutants). However, no studies were found that inves- Pratt, C. J., Mantle, J. D. G., and P. A. Schofield. Research into the tigated the nature of the correlation (linearly, asymptoti- Performance of Permeable Pavement, Reservoir Structures in Controlling Stormwater Discharge Quantity and Quality. Proc., cally, and others). Also, hydraulic residence usually is 2nd International Conference on Innovative Technologies in calculated simply by dividing the permanent pool volume Urban Storm Drainage (1995) pp. 337344. by the average outflow discharge rate of a BMP. The true Price, F. A., and D. R. Yonge. Enhancing Contaminant Removal in hydraulic residence time depends on the flow path through Stormwater Detention Basins by Coagulation. In Transportation the system, which requires some means of estimating the Research Record 1483, TRB, National Research Council, Wash- velocity field of the system, such as the use of tracers, ultra- ington, DC (1995) pp. 105111. sensitive velocity meters, or two-and three-dimensional Wada, Y., Miura, H., Tada, R., and Y. Kodaka. Evaluation of an hydrodynamic models. Improvement in Runoff Control by Means of a Construction of an Infiltration Sewer Pipe under a Porous Asphalt Pavement. Water Science and Technology, Vol. 36(89) (1997) pp. 397402. Water Environment Foundation and American Society of Civil 3.2.3.4. Primary References. Engineers. Urban Runoff Quality Management. Manual and Report, WEF Manual No. 23; ASCE Manual, Report and Engi- Backstrom, M., and A. Bergstrom. Draining Function of Porous neering Practices No. 87 (1998). Asphalt During Snowmelt and Temporary Freezing. Canadian Woodward-Clyde. City of Portland Stormwater Quality Facili- Journal of Civil Engineering, Vol. 27(3) (2000) pp. 594598. ties--A Design Guidance Manual. Report, Environmental Ser- Bond, P. C., Pratt, C. J., and A. P. Newman. A Review of Storm- vices, City of Portland, Clean Water Works (1995). water Quantity and Quality Performance of Permeable Pave- ments in the UK. Proc., 8th International Conference on Urban Storm Drainage (1999) pp. 248255. 3.2.4. Pollutant Retention Dempsey, B. A., and D. M. Swisher. Evaluation of Porous Pave- ment and Infiltration in Centre County, Pennsylvania. Water Pollution retention is another import criterion for evaluating World & Environmental Resources Congress 2003, Philadelphia, the performance of stormwater quality control facilities. Dur- PA (June 2003). ing large storm events, pollutants may be flushed out of sedi- Driscoll, E. D. Detention and Retention Controls for Urban Runoff. mentation systems, particularly in-line systems such as catch In Urban Runoff Quality: Impact and Quality Enhancement basin sumps, and be discharged inadvertently into receiving Technology, ASCE (1986) pp. 381393. waters. Changes in water chemistry also may have an effect on Galli, J. Analysis of Urban BMP Performance and Longevity in pollutant mobility. For instance a decrease in pH or a change in Prince George's County, Maryland. Final Report, Prince George's oxidation-reduction potential, or both, may cause solid-phase County Department of Environmental Resources, Watershed Pro- pollutants to become soluble, and therefore mobile. Dry tection Branch, MD (1992) 203 pp. weather flows into BMPs may have a different chemistry, or the Goforth, G. F., Diniz, E. V., and J. B. Rauhut. Stormwater Hydro- BMP, through biochemical processes, may alter water chem- logical Characteristics of Porous and Conventional Paving Sys- tems. U.S. Environmental Protection Agency, Washington, DC istry to the point that pollutants are released and remobilized. (1984) 302 pp. Resuspension of pollutants within stormwater BMPs can cause Greb, S. R., Corsi, S., and R. Waschbusch. Evaluation of Storm- the BMPs to be a source of pollutants, which may translate into ceptor and Multi-Chamber Treatment Train as Urban Retrofit negative percent removals in BMP evaluation studies if mobi- Strategies. Proc., National Conference on Retrofit Opportunities lization occurs during storm events. If pollutants are mobilized for Water Resource Protection in Urban Environments, Chicago, via dry weather flows, BMP studies that focus on stormwater IL (February 912, 1998). event monitoring alone would not detect this. Keblin, M. V., Barrett, M. E., Malina Jr., J. F., and R. J. Charbeneau. Potential research questions with regard to pollutant reten- The Effectiveness of Permanent Highway Runoff Controls: tion are Sedimentation/Filtration Systems. CRWR Online Report 97-4, Center for Research in Water Resources, Bureau of Engineering What is the potential for resuspension of previously cap- Research, The University of Texas at Austin, (1997) 126 pp. tured sediment?

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73 What conditions influence pollutant mobility in BMP 43.2 cm, respectively). Similarly, sediment accumulations in systems, and how can these conditions be reduced? the Stormceptor and Vortechs units were monitored. The What need is there for more continuous monitoring of Stormceptor unit showed consistent accumulation, except wet BMPs to assess the potential for pollutant remobi- 10.2 cm were lost during the large 87.6 mm rainfall event. lization between storm events? The Vortechs unit did not show sediment accumulation dur- How sequestered are captured pollutants in BMPs? ing the study, which was attributed to the unit not being installed properly. Smith (2002) investigated the sediment retention of oilgrit Because of the increased use of porous pavement systems separators and a deep-sumped catch basin. Despite the pres- in LID designs, the pollutant retention capacity of porous ence of bypass pipes at the inflows to the separators and the pavement is of particular interest to stormwater managers. A fact that the depth of bottom sediment retained in the catch study by Dierkes et al. (2002) investigated the pollutant reten- basin was less than 25% of the sump depth, previously cap- tion capabilities of four different systems of paving stones: tured sediments from the separators and the catch basin sump pavers with infiltration joints, porous concrete pavers with a were resuspended during several monitored storm events. filter-layer, greened (grass) porous pavers, and pavers with For the separators, resuspension of sediments was detected at greened infiltration joints. All four systems showed very high and above rainfall intensities of 0.04 in. per 5-min interval pollution retention capacities for cadmium, copper, lead and and flows >0.46 ft3/s. The amount of resuspended sediment zinc, but the greened systems and the porous pavers were estimated for the separators represented about 8% of the more efficient than the system with the infiltration joints. suspended-sediment loads retained at the end of the monitor- Copper and lead were retained more effectively than cad- ing period. The estimated quantity of suspended sediment mium and zinc in all of the pavement systems. In another that bypassed the separators was 1620% higher than the study, the pollutant retention of the subbase of porous con- amount of sediment resuspended (<0.062 mm in diameter). crete pavers was investigated. Differences in pollution reten- For the catch basin sump, the frequency of cases in which tion capacities between the subbase materials existed, with resuspension was detected did not increase with an increase the highest pollutant retention capacities being reached by in captured sediment. The estimated amount of resuspended crushed stones with high contents of CaCO3. Overall, the pH sediment represented 18% of the final mass of retained in the porous concrete effluent of all system configurations sediment in the sump. showed that the buffering capacities of concrete are very Results of experiments conducted by Clark et al. (2001) to high, so there is little danger of a mobilization of previously determine if four potential filter media (sand, activated car- captured metals from porous concrete paving systems. bon, peat moss, and compost) could retain previously trapped pollutants indicated that permanent retention of heavy met- als (copper, lead, iron, and zinc) may occur even in an anaer- 3.2.4.1. Identification of Research Needs obic environment. However, retention of nutrients may not occur under these conditions. When evaluating the performance of stormwater control In a BMP performance study by Yu and Stopinski (2001), facilities, it is important to consider not only the pollutant four ultra-urban BMPs--three oilgrit separators (Isoilater, removal capacity under a variety of hydrological and influ- Stormceptor, and Vortechs Stormwater Treatment System) ent quality conditions but also the pollutant retention capac- and a bioretention area--were evaluated. Monitoring results ity over long time periods and under both storm and low-flow indicated that resuspension of sediment from the bioretention conditions. Few studies were found that investigated the occurred during three of the larger monitored storm events, potential for leaching or resuspension of previously captured presumably because of minimal vegetation establishment pollutants. The studies that were found indicate that resus- before the study's onset. Negative removals for total nitro- pension of sediments in catch basin sumps and oilgrit sep- gen also were observed in three events of the Stormceptor arators may be significant. Resuspension also may occur in monitoring. However, these events did not correspond to bioretention areas before complete establishment of vegeta- large events. In fact, the largest negative removal occurred tion. Heavy metals do not appear to go easily into the dis- during the smallest storm event. The authors hypothesized solved phase once captured, but nutrients do, particularly if that the negative total nitrogen removals were due to a there is a change in the oxidation-reduction potential. The pH decrease in the amount of aeration inside the BMP, which of the stormwater likely has some effect on the solubility of would limit the oxidation of ammonia. Analysis of accumu- captured metals; however, concrete and other construction lated sediment depths in the oilgrit separators showed that materials containing high concentrations of CaCO3 have a the Isoilator unit lost captured sediment during 5 out of 15 high buffering capacity and tend to raise the pH of storm- storm events, with the highest loss of sediment (21.8 cm) water on contact. Therefore, a slight decrease in the pH of occurring during the largest monitored rain event (87.6 mm rainwater is not expected to cause a substantial increase in on 3/31/00). The observed sediment depth never reached the dissolved metals concentrations, especially if the stormwater manufacturer's recommended clean-out depth (34.5 cm and flows over or through porous concrete.

