Identification of Research Needs Related to Highway Runoff Management (2004)

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The purpose of research project NCHRP 25-20(02) is to identify and describe research projects that address priority needs in the area of highway runoff management and control. This project was designed to conduct a broader search of the available data and studies than was accomplished by NCHRP Project 25-20 and to identify state departments’ of transportation (DOTs) research activities and priority research areas for improving the quality of stormwater runoff. This report provides an updated and thorough list of research needs that can be used in the decision-making and prioritiza- tion processes regarding funding of future research. State DOT research directors and water quality professionals were contacted to locate existing research and research-in-progress and to list what they believed were the most important remaining needs related to research on water quality control and stormwater impacts on receiving waters. DOT water quality professionals—includ- ing engineers, National Pollutant Discharge Elimination System (NPDES) special- ists, and other program managers—from all 50 states participated. The DOTs expressed the strongest needs and interests in the area of cost and performance of stormwater control facilities or best management practices (BMPs). The DOTs’ top interests paralleled gaps in the literature in most cases; where such gaps were not identified in the literature but where DOT interest was high, mechanisms for infor- mation sharing are needed. A combined literature review, DOT preferences, and research team recommendations are itemized in Table 4-1 of this report. The research needs identified through the DOT survey, literature review findings, and the opinions of the stormwater experts involved in this investigation were prioritized by rank on a scale from 1 to 5. RESEARCH GAPS AND NEEDS The following paragraphs summarize the conclusions of the investigation. A brief discussion of the research gaps and needs identified in the literature is presented, fol- lowed by the literature review and itemized research statements identified by DOTs as research needs. Some research statements span multiple research categories and likely would be combined into single projects. SUMMARY IDENTIFICATION OF RESEARCH NEEDS RELATED TO HIGHWAY RUNOFF MANAGEMENT

BMP Maintenance and Longevity A compilation of the results of current studies on BMP operations and maintenance is needed, potentially in the form of a nationally applicable manual on BMP operations and maintenance, with guidance on estimating maintenance frequencies based on influ- ent characteristics and site conditions. The costs of BMP maintenance are not factored in during the initial planning and BMP selection phases of construction projects. Agen- cies often lack the tools to make good estimates of the staff-hours needed to adequately maintain BMPs. Guidance on estimating life-cycle costs of BMPs appears to be needed. Finally, there is a need for further development of methods to increase the longevity and minimize maintenance requirements of infiltration BMPs, such as the use of pre- settling basins or polyacrylamides to maintain infiltration rates. With regard to sedimentation, research is needed to evaluate sediment toxicity as a function of maintenance frequency and methods for disposing or reusing BMP maintenance-generated wastes. See section 3.2.11 for a discussion on BMP maintenance and longevity. Itemized Research Needs 1. Development of contract administration of BMP requirements and contractual methods to improve BMP implementation, 2. Compilation of BMP maintenance and lifetime effectiveness information, 3. Cost–benefit analysis of BMP maintenance practices, 4. Guidance for estimating life-cycle costs of BMPs that account for maintenance required for continually functioning and efficient BMPs, 5. Development of nationally applicable BMP operations and maintenance guidance (maintenance frequencies, logistics and personnel requirements, estimates based on influent characteristics and site conditions), 6. Development of methods for increasing longevity and minimizing maintenance requirements of infiltration BMPs, 7. Evaluation of sediment toxicity as a function of maintenance frequency, 8. Evaluation of issues and methods of disposing or reusing BMP maintenance- generated wastes, and 9. Evaluation of designs and maintenance of BMPs to reduce conflicts with endan- gered and threatened species. Information Sharing and Technology Exchange A compilation of major syntheses extracted from stormwater runoff research is needed to support, encourage, and facilitate a more efficient and comprehensive exchange of information among stormwater professionals. This report, and that of the National Highway Runoff Data and Methodology Synthesis (NDAMS), presents a good start- ing point for such a compilation. The International Stormwater BMP Database pri- marily contains BMP design and monitoring data but makes no direct links to published literature. A research project linking an extensive bibliographic database, such as a refined and value-added NDAMS database, to a water quality and BMP performance database, such as the International Stormwater BMP Database, would create a useful tool for stormwater practitioners. See section 3.1.1 in this report for a synthesis of recent major highway runoff research. 2

3Itemized Research Needs 1. Compilation of major syntheses extracted from stormwater runoff research and linkage into a master bibliographic database; 2. Development of a stormwater runoff research database and a BMP performance and design database specific to highways using the International Stormwater BMP Database as a model; 3. Development of an information-sharing system that links the two databases listed in item 2 into an online, user-friendly database for data entry and retrieval; and 4. Identification of and guidance on practical and accepted monitoring methods for highway runoff. Watershed Planning Although much literature supporting watershed management exists, there is still a need for the development and evaluation of techniques to integrate transportation-related runoff analysis with overall watershed management. Stream channels respond to changes in flow volume and sediment loading, which subsequently produce recognizable patterns and forms. Watershed change is known to have a corresponding effect on channels lead- ing to bank erosion and head cutting. Although these processes are well understood and descriptions of channel morphology are well developed, effective predictive models of channel geomorphic response are lacking. Correspondingly, research to support devel- opment of regulatory structure more appropriate to the episodic nature of runoff also is needed, along with identification of indices and indicators specific to transportation- related runoff. See section 3.3.1 for a discussion on watershed planning. Itemized Research Needs 1. Development and evaluation of techniques to integrate transportation-related runoff analysis into overall watershed management; 2. Development of standard methods, models, and data for establishing critical needs within a watershed to prioritize areas for retrofit and BMP implementation; 3. Development of geomorphologic models for estimating watershed development impacts on receiving streams; 4. Quantification or development, or both, of indices and indicators of the contribu- tion of state highway infrastructure relative to total impervious surface area in a watershed; 5. Evaluation of the ability of watershed or regionally based enhancements of wet weather storage capacity to improve baseline (high and low flow) hydrology and ecological productivity downstream; 6. Characterization of the availability and prioritization of sites on a watershed basis for constructed wetlands and wet ponds; and 7. Demonstration of the costs and benefits of alternative, off-site, and watershed- based stormwater mitigation. Economic Analysis and Assessment The review of literature pertinent to the economic analyses and assessment of BMPs revealed cost estimation information for nearly all proprietary BMPs and most of the common nonproprietary structural BMPs. For a number of BMP types, cost regression

equations have been developed that are primarily based on imperviousness, land use, and flow rates and volumes; however, life-cycle costs, opportunity costs, and external- ities often are neglected in cost estimation. There is a need to develop BMP cost esti- mation tools that account for land value, site constraints, construction, operations, and maintenance, as well as receiving waters protection, aesthetics, and infrastructure sav- ings on conventional drainage structures. Quantification of benefits from receiving waters protection requires the use of existing water quality, habitat, and bioassessment monitoring data for both the runoff and receiving waters. Costs associated with public education, catch basin maintenance, and roadside veg- etation control activities would aid in the optimization and adequate allocation of stormwater management funds. Cost evaluations and comparisons of BMP treatment trains, distributed BMPs, and large centralized regional BMP systems also are needed. See section 3.2.14 for a discussion on economic analysis and assessment of BMPs. Itemized Research Needs 1. Guidance on quantifying BMP life-cycle costs and benefits associated with receiving waters protection; 2. Evaluation of potential cost reductions of stormwater treatment through alterna- tive siting within the watershed; 3. Evaluation of the BMP benefits and constraints in highly urbanized corridors; 4. Cost comparisons of BMP treatment trains, distributed BMPs, and regional BMP systems; 5. Development of BMP cost-estimation tools that account for land value, site con- straints, construction, operations, and maintenance, as well as receiving waters protection, aesthetics, and infrastructure savings on conventional drainage struc- tures; and 6. Cost estimates for nonstructural BMPs. General BMP Evaluation and Selection There are many different ways to evaluate BMPs. The most common methods mon- itor the effluent and influent water quality with the primary goal of estimating BMP efficiency. Several methods exist for monitoring, analyzing, and reporting BMP effi- ciency. The most common methods include the efficiency ratio, summation of loads, regression of loads, mean concentration, efficiency of individual storm loads, reference watersheds, and before-and-after studies. More recent methods include effluent prob- ability, flow-dependent removal efficiency, minimum influent concentration, and the pollutant flux ratio. Of all these methods, the most promising for the consideration of standard BMP efficiency are the effluent probability method, which is recommended in Urban Stormwater BMP Performance Monitoring: A Guidance Manual for Meet- ing the National Stormwater BMP Database Requirements (GeoSyntec Consultants, 2002), and the minimum influent concentration removal efficiency methods, referred to in the Stormwater Best Management Practice Demonstration Tier II Protocol for Interstate Reciprocity (2001). The former provides greater detail on the actual perfor- mance of a BMP; the latter provides an easier way to understand the transferable measure of BMP efficiency. However, neither method can be used to estimate adequately the efficiency of BMPs without well-defined inlets and outlets, such as infiltration-type facilities or source controls. Reference watersheds and before-and- after studies are used often in these situations. 4

5Because there are so many methods available for evaluating and reporting the effi- ciency of BMPs, consensus and guidance clearly are needed. Selection of BMPs often is based on performance claims and reported efficiencies in relation to water quality goals, but many other factors are involved, including budgetary and site constraints, which include available land, climate, soil and vegetation conditions, topography, sur- rounding land use, and local regulatory issues. All of these factors make it difficult to select and design BMPs. As stated in the GKY Synthesis and Research Plan, “[p]rac- titioners need quick and ready access to BMP information and a means of quickly applying applicable portions of it to site-specific situations—an expert system.” This current effort agrees that such an “expert system” is needed to aid in the selection and design of highway stormwater BMPs. See section 3.2.1 for a discussion on general BMP evaluation. Itemized Research Needs 1. Development of standard performance measure(s) for BMP efficiency, 2. Development of an expert system for BMP selection and design, and 3. Assessment of and design guidance for ultra-urban BMPs. Low-Impact Development and Distributed BMPs Pilot projects conducted by several researchers have demonstrated the potential of low-impact development (LID) to meet regulatory requirements, but substantial work needs to be conducted on developing LID design strategies, performance standards, and specifications. LID’s decentralized approach to stormwater management technology has tremendous potential to supplement, or in some situations completely replace, con- ventional centralized stormwater BMP approaches. However, LID’s applicability, effi- cacy, and long-term economic sustainability for transportation systems have yet to be determined or documented. A long-term research need is to document the type of hydrologic losses that via LID can be achieved regionally and under various climatic, soil, slope, and vegetation conditions. See section 3.2.9 for a discussion on LID. Itemized Research Needs 1. Development of LID design strategies, performance standards, and specifications; 2. Documentation of LID’s applicability, efficacy, and long-term economic sus- tainability for transportation systems; 3. Evaluation of the type of hydrologic losses that can be achieved under various cli- matic, soil, slope, and vegetation conditions; 4. LID modeling and design guidance for accurately sizing end-of-pipe control sys- tems; and 5. Development of methods and technologies to promote the reuse of stormwater. Design Variables Affecting BMP Performance Primary design variables affecting BMP performance are those that control flow. These include outlet structures, baffles, berms, and vegetation density, in addition to the total volume a system is able to capture. Design features specific to individual types of BMPs, such as specific surface area for detention facilities and flow length for swales,

