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8Characterizing highway pollutants of concern supports the development of retrofit treatment objectives and selection and design of appropriate BMP strategies. 2.1 Retrofit Benefits of Water Quality Characterization Water quality characterization provides a basis for plan- ning, evaluation, and design of BMP retrofit projects. Retro- fit projects benefit from water quality characterization in the following ways: ⢠Appropriate representation of water quality issues: Many DOTs actively study or sponsor research on stormwater runoff, stormwater BMPs, and receiving water impacts. When water quality conditions and mitigation measures are studied and evaluated from a watershed perspective, DOTs gain a broader perspective of potential impacts from highway runoff, and a more accurate representation of the DOTâs contribution to watershed conditions and runoff controls. ⢠Basis for sensible retrofit objectives: Understanding receiv- ing water conditions and highway runoff characteristics provides a basis for evaluation and prioritization of ret- rofit projects, and supports development of sensible ret- rofit objectives that (1) address pertinent water issues, (2) attempt to balance costs and benefits, (3) achieve reg- ulatory compliance, and (4) strive for consensus among stakeholders. ⢠Basis for effective treatment strategies: Treatment BMPs are not equal in performance. Treatment effectiveness depends on the fundamental unit processes of the BMP as well as the BMP sizing and design. Defining and character- izing the target pollutants and their forms (e.g., particu- late-bound or dissolved) provides a basis for considered selection of retrofit BMPs. 2.2 Pollutants of Concern for Ultra-Urban Highway Retrofits Table 2.1 summarizes common pollutants of concern (POCs) for ultra-urban highways. Subsequent sections describe the pollutant characteristics, issues of concern in receiving waters, and potential implications for retrofit requirements and BMP design. Runoff from ultra-urban highways is a component of the regional urban runoff water quality. However, highways usually comprise a small fraction of the total watershed area, and therefore runoff quality from other urban land uses will tend to dominate the regional urban runoff quality. Using information compiled in the National Stormwater Quality Database, Table 2.2 shows median values of selected water quality parameters for various urban land uses (Maestre and Pitt, 2005). Comparisons in Table 2.2 indicate the water qual- ity of highway runoff is generally similar to the runoff from other urban land uses but tends to be somewhat higher in total suspended solids (TSS), oil and grease, and metals, and somewhat lower in nutrients and bacteria. 2.2.1 Runoff Volume and Discharge Characteristics in Highway Runoff High impervious cover in ultra-urban highway catchments dramatically increases runoff volumes and peak discharges in comparison to undeveloped conditions. Impervious cover also reduces infiltration and recharge to groundwater, reduces sediment supply to receiving streams, and accelerates the delivery of pollutants. These conditions cause changes to the hydrologic regime of receiving streams, including increased stream flows, increased frequency and number of erosive flow events, increased long-term cumulative duration of flows, and increased peak flows. These effects are referred to as hydromodification. S e c t i o n 2 Ultra-Urban Highway Runoff Characterization
9 Receiving Water Issues of Concern Hydromodification together with reduction in sediment supply can significantly intensify the erosion and sediment transport processes in receiving streams and often leads to stream channel adjustment, geomorphic impacts, and loss of habitat and associated riparian species. Retrofit Implications Hydromodification impacts to urban receiving streams are a regulatory issue of concern. NPDES permits are increasingly including hydromodification control requirements, particu- larly through implementation of Low Impact Development (LID) requirements. Retrofit BMPs to address hydromodifi- cation entail infiltration BMPs, including LID practices, and flow-duration control basins. These practices are difficult to implement in space-constrained settings and may necessitate evaluation of off-site BMPs or in-stream controls. 2.2.2 Sediments Characteristics in Highway Runoff Suspended sediments and solids are prevalent in highway stormwater runoff and urban runoff and are the most widely Condition/ Pollutant Potential Sources in Ultra-Urban Highway Environments Potential Receiving Water Impacts Runoff volume and discharge ⢠High impervious cover ⢠Hydromodification ⢠Increased erosion and sediment transport ⢠Stream channel adjustment, geomorphic impacts ⢠Loss of habitat and riparian species Sediment and particulates ⢠Vehicle abrasion, fall off, and wash off ⢠Pavement wear ⢠Wash off from landscape areas and construction sites ⢠Atmospheric deposition ⢠Sanding for traction control ⢠High turbidity ⢠Streambed occlusion due to deposition ⢠Loss of aquatic habitat ⢠Stream channel modifications ⢠Exceedance of water quality objectives Metals (copper, lead, zinc, cadmium, nickel, chromium) ⢠Tire wear ⢠Lubricating oils ⢠Brake lining wear ⢠Moving engine parts ⢠Fuels and fuel additives ⢠Automobile exhaust ⢠Metal plating and highway structures ⢠Atmospheric deposition ⢠Toxicity of aquatic organisms ⢠Behavioral effects on salmon ⢠Bioaccumulation in fish with potential health hazards to humans ⢠Contaminated sediments and associated impacts ⢠Exceedance of water quality and sediment quality objectives Organic compounds (polycyclic aromatic hydrocarbons, oil and grease, petroleum-related products) ⢠Lubricating oils ⢠Fuels and fuel additives ⢠Automobile exhaust ⢠Atmospheric deposition ⢠Toxicity and impairment of aquatic life ⢠Persistence in sediments ⢠Reduced diversity and abundance of benthic communities ⢠Exceedance of water quality and sediment quality objectives Litter and debris ⢠Intentional or inadvertent littering or dumping ⢠Windblown sources from outside the ROW ⢠Highway landscaping ⢠Impaired recreational benefits ⢠Loss of aquatic habitat ⢠Increased biochemical oxygen demand and contribution to eutrophication Nutrients ⢠Automobile exhaust ⢠Atmospheric deposition ⢠Roadside fertilizer applications ⢠Sediments ⢠Accelerated growth of vegetation ⢠Changes in algae, benthic, and fish communities ⢠Surface algal scum, water discoloration ⢠Exceedance of water quality objectives Chlorides ⢠Highway deicers ⢠Damaged or killed salt-intolerant vegetation ⢠Reduced plant and invertebrate diversity ⢠Impaired groundwater supplies ⢠Exceedance of water quality objectives Indicator bacteria ⢠Bird and wildlife droppings ⢠Road kill ⢠Transport of livestock or manure ⢠Human waste disposal ⢠Re-growth in storm drains ⢠Indicator of potential human health effects from body contact with receiving waters ⢠Exceedance of water quality objectives Table 2.1. Ultra-urban highway conditions and pollutants of concern.