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83 State DOT, Maryland State Highway Administration, Virginia Stormwater-Enhancing Programs at the Local Level, Chicago, DOT, and Caltrans expressed interest in piloting LID technol- IL (February 1720, 2003) pp. 497501. ogy because of its potential for addressing the escalating envi- Rushton, B. T. Low-Impact Parking Lot Design Reduces Runoff ronmental requirements that are projected. and Pollutant Loads. Journal of Water Resources Planning and NCHRP Project 25-26 will make recommendations on Management, Vol. 127, No. 3 (June 2001) pp. 172179. U.S. Environmental Protection Agency. Low Impact Development modeling programs that can simulate effectively LID sys- (LID): A Literature Review. Report EPA-841-B-00-005, Office tems and will develop an applicable model. Washington of Water (October 2000) 41 pp. State DOT has an insufficient amount of funding to update its MGSFlood continuous model to allow it to route water to structures in series. These structures could be modeled as 3.2.10. BMP Modeling leaky, where water losses can be incorporated. As time pro- gresses, LID practices can be input as significant leaky struc- Software modeling tools have become a vital part of storm- tures, but future field evaluations will be needed to quantify water management. The complexity of BMP models ranges accurately the water losses in LID systems. This will take a from simple spreadsheet calculations to multitiered combi- significant amount of time and money. The SeaStreets pro- nations of sophisticated models. Problems and limitations ject in Seattle found an approximate reduction of 40% in the that need to be overcome in this area include a lack of volume of runoff, which could be incorporated into assess- consistent BMP monitoring and performance data and a ments of BMP performance. better understanding of BMP treatment processes. Current The Friends of the Rappahannock and the Low Impact sources of data used in modeling applications include the Development Center (www.lowimpactdevelopment.org) are NWS, EPA, USGS, and the USDA. The monitoring and per- developing guidance and strategies for rural communities in formance data limitations encountered in the modeling of Virginia to incorporate LID into their local resource protec- BMPs could be overcome through the growth and the evolu- tion and regulatory programs (Weinstein and Tippett, 2003). tion of the ASCE/EPA BMP database. As more and more The first part of this effort includes evaluating state and local studies are added to the BMP database, BMP properties in codes to determine what, if any, necessary legislative, code, relation to various external variables can be determined more or local regulations need to be modified to include LID. Iden- accurately using statistical analysis and other mathematical tifying areas in the town and land uses that are appropriate techniques. Areas of interest and possible knowledge gaps for LID technologies follow this effort. The next step will be with respect to BMP modeling include to develop materials for developers and plan reviewers to help guide them through the development process when the Unit processes in BMPs (sedimentation, infiltration, use of LID is appropriate. The final step will be to design and absorption, adsorption, biodegradation, photolysis, vola- implement a small demonstration project that showcases LID tilization, etc.); features, such as rain gardens, soil amendments, permeable Prediction of BMP longevity and BMP effectiveness as pavers, and infiltration devices. a function of time; Modeling of BMP treatment trains; Modeling of distributed BMP systems such as LID; 3.2.9.1. Identification of Research Needs Development of stochastic BMP performance models; and Pilot projects conducted by several researchers have demon- Factoring the effects of maintenance on BMP perfor- strated the potential of LID to meet regulatory requirements, mance. but substantial work needs to be conducted on developing LID design strategies, performance standards, and specifica- Spreadsheets are simple programs that are used widely by tions. LID's decentralized approach to stormwater manage- engineers for a variety of technical applications. Numerous ment technology has tremendous potential to supplement or spreadsheet models have been created for water quality and in some situations to replace completely conventional cen- hydraulic models and, more recently, for BMP modeling. tralized stormwater BMP approaches; however, LID's appli- Hayes et al. (2003) discuss the application of the Integrated cability, efficacy, and long-term economic sustainability have Design and Evaluation Assessment of Loadings (IDEAL) yet to be determined or documented for transportation sys- model to BMP design and evaluation issues. IDEAL is a tems. One long-term research need is documenting the type process-based stochastic spreadsheet model that is capable of of hydrologic losses that can be achieved via LID regionally predicting and routing runoff and pollutant loadings. IDEAL and under various soil, slope, and vegetation conditions. is limited in the number of pollutants that can be simulated satisfactorily, though it can model vegetative strips, dry deten- 3.2.9.2. Primary References tion ponds, and wet detention ponds. New models are sometimes created to tackle new issues. Weinstein, N., and J. Tippett. Low Impact Development Strategies Likewise, existing models are often modified, updated, and for Rural Communities. Proc., National Conference on Urban enhanced to address new problems or to take advantage of

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84 new technologies and more efficient algorithms. SWMM, in Detention systems and infiltration systems appear to be the use since 1970, has been used primarily for hydrologic and most extensively studied BMPs in terms of BMP modeling. hydraulic modeling; however, the increasing emphasis on There are numerous studies that have used existing software water quality and environmental regulation compliance has packages like SWMM, HPSF, and BASINS to model deten- been a driver for the development of new tools and the tion ponds. Boyd et al. (1994) discuss the use of MOUSE in enhancement of old ones. Huber (1996) presented a discus- on-site detention design in the City of Wollongong, Aus- sion about the use of SWMM in BMP modeling, including a tralia. Wu and Ahlert (1985) discuss a trajectory model that discussion of model enhancements, and a case study to illus- is used to investigate sedimentation processes in detention trate the use of the model. According to Baxter (2002), both ponds. The model used a normal distribution of sediment SWMM and the Better Assessment Science Integrating Point particle sizes to calculate sediment trapping efficiencies for and Non-Point Sources (BASINS) have BMP modeling various length-to-width ratios. Wong et al. (1996) discussed capabilities. SWMM is particularly proficient at predicting the use of the P8 Urban Catchment Model and dynamic pro- pollutant loads. BASINS is based on an extensible, open gramming in detention pond design. The methodology was architecture. Integration with geographic information sys- applied to the Marley Creek watershed to obtain the most tems (GIS) enhances the visualization of input data and model economical system of detention ponds that would meet water results. BASINS version 3 includes PLOAD, HSPF, and quality and flow goals set for the watershed. The Detention SWAT, all of which are all capable of modeling various BMPs. Outlet Channel Dynamic Program (DOCP) is an optimiza- PLOAD is a nonpoint-source loading model. The HPSF BMP tion model that helps determine least-cost locations and sizes module can interface with an Access database of 34 standard of detention basins. Bennett (1983) demonstrates the capa- BMPs, including