also are significant factors to consider when evaluating and comparing BMP perfor- mance. These design variables are related directly to physical treatment mechanisms of sedimentation and filtration. Variables related to the biochemical and geochemical treat- ment mechanisms, such as vegetation and soil type, also may be important design fac- tors; however, no studies were found that compared BMP performance according to these variables, indicating a potential research gap. Before sufficient field data are available to make assessments of design variables that influence treatment, pilot-scale experiments can be conducted to ascertain some of the needed design and performance information. See section 3.2.7 for a discussion of design variables affecting BMP performance. Itemized Research Needs 1. Evaluation of design variables that are related to biochemical and geochemical treatment mechanisms, and 2. Execution of pilot-scale experiments that evaluate the relation of various design variables on BMP performance. BMP Modeling With regard to BMP modeling, the unit processes of sedimentation and infiltration appear to be well covered in the literature. However, other BMP water quality treat- ment unit processes such as sorption processes (absorption and adsorption), biodegra- dation and uptake, photolysis, and volatilization still need further study before reli- able BMP performance models can be developed. Information on the modeling of BMP treatment trains appears to be lacking as well. A better understanding of BMP longevity and the decrease in the treatment efficiency as a function of time are required so that the optimization models that are used to select cost-effective BMP systems can provide better estimates of the lifetime costs and benefits of BMPs. How sources of pollutants are represented in models also merits further exploration. Many models still use a “build-up–wash-off” approach as the only way the pollutants get into stormwater, which can lead to faulty results if the BMP acts directly on that func- tion. The development of a review of modeling approaches and guidance on their selection and application would be a useful resource for stormwater practitioners. See section 3.2.10 for a discussion of BMP modeling. Itemized Research Needs 1. Evaluation of modeling approaches and guidance on model selection and application; 2. Use of pilot experiments to collect data needed for parameter estimation and model calibration; 3. Development of unit treatment models that incorporate sorption, biodegradation and uptake, photolysis, and volatilization; 4. Development of models for simulation of BMP treatment trains; 5. Development of BMP treatment models that account for treatment efficiency losses over time; and 6. Development or evaluation of models that can be used for modeling pollution plumes in BMPs. 6

7Hydraulic Assessment Based on the review of literature with regard to the hydraulic assessment of storm- water control facilities in relation to BMP performance, the most pressing gaps appear in the evaluation of the characteristics and effects of short-circuiting and bypass or overflow (e.g., ponds or wetlands discharging over the low-flow outlet or bioswales when depths and velocities for good treatment are exceeded). The influ- ence of hydraulic residence time on BMP performance has been well studied, and it has been confirmed that detention time correlates positively with pollutant removal (at least for particulate-bound pollutants). However, no studies were found that inves- tigated the nature of the correlation (linearly, asymptotically, etc.). Also, hydraulic residence is calculated usually by dividing the permanent pool volume by the average outflow discharge rate of a BMP. The true hydraulic residence time depends on the flow path through the system, which requires some means of estimating the velocity field of the system such as the use of tracers, ultra-sensitive velocity meters, or two- and three-dimensional hydrodynamic models. See section 3.2.3 for a discussion on hydraulic assessment. Itemized Research Needs 1. Evaluation of the characteristics and effects of short-circuiting, bypass, and overflow; 2. Investigation of the nature of the correlation between detention time and pollu- tant removal; 3. Development of methods or models for estimating the true hydraulic residence in stormwater ponds; and 4. Development of methods to optimize detention basin design to maximize treatment. Methods to Improve Pollutant Removal in Existing Stormwater Systems One promising method to improve pollutant removal in existing stormwater systems is detailed design guidance that includes overall feasibility cost–benefit comparisons between retrofit alternatives and potential impacts to flood protection. Another need is to sponsor research to evaluate if other less-conservative flood control methods—such as the use of more-refined continuous simulation approaches to assess flood detention needs—could be employed safely. With regard to coagulants, the reviewed literature (as well as the plethora of litera- ture available in the area of wastewater management) suggests that further research in this area is unlikely to be a high priority. However, in selected locations, coagulant use may be necessary to achieve water quality goals, thus more detailed guidance on design for highway situations may be valuable. Potential impacts to receiving waters from coagulant use may warrant further research, particularly for stormwater treatment prod- ucts (either new products or ones not used widely—such as chitosan). Recommenda- tions for soil amendments to use in BMPs to more passively improve performance also are areas for potential research. See section 3.2.5 for a discussion on methods to improve pollutant removal in existing stormwater systems.

Itemized Research Needs 1. Cost–benefit analysis of alternative flood control retrofits with consideration of overall feasibility and potential impacts to flood control; 2. Risk assessment of alternative, less-conservative flood control methods through the use of continuous runoff simulation modeling; 3. Development of detailed design guidance for flood control retrofits; 4. Evaluation of the effectiveness of BMP retrofits; 5. Development of recommendations for soil amendments for use in BMPs to pas- sively improve performance; 6. Development of methods for improving or maintaining hydraulic conductivity of infiltration-based stormwater control facilities; 7. Evaluation of the effectiveness of combining sedimentation, filtration, and chem- ical addition for stormwater BMP construction projects; 8. Evaluation of the potential impacts of coagulants to receiving waters; 9. Detailed guidance for areas that require coagulant use to meet water quality objectives; and 10. Development of new technologies or improvements on existing designs to increase the removal of high-priority pollutants. Sedimentation and Turbidity Impacts With regard to sedimentation and turbidity impacts to fish in general and salmonids in particular, some of the significant research needs identified by Bash et al. (2001) include (1) developing new exposure metrics that account for sublethal effects (as opposed to direct mortality); (2) examining the effect of frequent short-term pulses of suspended sediment; (3) conducting additional research on correlations between parti- cle size, shape, and composition of sediments to fish sensitivity; (4) studying relation- ships between seasonal timing and effect of sediment load; and (5) determining whether knowledge of survival responses to turbid flows can be used to develop mixing zones, work windows, treatment systems, and buffers that will allow fish to perform their nec- essary life functions during project construction and operation. See section 3.5.3 for a discussion of sedimentation and turbidity impacts. Itemized Research Needs 1. Development of new exposure metrics that account for sublethal effects (as opposed to direct mortality); 2. Examination of the effects of frequent short-term pulses of suspended sediment; 3. Additional research on correlations between particle size, shape, and composi- tion of sediments to fish sensitivity; 4. Evaluation of the relationships between seasonal timing and the effect of sedi- ment load; 5. Evaluation of the applicability of the knowledge of fish survival responses to turbid flows to the development of mixing zones, work windows, treatment sys- tems, and buffers that will allow fish to perform their necessary life functions during project construction and operation; 6. Identification of practical means of controlling turbidity; and 7. Development of hydromodification measures (estimated downstream hydrolog- ical changes) and measures for assessing potential downstream channel and bank instability. 8

9Erosion and Sediment Control With regard to temporary vegetation controls, sufficient research appears to be avail- able on the erosion control effectiveness of compost and mulch, erosion control mats and blankets, and cellular confinement technologies. Adequate guidance exists as well (see Appendix B for a list of selected guidance manuals). Erosion control effectiveness for removing fine particulates does not seem to be covered adequately in the literature. However, the use of polyacrylamides or other flocculants in conjunction with tempo- rary vegetation controls holds promise for controlling erosion of fine particulates. More research may be needed on ways to increase germination and survival rates of native vegetation. With regard to bank protection, research is needed to investigate the feasibility and performance of vegetated riprap and alternative bank stabilization designs that minimize impacts to riparian habitat. NCHRP 24-19, expected in 2004, will help to fill this gap. See section 3.2.6 for a discussion of erosion and sediment control. Itemized Research Needs 1. Evaluation of the effectiveness of erosion controls at removing fine particulates; 2. Evaluation of the effectiveness of using polyacrylamides or other flocculants in conjunction with other sedimentation and erosion control practices; 3. Development of techniques to increase germination, soil coverage, and survival rates of native vegetation; 4. Evaluation and comparison of the different types of vegetation for riprap planting; 5. Research on the necessary top elevation for conventional riprap as a function of velocity, turbulence, and flow duration; 6. Comparison of terraced versus sloping riprap in terms of hydraulic performance and planted vegetation success; 7. Evaluation of alternative bank stabilization techniques that have a lesser effect on riparian and aquatic habitat than riprap; 8. More detailed inspection of riprap where vegetation is growing now or has grown previously to better understand its impacts on bank stability; 9. Guidance for seed mixes and effective establishment and maintenance of ero- sion control vegetation for short-term first growth and long-term establishment; 10. Evaluation of potential water quality impacts of soil stabilizers used in erosion control; 11. Development of standard, approved postconstruction erosion control inspection and enforcement programs; 12. Evaluation of slope and soil conditions necessary for vegetation establishment; 13. Evaluation of new and innovative erosion control technologies; 14. Evaluation of erosion control methods for arid regions; 15. Evaluation of the performance of nonvegetative permanent soil stabilizers for reducing erosion and potential water quality impacts; and 16 Development and evaluation of temporary nonvegetative soil stabilization techniques. General Constituent Characterization Many state DOTs have studied highway runoff, so there are several studies available that generally characterize highway runoff quality. The constituents sampled and the con- centrations detected do not appear to vary significantly among the studies; therefore, in

general, characterization of highway runoff does not represent a primary research need. However, there are gaps in this research for some chemical constituents, including trace elements—such as polycyclic aromatic hydrocarbons (PAH); benzene, toluene, ethyl- benzene, and total xylene (BTEX); methyl tert-butyl ether (MTBE); and platinum group metals—not normally included in characterization studies. See section 3.4.1 for a dis- cussion on general constituent characterization. Itemized Research Needs 1. Characterization of chemical constituents not generally monitored but believed to be frequently present in highway runoff, and 2. Evaluation of methods for monitoring and analyzing oil and grease and total petroleum hydrocarbons. Atmospheric Deposition Based on the review of literature, more studies that relate transportation systems to atmospheric deposition of pollutants are needed to quantify the contribution of atmo- spheric deposition to pollutant concentrations found in highway runoff and to gain a bet- ter understanding of the pollutant sources. Standard methods for evaluating the contribu- tion of atmospheric deposition to highway runoff should be developed. The contribution of organic and inorganic pollutants from atmospheric deposition likely differs between urban and nonurban areas. See section 3.4.3 for a discussion on atmospheric deposition. Itemized Research Needs 1. Studies that directly relate highways and transportation systems to atmospheric deposition, 2. Development of methods to evaluate the contribution of atmospheric deposition to highway runoff pollution, and 3. Evaluation of the fractions of pollutants contributed by atmospheric deposition for different land uses and classes of contaminants. First Flush Characterization There is seemingly a need for the adoption of a standardized method for defining and identifying first flush phenomena. Some parameters appear to exhibit a first flush, while others do not. Therefore, a comprehensive list of highway runoff pollutants that tend to exhibit a first flush may be useful for evaluating receiving water impacts and the fea- sibility of treating only the first flush of a storm. The current research effort did not find any studies that investigated specifically how the first flush effect was related to hydro- logical and watershed characteristics, indicating a potential research gap with regard to first flush characterization and assessment. See section 3.4.6 for a discussion on first flush characterization. Itemized Research Needs 1. Adoption of a standardized method for defining and identifying first flush phenomena, 10