10 addressed pollutant in urban stormwater. The primary sources of sediments and solids in highway runoff are pavement, tire, and vehicle abrasion (Oregon State University et al., 2006). Abraded pavement is reported to make up between 40â50% of the total particulate mass, and abraded tires account for 20â30% of the total particulate mass (Karamalegos et al., 2005). Other identified sources include salting and sand- ing, brake pad dust, aerial deposition, off-site tracking, and runoff from highway landscaping and construction sites (USEPA, 2005a). The particle size distribution (PSD) in highway runoff affects pollutant transport and treatability. There is consider- able variability in reported PSDs (Bent et al., 2003; Kim and Sansalone, 2008a), even on different shoulders of the same highway section (Sansalone and Tribouillard, 1999). Variabil- ity in measured PSD is due to spatial and temporal variability in runoff, wear of materials, and deposition, as well as differ- ences in the collection and measurement procedures (Bent et al., 2003; Kim and Sansalone, 2008a). Kim and Sansalone (2008a) measured event-based PSDs from paved surfaces and compared results to an extensive review of published PSDs. Measured PSDs were dominated by fine particles (<75 µm), which accounted for 25â80% of the particles on a mass basis. This is generally consistent with published PSDs from urban street surfaces. Other studies have reported a dominance of coarser particle sizes (>250 µm) in PSDs from highway and street runoff (Shaheen, 1975; Sansalone et al., 1998). Sediment concentration in runoff is commonly measured as TSS. The TSS method requires subsampling of the collected water sample, which has been found to result in the under- representation of the true sediment concentration (Bent et al., 2003). Alternatively, the suspended sediment concentration (SSC) method measures the sediment concentration of the entire water sample, which provides a more accurate measure- ment of the true sediment concentration (Guo, 2006). Although TSS is more commonly used, it has been sug- gested that TSS measurements are fundamentally unreliable for measuring sediment loads in runoff (Bent et al., 2003). Guo (2006) concludes that a more precise âmeasurement methodology would lead to a more reliable performance certification process and greater water quality benefits.â Accordingly, some testing and certification organizations of proprietary BMPs do require use of SSC measurements. On the other hand, Lenhart (2007) argues that both TSS and SSC should be used to measure BMP performance. He notes that SSC does not usually work within the framework of regu- Water Quality Constituent Parameter Residential Commercial Industrial Freeway TSS (mg/L) Number of samples 99 1 458 428 134 % above detection 98.6 98.3 99.1 99.3 Median value 49 42 78 9 9 Total dissolved solids (mg/L) Number of samples 861 39 9 413 9 7 % above detection 99.2 99.5 99.5 99.0 Median value 72.0 74 9 2 77.5 Oil and grease (mg/L) Number of samples 533 308 327 60 % above detection 57.8 70.8 65.1 71.7 Median value 3.9 4.7 5.0 8.0 Fecal coliform (MPN/100 mL) Number of samples 446 233 29 7 49 % above detection 88.3 88.0 87.9 100 Median value 8345 4300 2500 1700 Nitrate + nitrite (mg/L) Number of samples 927 425 418 25 % above detection 97.4 98.1 96.2 96.0 Median value 0.6 0.6 0.73 0.28 Total phosphorus (mg/L) Number of samples 963 446 434 128 % above detection 96.9 95.7 96.3 99.2 Median value 0.30 0.22 0.26 0.25 Total copper (µ g/L) Number of samples 79 9 387 416 9 7 % above detection 83.6 92.8 89.9 99.0 Median value 12 17 22 34.7 Dissolved copper (µ g/L) Number of samples 9 0 48 42 130 % above detection 63.3 79.2 90.5 99.2 Median value 7.0 7.6 8.0 10.9 Total zinc (µ g/L) Number of samples 810 39 2 433 9 3 % above detection 96.4 99.0 98.6 96.8 Median value 73 150 210 200 Dissolved zinc (µ g/L) Number of samples 88 49 42 105 % above detection 89.6 100 95.2 99.1 Median value 31.5 59 112 51 Source: National Stormwater Quality Database: http://rpitt.eng.ua.edu/Research/ms4/Paper/Mainms4paper.html Table 2.2. Comparison of highway runoff quality and water quality of other urban land uses.