detention ponds, infiltration systems, and bilities of DOCP in an application of the model to the Brays manufactured systems. HSPF also allows custom BMPs to Bayou Watershed in Houston. In a study presented by Lam be specified and modeled. SWAT is capable of simulating a and Palmer (1996), two existing detention facilities that variety of agricultural practices, including tillage and pesti- were constructed originally for flood-control purposes were cide application. retrofitted to provide water quality treatment. The sediment GIS is rapidly becoming an indispensable tool in storm- removal capabilities of the retrofitted ponds were analyzed water management. In the past, GIS packages were used using QUALHYMO and STORM. Dynamic wave routing mostly for the post-processing of model output, mainly as a was accomplished with OTTHYMO, QUALHYMO, and visualization tool. However, a number of researchers are BOSS-DAMBRK. Petterson et al. (1998) used data from an beginning to integrate stormwater modeling into GIS, taking open stormwater detention pond to verify the FEM model. advantage of the inherent libraries, routines, and underlying The authors found that the 2-dimensional and 3-dimensional programming interfaces of GIS packages. Xue et al. (1996) analysis was in agreement with the observed data. created a mechanism-based BMP model and successfully Modeling infiltration systems has been an active area of linked the model to Arcview 2.1 using ArcView's built-in study. James et al. (1997) present a discussion of the use of macro language AVENUE. The integrated model had a user- SWMM and HSPF shallow groundwater routines as a foun- friendly interface, and a sample simulation was provided to dation for developing alternate approaches to infiltration BMP illustrate the functionality of the tool. modeling. To gain a better understanding of the clogging Melancon et al. (2000) outline the application of a GIS- phenomena of infiltration BMPs, Gautier et al. (1999) inves- based BMP model to simulate the use of BMPs in the mitiga- tigated an infiltration basin and two groundwater recharge tion of bacteria-contaminated runoff. The model was devel- basins. Other studies that have made attempts to model porous oped using Arc/Info and ArcView GIS software packages. pavement structures include Loughreit et al. (1996) and Results from the model were used to determine the source of Goforth (1983). Morita et al. (1996) describe a conjunctive bacteria loads, and the model was found to be capable of esti- flow model that overcomes some of the simplifying assump- mating flow and load conditions with reasonable accuracy. tions made in the development of other models. The model BMP models are used mainly in the context of water qual- accounts for the interaction between surface flow and sub- ity or flood control design; however, there are models that surface flow, and the authors provide an example to demon- incorporate additional optimization parameters such as cost. strate the capabilities of the model. Debo (1994) discuss the Heatwole et al. (1985) present a model capable of analyzing development of a model used in the design and analysis of the cost and water quality implications of selectively apply- infiltration basins, infiltration trenches, dry wells, porous ing various BMPs throughout a basin and comparing differ- pavement, and vegetated swale with check dams. ent scenarios. Using the model, the authors discovered that Bishop and Scheckenberger (1994) describe the use of the cost for the maximum level of BMP treatment was four HPS-F in the design of a constructed wetland, which was to times as high as the cost for a 90% improvement in water serve as a BMP to mitigate runoff for a proposed freeway quality using the four most economical BMPs. interchange. The HPS-F analysis provided information that

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85 was used in the bathymetric design and also the plant species shown the pollutant reductions in outfall discharges that the distribution for the wetland. modeling of this type has--a result that is likely due to the SLAMM, a recently calibrated urban runoff model, was pollutant source assumption. used to compare the cost-effectiveness of using combina- tions of source area and regional stormwater treatment prac- tices (Bannerman et al., 2003). Model results indicated that 3.2.10.1. Identification of Research Needs individually the Delaware Perimeter Sand Filter, Stormcep- With regard to the modeling of BMP unit processes, sedi- tor, Multi-Chamber Treatment Tank, bioretention, porous mentation and infiltration appear to be well covered in the lit- pavement, and infiltration trenches could reduce the solids erature. However, other BMP water quality treatment unit load to Lake Wingra by 7 to 19% and that high-efficiency processes--such as sorption processes (absorption and adsorp- street sweeping could reduce annual solids load by 17%. By tion), biodegradation, photolysis, and volatilization--still modeling various street sweepingtreatment control prac- need to be studied further before reliable BMP performance tices, it was found that nine different combinations would be models can be developed. There also is a lack of information able to achieve the 40% reduction goal. For example, a 42% on the modeling of BMP treatment trains. A better under- reduction in solids load to Lake Wingra is estimated for the standing of BMP longevity and of the decrease in treatment combination of high-efficiency street sweeping on all the efficiency as a function of time is required, so that the opti- streets and Delaware Perimeter Sand Filters on all the park- mization models used in selecting cost-effective BMP systems ing lots. Alternatively, the 40% reduction could be achieved can provide better estimates of BMP lifetime costs and bene- by using regional detention ponds with a total of 20 acres of fits. Another area that could be explored addresses how the permanent pool area. However, it was estimated that the sources of pollutants are represented in models. Many models annual cost of the source area practices range from about still use a build-upwash-off approach as the only way the pol- $573,000 to $1,504,000, while the range for the detention lutants get into stormwater; however, this approach should be ponds is $963,000 to $1,840,000, assuming a 20-year life used with caution as it can lead to faulty results if the BMP span. The least expensive combination of source area prac- acts directly on that function. tices would only increase the annual stormwater utility bill With any attempt to predict or model environmental for the Madison taxpayers by about $6, while the most likely processes, there is a general need for accurate and represen- detention pond alternative will increase the utility bills by tative data for parameter estimation and model calibration. about $18. Thus, the ability to measure and analyze accurately unit treat- Through the use of the SIMPTM computer simulation, ment processes is essential for the development of reliable Kurahashi and Associates (1997) evaluated the effectiveness models that can evaluate or predict BMP performance. As of new high-efficiency pavement sweepers in combination Sansalone (2000) stated in his TRB Millennium Paper, ". . . with conventional sediment-trapping catch basins to determine the future for ecologically sustainable transportation will if the combination technology provided pollutant-reduction require the ability to gather sufficient temporal and spatial benefits that were comparable to those of wet vaults. The measurements for increasingly sophisticated and integrated results of the simulation study showed that pollutant removals hydrologic, hydraulic, and water quality treatment models." obtained with high-efficiency sweeping at a weekly frequency The development of a review of