11 2. Development of a list of highway runoff pollutants that tend to exhibit a first flush, 3. Evaluation of road surface runoff toxicity from different phases of a runoff event, 4. Correlation of toxicity with respect to pollutographs and hydrographs, 5. Evaluation of the effects of watershed characteristics on first flush phenomena, and 6. Evaluation of BMPs designed to capture the first flush. Impacts of Highway Construction and Vegetation Maintenance To evaluate erosion control practices, suspended sediment is the primary (and often the only) parameter monitored during highway construction runoff characterization studies. It is unclear in the literature whether total suspended solids (TSS) or suspended-sediment concentrations are being reported; these two terms often are used interchangeably but may yield vastly different results. There appears to be a need to evaluate the differences and consequences of using TSS for sediment load calculations and a need to make stormwater practitioners aware of this potential issue. Sediment particle-size distribution is an important parameter that is not monitored frequently. Particle-size distribution plays an important role in the transport and aquatic biota impacts of mobilized sediment, metals, nutrients, and trace organics. Since mon- itoring for particle size and other parameters may increase significantly the costs of a construction project, it would be beneficial to have an initial screening method for assessing the quality of site soils on a grain-size basis to determine if sediment and ero- sion controls are necessary to prevent impacts to receiving waters. Additional work also may be needed in the area of roadside vegetation management. The potential for herbicides to migrate from roadsides to receiving waters is strongly dependent on the type of chemical applied (i.e., depends primarily on solubility and hydrophobicity). Numerous herbicides—only a small number of which have been stud- ied for their mobility and potential toxicity to aquatic biota—are in use by DOTs throughout the country. More herbicide runoff characterization studies during storm conditions are needed as are toxicity studies of the concentrations found. Also, an analysis of the adsorption of herbicides to various grain sizes would aid in determining the potential for migration. When more information is available on the potential impacts of herbicides, a detailed cost–benefit comparison of using herbicides (as opposed to other vegetation control methods, such as manual clearing), should be considered. See section 3.4.8 for a discussion on the impacts of highway construction and vegetation maintenance. Itemized Research Needs 1. Standardization of suspended-sediment measurement and reporting methods, 2. Development of screening methods for assessing the quality of site soils on a grain-size basis so as to determine the level of monitoring as well as sediment and erosion controls necessary to prevent impacts to receiving waters, 3. Characterization of herbicide runoff and assessment of toxicity, 4. Guidance on maintenance facility BMP design, 5. Development of guidance for fertilizer utilization for seeding and turf establish- ment near sensitive water bodies (nutrient runoff prevention), and 6. Equipment testing methods and performance assessment of mechanical and mechanical/vacuum sweepers.

Stream Crossings Receiving waters are most vulnerable to highway runoff at stream crossings. Storm- water runoff or by runoff generated during maintenance activities such as bridge deck cleaning may cause direct impacts. Other bridge maintenance activities such as paint- ing, surface treatments, substructure repair, joint repair, drainage structures repair, and pavement repair or repaving also may impact receiving waters depending on storm event timing, duration, and intensity. With regard to highway runoff, potential impacts to receiving waters at stream crossings have been assessed by only a limited number of researchers. NCHRP Project 25-13 is the most extensive assessment to date on this topic. See section 3.5.2 for a discussion on stream crossings. Itemized Research Needs 1. Examination of the water quality effects of maintenance practices through field studies, 2. Development of a bridge deck runoff quality constituents database, 3. Examination of the potential risks associated with hazardous material spills, 4. Evaluation of how bridge design and average daily traffic affects runoff quality, 5. Assessment of potential receiving water temperature changes and mitigation, and 6. Development and evaluation of BMPs and standards for abating receiving water temperature impacts. Unit Treatment Processes Because NCHRP Project 25-20(01) was initiated to begin filling highway storm- water performance evaluation and assessment research gaps identified by earlier investi- gators and because the final report will include the identification of additional research gaps and needs with regard to unit process evaluations, it is premature to include such an identification here. Based on the opinion of the 25-20(01) project team, the most likely gap will be treatability data and information that can be used to characterize the fun- damental removal processes (unit processes) in action within a given BMP, as well as the simple lack of monitoring data of several different BMP types. See section 3.2.8 for a discussion on unit processes. Itemized Research Needs 1. Characterize and evaluate the fundamental treatment processes within different BMP types; 2. Conduct pilot-scale experiments for the collection of data on unit treatment processes for various BMP types; 3. Compile and assess available unit treatment processes data; 4. Research to obtain within-storm data on BMP effectiveness to assess short-term pollutant issues and collect unit treatment processes information; 5. Evaluate metals fractionation under anaerobic and anoxic conditions; 6. Develop the ability to measure accurately and analyze unit treatment processes; 7. Evaluate BMP design and performance with respect to particle-size distribution in stormwater runoff and associated metals; and 8. Evaluate the physical, chemical, and biological treatment processes of BMPs. 12

13 Toxicity and Bioassessment The top two research needs identified by GKY and Associates in the original NCHRP 25-20 report were (1) to identify and develop regional aquatic biological indi- cators for assessing impacts of highway runoff and (2) to conduct research methods for assessing the toxicity of highway runoff. This review supports this claim; bioassess- ment methods for assessing impacts of highway runoff on receiving water systems are inadequate, particularly for the time-scales typical of stormwater-runoff events. Also, a wide variety of assessment methods are currently in use by the few highway water quality researchers conducting toxicity and bioassessment studies. As a result, it is dif- ficult to compare quantitatively the existing data or to make any general assessment of the impacts of highway runoff on receiving water biota. In addition, more within-storm toxicity testing needs to be conducted to ascertain which parts of storm events are most toxic. Another research gap is comparison of runoff toxicity from different drainage systems (e.g., vegetated versus piped conveyance). Itemized Research Needs 1. Development of standardized bioassessment methods for assessing impacts of highway runoff on receiving water systems; 2. Evaluation of the parts of storm events most toxic to receiving waters; 3. Assessment of BMP performance in terms of toxicity reduction or other biolog- ical impact indicators; 4. Guidance on BMP selection based on toxicity; 5. Evaluation of chemical, physical, and toxicity impacts to aquatic biota of storm- water discharges; and 6. Evaluation of viral pathogen indicators and development of treatment options. Fate and Transport of Highway Runoff Constituents More detailed studies of sediment transport mechanics in relation to blockage of full and partly full conduits in various cross-sections may be needed. Comprehensive stud- ies on the effects of soils, topography, land use, and various storm hydrographs on sed- iment yield appear to be limited in number. In addition, the behavior of sediment at inlets, junctions, and transitions in the drainage system may require further study. Good predictive models that consider runoff–storm relationships, particularly storm scour and redeposition, are unavailable. Research on the speciation of pollutants has focused primarily on the dissolved and particulate fractions of the common metals found in highway runoff, cadmium, copper, lead, and zinc. However, there appears to be a need for better characterization of the bioavailability of dissolved metal complexes, as well as trace organics, in highway runoff. Sorption plays an important role in the speciation and bioavailability of pollutants; however, the factors controlling sorption—such as cation exchange capacity and spe- cific surface area—are poorly understood. There is a general need for more research on the sorption of pollutant to sediment in highway runoff. It would be beneficial to high- way agencies to have information on the sorption capacity of roadside soils for the pur- poses of prioritizing retrofits and installations of treatment control practices. See sec- tion 3.4.5 for a discussion on fate and transport of highway runoff constituents.

Itemized Research Needs 1. Identification of sediment sources and evaluation of transport rates and residence time of sediment in highway runoff, treatment facilities, and receiving waters; 2. Evaluation of sediment transport mechanics and blockage at inlets, junctions, and transitions in full and partly full conduits; 3. Comprehensive studies on the effects of soils, topography, land use, and various storm hydrographs on sediment yield; 4. Evaluation of nutrient leaching and the sorption and desorption processes of road- side soils; 5. Development of predictive models that consider runoff–storm relationships, par- ticularly storm scour and redeposition; and 6. Characterization of the bioavailable fraction of dissolved metals and trace organ- ics in highway runoff. Market-Driven Approaches: BMP Asset Management and Pollutant Trading Because market-driven, watershed-based stormwater management approaches are relatively new, further research into the practicality of such approaches is needed, par- ticularly for the application to highway runoff management. A current study funded by the Water Environment Research Foundation, when completed, should provide infor- mation and guidance on how a market-driven, watershed management system could be applied to the highway environment. See section 3.3.2 for a discussion on market- driven approaches. Itemized Research Needs 1. Research into the practicality of pollutant trading as a viable approach to high- way runoff management, and 2. Enhancement of maintenance management systems to facilitate asset manage- ment of BMPs. Gross Pollutant Removal and Drain Inlet Studies The effectiveness of gross pollutant source controls—such as street sweeping, pub- lic education, and catch basin cleaning, particularly with regard to the overall effects of catch basin bypass—have not been demonstrated clearly. Most researchers quantify gross pollutants by either weight or volume. Some segregate according to material type, such as plastic and metals. For the purposes of data transfer, development of standard methods for quantifying gross pollutants is needed. A uniform definition of gross solids (and its components) needs to be identified for purposes of standardizing the reporting of data. An ASCE/EWRI committee is working on this issue, but protocols developed for highway situations may be appropriate to help standardize BMP performance. The effects of gross solids on receiving waters are not well documented in the liter- ature. These effects need to be ascertained for the purposes of assessing future total maximum daily load (TMDL) requirements potentially faced by highway runoff man- agers. Modeling and estimation techniques for gross solids need to be developed espe- cially in relation to TMDLs. Leaching and sorption capacity of pollutants captured in catch basins represents another potential research gap. See section 3.2.2 for a discus- sion on gross pollutant removal and drain inlet studies. 14

15 Itemized Research Needs 1. Evaluation of the effectiveness and limitations of source controls at reducing gross solids in highway runoff (e.g., public education, catch basin cleaning, and street sweeping); 2. Development of a standard method for measuring and reporting gross solids; 3. Development of modeling and estimation techniques for gross solids; 4. Evaluation of the impacts of gross solids in highway runoff; 5. Evaluation of leaching or sorption capacity, or both, of pollutants captured in catch basins; and 6. Guidance on gross solids removal device design and performance. Pollutant Retention Some studies investigated the potential for leaching or resuspension of previously captured pollutants. They indicate resuspension of sediments in catch basin sumps and oil/grit separators may be significant. Resuspension also may occur in bioretention areas before the complete establishment of vegetation. Once captured, heavy metals do not appear to go easily into the dissolved phase, but nutrients do, particularly if there is a change in the oxidation-reduction potential. The pH of the stormwater affects the solubility of captured metals. With regard to pollution retention, it appears that the pri- mary research needs and gaps are in identifying the conditions—such as pH, oxidation- reduction potential, hardness, and organic content—that affect desorption or dissolu- tion, or both, of captured pollutants in stormwater treatment systems. See section 3.2.4 for a discussion on pollutant retention. Itemized Research Needs 1. Investigation of the potential for leaching or resuspension of previously captured pollutants; 2. Investigation of how changes in pH, oxidation-reduction potential, hardness, and organic content may affect desorption or dissolution, or both, of captured pollutants; 3. Assessment of the long-term ability of BMPs to keep pollutants sequestered; and 4. Bioavailability of pollutants in the sediments of wet ponds and wetlands used for highway stormwater treatment. Water Quality Runoff Modeling As with BMP modeling, there is a general need for accurate and representative data for parameter estimation and model calibration and for stochastic models and model development. There also is a need for data collection efforts that focus more on new constituents that may be required by models of the future. Additionally, hybrid models that take advantage of both stochastic and deterministic methods need to be developed. Adaptation of agricultural models for herbicide and pesticide modeling for highway runoff management could provide insights into the transport of pesticides and herbi- cides. Finally, existing models need to be extended and enhanced to simulate a wider range of contaminants in highway runoff. See section 3.4.7 for a discussion on water quality runoff modeling.