11 l atory requirements, which often specify TSS. Furthermore, SSC measurements can skew BMP performance by show- ing high mass load reductions when there is diminished or ineffective treatment of the smaller particles that are more strongly associated with some pollutants and are mobilized by smaller, more frequent storms. Lenhart (2007) suggests SSC measurements to assess BMPs that target heavy sediment loads and TSS measurements to assess filtration-type BMPs. For any online BMP and likely many offline BMPs that effec- tively remove larger materials, the question of TSS versus SSC is likely a non-issue for effluent quality. Receiving Water Issues of Concern Excessive levels of sediments and solids in highway runoff contribute to receiving water impacts from high turbidity, sedimentation, loss of aquatic habitat, and channel modi- fication. Sediments in highway runoff also transport other pollutants that adhere to them, such as trace metals, poly - cyclic aromatic hydrocarbons (PAHs), polychlorinated biphe- nyls (PCBs), and phosphorus. Particulate-bound pollutants can accumulate in receiving waters and have been associated with impacts on aquatic life near highway discharge points (Buckler and Granato, 2003). Trace metals are of particular concern for highway runoff because they can strongly parti- tion to sediments, contributing to exceedance of water qual- ity objectives in receiving waters: Retrofit Implications Almost all highway retrofit projects will consider the effects of sediment loadings and sediment treatability due to one or more of the following issues: ⢠Sediment impairments may trigger BMP retrofits. Water quality impairments caused by sediments or particulate- bound pollutants are common in urban receiving waters. Sediment and turbidity TMDLs make up more than 10% of all approved TMDLs, and TMDLs associated with particulate-bound pollutants such as metals, phosphorus, and organics comprise more than a third of all approved TMDLs (USEPA, 2010a). Highway facilities located in TMDL watersheds are likely to be identified as contrib- uting sources and assigned wasteload allocations for the impairing pollutants. This can potentially necessitate ret- rofit treatment requirements in order to meet the waste- load allocations. ⢠Sediment removal is a common performance metric. Because sediments are surrogates for other pollutants, sed- iment removal criteria are often performance measures for BMPs and/or programmatic effectiveness. Retrofit treat- ment objectives may be based solely on meeting regulatory criteria for sediment removal (e.g., 80% TSS removal). In addition, some states use TSS removal as a criterion for evaluating and certifying proprietary BMPs. ⢠Sediments in highway runoff influence BMP design. Because of the prevalence of sediments in highway runoff, all treatment BMPs must be designed to manage the effects of sediment loadings on BMP performance and maintenance. Design considerations include sedimentation mechanisms, storage, trapping and resuspension, clogging of filtration BMPs, maintenance frequency, and access. The PSD is also a consideration in assessing candidate ret- rofit BMPs. Coarser particles (>75 µm) are removed relatively easily in BMPs through gravitational settling. For example, Smith (2002) found the vast majority of sediments retained in deep sumped catch basins and oil and water separators are greater than 62 µm. Finer particles, on the other hand, are more difficult to treat, requiring longer settling times or the use of filtration processes. In addition, some pollutants tend to be more strongly associated with finer particles on a particle mass basis due to larger surface area (Grant et al., 2003; Lau and Stenstrom, 2005; Smith, 2002; Wilson et al., 2007). Consequently, treatment effectiveness of particulate- bound pollutants can be constrained by the ability to cap- ture fine particles. Retrofit BMPs that utilize media filtration processes, such as sand filters, are likely to be more effective at reducing fine particulates than sedimentation BMPs such as extended-detention basins (Karamalegos et al., 2005). In addition, testing organizations and regulatory agencies that certify and approve BMPs sometimes require PSD measure- ments to address uniformity in evaluation results and repre- sentativeness to highway conditions. 2.2.3 Metals Characteristics in Highway Runoff Metals are ubiquitous in highway and are common pol- lutants of concern. Copper, lead, zinc, and cadmium are the most routinely monitored and most prevalent metals in high- way and urban runoff (Oregon State University et al., 2006). There are numerous sources of metals in ultra-urban high- way runoff, including vehicles, highway infrastructure, and atmospheric deposition. A key attribute of metals is the form in which they are characterized. Metals in highway runoff and receiving waters are commonly measured as total metals (particulate-bound forms plus soluble forms) or as âdissolvedâ metals based on an operational definition of filtration through a 0.45-micron filter. The partitioning between particulate and dissolved forms depends on chemical and physical factors including pH, alkalinity, temperature, the amount of particulates avail- able, and dissolved and particulate organic carbon (Breault
12 and Granato, 2003; Oregon State University et al., 2006). Thus, considerable variability in particulate and dissolved concen- trations has been reported (Grant et al., 2003; Breault and Granato, 2003). Lead, chromium, and copper generally have the highest particulate phase fractions but reported ranges are large (Grant et al., 2003; Breault and Granato, 2003; Barber et al., 2006). In addition, particulate-bound metals are often associated with small and fine particle sizes (Sutherland, 2003; Grant et al., 2003; Pitt et al., 2004; Lau and Stenstrom, 2005; Wilson et al., 2007). Receiving Water Issues of Concern Metals in highway runoff can accumulate in receiving water sediments and can contribute to the exceedance of aquatic life standards. At elevated levels, metals can impact aquatic life and potentially contribute to toxicity of aquatic organisms (Grant et al., 2003; Breault and Granato, 2003). Metals can also bioaccumulate in fish tissues, posing potential health risks to humans. Some dissolved metals have been associated with neurophysiological and behavioral responses in salmon, which may cause them to be more susceptible to predation (Sandahl et al., 2007). Receiving water objectives for aquatic life protection are typically developed for dissolved concen- trations, with conservative conversion factors (i.e., trans- lators) included for total concentration measurements. In addition, water quality objectives for some metals are a function of hardness, which varies regionally. Increasingly, stormwater sources of metals are being identified as signifi- cant contributors to sediment contamination, which could lead to Superfund implications. Retrofit Implications Listed impairments and TMDLs are a significant issue of concern for DOTs. Because of the numerous sources of metals in urban areas, urban streams are susceptible to exceedance of aquatic life protection standards for metals, sediment con- tamination, and toxicity issues. TMDLs for metals account for more than 17% of all TMDLs (USEPA, 2010a). Highway facilities located in such watersheds are likely to be identified as contributing sources. This can potentially trigger retrofit treatment of highway facilities in order to meet DOT waste- load allocations. In the Pacific Northwest, the potential effects of very low levels of dissolved copper in highway runoff on endangered salmon is a primary concern of resource agen- cies, which can also trigger BMP retrofit requirements. A primary consideration in the design of BMP retrofits is the treatability of dissolved and particulate phase metals. Dif- ferent treatment processes are needed to reduce dissolved and particulate concentrations in highway runoff. Sedimentation BMPs that are effective for metals associated with medium and coarse particles will be largely ineffective for dissolved metals and metals associated with very fine particulates. The latter may require use of infiltration BMPs or BMPs that include sorption and fine filtration processes. These consid- erations have implications for retrofit treatment objectives, BMP selection, design, maintenance, and overall costs. 