modeling approaches and in combination with normal catch basin inlets cleaned annu- guidance on their selection and application is a potential ally are comparable to removals obtained by wet vaults. How- research topic. ever, the model assumes that all of the sources of pollutants can be described by a build-upwash-off function, which is not true. Therefore, the findings regarding a BMP that acts 3.2.10.2. Primary References on this function can overstate grossly the BMP performance. In fact, high-efficiency sweeping appears to be more effec- Bannerman, R., Fries, G., and J. Horwatich. Source Area and tive than wet vaults in the removal of highly dissolved pol- Regional Stormwater Treatment Practices: Options for Achiev- lutants (copper, zinc, and phosphorus), but wet vaults appear ing Phase II Retrofit Requirements in Wisconsin. Proc., of the more effective than high-efficiency sweeping in the removal National Conference on Urban Stormwater-Enhancing Pro- of TSS and sediment-bound pollutants such as lead. The use grams at the Local Level, Chicago, IL (February 1720, 2003) pp. 1219. of high-efficiency pavement sweepers in combination with Baxter, R. Modeling Tools for the Stormwater Manager: An conventional sediment-trapping catch basins would result in Overview of EPA's Widely Used Modeling and Assessment substantial savings for the Port of Seattle compared to the use Tools. Stormwater (MarchApril 2002) www.forester.net/sw_ of wet vaults (estimated lifecycle costs of $2 million for 0203_modeling.html high-efficiency sweepers in combination with conventional Bennett, M. S. Dynamic Programming Model for Determining sediment-trapping catch basins versus $18 million for wet Optimal Sizes and Locations of Detention Storage Facilities. vaults) if their treatment were equal. Street sweeping has never Proc., from the University of Kentucky Urban Hydrology,

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86 Hydraulics and Sediment Control Symposium (July 1983) pp. tee on Hydrology, Hydraulics, and Water Quality, TRB, National 461468. Research Council, Washington, DC (2000). Bishop and Scheckenberger. HSP-F Simulation of a Constructed Wong, K. S., Schaeffer K., and T. Tapley. A Dynamic Program- Wetland Stormwater BMP for Urban Highway Runoff. Proc., ming Approach to Stormwater Management Systems Design. 1994 Stormwater and Water Quality Management Modeling Con- Proc., Watershed 1996 (1996) pp. 435439. ference (March 1994). Wu, J., and R. C. Ahlert. A Trajectory Model for Analyzing Sedi- Boyd, M. J., Carr, R., and G. Mackintosh. Modeling On-site Deten- ment Trapping Efficiencies in Stormwater Detention Basins. tion Storage in an Urban Catchment Using MOUSE. Proc., Inte- Proc., U.S. EPA Sanctioned Stormwater and Water Quality Man- grated Urban Storm Runoff-7th European Junior Scientist Work- agement Modeling Conference, Toronto, ON (December 1985) shop (June 1994) pp. 173178. pp. 257284. Debo, T. N. Computer Model for Infiltration System Design. Com- Xue, R. Z., Bechtel, T. J., and Z. Chen. Developing a User-Friendly puting in Civil Engineering, American Society of Civil Engi- Tool for BMP Assessment Model Using a Geographic Informa- neers, New York, NY (1994) pp. 241248. tion System. Proc., Symposium on GIS and Water Resources, Gautier, A., Barraud, S., and J. P. Bardin. An Approach to the Char- Fort Lauderdale, FL (September 1996). acterization and Modeling of Clogging in Stormwater Infiltration Facilities. Proc., 8th International Conference on Urban Storm Drainage (August 1999) pp. 10071015. 3.2.11. Maintenance and Longevity Goforth, G. An Advancement in Hydraulic Modeling of Porous Pavement Facilities. U.S. Environmental Protection Agency (Jan- Twelve state DOTs have conducted studies and prepared uary 1983) pp. 237254. reports on the maintenance aspects of stormwater management Hayes, J. C., Barfield, B. K., Holbrook, F., Gillespie, J., Fersner, J., measures during construction, as well as at DOT facilities. and B. Bates. A Model for Assessing the Impact of BMPs on A review of the literature pertinent to the role of mainte- Water Quality. Stormwater (SeptemberOctober 2003). nance in BMP performance and longevity shows that Heatwole, C. D., Bottcher, A. B., and L. B. Baldwin. A Model for numerous studies have attempted to link BMP performance Assessing the Cost-Effectiveness of Agricultural BMP Imple- and longevity to maintenance practices. All BMPs, both pro- mentation Programs on Two Florida Basins. U.S. Environmen- prietary and nonproprietary, require regular and nonroutine tal Protection Agency (January 1985) pp. 257264. maintenance in order to perform well. The frequency and Huber, W. C. BMP Simulation Using the U.S. EPA Stormwater Management Model (SWMM). Proc., 7th International Confer- extent of maintenance depend on pollutant loading and the ence on Urban Storm Drainage (September 1996) pp. 16291634. availability of pretreatment. Regular maintenance activities James, W., and J. A. Ulan. Towards a Shallow Groundwater Routine include removing accumulated materials and cleaning inlets for Modeling Infiltration BMPs in Urban Stormwater Models. and outlets. Nonroutine maintenance may include structural Advances in Modeling the Management of Stormwater Impact, repairs and revegetation (Livingston et al. 2000). The Vol. 6 (February 1997). Watershed Management Institute, in cooperation with the Kurahashi and Associates, Inc. Port of Seattle-Stormwater Treat- EPA, published the document Operation, Maintenance, and ment BMP Evaluation. Port of Seattle, Pier 66 (1997). Management of Stormwater Systems that includes guidance Lam, A. S., and R. M. Palmer. Modeling Retrofitted Extended- on BMP maintenance practices and costs as well as design Detention Wet Ponds and Wetland Pockets. Proc., 1996 Storm- information (Livingston et al., 1997). This document is one water and Water Quality Management Modeling Conference of the most comprehensive BMP maintenance guidance (February 1996) pp. 407428. documents available. However, because of the large number Loughreit, F., Barraud, S., Cres, F. N., and E. Alfakih. A Concep- tual Model for the Design and Simulation of Porous Pavements. of new and innovative BMPs, as well as variations to exist- Proc., 7th International Conference on Urban Storm Drainage ing BMPs, there have been several more case studies since (September 1996) pp. 533538. its publication. Therefore, the topic area of BMP mainte- Melancon, P. A., Maidment, D. R., and M. E. Barrett. Modeling nance practices and costs is in need of further research. Non-Point Source Pollution and the Impact of Best Management Areas of interest and possible knowledge gaps related to the Practices Using a Geographic Information System. Proc., Water- effects of maintenance on BMP performance and longevity shed 2000 Management Conference (July 2000). include Morita, M., Nishikawa, R., and B. C. Yen. Application of Con- junctive Surface-Subsurface Flow Model to Infiltration Trench. Recommended frequency of maintenance for various Proc., 7th International Conference on Urban Storm Drainage BMPs, (September 1996) pp. 527532. Determination of the most cost-effective maintenance Petterson, T. J. R., German, J., and G. Svensson. Modeling of Flow activities, Pattern and Particle Removal in an Open Stormwater Detention Maintenance requirements for natural and constructed Pond. Hydra Storm 1998 Proceedings, Combining 3rd Interna- tional Symposium on Stormwater Management and 6th Interna- wetlands, tional Conference on Hydraulics in Civil Engineering (Septem- Cost and benefit analysis of BMP maintenance, ber 1998) pp. 6370. Sediment toxicity as a function of type and frequency of Sansalone, J. J. The Role of Water in Ecologically Sustainable Trans- maintenance, and portation. In Transportation in the New Millennium, Commit- Disposal of maintenance waste products such as sediment.

