Evaluation of Best Management Practices for Highway Runoff Control (2006)

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62.1 Introduction Performance evaluation and selection of a stormwater management system from among the many available options can be characterized in different ways. Control of water quantity, that is, control of the flow and volume of the runoff is always an aspect of stormwater management, if for no other reason than that flood control and management of drainage volumes and peak flows will always be a part of the drainage engineer’s job. Because a pollutant load is the product of flow and concentration, hydrologic (and some- times, hydraulic) controls offer fundamental mechanisms by which to reduce pollutant loadings to off-site locations. Hence, hydrologic controls are discussed in the first part of this chapter. Unit processes for removal of pollutants from waste streams are the building blocks of environmental engineer- ing. Pollutant removal within BMP and LID facilities follows the same principles of physics, chemistry, and biology as pol- lutant removal within municipal or industrial wastewater treatment systems. Hence, the second part of this chapter dis- cusses the performance of stormwater management facilities in the context of unit treatment processes. Finally, it is often convenient to group BMPs with simi- lar characteristics, such as storage, filtration, biological removal, etc., as well as to consider nonstructural options for stormwater management. This kind of characterization is discussed in the third part of this chapter. Throughout, this chapter emphasizes principles and back- ground information for control of stormwater runoff. These principles as well as the application of these principles are included in the Guidelines Manual of this project. The sec- tions of this chapter that follow discuss the fundamental prin- ciples and characteristics of stormwater management facilities with regard to hydrologic controls, unit treatment processes, and BMP types. These topics are also discussed in Chapters 4 and 5 of the Guidelines Manual. 2.2 Characterization by Hydrologic Control 2.2.1 Hydrologic Basis Flow alteration is a significant unit operation for stormwa- ter treatment and has been the single major unit operation for stormwater management for decades in the United States and many parts of the world. Water quality and quantity cannot be separated; alterations to the hydrograph affect water qual- ity. In large part, flow alteration is implemented as a hydro- logic control. Flow alteration includes modifications to components of the hydrologic cycle such as runoff, infiltra- tion, detention, storage, and evaporation. In general, the goals of these physical operations (recognized as hydrologic con- trols) have been to reduce volume, reduce peak flows, gener- ate more uniform flow rates, and attenuate temporal aspects of flow. To varying degrees, these hydrologic controls can have a significant impact on water quality. Applications of hydro- logic modification are ubiquitous in the built environment and are intentional or inadvertent, as well as beneficial or detrimental. Examples of intentional applications that have potential water-quality and -quantity benefits include infil- tration, detention, and flow equalization; detrimental appli- cations include impervious paving or loss of vegetation. The sections that follow discuss the two fundamental hydrologic unit operations: flow attenuation and volume reduction (or minimization of volume increases). Flow attenuation refers to the hydrologic operations responsible for reducing peak-event discharges (e.g., “peak shaving,” see 10.2.1). The primary mechanisms involved in flow attenua- tion include interception, conveyance, and detention, and, to a lesser degree, infiltration. Volume reduction hydrologic operations are responsible for reducing the total volume of runoff via retention, infiltration, and ET. Runoff can also be detained in storage vessels such as underground tanks and vaults and reused (e.g., irrigation water). If pollutant loads are a primary concern, then volume reduction should be a C H A P T E R 2 BMP/LID Characterization

7major unit operation in any selected treatment system design. Volume reduction is the essence of most LID approaches to stormwater management. Finally, down- stream hydrologic impacts often depend upon the similarity of hydrographs between predrainage and postdrainage conditions. One method for this type of analysis is to eval- uate flow-duration curves, discussed more extensively in Section 10.2.3. 2.2.2 Flow Attenuation 2.2.2.1 Interception Interception is a form of detention storage that occurs when leaves, stems, branches, and leaf litter temporarily store rainfall. Interception is considered to be detention storage if raindrops drain off vegetation by “throughfall” (dripping off a leaf onto the ground) or by stemflow (flowing down stems or trunks). Throughfall accounts for the majority of the movement of intercepted rainfall. Intercepted rainfall that is retained is lost to the atmosphere by evaporation from the surface of leaves. The retained and evaporated fraction of rainfall is considered a volume-reduction operation and is discussed in more detail in Section 2.2.3. The percentage of rainfall that is intercepted increases with the density of vegetation, including all vertical layers from canopy to leaf litter. At maximum density, both trees and grasses may intercept 10 to 20% of precipitation from an indi- vidual storm. Per unit of ground area, some grass species have the same leaf area as many trees (Dunne and Leopold 1978). 2.2.2.2 Conveyance Conveyance is the transport of surface runoff and includes the entire flow path from where a raindrop falls to where it enters the receiving body of water. In conventional storm- water designs, conveyance is synonymous with the efficient drainage of runoff. By contrast, decentralized controls, like LID, which provide conveyance, also promote infiltration, improve water quality, and increase runoff travel time, or time of concentration (Tc). These controls are often critical components of the treatment train approach. In this guid- ance, “conveyance” refers to the act of transporting runoff, rather than the carrying capacity of a treatment system or other structure. 2.2.2.3 Detention Detention is the temporary storage of stormwater, which is then released over a period that can generally range from hours to days after rainfall ceases. (This is as opposed to retention, in which stormwater is captured and not released downstream.) Detained stormwater may exist as ponded free water or can be held within moist soil. In highly urbanized environments, detained runoff ultimately enters the storm drain system. In a vegetated system, ponded water and any soil moisture above the field capacity are detained, rather than retained, because that portion of the stormwater slowly percolates by gravity through the soil column into the under- drain. For small, frequently occurring storms, the release of detained water will not usually cause flooding because the stormwater will enter the system over a much longer period of time and at a lower rate than it would if detention storage controls were not in place. 2.2.3 Volume Reduction/Minimization of Volume Increases 2.2.3.1 Retention Retention captures stormwater permanently. The volume of retained runoff that may never enter the storm drain sys- tem can include vegetative interception, evaporation, tran- spiration of soil moisture, and reuse. The combination of evaporation and transpiration is called “evapotranspiration” (ET), and may occur at differing rates and extents from soil, vegetation, or hard surfaces such as pavement. Transpiration reduces the water volume within the root zone of soil. As stormwater enters a treatment system, infiltrating water will be retained up to the point that the soil moisture content equals the field capacity. If the rainfall is sufficiently light that the soil moisture content in a vegetated system never reaches field capacity, ET alone will eliminate the volume of storm- water in the soil. 2.2.3.2 Infiltration Infiltration is the downward movement of water into the soil after surficial entry and percolation through pore spaces. In an open system such as a meadow, this movement is unre- stricted, and water can infiltrate down to and recharge the groundwater table. In urban areas and areas near highways, the soil is always disturbed and, when compacted, may inhibit easy movement of water into the ground, even in sandy soils. Groundwater recharge is a basic component of the natural hydrologic cycle. In urban areas, unrestricted infiltration may exacerbate infiltration and inflow (I/I) problems in both separate and combined sewer systems; the likelihood of this scenario must be evaluated before constructing unlined infiltration systems. Some of the infiltrated stormwater will be retained and its volume permanently taken “out of the system”through ET,deep percolation, or both. Another component of the infiltrated stormwater may simply be detained,which temporarily reduces