2.2.4 Organic Compounds Characteristics in Highway Runoff Many organic compounds are used for vehicle operation, including fuels, oils, and lubricants. Consequently, ultra- urban highways, which have high ADT, are potentially sig- nificant sources of organic compounds in runoff due to accidental spills and drips of fuels and lubricants, deposition from exhaust, and tire wearing. Other potential sources are atmospheric deposition, leachate from asphalt roads and treated lumber such as utility poles, and pesticides and her- bicides from highway landscaping. A large variety of organic compounds with varying physi- cal, chemical, and toxicological properties are potentially found in highway runoff. Semivolatile organic compounds (SVOCs) and volatile organic compounds (VOCs) are two classes of organic compounds that have been studied in highway runoff (Lopes and Dionne, 2003). SVOCs are more likely to be detected in highway runoff. Commonly reported SVOCs include oil and grease, PAHs, and total petroleum hydrocarbons. As a class of compounds, SVOCs are strongly associated with particulates. VOCs such as toluene, xylene, and benzene are common components of fuels but are less commonly monitored and less frequently detected in high- way runoff than SVOCs. In fact, most organic constituents are below laboratory detection limits in samples of highway runoff (Smith, 2002; Smith and Granato, 2010). Oil and grease are used as vehicle lubricants and are there- fore common constituents in highway runoff. Runoff con- centrations are variable, but are typically less than 10 mg/L, and sometimes spike to 20 mg/L or more (CalEPA, 2006; Cal- trans, 2003a). Highest concentrations have been associated with parking lots, urban highways, and industrial land uses (CalEPA, 2006). Oil and grease are composed of many com- pounds, which individually have different physical, chemical, and toxicological properties. Many components will tend to adsorb and are associated with sediments. Monitoring of oil and grease is typically accomplished with grab samples due to interactions with tubing and pumps for automated samplers. Stenstrom and Kayhanian (2005) found a high degree of cor- relation between measured dissolved organic carbon (DOC) and oil and grease in highway runoff. They suggested that DOC, which can be reliably measured by automated samplers, can be used as a surrogate for oil and grease measurements.
13 Caltrans Litter Research Program Caltrans has an ongoing litter research program to evaluate litter management strategies and the effectiveness of various education and treatment BMPs (Caltrans, 2000). ⢠Measured annual trash loadings from highway monitoring stations are variable, ranging from about 3 to 7.5 kg/area on an air-dried mass basis, or about 20 to 60 L/acre on a volume basis. ⢠The composition of litter and debris is domi- nated by vegetative material, accounting for 75% to 87% by weight of all material collected. ⢠A high proportion of the litter composition was from smoking- and food-related waste (20% to 30% by weight and volume). Receiving Water Issues of Concern Toxic SVOCs that strongly partition to particulates can accumulate in receiving water sediments, potentially to lev- els that can impair aquatic life (Lopes and Dionne, 2003; Buckler and Granato, 2003). PAHs, in particular, are of con- cern because they are often present in urban runoff, they partition to particulates and can accumulate in sediments, and certain PAHs have a high potential for adverse impacts on aquatic health (Grant et al., 2003). Other potentially toxic organic compounds in ultra-urban runoff are PCBs from atmospheric deposition and older pavement joint compounds, and herbicides and pesticides that are applied to highway landscaping. VOCs are generally considered to have low environmental toxicity at concentrations found in urban stormwater (Lopes and Dionne, 2003). A significant concern of VOCs on high- ways is the possibility of large fuel spills that can potentially contaminate drinking water supplies. Excessive levels of oil and grease in highway stormwater discharges can potentially impair aquatic and recreational beneficial uses. Receiving water objectives for oil and grease are often qualitative, requiring the waters to be free of visible floating oils and grease. Certain components of oil and grease are highway pollutants of concern and may have numeric objectives, metals and PAHs, in particular, which can accu- mulate in receiving water sediments and potentially contrib- ute to aquatic toxicity. Retrofit Implications Receiving water impairments and TMDLs that address organics and toxicity, notably from PAHs and oil and grease, can potentially trigger highway BMP retrofits. Many organics have low solubility and will tend to partition to sediments with high organic content. Effective treatability of organics with retrofit BMPs may require filtration and sorption processes. 2.2.5 Litter and Debris Characteristics in Highway Runoff Litter and debris are general waste products on the land- scape. Litter is composed of manufactured materials such as paper, plastic, wood, cigarette butts, Styrofoam, metal, and glass. Debris is biodegradable organic material such as leaves, grass cuttings, and food waste. Litter and debris are common on ultra-urban highways from intentional and inadvertent littering or dumping, vegetative litter from highway land- scaping, and deposition of windblown trash and debris from adjacent urban areas. Several studies have found the compo- sition of litter and debris in highway runoff is dominated by vegetative debris and includes a high proportion of plastics and cigarette butts (Caltrans, 2000; Smith, 2002). Receiving Water Issues of Concern The presence of excessive litter and debris in receiving waters can result in the impairment of recreational uses and can increase the biochemical oxygen demand. Litter and debris can also impact aquatic habitat by inhibiting growth of aquatic vegetation, decreasing spawning areas, or directly impacting wildlife that ingest or become entangled in trash. Retrofit Implications The USEPA has listed trash impairments in several states, and trash TMDLs are established in California (USEPA, 2010a). In watersheds with established TMDLs, DOTs are required to meet TMDL wasteload allocations for highway facilities. This requirement can potentially necessitate retro- fit BMPs. For example, the trash TMDL for the Los Angeles River Watershed has a wasteload allocation of âzeroâ trash in all municipal separate storm sewer system (MS4) discharges, to be achieved over a 10-year implementation period. Treat- ability of trash and debris requires screening and capture processes. In watersheds with comparatively large trash loads, effective treatment of trash will require greater BMP storage capacity, and/or more frequent maintenance.