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87 The current state of maintenance practices of various munic- that in most cases infiltration BMP performance can be ipalities have been the subject of a number of studies. An improved significantly with maintenance. A comparison of extensive survey of more than 800 stormwater structures was infiltration facilities that had been in use for up to 10 years conducted in four North Carolina cities in an effort to evaluate showed a decline of infiltration capacity with time. To evalu- stormwater maintenance practices and needs (Roenigk et al., ate the effects of washing and cleaning, infiltration facilities in 1992). Culverts, inlet devices, channels, detention ponds, wet four municipalities (Tokyo, Chiba, Nagoya, and Hamamatus) ponds, and infiltration systems were evaluated. Stormwater that were more than 10 years old were maintained and officials in 88 North Carolina cities were interviewed on the assessed. With the exception of one infiltration trench in phone about maintenance issues. The results of the study Nagoya, all of the infiltration facilities showed a marked indicated that maintenance practices were adequate, with the improvement in infiltration capacity. exception of detention facility maintenance. Most of the A study of four porous pavement systems, including pavers surveyed systems were designed to operate primarily at with infiltration joints, porous concrete pavers with filter lay- flood control capacities; hence, maintenance requirements ers, greened porous pavers, and pavers with greened infiltra- are expected to increase with the increasing prominence of tion joints was performed to determine the pollutant retention water quality issues. abilities of the various systems (Dierkes et al., 2002). All four The phone interviews revealed that the maintenance activ- systems demonstrated high pollutant retention capabilities, ities performed, in order of frequency from highest to low- but the system with infiltration joints was relatively less effi- est, were mowing, inlet cleaning, facilities inspections, and cient. Field evaluation of a 15-year-old piece of porous pave- then sediment removal at detention facilities. The final com- ment revealed no impact to soil or groundwater. A device to ponent of the study was conducting interviews with 25 storm- alleviate clogging in porous pavement was tested successfully water management experts, who all agreed on the importance in a school yard. As a result of the cleaning, infiltration rates of inspections, mowing, and sediment removal, but who did were increased from 1L/(s.ha) to 1500 L/(s.ha). Researchers not agree as to how frequently these maintenance activities concluded that porous pavements do get clogged and that the needed to be performed. device developed in this study seemed suitable for mainte- King County performed a survey of 17 wet ponds and nance of porous pavers. 33 biofiltration swales to assess the state of these water qual- To evaluate the impacts of accumulated sediment on nutri- ity facilities (King County, 1995). Results of the survey indi- ent removal efficiencies in a pond, a field study was con- cated that because of poor design, construction problems, ducted on a submerged biofilter (Mothersill et al., 2000). and inadequate maintenance practices, only 35% of the wet- Substantial removal of suspended solids (97%) from the ponds and 28% of the biofiltration swales were working prop- influent stormwater resulted in a significant accumulation of erly. The study attributed the unsatisfactory condition of the sediment in the biofilter, which interfered with the system's BMPs to the novelty of the stormwater facilities and to a lack main treatment objective--removing soluble nutrients through of understanding about the effort required to sustain water bacterial assimilation. Removal efficiencies of total organic quality facilities in decent working condition. carbon and suspended orthophosphate were found to decrease The Caltrans BMP Retrofit Program evaluated the costs of with time; however, the removal efficiency for ammonium acquisition, operation, and maintenance of 39 BMPs from nitrate (64%) appeared independent of time or sediment accu- 12 different BMP categories (Currier et al., 2001). The study mulation. Sediment accumulation was attributed to infrequent estimated the annual maintenance requirements for sand fil- backwashing of the filtration unit. ters, extended detention basins, infiltration basins, biofil- Other studies have examined and compared the effects of tration strips, swales, and wet basins at 93 hours, 136 hours, maintenance on different classes of BMPs. A paper by Botts 193 hours, more than 200 hours, and 570 hours, respectively. et al. (1996) presented maintenance requirements and lon- The private sector increasingly is adopting the use of BMPs gevity estimates for four standard BMPs, including a water for commercial, industrial, and residential development quality inlet, an infiltration trench, a wet detention pond, and applications. Inspection and maintenance of BMPs are often a sand filter. Wet detention ponds and water quality inlets are the responsibility of the property owner. The City of Lacey shown to have long life spans, with well-designed detention in Washington State attests to the effectiveness of inspec- ponds operating as designed for 20 years or more and 95% tions and education coupled with field activities as opposed of water quality inlet installations operating as designed for to pure enforcement (Hielema, 2001). up to 5 years. Infiltration trenches have short life spans with Infiltration facilities appear to be the most prone to failure less than 50% of installation failing in fewer than 5 years. because of inadequate maintenance practices. Consequently, Proper design and regular maintenance can prolong the life many of the studies on BMP maintenance have examined of infiltration facilities to well over 5 years. infiltration facilities. The study by Nozi et al. (1999) exam- A study by Galli (1992) in Prince George's County, Mary- ined various infiltration facilities (infiltration inlet, infiltration land, evaluated the performance and longevity of 11 types of trench, porous asphalt pavement, and an infiltration well) in BMPs. The BMPs studied included infiltration trenches and Japan, evaluated the effects of maintenance, and demonstrated basins, dry wells, porous pavement, vegetated swales, extended

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88 detention dry ponds, wet ponds, constructed marshes, pocket 3.2.11.2. Primary References wetlands, oil and grit separators, and dry ponds. Assessment criteria used in the study included design strengths and weak- Botts, J., Allard, L., and J. Wheeler. Structural Best Management nesses, maintenance issues, and environmental considera- Practices for Storm Water Pollution Control at Industrial Facili- ties. Proc., Watershed 1996 (1996) pp. 216219. tions for each of the 156 sites included in the study. The Colwell, S. R., Horner, R. R., and D. B. Booth. Characterization of results of the study suggested that infiltration basins, porous Performance Predictors and Evaluation of Mowing Practices in pavement, grass filters, swales, and "pocket" wetlands gen- Biofiltration Swales. King County Land and Water Resources erally required modifications or improvements in order to Division Report, Center for Urban Water Resources Manage- provide reliable pollutant removal. ment, Department of Civil and Environmental Engineering, Uni- A King County study evaluated the effects of mowing on versity of Washington, Seattle (2000). the performance of vegetated swales (Colwell et al., 2000). Currier, B., Taylor, S. M., Borroum, Y., Friedman, G., Robison, D., Two mowing regimes--mowing at both the beginning and at Barrett, M., Borroum S., and C. Beitia. California Department of the end of the growing season and mowing only at the end of Transportation BMP Retrofit Pilot Program. Presented at 80th the growing season--were evaluated to determine impacts to Annual Meeting of the Transportation Research Board, Wash- ington, DC (January 711, 2001). swale treatment efficiencies. TSS and turbidity mitigation Dierkes, C., Kuhlmann, L., Kandasamy, J., and G. Angelis. Pollu- were significantly higher for the unmowed swale showing tion Retention Capability and Maintenance of Permeable Pave- that mowing did not provide increased treatment. The two ments. Global Solutions for Urban Drainage, Proc., 9th Interna- mowing strategies were found to be equivalent with respect tional Conference on Urban Drainage, Portland, OR (September to water quality benefits. The authors cautioned that the test 813, 2002) pp. 444445. systems may not be representative of all swales. Galli, J. Analysis of Urban BMP Performance and Longevity in The performance of BMPs commonly used in public works Prince George's County, Maryland. Report, Prince George's practices--such as water quality inlets, sedimentation man- County Department of Environmental Resources, Watershed holes, and catch basin inserts--also are dependent on the Protection Branch, MD (August, 1992) 203 pp. extent and frequency of maintenance. Maintenance-related Hielema, E. J. Private Facility Inspection and Maintenance: `Deluxe With Bacon' or `Maintenance Lite'? Stormwater (September information on public works practices are included under the October 2001) www.forester.net/sw_0109_private.html#what Gross Pollutant Removal and the Drain Inlet/Gross Pollutant King County Department of Public Works. Evaluation of Water Studies sections of this report. Details of studies presented Quality Ponds and Swales in the Issaquah/East Lake Sam- by England and Rushton (2003), Sedrak et al. (2001), Lipp- mamish Basins. Surface Water Management Division, Seattle, ner and Moeller (2000), and Dammel et al. (2001) are all rel- WA (1995) 75 pp. evant to maintenance of public works-related BMPs. Livingston, E. H., Baldwin R., and B. Clevenger. Lessons Learned About Successfully Using Infiltration Practices. Proc., National Conference on Tools for Urban Water Resource Management 3.2.11.1. Identification of Research