Itemized Research Needs 1. Forward-looking data collection efforts that focus more on the new parameters that may be required by models of the future, 2. Development of hybrid models that take advantage of both stochastic and deter- ministic methods, 3. Adaptation of agricultural models for herbicide and pesticide transport, 4. Extension and enhancement of existing models to simulate a wider range of con- taminants, and 5. Evaluation of the validity of build-up and wash-off as a method of estimating pol- lutant loads. Cold Weather Studies and Deicing Agent Impacts Based on the literature review, a clear need exists for more monitoring and charac- terization of snowmelt runoff from highways. The reviewed studies indicate that snowmelt runoff—especially during the first major snowmelt runoff events of the year—often has highly elevated pollutant concentrations. Some guidance is available from the Center for Watershed Protection (http://www.cwp.org/cold-climates.htm), but more guidance is needed. Models that can be used to predict the occurrence of a snowmelt runoff event could be helpful to determine when monitoring should take place. The performance and feasibility of stormwater BMPs during cold weather also need to be evaluated, along with the management of removed urban highway snow. With regard to the receiving water impacts associated with deicing agents, a data- base containing an evaluation of the human health and receiving water impacts along with toxicity test results for all existing deicing agents is needed to aid in the selection of deicing agents. Other potential research needs include the evaluation of the persis- tence and implications of various deicing agents in roadside soils, the evaluation of the factors that influence or compound receiving water impacts, and the development of strategies to minimize those impacts. Recommendations suggested by Fischel (2001) include the development and implementation of deicing strategies for reducing the amount of chemicals required and the development of decision support systems based on weather conditions to optimize deicing operations. Finally, there is also a need for guidance and methods for applying the minimum amount of deicing chemicals neces- sary to maintain safe road conditions. See section 3.4.10 for a discussion on cold- weather studies and deicing agent impacts. Itemized Research Needs 1. Guidance on monitoring roadside snow as well as snowmelt runoff; 2. Development of modeling methods for estimating snowmelt runoff events; 3. Evaluation of the performance and feasibility, as well as maintenance issues, of stormwater BMPs during cold weather; 4. Assessment of deicing agent and traction materials impacts on receiving waters; 5. Guidance on the management and storage of snow removed from urban highways to minimize impacts of snow storage area runoff; 6. Development of deicing agent selection criteria based on cost, effectiveness, and potential environmental impact; and 7. Guidance and methods for applying the minimum amount of deicing chemicals and traction sand necessary to maintain safe road conditions. 16

17 Modeling of Water Quality Impacts to Receiving Waters The category of water quality modeling shares research gaps identified under the BMP Modeling and Water Quality Modeling section of this report. These research gaps include the availability of data for accurate and representative parameter estimation, the ability to measure accurately and analyze unit processes, and model calibration, as well as the need for an expert model evaluation and selection system. Other potential knowledge gaps pertinent to water quality modeling include guidance on modeling temperature change impacts from pavement runoff, further development and enhance- ment of stochastic water quality models, evaluation of the limitations imposed by snow on water quality modeling methodologies, and the development of solutions for more accurate simulation of the effects of snow in water quality models. See section 3.5.5 for a discussion on modeling of water quality impacts to receiving waters. Itemized Research Needs 1. Research and data collection to support model parameter estimation, 2. Guidance on water quality model selection, 3. Development of stochastic water quality models, and 4. Development of models that predict pollutant bioavailability and toxicity. BMP Vector Control The potential for vectors, particularly mosquitoes, to inhabit and breed in stormwater control facilities is of increasing concern to stormwater management practitioners. The evident scarcity of studies and literature pertaining to the incidence of vectors in stormwater BMPs makes this whole category a research need. Research needs include the development and evaluation of maintenance and design practices that deter vectors. Poor water quality has been linked to the mosquito proliferation. Mosquito larvae thrive in stagnant and nutrient-rich waters, as nutrients provide food for the bacteria and algae on which mosquito larvae feed. A better knowledge of the relationship between mos- quitoes and water quality and flow rate may aid in assessing opportunities for vector control in highway BMPs. See section 3.2.15 for a discussion on BMP vector control. Itemized Research Needs 1. Evaluation of public health impacts of various stormwater management alterna- tives, and 2. Evaluation of maintenance and design methods for controlling mosquitoes and other vectors in highway BMPs. Runoff Characterization with Independent Variable Correlation With regard to suspended sediment and particle-size distribution, better characteri- zation of constituents associated with different-sized particles in highway runoff, par- ticularly heavy metals, nutrients, and hydrocarbons is needed. Average daily traffic does not appear to be a consistently good predictor of pollutant concentrations and loads. Vehicles during a storm may be a better predictor for some metals and nutrients, as well as TSS, chemical oxygen demand, biochemical oxygen demand, and oil and grease. There was only one study found that investigated the effects of antecedent dry

period traffic count; this may be indicative of another possible research gap. In general, traffic volume studies need to be evaluated according to their statistical significance. Runoff volume, rainfall volume, intensity, and duration are hydrological factors that have been shown by a few researchers to affect runoff constituent levels. Total storm volume affects loads of some water quality parameters—such as TSS and oil and grease—but does not appear to significantly affect concentrations. Correlations between intensity and duration with constituent levels are sparse in the literature reviewed, indi- cating this may be another research gap. Land use appears to affect average stormwater runoff concentrations, yet no studies have been found that show statistically significant differences in concentrations based on land use type alone. Characterization of runoff quality according to the various high- way classifications, especially urban versus rural, on-ramps and off-ramps, and percent impervious area appears to be an area needing further research. Staff at the Center for Watershed Protection, together with Dr. Robert Pitt, are com- piling and summarizing the available national data on urban runoff water quality and conducting data explorations to ascertain potential explaining factors. See section 3.4.2 for a discussion on runoff characterization with independent variable correlation. Itemized Research Needs 1. Better characterization of constituents associated with different-sized particles in highway runoff, particularly heavy metals, nutrients, and hydrocarbons; 2. Evaluation of statistically valid traffic volume-related studies; 3. Development of correlations between storm event intensity and duration with constituent levels; 4. Identification of statistically significant differences in concentrations in relation land use type alone; 5. Runoff quality characterization according to the various highway classifications (e.g., urban, rural, on-ramps, off-ramps, and total impervious area); and 6. Establishment of traffic thresholds beneath which certain pollutants in highway runoff can be considered negligible or irreducible. Wetland Impacts Based on the review of literature, the potential highway runoff impacts on natural and mitigated wetlands appear to be well documented. The tendency for many high- way runoff pollutants to accumulate in wetland sediments and vegetation raises some concern with regard to long-term impacts on wetland biota. There are various sediment toxicity methods available for assessing impacts to both freshwater and estuarine ben- thic organisms. Indicators may be needed for assessing impacts to wetlands from high- way runoff; this may require a detailed analysis of currently available data on wetlands receiving runoff from highway facilities. See section 3.5.9 for a discussion on wetland impacts. Itemized Research Needs 1. Compilation and analysis of available water quality, sediment quality, and bio- assessment data for wetlands receiving runoff from highway facilities, and 2. Development of bioindicators for assessing impacts to wetlands from highway runoff. 18

19 Public Perception and Aesthetics A limited amount of research has focused on the use of aesthetics and public per- ception as a BMP evaluation measure. Public opinion surveys that attempt to assess BMPs with respect to public perception may aid in selecting and improving the aes- thetics of BMPs and may provide insight on how to improve the public’s opinion of stormwater management. Furthermore, research that quantifies the impacts of various types of BMPs on property values may aid in the development of tools for evaluating the actual costs of BMPs. Such tools also could be used in public education and out- reach programs. Finally, methods and guidance are needed for maximizing the net ben- efit of BMPs by increasing their multi-use functionality, such as making treatment wet- lands safe and accessible to the public for bird watching or using porous pavements in pullout areas and rest stops. See section 3.2.13 for a discussion on public perception. Itemized Research Needs 1. Development of public opinion surveys to assess the public’s perception of storm- water management, in general, and BMPs in particular; 2. Guidance on how to improve public perception of various types of BMPs; and 3. Quantification of BMP impacts to property values and evaluation of methods to improve aesthetics and multi-use functionality. Impacts of Highway Construction and Repair Materials A literature review on highway construction and maintenance materials as a source of runoff contaminants reveals a limited number of studies on the subject (NCHRP Project 25-09 is the most comprehensive to date) and a significant amount of research in progress. Currently, potential gaps include the availability of materials properties data; sorption and desorption processes in roadside soils; a better understanding of the speciation, bioavailability, and toxicity of metals in highway construction material leachate; the effects and influence of temperature on the leaching of pollutants from construction materials; and a better understanding of the capabilities of existing BMPs to mitigate impacts from highway construction material contamination. See section 3.4.4 for a discussion on the impacts of highway construction and repair materials. Itemized Research Needs 1. Compilation of properties data for highway construction and repairs materials; 2. Evaluation of the speciation, bioavailability, and toxicity of metals in highway construction material leachate; 3. Evaluation of the effects and influence of temperature on the leaching of pollu- tants from construction materials; and 4. Evaluation of the capabilities of existing BMPs to mitigate impacts from high- way construction materials contamination. Groundwater Quality Analysis and Impacts Based on the research review, information is needed on the potential impacts to groundwater caused by infiltration of stormwater runoff. The methods used to assess impacts are difficult to implement and the results are difficult to assess. State DOTs

need a procedure to estimate the potential extent and magnitude of groundwater qual- ity degradation from transportation BMPs, particularly those that rely on infiltration as their primary treatment mechanism. Guidance would include procedures for identify- ing and evaluating current and potential uses of groundwater and water quality require- ments that could be affected by transportation BMPs. See section 3.5.8 for a discussion on groundwater quality analysis and impacts. Itemized Research Needs 1. Development of a standardized procedure for monitoring and assessing soil and groundwater impacts caused by infiltration facilities; 2. Evaluation of the pollutant retention capacities of different soil types and geo- logical conditions; 3. Evaluation of the potential groundwater impacts of soluble highway runoff pol- lutants such as herbicides, nutrients, deicing agents, petroleum hydrocarbons (e.g., BTEX), and gasoline oxygenates; 4. Determination of the sources of MTBE in groundwater; 5. Development of infiltration guidance to prevent groundwater contamination; and 6. Development of approaches addressing groundwater pollutants introduced to sur- face waters from dewatering operations. Water Quality Impacts of Combined Sewer Overflows Combined sewer overflow (CSO) systems are widely variable, and water quality impacts depend on a host of site-specific parameters. Assessment of impacts is based primarily on computer simulations. There are needs for better monitoring of CSO efflu- ent quality in relation to meteorological factors, identification of the prevailing condi- tions or factors for increasing or decreasing CSO impacts, and methods for mitigating impacts (structural and nonstructural). However, CSOs are being phased out slowly through retrofits and new construction, so CSO research needs are considered a low pri- ority for highway runoff control and management. See section 3.5.6 for a discussion on the water quality impacts of CSOs. Itemized Research Needs 1. Hydraulic assessment of highway runoff contributions to CSO impacts to receiv- ing waters, 2. Better monitoring of CSO effluent quality in relation to meteorological factors, 3. Evaluation of the prevailing conditions or factors that increase or decrease CSO impacts, and 4. Development and evaluation of practices (structural and nonstructural) to miti- gate CSO impacts. DETAILED PROJECT STATEMENTS The research statements identified in this document are a subset of the numerous research gaps that exist in the area of highway runoff management. The combination of a detailed survey, a literature review, and the opinions of stormwater experts has resulted in identifying the most pressing research gaps in the area of highway runoff management. The individual research gaps identified in Table 4-1 were combined to 20