14 2.2.6 Nutrients Characteristics in Highway Runoff Nutrients are inorganic forms of nitrogen (nitrate, nitrite, and ammonia) and phosphorous. Organic forms of nitro- gen are associated with vegetative matter such as particulates from sticks and leaves. Total Kjeldahl nitrogen (TKN) is a measure of organic nitrogen plus ammonia. Phosphorus in runoff occurs in dissolved and particulate forms. Particular matter includes organic debris and phos- phorous adsorbed to soil particles. Phosphorus is measured as total phosphorus (TP), orthophosphate (the biologically available form), and soluble phosphate (orthophosphate and organic phosphorus) (Oregon State University et al., 2006). Nutrients are commonly present in highway runoff and are generally more prevalent in runoff from other urban land uses (Table 2.2). The sources of nutrients include automobile exhaust, atmospheric deposition, and runoff from highway landscaping and cut slopes. Groundwater inflow into storm drains/slope drains has also been identified as a source of phosphorus in areas where phosphorus is naturally high in groundwater or historical uses (e.g., farming) have contrib- uted to elevated phosphorus. Receiving Water Issues of Concern Nutrients are biostimulatory substances that can cause excessive or accelerated growth of vegetation, such as algae, in receiving waters. Eutrophication due to excessive nutrient input can lead to changes in algae, benthic, and fish commu- nities; extreme eutrophication can cause hypoxia, resulting in fish kills. Surface algal scum, water discoloration, and the release of toxins from sediment can also occur. Retrofit Implications Listed impairments and TMDLs for nutrients are a signifi- cant issue of concern for DOTs in nutrient-sensitive areas and can potentially initiate retrofit treatment requirements, for example in the Chesapeake Bay watershed. Nutrient TMDLs account for about 10% of all approved TMDLs (USEPA, 2010a). Retrofit treatability of nutrients can be difficult and may require multiple treatment processes. Particulate-bound nutrients including phosphorus and organic nitrogen are removed by sedimentation and filtration processes, whereas soluble nutrients including orthophosphate and nitrate are more difficult to remove requiring sorption and/or biologi- cally mediated processes. Some DOTs are actively studying BMP processes and designs for enhanced treatment of nutri- ents (SHA, 2009). 2.2.7 Chlorides Characteristics in Highway Runoff In many parts of the nation, deicing activities are the pri- mary source of chloride in highway and urban runoff. Deic- ing activities are routinely conducted in cold weather regions for public safety, and urban areas in particular receive greater and more responsive deicing activities due to large ADT. Sodium chloride is the most commonly used deicer due to low cost of the material. Alternatives to sodium chloride are traction sanding and more costly chemical deicers, including calcium chloride, magnesium chloride, and calcium magne- sium acetate. Sodium chloride readily dissolves into sodium and chlo- ride ions in runoff. Chloride ions are very mobile in the envi- ronment and are conservative; they do not degrade, adsorb to solids, or volatilize. Thus, chloride is readily transported with highway runoff to surface receiving waters and can infil- trate and migrate to groundwater (Kunze and Sroka, 2004). Sodium ions are less mobile and will tend to accumulate on sediments but can leach to groundwater supplies (MassHigh- way, 2006). The concentration of chloride and sodium ions in receiving waters is diminished by mixing and dilution, espe- cially in surface waters. Maryland SHA Nutrient Management The Maryland State Highway Administration (SHA) is implementing nutrient reduction initia- tives to address exceedances in TMDL nutrient objectives. Measures include: ⢠Detailed geographical information system (GIS) mapping of nutrient and sediment impair- ments overlaid with SHA impervious surfaces and stormwater BMPs to assist in prioritiza- tion and deployment of BMPs ⢠BMP research and development, including: swale design and effectiveness monitoring studies; optimization of bioretention media for nutrient removal through laboratory mea- surement of sorption isotherms and vegetated column studies; and wet infiltration basin transitional performance assessment studies ⢠Reduction in fertilizer use through active nutrient management programs based on soil testing ⢠Pilot projects on the use of native meadow vegetation that have reduced mowing and fertilizer requirements
15 Receiving Water Issues of Concern DOT studies have found that road salting is not a wide- spread environmental threat and that impacts from road salting are site specific with greatest impacts occurring near the place of application where concentrations are greatest (MIDOT, 1993). Other reports have found that chloride concentrations in receiving waters may be diluted to con- centrations for which there are little measurable effects (MDT, 2004). However, elevated chloride concentrations in highway runoff and splash zones do cause damage or kill roadside salt-intolerant vegetation, reduce plant and inver- tebrate diversity, and impair groundwater supplies and sur- face receiving waters. Road salt may also have impurities such as nitrogen, phosphorus, copper, and cyanide that are discharged to receiving waters with snow melt. The United States Geological Survey (USGS) found that levels of chlo- ride are elevated in many urban streams and groundwater across the northern United States, and that increases in chlo- ride levels in streams during the last two decades are consis- tent with overall increases in salt use in the United States for deicing (Mullaney et al., 2009). Road salting has been linked to exceedances of drinking water standards for sodium in ground- water supply wells (MassHighway, 2006). Listed impairments and TMDLs for chloride due to deicing activities can poten- tially trigger changes in snow removal practices. Retrofit Implications Chloride is not effectively removed with traditional treat- ment BMPs. Control of chloride requires reducing sources via snow removal practices, including less frequent deicing, and use of alternative deicers. Many alternative deicers, how- ever, can increase loadings of biochemical oxygen demand (BOD) to BMPs and receiving waters. 