Gaps and Protection, Chicago, IL (February 710, 2000) pp. 141161. and Needs Livingston, E. H., Shaver, E., Skupien, J. J., and R. R. Horner. Operations, Maintenance, and Management of Stormwater Sys- Based on the literature review, it is apparent that there is tems. U.S. Environmental Protection Agency Report CS82361- substantial information on maintenance practices of BMPs 01-0, Watershed Management Institute, Washington, DC (1997). and how these practices affect performance. A compilation of Mothersill, C. L., Anderson, B. C., Watt, W. E., and J. Marsalek. Biological Filtration of Stormwater: Field Operations and Main- the results of these studies in an updated BMP operations tenance Experiences. Water Quality Research Journal of Canada, and maintenance manual, such as the document by Livingston Vol. 35, No. 3 (2000) pp. 541562. et al. (1997), may be a potential research need. Another need Nozi, T., Mase, T., and K. Murata. Maintenance and Management is for guidance on estimating maintenance frequencies based Aspects of Stormwater Infiltration Systems. Proc., 8th Interna- on influent characteristics and site conditions. tional Conference on Urban Storm Drainage, Vol. 3, Sydney, BMP maintenance costs frequently are not factored in dur- Australia (August 30September 3, 1999) pp. 14971504. ing the initial planning and BMP selection phases of construc- Roenigk, D. J., Paterson, R. G., Heraty, M. A., Kaiser, E. J., and R. J. tion projects. There appears to be a need for guidance on esti- Burby. Evaluation of Urban Stormwater Maintenance in North mating the lifecycle costs that account for the maintenance Carolina. Report No. 267, Department of City and Regional Plan- required for continually functioning and efficient BMPs. A ning, University of North Carolina, Chapel Hill (1992) 197 pp. final need is for further development of methods to increase the longevity and to minimize maintenance requirements of 3.2.12. Use of Toxicity and Biological infiltration BMPs--such as the use of presettling basins or Indicators in Performance Evaluations the use of PAMs to maintain infiltration rates. Evaluations of sediment toxicity as a function of mainte- Toxicity and other biological indicators--such as bioavail- nance frequency and methods for disposing or reusing BMP ability, species diversity, and biomass--are underused meth- maintenance-generated wastes would be helpful. ods for evaluating BMP performance. This subject is not to

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89 be confused with a full receiving waters biological impact were capable of sufficiently reducing toxicity only in the analysis. Rather, use of indicators entails applying toxicity, lowest concentrations tested; some media appeared to gener- bioavailability, and biological communities as metrics for ate toxicity (which may have been due to reductions in pH or BMP performance. Potential research questions include hardness). By studying algal communities in two stormwater man- How can biological indicators be used to assess BMP agement ponds, Rouge Pond and Harding Pond, Olding (2000) performance, and what are the limitations to their use noted that impacts to aquatic biota decreased as stormwater and interpretation? passed through ponds. The greatest disturbances to biologi- How are toxicity and bioavailability reductions related cal communities were observed in the sediment forebay area to the reduction or speciation of chemical constituents? for both ponds. The author attributed the absence of blue- Which indicator organisms are most appropriate for green algae populations in both ponds to the hydraulics of the evaluating BMP performance? ponds, despite nutrient-rich conditions, and suggested that stormwater ponds can be engineered to limit nuisance algal Pitt et al. (1991) investigated the control of stormwater communities. The reduction of impacts to biological com- toxicants through conventional treatment processes. Twelve munities observed in the pond translated to a reduction of sheet flow samples were collected from the source areas that impacts to receiving water biological communities. were found previously to produce the most toxic storm runoff The use of biological indicators for evaluating BMP per- waters. These areas were automobile service areas, industrial formance is a relatively new method that is gaining popular- parking and loading dock areas, and automobile salvage yards. ity among stormwater regulators. Biological indicators such The samples were subjected to a variety of benchscale treata- as those used in toxicity bioassays have been used exten- bility tests, including settling columns, sieving screens, mem- sively for evaluating potential impacts to and the contamina- brane filters, aeration, photo degradation, aeration and photo tion of receiving water systems (see EPA's Biological Indi- degradation combined, floatation, and alum addition. cators of Watershed Health: www.epa.gov/bioindicators). It Toxicity changes were monitored using the Microtox bio- is not difficult to extend this knowledge base to the evalua- assay test. The benefits of the treatment processes varied tion of BMP performance, but some issues with traditional for the different samples. However, some of the treatment toxicity testing methods should be considered. Burton et al. processes consistently provided the greatest toxicity reduc- (2000) points out that traditional toxicity tests may not pro- tion. The most beneficial treatment tests included settling for duce reliable conclusions when used to detect the adverse at least 24 hours (generally 4090% reduction), screening effects of fluctuating stressor exposures, nutrients, suspended through at least 40-micron screens (2070% reduction), and solids, temperature, ultraviolet light, flow, mutagenicity, car- aeration or photo degradation for at least 24 hours (up to 80% cinogenicity, teratogenicity, endocrine disruption, or other reduction). The floatation tests produced floating sample lay- important subcellular responses. This inability to predict effects ers that generally decreased in toxicity with time. However, is largely a result of the complex biological response patterns the benefits were quite small (less than 30% reduction). Alum that result from various combinations of stressor magnitudes, additions substantially reduced the turbidity of the samples duration, and frequency between exposures, as well as from but the changes in toxicity were highly irregular. the interactions of stressor mixtures, such as synergistic effects The Port of Seattle tested four filtration media in con- of certain pesticides, metals, and temperature. In watersheds trolled laboratory experiments to determine their effective- receiving multiple sources of stressors, accurate assessments ness for concurrent metals removal and toxicity abatement in should define spatialtemporal profiles of exposure and effects synthetic stormwater (Tobiason et al., 2003). Media tested using a range of laboratory (such as WET tests) and novel in included commercially available leaf compost (CSF) media, situ toxicity and bioaccumulation assays, with simultaneous a zeoliteperlite mix, and a polyamine sponge, as well as the characterizations of physicochemical conditions and indige- recently developed citric acid modified soybean hull media. nous communities. Toxicity was assessed using acute Ceriodaphnia dubia Beginning in May 2003, the Louisiana DOT embarked on (48-hr) bioassays. Results indicated that the CSF media the research project Transport, Treatability, and Toxicity removed up to 75% of the zinc and reduced toxicity signifi- of Highway Stormwater Discharged into Receiving Waters cantly for influent concentrations of up to about 300 ppb zinc. across Louisiana (http://rip.trb.org). The primary objectives The soybean hull material removed 8099% of the zinc over of this research are (1) characterization of highway storm- all influent concentrations, though it reduced pH to toxic lev- water based on hydrology, pollutant loadings, toxicity, and els. After pH adjustment, the effluent from the soybean mate- rainfall quality; (2) comparison of standard tests for storm- rial was nontoxic over all concentrations tested (survival was water characterization; (3) quantification of pollutant load- 100% in pH-adjusted effluent samples). Augmenting the soy- ings as a function of hydrologic parameters and traffic char- bean material with leaf compost media or activated carbon acteristics; and (4) assessment of treatment alternatives. Part effectively buffered effluent pH to circumneutral ranges. of this research also will include documenting toxicity reduc- Other media tested removed modest amounts of zinc and tions at three experimental sites: a site near Shreveport at the

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90 I-220 bridge over Cross Lake, a site in Baton Rouge at the children for fear of exposing their children to the risk of I-10 bridge over City Park Lake, and a site in New Orleans drowning. Poorly maintained ponds can become unsightly at the I-10/I-610 junction over the 17th Street canal. and odorous and provide a breeding ground for mosquitoes and other parasites. On OCR for page 63