21 form the various projects described below with tasks that will address the identified objectives. BMP Maintenance, Costs, and Longevity There is little information available that has been based on actual field data regard- ing the amount and frequency of the BMPs maintenance needed to maintain the pollu- tant removal effectiveness and the related costs. The purpose of this research would be to develop better information on the types and frequencies of BMP maintenance needed and on the cost-effectiveness of such maintenance. Guidance on how to factor BMP maintenance costs into life-cycle costs for BMP selection also would be developed. In addition, this project would investigate the state of the practice in how costs of BMP maintenance are being tracked in maintenance management systems, as well as how such information could be compiled and shared nationally on a more continuous basis. See sections 3.2.11 and 3.2.14 for discussions about BMP maintenance, costs, and longevity. The research gaps addressed under this research statement are listed under the BMP Maintenance and Longevity and the Economic Analysis and Assessment of BMPs sections of Table 4-1. Research Objectives Specific research topics addressed by this proposed project include the following: • Develop BMP maintenance versus performance information, including how to maintain infiltration BMPs to extend the effective life of these facilities and how maintenance (pollutant removal) can reduce potential build-up of pollutants to hazardous levels; • Identify the state of the practice in how BMPs cost information is being tracked within maintenance management systems as well as how such information could be compiled and shared nationally on a more continuous basis; • Identify how maintenance costs should be incorporated into life-cycle costing for BMP selection for highways (for overall evaluation of cost-effectiveness); • Develop nationally applicable BMP operations, inspection, and maintenance guid- ance (maintenance frequencies, logistics and personnel requirements, and esti- mates based on influent characteristics and site conditions), including suggestions on contractual methods to improve BMP implementation, removal, and disposal of BMP captured pollutants, and environmentally sensitive BMP maintenance (to reduce effects on endangered and threatened species, wetlands, and pond treatment systems); and • Identify whether maintenance data could be overlaid with other geographic infor- mation system information to determine cost factors across various landscapes. Determine cost factors across landscapes using pilots in several locations selected to tease out patterns in cost differences in rural, suburban, and urban landscapes and different climatological zones. Tasks 1. Search Literature and Gray Literature: Reassess available research on BMP maintenance from highways and other urban areas for information on BMP main- tenance versus performance and for the other research questions above. Critically

assess what information still needs to be collected via field studies. Recommend two BMP types for an example study that could be performed to assess mainte- nance needs; include a biofiltration-type BMP (e.g., wetland, pond, bioswales) and a more structural-type BMP (e.g., underground vault or filtration type BMP). 2. Survey DOTs to Identify the State of the Practice in How BMP Cost Information is Tracked: Survey DOTs to identify the state of the practice in how BMP cost information is tracked in construction budget reporting and within maintenance management systems and to determine how such information could be compiled and shared nationally on a more continuous basis. 3. Develop a Field Evaluation Plan for BMP Maintenance Testing: Develop a field testing plan for a 6-year study of maintenance versus performance for the initial two BMP types. The purpose of this task is to test an initial set of BMPs, so that testing on other BMP types could be recommended and completed based on what is learned from the program. For each type of BMP, 6 facilities should be tested (i.e., a total of 12 for two BMP types). The testing should include a minimum of six storms per year in which storm flow-weighted composite samples and selected grab samples for a suggested suite of parameters (minimum of TSS, dissolved and total heavy metals, nutrients, and oil and grease) are collected during a storm. The test plan should suggest how to alter maintenance strategies so that the perfor- mance of the BMP can be evaluated (e.g., alternating maintenance levels for 3 years for each site or comparison sites). 4. Conduct Field Evaluation: Under this task the above study plan will be imple- mented. Produce interim reports at 2 and 4 years and a final report at 6 years. The final report should recommend additional testing of BMP types and improve- ments to the testing plan and strategies. 5. Develop Recommended Draft National Guidance: Based on the literature review and the results of the field evaluation, prepare guidance on BMP operations and maintenance that addresses the above research questions. The Draft National Guidance would be updated as additional BMPs are tested. Estimated Project Budget: $800,000 to $1,200,000; 36 months to complete. Highway Runoff Information Sharing and Technology Exchange Systems Although there are still many research needs regarding highway runoff, a great deal of information exists on highway runoff characterization impacts to receiving systems, construction and post-construction water quality control technologies, and nonstruc- tural practices. Many states have made significant investments in new technology eval- uations and highway runoff studies. Nevertheless, this information has not been read- ily available to practitioners in forms that are useful for assisting in highway runoff planning and design efforts. DOTs need ways to better access the fragmented and scat- tered information that already exists. The purpose of this project would be to establish an Internet site that would be developed in conjunction with other NCHRP best prac- tice information collection efforts, such as NCHRP 25-25(04), and water quality BMP evaluation projects, such as NCHRP 25-20(01). The Internet site could speed and expand availability of relevant transportation runoff water quality programs and practices so that the findings of research efforts could be used. Electronic exchange through the Internet, list servers, or electronic subscriptions or newsletters has been noted as the preferred method for information exchange by DOTs; there is a need for a central repository of this information. Practitioners manually review 22

23 many paper products and develop their own information networks. Beyond AASHTO’s Environmental Technical Assistance Program, no formal system keeps practitioners aware of new information. Timeliness of getting new information is a function of data availability, and practitioners are frustrated with the difficulty of finding relevant infor- mation. The purpose of this project is to recommend an information exchange mecha- nism for sharing and disseminating information that supports more effective water qual- ity management at DOTs. See section 3.1 and Chapter 3 of the GKY report (NCHRP Web Document 37) for a summary of major runoff management information sources. The research gaps addressed under this research statement are listed under the Information Sharing and Technology Exchange Systems section of Table 4-1. Research Objectives The Internet-based resource would extend highway runoff characterization, impacts, and best practice collection and technology transfer by including • The community’s best bibliography of titles and authors, with as many annotations and cross-references as possible; • An extensive online library of as many complete sources as possible (or links to sites where such information is available) to shorten the acquisition time for infor- mation on subjects covered; • An area for postings and roundtable discussions of research updates and progress reports on work of interest to the community [The discussion forum would facili- tate professional exchange of information publicly (through the forum) and indi- vidually (off line)]; and • A strategy for publicizing the site’s existence and incorporating access to it into appropriate online resources. Tasks 1. Develop a Conceptual NCHRP Highway Runoff Information Sharing Plan: Develop a formal list of information needs based on the NCHRP 25-20, 25-25(04), and 25-20(01) research efforts. Review existing literature, on line and published, for supplementary information. Develop a recommended plan for implementa- tion, to include • Development of a bibliographic database for compilation of major syntheses addressing stormwater runoff research and guidance; • Development of a stormwater runoff and BMP performance and design data- base specific to highways or identification of suitable databases with relevant links (e.g., the National Stormwater BMP Database); • Development of an information sharing system for highway runoff research documents and monitoring data; • Development of an online chat area for discussion of highway runoff issues; • Development of links to other best practices information collection efforts and online resources; and • Development of links to guidance available on line for highway runoff related issues, including monitoring procedures, impact analyses, and BMP design. 2. Finalize Implementation Plan: Finalize the implementation plan based on meet- ings with the project committee and on discussions with NCHRP staff.

3. Implement Information Sharing Program: Implement the information sharing program, including instructions for continued operation of information sharing tools by an appropriate entity. Estimated Project Budget: $200,000 to $300,000, 18 months for implementation. $40,000 to $50,000 per year to maintain information sharing tools (not included in this project). Watershed-Based Highway Runoff Mitigation Approaches and Guidance There is a high potential to increase the cost-effectiveness of highway stormwater management through greater DOT participation in watershed planning and information collection and sharing efforts. Such participation has the potential to result in alterna- tive BMP siting within a watershed and increased cost-effectiveness of stormwater con- trols. Transportation project delivery often is impeded by regulatory requirements for the mitigation of water quality, hydrology, and habitat impacts within or adjacent to the transportation system right-of-way (in-ROW mitigation) or by DOT reluctance to use something other than onsite BMPs. Onsite BMPs are problematic in some cases and may be less effective than desired because of site-specific constraints such as a lack of available land, presence of protected natural resources, unstable slopes, shallow water tables, excessive costs of construction or maintenance, or marginal environmental ben- efits of the technology employed. Additionally, the limits inherent in in-ROW mitiga- tion can impede the ability to address a community or watershed’s most critical water quality needs. On the other hand, opportunities may exist in areas with substantial ROW to boost treatment of runoff from adjacent areas, again enhancing the public sec- tor’s ability to address water quality issues on a watershed basis. Many states and local municipalities currently support projects that are assessing sur- face water quality, groundwater quality, floodplain impacts, wetland protection, and streambank–shoreline erosion at the watershed–ecosystem level. Transportation sys- tems potentially could use the flexibility that watershed-based mitigation provides to reduce project costs, maximize environmental benefits, and address multiple ecological needs and functions. The more piecemeal approach typically employed today can result in less cost-effective mitigation strategies and inequitably burdened mitigation costs. New approaches and flexibility for mitigation are needed if transportation systems are to improve project delivery and to maximize the benefits of environmental invest- ments. The purpose of this project would be to develop a guidance document, with case studies, of how transportation agencies could participate in watershed efforts to miti- gate highway runoff impacts. See section 3.3 for a discussion on watershed-based approaches. The research gaps addressed under this research statement are listed under the Watershed Planning section of Table 4-1. Research Objectives • Evaluate pilot watershed-based transportation approaches undertaken in Wash- ington State, North Carolina, and other model states. • Identify protocols that have been developed for establishing critical needs and local priorities (including methods developed under the North Carolina Ecosystem Enhancement Program, Center for Watershed Protection, and EPA) and for identi- fying opportunities for mitigation banking, retrofits, offsite mitigation develop- ment, pollutant abatement trading, and watershed-based BMP design and place- 24

25 ment. Identify gaps and needs for a protocol that could be used by transportation agencies on a national basis. • Develop a model by which to consider the relative costs and benefits of watershed- based mitigation versus site-specific mitigation from a cumulative effects per- spective, linked to loss of function from impacted sites. Assess quantitatively and qualitatively the monetary and ecological benefits of watershed-based mitigation. • Develop guidance on watershed-based approaches for transportation agencies with case studies. Tasks 1. Conduct a Review of Watershed-Based Approaches: Conduct a critical review of watershed-based approaches and protocols that have been implemented and assess their potential for application to transportation agencies. The results of this review should include overall synopses of potential approaches and particular case studies of potential models. The review should include water quality and hydromodification (hydrology changes) in addition to a review of pollutant trad- ing, as applied to surface waters. The review also would include an evaluation of existing regulatory policies and impediments to implementation. Gaps and needs for a protocol that could be used by transportation agencies on a national basis would be identified. 2. Develop Potential Protocols: Based on Task 1, develop potential protocols that could be used by transportation agencies for addressing impacts of highway runoff via a watershed-based approach. Models for considering the relative ben- efits and costs of watershed approaches versus onsite approaches should be included in the protocol. The product of this task would be a report on the sug- gested protocol that the project committee could discuss. 3. Develop Watershed-Based Highway Runoff Mitigation Guidance: Based on proj- ect committee feedback, develop a guidance document on how watershed-based approaches can be applied to mitigation of highway runoff. Include guidance on how “over-mitigation” of highway runoff or adjacent land uses, or both, might provide value to a transportation agency. Estimated Project Budget: $200,000 to $250,000, 18–24 months to complete. Economic Analysis and Assessment of BMPs The costs for selecting, designing, and installing (and maintaining—an emphasis of RS-1) BMPs are not well understood and in many cases are not tracked. Costs also often can be misleading. For example, frequently land-intensive BMPs are placed in areas that would have been landscaped in any case, and therefore no additional land costs are pro- cured with BMP implementation; yet, these land costs are sometimes included in assess- ing BMP costs. In order to improve the information available on BMP costs, a more uniform set of protocols is needed for recording and tracking such information. In addi- tion, guidance is needed on how to use available and improved cost information, so that BMPs can be compared and selected on a cost-effectiveness basis. See section 3.2.14 for a discussion on the economic analysis and assessment of BMPs. The research gaps addressed under this research statement are listed under the Economic Analysis and Assessment of BMPs section of Table 4-1.