2.2.8 Indicator Bacteria Characteristics in Highway Runoff Pathogens are viruses, bacteria, and protozoa that can cause gastrointestinal and other illnesses in humans through body contact exposure. Identifying pathogens in water is dif- ficult as the number of pathogens is exceedingly small. Tradi- tionally, water managers and regulatory agencies have relied on measuring âfecal indicator bacteria,â such as fecal coli- form bacteria, as indirect measures of the presence of human pathogens and, by association, human illness risk. However, indicator bacteria are not reliable markers of actual human pathogens in highway runoff due in part because there are many non-human sources of indicator bacteria in highway runoff including bird and wildlife droppings, roadkill, trucks hauling livestock and livestock waste, and sediments from highway landscaping. Highway monitoring studies have found variable and elevated levels of indicator bacteria in highway stormwater runoff, often above receiving water objectives (Barrett et al., 1995b; Smith, 2002; Caltrans, 2003a; Herrera Environmental Consultants, 2007). However, in a detailed monitoring study conducted by the California DOT (Caltrans), actual human pathogens were infrequently detected in runoff from exclu- sive highway drainages and mixed use drainages (Caltrans, 2002b). The Caltrans study supports the common belief that highway facilities are generally not a significant source of human contamination and human pathogens. Receiving Water Issues of Concern Although there is ongoing debate on the health effects of exposure to receiving waters of direct and recent stormwater runoff (WERF, 2007), indicator bacteria are ubiquitous in urban runoff and concentrations frequently exceed receiv- ing water objectives. Consequently, receiving water impair- ments and TMDLs for indicator bacteria are widespread in urban centers. Bacteria TMDLs account for almost 20% of all approved TMDLs (USEPA, 2010a). Retrofit Implications Highway facilities in urban centers that discharge into TMDL-listed receiving waters may be assigned waste load allo- cations that trigger retrofit treatment requirements. Because highways are not common sources of human pathogens, initial retrofit studies should include source identification efforts of indicator bacteria, such as illicit connection testing, identifi- cation of off-site contributions, wildlife sources in landscaped areas, and possibly highway sources such as trucks hauling livestock. If specific sources are identified, then source control efforts may be sufficient to meet retrofit objectives. If needed, effective retrofit treatment of indicator bacteria requires media filtration processes or advanced disinfection systems. 2.3 Ultra-Urban Influences on Highway Runoff Quality 2.3.1 ADT and Adjacent Land Use Pollutant levels in ultra-urban highway runoff are gen- erally greater than in runoff from other highway facilities. Dense urban development and high ADT are primary factors that are associated with higher pollutant levels in urban high- way runoff. Their influence, however, is difficult to separate as both are found in dense urban areas (Driscoll et al., 1990; Irish et al., 1995; Smith and Granato, 2010).
16 Concentrations of contaminants in highway runoff have been found to increase as the adjacent land use becomes increasingly urban (e.g., Driscoll et al., 1990; Kayhanian et al., 2003, 2007), in particular industrial and commercial land uses (Driscoll et al., 1990; Caltrans, 2003a). Recently, Smith and Granato (2010) monitored highways with similar ADT and different total impervious fractions within a 1-mi radius. They found an order of magnitude difference in concentra- tions at highways with similar ADTs but with impervious- ness in the 20â30% and 41% ranges, which suggests that surrounding land use (airborne deposition) may be a major source of constituents found in highway runoff from these areas. The surrounding land use affects the amount of pol- lution in dustfall deposited on a highway, which affects the ensuing quality of highway runoff (Barrett et al., 1995b). There is also an association between ADT and increasing levels of pollutants in highway runoff (Driscoll et al., 1990; Barrett et al., 1995a; Caltrans, 2003). Highway monitoring studies have shown greater runoff concentrations at sites with higher ADT, with a consistent pattern for conventional constituents and trace metals with few exceptions (Barrett et al., 1995a; Caltrans, 2003). Caltrans (2003) noted that ADT is an important predictor of pollutant concentration and an important factor in prioritizing management alternatives. Other studies have found that runoff concentrations do not correlate directly with ADT, and there are contributing co-factors (Driscoll et al., 1990; Barrett et al., 1995a; Kayhanian et al., 2003). Pollutant concentrations correlated to ADT only in conjunction with numerous other factors, including total event rainfall, seasonal cumulative precipitation, antecedent dry period, surrounding land use, vegetation, soil character- istics, pervious versus impervious area, and rainfall intensity (Barber et al., 2006). Greater pollutant concentrations expected in ultra-urban highway runoff pose challenges and constraints as well as opportunities for BMP selection and treatment performance, especially in areas with established loading limits (TMDLs) to receiving waters. Because land use and ADT only partially explain elevated pollutants concentrations, other factors must be considered for the estimation of runoff concentrations when there is an absence of site-specific monitoring data. 2.3.2 First Flush Phenomena First flush is the concept that the highest pollutant con- centrations and loads occur in the first portions of the run- off hydrograph. Many monitoring studies have noted first flush for a variety of constituents and land uses; however, first flush is not always present for all constituents or for all land uses, or may not be significant (e.g., Roseen et al., 2006; Flint, 2004; Strecker et al., 2005; Sansalone and Cristina, 2004). A number of highway monitoring studies have reported first flush