91 3.2.14. Economic Analysis and Assessment Vortechs treatment systems). A comprehensive cost analysis was presented for each system. In order to compare the cap- Evaluating BMPs on the basis of cost is an integral part of ital costs of each treatment system, researchers evaluated the the BMP selection process. Initial capital costs and some- cost-per-volume served, the cost-per-volume served per year, times operation and maintenance costs can become the key and the cost-per-percentage of TSS removal. The bioreten- controlling factors that dictate which BMPs are selected and tion basin was found to be the most economical system of the whether projects get constructed. The tools and methods used four tested. The Vortechs system was not installed properly in cost estimates therefore play a prominent role in stormwater and hence provided unreliable results. management. According to Heaney et al. (2002), most of the Scott et al. (1999) evaluated and compared two flood con- cost-estimation methods are based on regression equations. trol mitigation systems: on-site detention (OSD) and on-site Cost-estimation techniques can be improved through addi- retention (OSR). Both flood control systems provided flood tional funding and research and through the use of available protection by attenuating peak flows of in-coming storms and technology. Potential research needs and knowledge gaps in discharging the effluent at lower flow rates. OSR outper- the economic analysis and assessment of BMPs, some of formed OSD in terms of cost efficiency and environmental which were suggested by Heaney et al. (2002), include benefits. Cost efficiency was based on the volume of site storage required to attain peak flow attenuation. Quantification of the benefits of urban storm systems; Caltrans has assessed and approved more than 110 BMPs Consideration of receiving water impacts in costbenefit for use since 1996 and has included costbenefit analysis analysis; as part of their assessment. Caltrans assessed the cost- Development of costbenefit evaluation methodologies effectiveness for each BMP in terms of its EUAC relative to for nonstructural BMPs; The availability and application of flow and water qual- a detention basin (Caltrans, 2003). A four-quadrant system was used as a tool to rate each BMP. The cost estimates were ity data to BMP costbenefit analyses; Inclusion of land use data into cost optimization analy- defined first by calculating the typical range of costs for con- ses; and structing or operating a BMP on a per-acre basis. The acre Costbenefit analysis of BMP treatment trains. represented the drainage area served by the BMP. Operation and maintenance costs then were added, based on the design Another important consideration is the assessment of cost life of the BMPs. The EUAC for a particular BMP was esti- differentials. Costs for similar needs (e.g., landscaping and mated and compared qualitatively to that of a detention maintenance) for many BMPs may have been incurred already basin. If the EUAC was higher for the BMP than for a deten- or may be avoided (e.g., reduction of pipes and inlets via the tion basin, it was marked as a higher cost using the quadrant use of biofilters). Cost studies may be misleading if these rating key. The benefit of the BMP was evaluated relative to potential cost offsets are not assessed. the performance of a typical detention basin. If the con- A review of the literature pertaining to BMP cost estima- stituent removal was greater than that of a detention basin, tion reveals a number of documents that provide BMP cost the BMP was marked as having a greater benefit. information. FHWA (2000) provided a table of relative BMP Using published literature and cost estimation guides, costs. Structural BMPs that were assigned relatively high Sample et al. (2003) synthesized methods for estimating capital costs included underground sand filters and organic costs for BMPs such as detention, retention and infiltration media filters; detention tanks, underground sand filters, organic basins, infiltration trenches, sand filters, and vegetated swales. media filters and oil-grit separators were assigned high rela- According to the study authors, costbenefit analysis method- tive operation and maintenance costs. Low-cost structural ologies can be improved by considering additional parame- BMPs included treatment systems like bioretention, deten- ters such as flow monitoring data, receiving water impacts, tion ponds, vegetated swales, vegetated filter strips, and porous and the effects of streets and parking lots. pavement. Relative costs for new innovative BMPs range Sear et al. (1996) explained the development of equations from moderate to high for systems such as alum injection, used to estimate BMP cost as a function of pollutant removal. MCTT, biofilters, and vegetated rock filters. The functions were used to evaluate nine alternatives for five Heaney et al. (2002) present a comprehensive collection of stormwater treatment technologies in Lakeland, Florida. Pro- tables and equations for estimating the cost of drainage struc- duction cost functions, with respect to TSS removal percent- tures, including BMPs. 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92 A number of simplifying assumptions are made and some estimation. As mentioned under the section Maintenance and external variables are overlooked in a lot of the available Longevity, lifecycle costs account for the operations and cost-estimation methods. Maintenance costs and longevity of maintenance requirements necessary to maintain lifetime BMPs are not considered in some BMP evaluation literature. BMP functionality and efficiency. Opportunity costs are the According to England (1998), maintenance costs for retrofit costs of land taken out of other uses and the costs of an alter- projects often are neglected or underestimated. An evalua- native conveyance system (which actually may be a net sav- tion of maintenance costs of BMPs--such as wet ponds, dry ings in some cases). Externalities are the effects of produc- ponds, exfiltration and infiltration trenches, porous pave- tion and consumption activities not directly reflected in the ment, baffle boxes, inlet baskets, and sediment sumps--is market, such as receiving waters protection and aesthetics presented by the authors, who recommended that mainte- (Willis and Finney, 1999). There is a need to develop BMP nance needs be considered in the design and construction of cost-estimation tools that account for land value, site con- retrofit projects in order to ensure that retrofits provide long- straints, construction, operations, and maintenance, as well term pollutant removal. as receiving waters protection, aesthetics, and infrastructure After evaluating baffle boxes and inlet devices, England savings on conventional drainage structures. Quantification (1998b) concluded that the tradeoff for the low initial cost of of receiving waters protection requires the use of existing the evaluated BMPs is the perpetual maintenance expense. water quality, habitat, and bioassessment monitoring data for Baffle boxes are recommended for small to medium-sized both the runoff and the receiving waters. drainage basins, while inlet devices are recommended for With the possible exception of street sweeping, nonstruc- small flows and small drainage basins. tural BMPs have been primarily overlooked. Costs associ- BMPs can affect the market values of neighboring proper- ated with public education, catch basin maintenance, and ties, so another way to evaluate BMPs is to assess the eco- road side vegetation control activities would be helpful for nomic benefits of implementing BMPs in residential and the optimization and adequate allocation of stormwater man- commercial areas. Frederick et al. (1996) presented a discus- agement funds. sion about the potential increase in property value that can be Finally, although there is an abundance of cost-evaluation gained through the construction of detention-type BMPs. methodologies for individual standard structural BMPs, BMP Prices of homes situated close to a body of water tend to be treatment trains and distributed BMP systems appear to have been neglected. There is a need for cost evaluations and com- significantly higher than comparable properties that are not parisons of BMP treatment trains, distributed BMPs, and near a body of water. In addition to environmental benefits, large centralized regional BMP systems. aesthetically pleasing BMPs can improve property values, lower vacancy rates of rental properties, and make properties easier to sell. 3.2.14.2. Primary References As described in section 3.2.10., BMP Modeling, Bannerman et al. (2003) were able to find the most cost-effective combi- Bannerman, R., Fries, G., and J. Horwatich. Source Area and nation of high-efficiency street sweeping and treatment con- Regional Stormwater Treatment Practices: Options for Achiev- trol practices by using the SLAMM model to meet the TSS ing Phase II Retrofit Requirements in Wisconsin. Proc., National reduction goal of 40%. The annual cost of the source area Conference on Urban Storm Water: Enhancing Programs at the practices was estimated (assuming a 20-year life span) to Local Level, Chicago, IL (February 1720, 2003) pp. 1219. Caltrans. Caltrans New Technology Report. Report CTSW-RT- range from $573,000 to $1,504,000, while the range for deten- 03-010 (February 2003) 130 pp. tion ponds was $963,000 to $1,840,000. The least expensive England, G. Maintenance of Stormwater Retrofit Projects. Water combination of source area practices would only increase the Resources and the Urban Environment, Proc., 25th ASCE Water annual stormwater utility bill for the Madison taxpayers by Resources Planning and Management Conference, Chicago, about $6, while the most likely detention pond alternative IL (1998). would increase the utility bills by about $18. England, G. Baffle Boxes and Inlet Devices for Stormwater BMPs. Water Resources and the Urban Environment, Proc., 25th ASCE Water Resources Planning and Management Conference, Chi- 3.2.14.1. Identification of Research Needs cago, IL (1998b). Federal Highway Administration. Stormwater Best Management Based on the literature review addressing the economic Practices in an Ultra-Urban Setting: Selection and Monitoring. analyses and assessment of BMPs, it is evident that there is Report No. FHWA-EP-00-002, U.S. Department of Transporta- tion, Washington, DC (May 2000). cost estimation information for nearly all proprietary and Frederick, R., Goo, R., Corrigan, M. B., Bartow, S., and M. Bil- most of the common nonproprietary structural BMPs. Cost lingsley, M. Economic Benefits of Urban Runoff Controls. Proc., regression equations have been developed for a number of Watershed 1996 (1996) pp. 389392. BMP types that are based primarily on imperviousness, land Heaney, J. P., Sample, D., and L. T. Wright. Costs of Urban use, and flow rates and volumes. However, lifecycle costs, Stormwater Control. U.S. EPA Report No. 600/R-02 (January opportunity costs, and externalities often are neglected in cost 2002) 105 pp.