Research Objectives • Development of highway runoff BMP cost determination protocols that account for land value (if land had to be purchased or eliminated from another use), con- struction, operations, and maintenance, as well as receiving waters protection, aes- thetics, and potential infrastructure savings on conventional drainage structures; • Guidance on quantifying and comparing BMP lifecycle costs and benefits associ- ated with receiving waters protection; • Cost comparisons of BMP treatment trains, distributed BMPs, and regional BMP systems; and • Cost estimates for nonstructural BMPs. Tasks 1. Conduct a Review of BMP Costs and Benefits: A literature review and survey of available state DOT BMP cost and benefits data and methodology should be con- ducted. Available data on BMP costs should be summarized in a concise report. 2. Develop BMP Cost and Benefit Protocols: A protocol by BMP type (e.g., National BMP Database Classifications or other suitable BMP classification scheme) will be developed for recording BMP cost information and the benefits to receiving waters. The protocol will include costs for BMP design and construction; BMP mainte- nance; land costs, including whether they were additive or not; avoided costs (such as reduced piping or other conveyance structures); and estimates of the monetary benefits of receiving waters protection from pollutants and hydrological impacts (such as avoided fines and required restoration). The protocol also will include a tool for assessing the lifecycle costs and benefits of highway runoff BMPs. 3. Compile and Summarize Available Data on BMP Costs and Benefits: Data from the sources in Task 1 will be compiled and placed into the protocols where appro- priate. Establish identification of different qualities of data. Subsequently sum- marize by BMP type the cost and benefit data. Assess the value of available cost- ing data. 4. Develop Guidance on BMP Costs and Benefits: Develop a guidance manual on how to track costs and benefits of highway runoff BMPs based on the previously listed data. The guidance will be by BMP type and activity, based on the proto- cols developed above. In addition, the guidance will include the information available on costs and benefits of BMPs by BMP type as established in Task 3. Once it is implemented, cost and benefit information could be exchanged in the Information Sharing System described in RS-2. Estimated Project Budget: $200,000 to $275,000; 24 months to complete. BMP Evaluation and Design Expert System—Construction Sites The selection and design of construction site BMPs is very dependent on site specifics. To productively use existing BMP effectiveness studies, practitioners must determine the aspects applicable to a specific site under consideration and how that information applies to the site. Such a system could be developed as an automated interface and database, which users could access on desktop systems or via the Internet. Better guid- ance on the selection and design of construction site BMPs would be helpful in improv- ing the cost-effectiveness of construction site BMPs. See section 3.2.6 for discussions on erosion and sediment control. The research gaps addressed under this research state- 26

27 ment are listed under the General BMP Evaluation and Selection section of Table 4-1 of this report and Chapter 6 of the GKY report (NCHRP Web Document 37). Research Objectives • Development of guidance on highway construction site factors versus construc- tion site BMP selection and design, and • Development of an expert system for use in construction site BMP selection and design based on developed guidance. Tasks 1. Conduct a Review of Existing Highway Construction Guidance Documents: Con- duct a critical literature review on available construction site BMP selection and design; make recommendations on how the available guidance documents can be used to develop a national-level guidance document. 2. Develop a National-Level BMP Selection and Guidance Document for Highway Construction Projects: Develop a national-level guidance document targeted toward transportation agencies. The guidance document should include how spe- cific site factors, receiving water sensitivity, and other factors should be used in selecting and designing construction site BMPs. 3. Develop an Expert System: Incorporate the guidance document into an expert sys- tem that can be used to guide construction site BMP selection and design. The expert system will be able to be customized by individual agencies to meet their respective needs. Estimated Project Budget: $250,000 to $300,000; 24 months to complete. Evaluation and Design Expert System—Post-Construction The selection and design of post-construction site BMPs is very dependent on site specifics. State DOTs have indicated that having better guidance on the selection and design of post-construction site BMPs would be helpful for improving the cost- effectiveness of BMPs. To effectively use existing BMP effectiveness studies, practi- tioners currently must determine which portions are applicable to a specific BMP under consideration and how that information applies to a particular site. Practitioners could benefit from accessible BMP information and a means of quickly applying applicable portions of it to site-specific situations—hence, an expert system. Such a system could be developed as an automated interface and database, which users could access on desktop systems or via the Internet. A critical component of this work is the development of a rec- ommended set of protocols for assessing BMP performance for highway BMPs. The use of percent removals of pollutants has been found to be very problematic. See section 3.2.1 for a discussion on general BMP evaluation. The research gaps addressed under this research statement are listed under the General BMP Evaluation and Selection section of Table 4-1 in this report and Chapter 6 of the GKY report (NCHRP Web Document 37). Research Objectives • Development of a recommended set of protocols for characterizing the performance of BMPs. The National BMP Database team has suggested that post-construction

BMPs should be characterized by (1) how much runoff is prevented, (2) how much of the runoff that occurs is treated by the BMP, and (3) what are the effluent qual- ity characteristics of the runoff that is treated. There are many other suggested per- formance measures that should be considered in the development of a set of high- way BMP performance protocols. • Development of overall guidance on the selection and design of highway run- off BMPs. • The term “ultra-urban” has been used to describe areas of the country where space for stormwater BMP implementation in urban areas is limited. The goal of ultra- urban technology is to provide cost-effective, low-maintenance solutions to storm- water management problems in the ultra-urban environment. A number of ultra- urban BMPs were identified in a national study by FHWA on ultra-urban BMPs, the purpose of which was to provide a planning-level review of the applicability and use of new and more traditional BMPs in ultra-urban settings. A specific research objective of this work is to develop guidance on the selection and design of such measures. – Collect existing research on ultra-urban BMPs and identify pilots or demonstra- tions that are still needed. Demonstrate and evaluate a number of case studies for practicality, so that initial comprehensive design guidance may be undertaken. – Evaluate likely ultra-urban BMP designs in ultra-urban settings for operational effectiveness as a function of maintenance requirements. – Identify regional and site-specific issues and guidance. – Incorporate results into the design guidance manual. • Development of an expert system for use in post-construction site BMP selection and design based upon developed guidance. Tasks 1. Conduct a Review of Existing Highway Post-Construction BMP Selection and Design Guidance Documents: Develop a critical literature review on available post-construction BMP selection and design, including recommendations on how the available guidance can be used to develop a national-level guidance document. 2. Develop a Set of Proposed BMP Effectiveness Protocols: Develop a recom- mended set of protocols for assessing the performance of highway runoff BMPs, so that the performance of various BMPs could be compared to one another and contrasted with receiving water goals and standards. 3. Develop a Research Report on Ultra-Urban BMPs: – Collect existing research on ultra-urban BMPs and identify pilots or demon- strations that still are needed. A number of case studies need to be demonstrated and evaluated for practicality so that initial comprehensive design guidance can be undertaken. – Evaluate likely ultra-urban BMP designs in ultra-urban settings for operational effectiveness as a function of maintenance requirements. – Identify regional and site-specific issues and guidance. 4. Develop a National-Level BMP Selection and Guidance Document for Highway Post-Construction BMPs: Develop national-level guidance targeted to transporta- tion agencies. Include how specific site factors, receiving water sensitivity, and other factors should be used in selecting and designing post-construction BMPs and include in-depth exploration of ultra-urban BMPs. 28

29 5. Develop an Expert System: Incorporate the guidance document into an expert sys- tem that can be used to guide post-construction BMP selection and design. The expert system would be able to be customized by individual agencies. Estimated Project Budget: $450,000 to $550,000; 30 months to complete. Low Impact Development/Distributed BMPs LID/Distributed BMPs are gaining increasing attention in the stormwater manage- ment field and among selected DOTs; however, their functioning in various geographic conditions and environments is not well known, particularly as applied to highway environments. LID design guidance is being prepared under NCHRP 25-26. In the course of this work, additional research needs regarding LID/Distributed BMPs will be identified. It is likely that these will include • Documentation of LID’s applicability, efficacy, and long-term economic sustain- ability for transportation systems; • Evaluation of the type of hydrologic losses that can be achieved under various cli- matic, soil, slope, and vegetation conditions; and • Development of methods and technologies to promote the reuse of stormwater. This area of research was ranked as a relatively high priority by state DOTs and was included here for that reason; it should be modified based on the results of the ongoing 25-26 project. See section 3.2.9 for a discussion on LID. The research gaps addressed under this research statement are listed under the Low Impact Development/Distributed BMPs section of Table 4-1. Research Objectives • Conduct a review of LID/Distributed BMPs that are in the ground and operating. • Conduct an assessment of the types of hydrological losses that can be achieved under different scenarios of climates, soils, slopes, and vegetative conditions and an assessment of how the losses will reduce or eliminate downstream impacts of increased runoff from highway environments. • Determine feasibility of and conditions for re-use of captured stormwater. Tasks 1. Perform a Survey and Review of In-the-Ground Applications of LID/Distributed BMPs: Critically review constructed LID-type projects to assess potential bene- fits and costs. Interview state DOTs to ascertain what monitoring (water quantity and quality, as well as visual, maintenance, and problem) has been completed. Develop a report of case studies and include recommendations for future designs. 2. Conduct a Hydrological Loss Assessment: Unless Task 1 includes some detailed hydrological monitoring that can be used to ascertain losses from LID-type BMPs, base the hydrological assessment primarily on the use of existing continuous simulation models (such as SWMM) to ascertain on regional and national lev- els what types of runoff hydrological losses can be achieved and how these reductions in runoff affect potential downstream channel erosion levels. Include

in the the assessment an evaluation of the site conditions (such as climatic, soil, slopes, and downstream water body sensitivity) and LID types and level of imple- mentation necessary to fully or partially mitigate downstream erosional impacts. 3. Develop Potential Water Re-Use Methods and Technologies: Review potential highway runoff water re-use applications. Develop potential conceptual water re-use scenarios and evaluate them for potential costs and benefits. Perform long- term simulation modeling to ascertain the storage size that may be required; eval- uate the potential for draining the storage to ascertain the costs and benefits. Estimated Project Budget: $150,000 to $250,000; 18 months to complete. BMP Design Variables The effect of BMP design variables on BMP performance is not well understood. To date, major efforts such as the National Stormwater BMP Database have been able to confirm only a few of the major design variables that can be shown statistically to affect BMP performance. The BMP design guidance developed to date has been based primarily on “good engineering judgment” and on some limited modeling studies. This research project would develop an approach for conducting more rigorous evaluation of BMP design versus performance on one or two selected BMP types and would make recommendations for future evaluations. Research Project 25-20(01) is developing a unit processes evaluation of BMP performance that should indicate some of the impor- tant design variables for BMP performance. This research is intended to build on that effort by conducting more in-depth evaluations of the design parameters that appear to affect BMP performance but are not well understood. See section 3.2.7 for a discus- sion on BMP design variables. The research gaps addressed under this research state- ment are listed under the Design Variables Affecting BMP Performance section of Table 4-1. Research Objectives • Evaluation of design variables that are related to biochemical and geochemical treatment mechanisms in BMPs, and • Implementation of pilot-scale experiments that evaluate the relation of various design variables to BMP performance. Tasks 1. Review the Findings of NCHRP 25-20(1) and Select Design Variables: Select one or two BMP types and design variables, based on the findings of NCHRP 25-20(1), to further evaluate design versus performance. Prepare a study plan to conduct a laboratory or field program (preferable), or both, to explore these design variables. 2. Conduct BMP Design Variables Evaluation: Conduct a laboratory or field pro- gram evaluation, or both, of BMP design parameters. Report on findings in terms of recommendations for improving designs. Develop recommendations regard- ing future testing and evaluation efforts. Estimated Project Budget: $250,000 to $400,000; 24–36 months to complete. 30