in highway runoff (Barrett et al., 1995a; Irish et al., 1995; Sansalone and Buchberger, 1997; Oregon State University et al., 2006; Caltrans, 2003a; Stenstrom and Kayhanian, 2005). One way to determine first flush is to plot runoff versus mass load for individual storm events and pollutants, as shown in Figure 2.1. First flush is indicated when a large frac- tion of the total pollutant load occurs disproportionately in the early runoff. Quantitative measures of mass first flush have been developed, for example, 50% of the mass load in the first 25% of runoff (Wanielista and Yousef, 1993) or, more generally, the mass first flush ratio (Stenstrom and Kayha- nian, 2005). Maestre et al. (2004) found that first flush occurs with greater frequency from land uses with high impervious cover and in simple watersheds where the peak intensity is near the beginning of the storm. Such conditions are typical of ultra- urban highway environments. Therefore, first flush is more likely in ultra-urban highway environments than in other land use types due to: ⢠Small catchment areas and simple watersheds, which have been associated with high pollutant concentrations (Caltrans, 2003a; Kang et al., 2008b); ⢠High fractions of impervious cover that can produce rapid runoff response and high flow rates that mobilize pollutants; and ⢠Greater and more widely distributed pollutant sources from tire/road wear, cars, highway infrastructure, run-on from adjacent urban areas, and atmospheric deposition. First flush in highway runoff affords opportunities for more efficient or effective BMP design (Stenstrom and Kayhanian, Caltrans First Flush Characterization Study Caltrans conducted a comprehensive first flush characterization of highway runoff from ultra highway catchments in southern California (Stenstrom and Kayhanian, 2005). ⢠ADT ranged from 260,000 to 328,000. ⢠Monitoring data showed significant and generally consistent first flush behavior for many dissolved and particulate-bound pollutants. ⢠Between 30% to 50% of the pollutants in highway runoff from a single storm event were contained in the first 10% to 20% of the runoff volume.
17 2005; Kayhanian and Stenstrom, 2008; Kang et al., 2006; Tucker, 2007). Some examples of first flush BMP designs are: ⢠Inlet control devices to limit mixing and dilution with bypass flows, ⢠Outlet controls to operate detention facilities in batch mode, and ⢠Two-compartment basin designs. Using first flush as a basis for BMP design is generally not a reliable practice because first flush is not always present or can be overwhelmed by periods of high rain intensity in the later portions of the storm (Strecker et al., 2005). However, in ultra-urban retrofit situations BMP design options are likely to be limited by space and budget constraints. In this case, first flush as a basis for BMP design is suitable and appropri- ate provided data support first flush behavior of the primary target constituents, and space and budget constraints justify reduced BMPs sizing. 2.3.3 Climatic and Hydrologic Factors There is an association between runoff quality and ante- cedent dry period. In the arid west where there is a distinct wet and dry season, highway monitoring studies have mea- sured greater pollutant concentrations in the early season storms, greater concentrations with increasing duration of antecedent dry period, and decreasing concentration with increasing cumulative rainfall during the wet season (Sten- strom and Kayhanian, 2005; Caltrans, 2003a). This evidence has led to the concept of a âseasonal first flush.â Stenstrom and Kayhanian (2005) suggest seasonal first flush affords opportunities for designing BMPs that target the early season storms, for example, designing and operat- ing infiltration basins that have dried out over the dry season to capture and retain the first few storms of the wet season. Another option is seasonally focused source control efforts to remove accumulated pollutants from surfaces and drainage systems prior to the onset of winter storms. Storm characteristics (depth, duration, intensity, interevent time, etc.) can also influence retrofit design and performance. Runoff and associated loadings increase with storm depth and impervious fractions that are characteristically large in ultra- urban catchments. Storm duration and rainfall intensity often have an inverse relationship with runoff concentrationsâ shorter storm durations and lower rainfall intensity produce higher runoff concentrations (Caltrans, 2003a). However, this relationship is not consistent for all parameters. For example, increasing rainfall intensity has been found to significantly increase sediment concentrations in runoff from highway construction sites (Pitt, 2001). It would also be expected to increase sediments arising from landscaped areas when they begin to contribute to runoff. Local storm characteristics will normally be reflected in DOT sizing and design criteria for BMPs. However, in space-constrained retrofit situations, site- specific sizing and design may be warranted. 2.3.4 Cold Climate Factors In cold climate regions, snow accumulation and snow removal practices affect highway runoff volumes, highway runoff quality, and BMP performance. (a ) 0. 0 0 .1 0. 2 0 .3 0. 4 Cu m ul at ive M as s [g] 0 50 100 150 200 (b ) 0. 0 0 .1 0. 2 0 .3 0. 4 0 40 80 120 (c ) 0. 0 0 .1 0. 2 0 .3 0. 4 0 20 40 60 80 (d ) 0. 0 0 .1 0. 2 0 .3 0. 4 0 10 0 20 0 30 0 0. 0 0 .1 0. 2 0 .3 0. 4 0 20 40 60 80 (f ) 0. 0 0 .1 0. 2 0 .3 0. 4 0 60 12 0 18 0 24 0 SSC R 2 = 0 .9 9 Cumu la tive V ol ume [m 3 ] Cumu la tiv e Vo lu me [ m 3 ] (e ) COD p R 2 = 0.9 9 TD S R 2 = 0 .9 9 COD d R 2 = 0.9 9 VSSC R 2 = 0.9 9 COD T R 2 = 0.9 9 Cu m ul at ive M as s [g] Cu m ul at ive M as s [g] Fi rs t-f lu sh : ⪠SSC (plo t a) ; ⪠VSSC ( pl ot b ); ⪠CO D T (plo t d) ; ⪠CO D p (p lo t f) . No firs t-f lu sh : ⪠TD S (p lo t c) ; ⪠CO D d (p lo t e) . SSC = S us p ended s edim ent c on ce nt r ation C OD T = C hem ic al o xyg en d em and - t ot al VSSC = V olat ile s us p ended se di me nt c onc en tr at ion CO D p = C hem ic al ox y gen d em and - par ti c ulate TD S = Total di sso lv ed s ol ids C OD d = C hem ic al ox y gen d em and - d isso lv ed Source: WERF (2005) Figure 2.1. Illustration of first flush.