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93 Sample, D. J., Heaney, J. P., Wright, L. T., Fan, C. Y., Lai, F. H., and mendations for pond-type BMPs included stocking permanent R. Field. Costs of Best Management Practices and Associated pools with Mosquito Fish (Gambusia affinis) and providing Land for Urban Stormwater Control. Journal of Water Resources steep sideslopes to create a less desirable habitat for vectors. Planning and Management, Vol. 129, No. 1 (2003) pp. 5968. The combination of vegetation and permanent pools of Scott, P., Santos, R., and J. R. Argue. Performance, Environmental stagnant water in wetlands makes wetlands prone to vector and Cost Comparisons of On-Site Detention (OSD) and On-Site infestations. As a result numerous vector-related studies have Retention (OSR) in Re-Developed Residential Catchments. targeted wetland locations. Studies that have evaluated the Water Science and Technology, Vol. 39, No. 2 (1999) pp. 3341. Sear, T. R., Bays, J. S., and W. G. Medley. Development of Cost- incidence and implication of vectors in wetlands include Effective Stormwater Treatment Alternatives. Proc., Watershed studies by Russell (1999a) and Russell et al. (1999b). 1996 (1996) pp. 870873. Walton et al. (1999) examined the dispersal, survival, and Yu, S. L., and M. D. Stopinski. Testing of Ultra-Urban Stormwater host-seeking behavior of mosquitoes from a constructed wet- Best Management Practices. Final Report VTRC 01-R7, Vir- land in Southern California. The study showed that the lim- ginia Department of Transportation, Washington, DC (January ited dispersal and the long survival of Culex erythrothorax 2001) 48 pp. were important factors in the development of large popula- tions at constructed wetlands. A study in Adelaide, Australia, 3.2.15. Vector Control evaluated 12 constructed wetlands in an attempt to under- stand the breeding habits of mosquitoes, especially those in The potential for structural BMPs to harbor and breed urban constructed wetlands (Sarneckis, 2002). The study nuisance- and disease-causing organisms had received little showed that wetlands with standing water, steep edges, and attention until recently. A vector, as used in this section, little emergent vegetation typically had fewer or no larval refers to any organism that can transmit an infectious disease- mosquitoes. Wetlands that supported large mosquito popula- causing organism to another living thing (Metzger et al., tions typically had sheltered shallow water, isolated pools 2003). Because mosquitoes are ubiquitous and their life-cycle that limited predator access, poor water quality, and low mar- depends on humans and other warm-blooded animals, their croinvertebrate diversities. The study concluded that well- potential to transmit infectious disease is high. Therefore many designed wetlands were less likely to produce mosquitoes. of the studies related to vectors in BMPs focus on mosquitoes MacLean (1995) presented mosquito management strate- (Metzger et al., 2003; VBDS, 2001). With the ever-increasing gies for wetlands. The author suggested mosquito control demand for BMPs, vector issues associated with BMPs could strategies that included the use of bacteria, chemical larvi- result in exponential increases in vector populations if not cides, insect eating fish, copepods, and other animals. The addressed. A review of the literature pertinent to the incidence efficiency and availability of selected controls are presented. of vectors in BMPs shows that wetlands appear to be the most The author inferred that wetlands designed to optimize sur- targeted BMPs, and mosquitoes are the most targeted vector in face area and plant growth without excessive mosquito pro- studies. duction result in cost savings. Caltrans established a comprehensive vector surveillance and monitoring study in cooperation with the Vector-Borne 3.2.15.1. Identification of Research Needs Disease Section (VBDS) of the California Department of Health Services. The objectives of the 2-year study were to The potential for vectors, particularly mosquitoes, to inhabit develop vector abatement protocols and to recommend mod- and breed in stormwater control facilities is of increasing ifications to Caltrans BMPs that would minimize their poten- concern to stormwater management practitioners. The evi- tial to harbor vectors. VBDS monitored 37 structural BMPs dent scarcity of studies and literature pertaining to the inci- at 31 sites with emphasis on mosquitoes. The study showed dence of vectors in stormwater BMPs makes this whole cat- that BMP technologies that maintained permanent pools of egory a research need. Details on the kind of research that is standing water (i.e., multichambered treatment trains, contin- needed include the development and evaluation of mainte- uous deflector separators, and wet basins) were more likely nance and design practices that deter vectors. Poor water to support a large mosquito population. BMP technologies quality also has been linked to the mosquito proliferation in that drained completely (i.e., biofiltration swales, biofiltration wetlands. It is thought that nutrients provide food for the bac- strips, sand media filters, infiltration basins, infiltration teria and algae on which mosquitoes feed. A better knowl- trenches, drain inlet inserts, extended detention basins, and edge of the relationship between vectors and water quality is oilwater separators) were less likely to harbor vectors. Fac- a necessary addition to the existing literature on vectors. tors that contributed to the incidence of vectors in BMPs include BMP design, BMP location, immediate and large- scale surroundings, nonstormwater discharges (such as irri- 3.2.15.2. Primary References gation), and site maintenance. BMP design features to be avoided include the use of sumps, catch basins, or troughs McClean, J. Mosquitoes in Constructed Wetlands-A Management that do not completely drain; loose riprap; automatic pumps Bugaboo? Watershed Protection Techniques, Vol. 1, No. 4 (1995) or motors; and orifices that are prone to clogging. Recom- pp. 203207.