31 BMP Modeling Tools BMP modeling tools are important to transportation agencies, particularly when highway runoff is draining into sensitive receiving waters. Increased understanding of BMP performance, the factors and designs that affect performance, and how BMPs ulti- mately affect receiving water quality has increased the need to identify and to improve BMP modeling capabilities. Several NCHRP projects, as well as other projects, have resulted in increased understanding of BMP performance. The purpose of this project is to develop guidance on BMP performance modeling. See section 3.2.10 for a dis- cussion on BMP modeling. The research gaps addressed under this research statement are listed under the BMP Modeling section of Table 4-1. Research Objectives • Evaluation of state-of-the-art BMP modeling approaches and development of guidance for model selection, needed model improvements, and application; • Development of unit treatment models that incorporate sorption, biodegradation and uptake, photolysis, and volatilization; • Development of simulation models for BMP treatment trains; • Implementation of pilot experiments to collect data needed for parameter estima- tion and model calibration; and • Development or evaluation of models that can be used for simulating pollution deposition within BMPs to assess potential impacts to wildlife. Tasks 1. Review the Findings of NCHRP 25-20(1) and BMP Modeling Research: Evalu- ate the results of the NCHRP 25-20(01) project to develop unit process descrip- tions of BMP performance and other relevant BMP modeling papers and to com- pile a listing or description of potential needs for BMP modeling improvements. Identify model approaches that may apply faulty logic (such as attributing all pol- lutant generation to model “build-up–wash-off” functions that then result in faulty street sweeping effectiveness estimates). 2. Review Available BMP Models and Approaches: Review the available models that typically are employed to model BMP performance and suggest a detailed list of needed improvements to such models, in order to address the stated research objectives. 3. Select One Publicly Available Model and Develop Model Improvements: Select a public domain model, such as SWMM, that typically is used for BMP model- ing and implement recommended improvements. 4. Develop a Pilot Experiment Plan: Develop a pilot experiment plan to collect the data needed for the adapted model’s parameter estimation and calibration. Develop a case study and guidance on the model’s application. Estimated Project Budget: $200,000 to $275,000; 18–24 months to complete. Stormwater Detention Hydraulic Performance and Retrofit Options The hydraulics of stormwater detention systems (ponds and vaults), including wet ponds and dry extended detention systems and the various combinations of these two

(including stormwater wetlands) are thought to greatly influence water quality. The purpose of this proposed research project is to evaluate how the performance of ponds and other types of detention-based BMPs can be optimized using the careful consideration and design of hydraulic characteristics of flows routed to and within these types of BMPs. Existing transportation infrastructure includes drainage management facilities such as detention ponds and storm sewers, which must be used, enhanced, and potentially extended to respond in a cost-effective manner to the increasing requirements of NPDES, TMDL, and the Endangered Species Act. Practitioners need practical guid- ance for capturing water quality benefits from BMP infrastructure already in place. Currently, there are no coordinated programs to address how existing infrastructure can be modified to benefit water quality. Existing infrastructure removes water from the roadway, and many hydraulic facilities can be retrofitted to provide water quality ben- efits; examples include catch basin–inlet modifications, detention pond retrofit to embankments and outlet works, riser structures added to culvert–embankment systems, and the fostering of pipe storage in storm drains. The overall potential of retrofit depends on the incidence of existing drainage systems available and on the receptivity to retro- fits. See section 3.2.3 for a discussion on hydraulic assessment of BMPs. The research gaps addressed under this research statement are listed under the Hydraulic Assessment of BMPs and the Methods to Improve Pollutant Removal in Existing Stormwater Sys- tems sections of Table 4-1. Research Objectives • Evaluation of the characteristics and effects of short-circuiting, bypass, and overflow; • Evaluation of the nature of correlation between hydraulic residence time and performance; • Development of methods or models for estimating the true hydraulic residence in stormwater ponds; • Development of methods to optimize detention basin design to maximize treat- ment; and • Assessment of retrofit options for flood-control basins and systems that maximize water quality while maintaining adequate flood-control protection. Tasks 1. Review the findings of NCHRP 25-20(1) and BMP Modeling Research: Evaluate the results of 25-20(01) to develop unit processes descriptions of BMP perfor- mance and other relevant BMP modeling papers and to prepare a review of the potential characteristics that can be used to optimize performance of detention systems. Based on this review, develop a detailed study plan for modeling and field analyses to assess the hydraulic characteristics that lead to improved storm- water detention BMP design. 2. Develop and Apply Models for Residence Times: Develop or adapt available mod- els to evaluate the effects of short-circuiting and the relationship between resi- dence time and performance. Apply models that evaluate the effects of by-pass and overflow on overall performance on a regional basis. This evaluation will examine the effects of larger detention sizes versus performance and will include an assessment of the trade-offs between longer residence times and increased by-pass or overflow, or both. The results of the task will create a report on how 32

33 short-circuiting can be minimized in ponds via optimization of pond design and on sizing and drain times versus overall water quality performance. Based on the modeling work, develop a relationship between pond design factors and hydraulic residence times for use in design work. 3. Evaluate How Existing Flood-Control Systems May Be Altered to Improve Water Quality: The purpose of this task is to develop an approach that evaluates sys- tematically existing flood-control facilities for the feasibility of retrofits to improve water quality. Identify elements of typical existing hydraulic facilities that may be modified or enhanced to provide water quality benefits. Review operation and design principles that can enable feasible and cost-effective modifications—hard design, extension of the treatment train, and management change. Find and eval- uate existing retrofits of detention facilities reported in the literature and report on their cost-effectiveness. Develop water quality retrofit conceptual designs to enhance water quality elements for as many drainage element types as possible— detention systems are the primary focus, but also include storm sewers, roadside channels, trenches, and drainage swales. An assessment of the potential to reduce flood protection, based on long-term simulations of flooding versus design, should be made to determine if some systems may be overdesigned and, therefore, whether some detention volume may be available for water quality improvement. 4. Develop a Guidance Manual: Develop a practical guidance manual for trans- portation practitioners to select, design, and implement water quality retrofits for existing highway drainage systems. Estimated Project Budget: $225,000 to $275,000; 18 months to complete. Assessment of the Effects of Hydromodification, Sedimentation, and Turbidity Changes in runoff volumes due to increased impervious surface from development have been receiving increasing attention for effects on downstream erosion, sedimen- tation, and turbidity. Sedimentation and turbidity from the highway runoff can be an issue, particularly in areas where sanding is employed. Highway construction sites can be a source of sediments and particulates during the construction phase. The purpose of this project is to assess the potential for highway sites to contribute to these prob- lems and to examine how they can impact receiving systems. The project also would evaluate and produce recommended methods for reducing sources of turbidity and sed- iments and would attempt to describe where vegetated swales may treat or reduce ade- quately runoff from highways. See section 3.5.3 for a discussion on sedimentation and turbidity. The research gaps addressed under this research statement are listed under the Sedimentation and Turbidity Impacts section of Table 4-1. Research Objectives • Examination of the effects of frequent short-term pulses of suspended sediment; • Identification of additional research needs on the correlation between particle size, shape, and composition of sediments to fish sensitivity; • Evaluation of the relationships between seasonal timing and the effect of sedi- ment load; • Evaluation of how the knowledge of fish survival responses to turbid water flows can be applied to the development of mixing zones, work windows, treatment systems,

and buffers that will allow fish to perform their necessary life functions during project construction and operation; • Identification of practical means of controlling turbidity; and • Development of hydromodification measures (estimated downstream hydrologi- cal changes) from highway runoff and, subsequently, development of measures for assessing potential downstream channel and bank instability. Tasks 1. Prepare a Review of How Sedimentation, Turbidity, and Hydromodification Can Affect Receiving Waters: Conduct a literature review on the potential impacts of sedimentation, turbidity, and hydromodification on receiving waters, with an emphasis on highway runoff. Include a characterization and evaluation of parti- cle sizes, shapes, and composition from construction sites as well as from com- pleted highways for different drainage system configurations. Make recommen- dations for data gathering on needed information. 2. Conduct a Hydromodification Assessment: Using long-term simulation models, perform an assessment of the conditions under which highway runoff (e.g., high- way area as compared to watershed area of receiving waters, soils, drainage sys- tem types, BMP effects, climate, etc.) might either cause or contribute signifi- cantly to downstream erosion and sedimentation issues. Estimate where a highway contribution might be negligible as compared to other watershed sources of runoff and where vegetated swales may treat or reduce adequately runoff from highways. Prepare guidance on how to conduct more local assessments. 3. Develop Guidance on Minimizing Impacts on Receiving Systems from Turbidity and Sedimentation: Make recommendations on the relationships between sea- sonal timing and effect of sediment load for various geographic regions targeted to important species. Include recommendations on the development of mixing zones, work windows, treatment systems, and buffers that will allow fish to per- form their necessary life functions during project construction and operation. Identify practical means of controlling turbidity to reduce impacts. Estimated Project Budget: $125,000 to $175,000; 18 months to complete. Methods to Improve Performance of BMPs The need for potential enhancements to existing BMPs as well as potential additions to the design of new BMPs has been identified as a research need. For example, although BMPs have been shown to be able to achieve certain levels of phosphorus in their efflu- ent, sometimes there is a need to achieve lower levels. This research project is designed to explore potential enhancements to BMPs to improve their performance through such methods as passive chemical additions, real-time chemical additions, and improving and maintaining the ability to infiltrate while protecting groundwater and other enhancements. See section 3.2.5 for a discussion on methods to improve pollutant removal in BMPs. The research gaps addressed under this research statement are listed under the Methods to Improve Pollutant Removal in Existing Stormwater Systems section of Table 4-1. Research Objectives • Development of soil amendment recommendations for use in BMPs to passively improve performance; 34