18 During cold weather, treatment systems can experience periods of no runoff followed by large volumes of runoff due to rapid snowmelt and/or rain-on-snow events. In other cases, melts can provide slow steady flows with low TSS. Thawing of accumulated roadside snow packs can lead to significant runoff periods and runoff volumes. Rain-on-snow events can produce extreme runoff volumes. The hydrologic load- ing from snowmelt, however, is difficult to predict. Snowmelt processes depend on many factors including the volume and nature of the accumulated snow pack, snow removal prac- tices, and environmental factors including temperature, pre- cipitation, and freeze-thaw cycles. Pollutants from vehicles, vehicle exhaust, and atmospheric deposition partition into and accumulate in snow banks over extended periods. Consequently, snowmelt typically has elevated pollutant concentrations in comparison to rainfall runoff. Snow removal practices such as plowing and removal of snow and use of chemical deicers and traction sand also affect runoff concentrations or are direct sources of pollutants. Pollutants most likely to be elevated in snow- melt are sediment, particulate-bound pollutants particularly metals and PAHs, salts from chemical deicers, chemical oxy- gen demand (COD), and oil and grease (Driscoll et al., 1990; Sansalone and Buchberger, 1996; Glenn, 2001). pH levels in snow are often low, which can change the portioning of pol- lutants with particulates. Cold weather conditions pose challenges for sizing and design of retrofit BMPs in space-constrained settings, including: ⢠Greater hydrologic and pollutant loading; ⢠Reduced treatment performance due to reduced infiltra- tion rates, reduced biological activity, and reduced settling velocities; ⢠Ice cover on permanent pools; and ⢠Pipe freezing and inlet clogging. Targeted snow removal may be a method to reduce load- ings associated with snowmelt. For example, in Lake Tahoe, snow is moved to specific snowmelt areas that drain to BMPs Organization Topics/Description References FHWA In 1990 FHWA published results of a nationwide highway stormwater monitoring study from 31 highway sites in 11 states. Site event mean concentrations were developed and factors influencing highway pollutant loads were investigated. The database includes a computer program to evaluate highway pollutant loadings and the associated receiving water impacts. http://ma.water.usgs.gov/fhwa/90M odel/ Driscoll et al. (1990) USGS stormwater database Comprehensive database of 103 highway-runoff monitoring sites in the conterminous United States, as documented in seven selected highway-runoff data sets. These data include the 1990 FHWA runoff-quality model data compilation and results from six other data sets collected during the period 1993â2005. http://ma.water.usgs.gov/fhwa/SEL DM.htm NSQD The National Stormwater Quality Database (NSQD) was developed by the University of Alabama and the Center for Watershed Protection in 2004. The database consists of nearly 10 years of stormwater outfall data collected by MS4 permit holders throughout the United States. http://rpitt.eng.ua.edu/Research/ms4 /mainms4.shtml Caltrans Caltrans publications listing for monitoring and applied studies http://www.dot.ca.gov/hq/env/storm water/ongoing/index.htm Litter research program â publications listing http://www.dot.ca.gov/hq/env/storm water/ongoing/litter_management/in dex.htm Statewide runoff characterization for DOT facilities Caltrans (2003a) Toxicity associated with particles in highway runoff Grant et al. (2003) First flush characterization Stenstrom and Kayhanian (2005) Texas DOT Highway runoff characterization in the Austin area Barrett et al. (1995a) Investigation of factors affecting highway runoff Irish et al. (1995) Receiving water impacts of bridge deck runoff Malina et al. (2005) Washington State DOT Publications listing of stormwater research reports http://www.wsdot.wa.gov/Environm ent/WaterQuality/Research/Reports. htm Heavy metals in highway runoff Barber et al. (2006) Summary of 5-year statewide highway monitoring program Mar et al. (1982) Research Organizations UT Austin - Center for Transportation Research, Searchable library. http://www.utexas.edu/research/ctr/ Table 2.3. Sources of highway runoff information.
19 versus allowing the snow to stay in areas where it cannot be treated effectively. 2.4 Sources of Water Quality Information to Support Retrofit Planning Compiling and evaluating existing runoff data is the first step in characterizing highway runoff and receiving water quality. The most common sources of runoff data follow: ⢠DOT monitoring data: DOTs are the best source of high- way runoff data. Most DOTs have historical or ongoing stormwater monitoring programs to characterize runoff from their facilities. Ideally, site-specific or regional DOT runoff data will be available that can be used to character- ize DOT contributions to receiving waters issues of con- cern and to support retrofit BMP selection and design. DOTs also sponsor research on highway runoff and receiv- ing water impacts. Table 2.3 includes references to selected DOT runoff data and sponsored research studies. ⢠Municipal stormwater programs: Metropolitan areas adjacent to ultra-urban highways are likely to be permit- ted under Phase I NPDES rules and in some cases the DOTs may be co-permittees with the municipal programs. Municipal stormwater programs routinely collect water quality monitoring data of MS4 discharges. ⢠Regulatory agencies and studies: State and federal envi- ronmental agencies routinely compile runoff monitoring data, often for the Section 303(d) water quality impair- ments designations and semiannual reports, and for source analysis in TMDL documents. ⢠Regional databases: Regional and national highway run- off quality data that have been subjected to rigorous sta- tistical testing can serve to fill data gaps. Several of these databases are listed in Table 2.3.