35 • Development of methods for improving or maintaining hydraulic conductivity of infiltration-based stormwater control facilities; • Evaluation of the effectiveness of sedimentation, filtration, and chemical addition combinations for stormwater BMP construction projects; • Evaluation of the potential impacts of coagulants on receiving waters; • Detailed guidance for areas that require coagulant use to meet water quality objectives; and • Development of new technologies and improvements on existing designs to increase the removal of high-priority pollutants. Tasks 1. Review the Findings of NCHRP 25-20(1) and BMP Modeling Research: Evalu- ate the results of NCHRP 25-20(1) to develop unit process descriptions of BMP performance and other relevant BMP-modeling papers and review other litera- ture on BMP enhancement options. Prepare a review of the potential enhance- ments that could be developed for BMPs, including passive and active chemical addition, infiltration capabilities maintenance and enhancement, reductions in human pathogens, and other enhancements to improve the removal of additional targeted pollutants. Potential factors that should be evaluated for enhancing infil- tration capabilities may include soil amendments, transitional underground stor- age areas, check dams, and French drains. 2. Prepare a Recommendations Report: Based on the review in Task 1, prepare a highway BMP enhancements recommendations report that describes potential enhancements and how they might be applied to BMPs for highway runoff treat- ment. The enhancements should focus specifically on improvements to targeted pollutant types. 3. Develop a Testing Plan for BMP Enhancements: Develop a testing plan to con- duct field evaluations of highway BMP enhancements based on the recommen- dations report and Task 1. The testing plan should recommend at least two passive and two active chemical addition technologies, along with other higher potential BMP enhancements. Estimated Project Budget: $125,000 to $150,000; 12 months to complete. Assessment of Potential Impacts on Biota Using BMPs That Incorporate Habitat Concern has been expressed increasingly with regard to BMPs that include significant habitat components (primarily stormwater wetlands) and whether such BMPs expose wildlife to captured pollutants. Because there is little data to evaluate whether this is in fact a problem, many turn to examples of wetlands (e.g., Kesterson Marsh in the Central Valley of California, an agricultural marsh) where the problems have been created from other pollution sources to draw conclusions. Consequently, potential impacts may be far overstated. In addition, some regulatory agencies have mandated that mitigation wetlands not be used for water quality enhancement. Such restrictions may result in suboptimal use of water resources as well as cost and environmental effectiveness inefficiencies, since the option of using larger wetlands to satisfy both requirements of habitat replace- ment and water quality enhancement would not be available, therefore, resulting in the employment of potentially less effective BMPs. Finally, good design guidance is needed for minimizing potential negative impact to biota when BMPs that incorporate habitat

areas are used. See section 3.5.4 for a discussion on the potential toxicity of highway runoff and section 3.5.9 for a discussion on wetland impacts. The research gaps addressed under this research statement are listed under the Toxicity and Bioassessment and the Wetland Impacts sections of Table 4-1. Research Objectives • Critical assessment of available information on the potential for highway runoff BMPs to cause problems for biota and on what habitat values can be achieved safely with such systems, and • Development and implementation of a monitoring plan to assess the potential impacts on biota and the value of habitat created by highway runoff stormwater treatment wetlands. Tasks 1. Conduct a Review of Literature on Impacts on Biota from Use of Highway Runoff BMPs and Constructed Wetlands in Particular: Conduct a literature review on impacts on biota from the use of constructed wetlands and other BMPs as appro- priate. Assess the availability of information on highway runoff BMPs and con- structed wetlands. Evaluate the available information on the value of wetlands created by highway runoff as well as those designed specifically to treat highway runoff. Develop a detailed study plan to assess two wetland systems for the poten- tial impacts and habitat benefits provided. 2. Conduct a Biological Impacts and Values Study: Conduct a study on the two wet- lands selected that assesses the risk to the biota using these BMPs as habitat and foraging areas. Include sampling and reporting on water column, sediment, plant tissue, and invertebrate tissue pollutant testing in areas near inlets and outlets and in the middle of systems. Perform an ecological risk assessment on the potential for such systems to impact wildlife and under what conditions and circumstances. Assess the biological values provided at these two systems by a comparison with natural wetlands as well as with unintentionally created wetlands with hydrology from past highway or urban runoff development. Prepare a report that documents the results of the study. 3. Develop Guidance: Based on the above tasks, develop guidance on how to design highway runoff BMPs that protect biota and on how to maximize the habitat value. Estimated Project Budget: $300,000 to $350,000; 24 months to complete. Evaluation of BMP Design and Performance with Respect to Particle-Size Distribution in Stormwater Runoff and Associated Metals Practitioners need effective ways to confine heavy metal toxins to rights-of-way and to prevent contamination of offsite waters. Suspended solids may be used as indicators of overall water quality if correlations can be developed with other parameters. Moni- toring costs can be reduced if the number of individual analyses can be reduced by cor- relating water quality to suspended solids concentrations. Improving the accuracy of measurements and representativeness of samples will increase the confidence in load- ing calculations and correlations. Most of the contaminants found along highways are associated with particles of a wide range of sizes (<0.1 m to >2 mm), but regulatory requirements and biological 36

37 effects typically center on the dissolved constituents. Particle-associated contaminants are critical to environmental protection and regulatory compliance because • Contaminants can move between particle bound and dissolved states (e.g., during a runoff event or during settling in a detention basin), • Particle-associated contaminants <0.45 mm are counted operationally as dissolved but could be removed by efficient coagulation and filtration processes in ways that dissolved contaminants cannot, and • Future regulations may target directly sediment quality. DOTs are faced with the technical challenge of developing or modifying existing BMP designs, so that they also can effectively capture dissolved-phase heavy metals. Most BMPs aim to treat runoff by removing particles and their associated contaminant load, but currently there is little understanding of which particles are retained by particular BMPs and what conditions favor particle retention. At least a portion of the variability of BMP performance may be attributed to poor understanding of particle dynamics. This project will address inconsistencies in the performance evaluation of stormwater runoff BMPs through less costly and more effective particle monitoring compared with cur- rent wet flow monitoring techniques. In addition to the proper evaluation of BMPs, the proposed project has the potential to eliminate most pollutant chemical (dissolved and particulate) analyses by replacing them with particle concentration-based correlations, saving DOTs significant analytical costs. See section 3.4.2.1 for a discussion on sus- pended solids and particle-size distribution. The research gaps addressed under this research statement are listed under the Unit Treatment Processes and the Runoff Char- acterization with Independent Variable Correlation sections of Table 4-1. Research Objectives • Identification and evaluation of accurate and applicable methods for monitoring particle-size distribution of suspended sediment concentrations; • Correlation of heavy metals concentrations to suspended sediment in highway runoff; • Evaluation of BMPs performance, according to suspended sediment removal and changes in particle-size distribution; • Recommendation of improvements for existing stormwater BMPs to improve sed- iment capture and retention; and • Recommendation of future research projects for evaluating the potential to treat metals concentrations not associated with sediment. Tasks 1. Develop, Test, and Refine Particle Measurement and Collection Methods: Include hydrographic measurement of particles (first flush, peak, and end-stage of storm events), testing and development of online particle-size analysis systems, com- parison of particle sizing methodologies, and study of the impacts of storage and detention time on particle coagulation and removal, 2. Test the Efficiency and Variability of Particle Removal in Selected BMPs in the Field: Determine the particle-size distribution of runoff entering and exiting exist- ing BMPs (or those under development) to characterize removal efficiencies and

to determine conditions that degrade removal. The dynamics of the particle wash- off process during storm events also would be examined as degradation in BMP performance may be related to fine particle breakthrough that will be detected by this study. 3. Test Measures to Improve Particle Retention and BMP Performance: Based on the results of the first two tasks, propose and test corrective actions to improve BMP performance. A critical result of the year-three activities would be to develop correlations between particle concentrations and dissolved contaminant concen- trations, which may be useful in replacing chemical analyses by particle-size measurements in most (or possibly all) applications. 4. Identify Modifications to Basic Treatment Systems to Improve Their Dissolved Met- als Removal Capability: Undertake a thorough literature review of available treat- ment technologies that could be applied to highway runoff for reducing metals lev- els not associated with particulates. Options to be examined should include, but not be limited to, carbonate/hydroxide/sulfide precipitation, stone/gravel biofilters, compost or humic filters, and engineered fabric filters. This task would result in a list of recommended research projects that focus on dissolved metals removal. Estimated Project Budget: $250,000 to $350,000; 36 months to complete. Implications and Impacts of TMDLs on DOTs and Guidance in Responding The Federal Clean Water Act requires that TMDLs for pollutants of concern are established for Section 303(d)-listed impaired water bodies. Once TMDLs are estab- lished, waste load allocations (WLAs), or maximum allowable pollutant loads, from each identified source are set by the state regulatory agency responsible for issuing NPDES permits. State DOTs are increasingly being subject to the regulatory require- ments of NPDES permits, including WLAs, for stormwater discharged from their facil- ities. The issue of TMDLs is a concern to DOTs because meeting WLAs and numeric effluent limits may increase significantly the cost of stormwater management. Some DOTs are ill-prepared to make the paradigm shift that the implementation of TMDLs requires. In order to comply with newly established NPDES regulatory requirements, DOT stormwater practitioners need to be able to accurately identify pollutant sources, estimate loads, and know of methods to reduce those loads if necessary. DOTs also must be aware fully of the legal and economic implications of TMDL development, implementation, and enforcement. There appears to be a need to provide guidance spe- cific to DOTs on the effect that TMDLs will have on DOT stormwater management activities. To this end, the following research objectives and associated tasks have been developed for consideration. See section 1.1.1 for an additional discussion of TMDLs. The research gaps addressed under this research statement are listed under the NCHRP Panel Recommended Research Needs section of Table 4-1. Research Objectives • Identification and evaluation of the impact of TMDLs on DOTs; • Development of guidance on new and improved BMP technologies for runoff management in the context of TMDLs; • Provision of guidance on runoff and BMP monitoring to facilitate compliance with TMDLs; • Development of recommendations on the types of effluent limits most appropri- ate and feasible for DOTs in complying with TMDLs; and 38

39 • Identifications and evaluation of suitable tools for runoff and BMP modeling for prediction of pollutant loads and BMP performance in relation to TMDLs. Tasks 1. Literature and Practitioner Survey to Determine the State of the Practice and to Identify Available Sources of Information: Distribute survey forms to and con- duct phone interviews with practitioners to document the needs and challenges facing practitioners with regard to TMDL compliance. The literature review will provide insights into technologies and management strategies as well as ongoing research on TMDLs. 2. Evaluate the Use of BMPs and New Management Strategies as a Means of Meet- ing TMDLs: Evaluate BMPs in the context of TMDL compliance. The result of this phase will be a short list of BMPs and target pollutants typically regulated by DOT NPDES permits and strategies for compliance. 3. Synthesize Guidance for Runoff and BMP Monitoring Necessary for Demonstrat- ing Compliance with TMDL: Evaluate monitoring methods and provide standard- ized monitoring recommendations that can be used to demonstrate TMDL compliance. 4. Develop Guidance for Pollutant-Load Modeling and Estimation Techniques for TMDLs: Examine pollutant-load modeling and estimation techniques and provide analytical methods to be used as planning tools. Examine and test alternative methods of demonstrating compliance with TMDLs. Emphasize production of reliable, reproducible, and defensible modeling results. 5. Develop Guidance or Tools, or Both, for Cost Analysis of TMDL Implementation: Examine methodologies for estimating the costs that may be incurred through TMDL implementation and compliance. Results of the DOT stormwater practi- tioner survey likely would be a major component in this research. 6. Develop a Synthesis of Guidance Methods for TMDL Determination and Imple- mentation: Combine the results of the first four tasks into a guidance document that can be used by practitioners for TMDL and WLA determination in addition to stormwater runoff and BMP modeling and monitoring. Estimated Project Budget: $120,000 to $150,000; 10–12 months to complete.