8the amount of stormwater that would otherwise be in the storm drain system and allows it to enter the system over an extended period of time. The soil moisture content determines the volume of stormwater that is retained and detained. In a given treatment system, the volume of retained water is the volume for which the soil moisture content equals the soil’s field capacity. The retained water leaves the soil through ET. The field capacity is the point at which free drainage by gravity ceases and the remaining water is held in the soil pores by capillary and osmotic forces. At this moisture content, the soil is unsatu- rated. The volume of additional stormwater that causes the soil moisture content to exceed the field capacity will be detained and will drain by gravity into underdrains over a period of several hours or days. Infiltration is influenced by factors such as soil type, vege- tative cover, and groundwater conditions at the site (Urbonas and Stahre 1993). Some common BMP systems that rely on infiltration include infiltration trenches and basins and a number of LID installations, such as porous pavement, lawns, green roofs, and swales. Other BMP systems using some aspect of infiltration within their removal processes include wet ponds, wetlands, and bioswales. 2.2.3.3 Evapotranspiration Evapotranspiration (ET) refers to the combined effects of evaporation and transpiration in reducing the volume of water in a vegetated area during a specific period of time. The volume of water in the root zone of soils is taken up by roots and then transpired by being diffused through leaves. (Uptake by roots may also remove a variety of pollutants from stormwater.) For the first 2 to 3 days after a rainfall, ponding and infil- tration control (i.e., detain) a large proportion of the stormwater volume, even when ET is occurring. After this time, gravitational drainage into the underdrains effectively ceases, and the field capacity is reached. ET becomes the dom- inant process because the volume of water present in the soil at field capacity will be lost to the atmosphere through ET alone. The following equation gives the maximum volume of water that ET can potentially remove once the soil moisture content equals the field capacity. where Vtrans = Transpired volume; Dr = Rooting depth; A = Soil surface area; FC = Field capacity, dimensionless; and WP = Wilting point, dimensionless. V D A FC WP -trans r= ⋅ ⋅ −( ) ( )2 1 The wilting point is the soil moisture content (volume of water per volume of soil plus voids) beyond which plants cannot exert enough suction to draw more water out of the soil. The difference between the field capacity and the wilting point is the moisture content available for transpiration. The field capacity of urban stormwater treatment systems can be designed to meet desired drainage characteristics. The connectivity to underlying soils, including the presence of underdrains and gravel bedding, also affects the field capac- ity. Many vegetated systems, such as rain gardens, have a low field capacity in order to maximize free drainage and filter pollutants. 2.2.4 Flow Duration Flow duration control is an extension of volume control, but is more accurate for sizing controls because matching flow duration maintains runoff volume for the full distribu- tion of flows, as opposed to a single storm event. The concept is illustrated in detail in Section 10.2.3. When one matches the pre-urban flow duration curve, the total number of hours that flows persist at any given magnitude is maintained, and thus the total work on downstream channel boundaries is maintained. Flow duration control can be used on-site or for mixed regional solution strategies. Flow duration control may also be effective at maintaining the erosion potential of receiving streams. 2.3 Characterization by Unit Processes 2.3.1 Fundamental Process Categories Fundamental process categories (FPCs) are often used as one method of classifying BMP technology, and these processes influence a respective system’s pollutant-removal mechanisms and efficiency. FPCs incorporate both unit oper- ations (treatment in which the application of physical forces predominates) and unit processes (treatment in which chem- ical or biological processes predominate) (Metcalf and Eddy 2003). A thorough evaluation, analysis, and categorization of FPCs, originating from principles associated with water and wastewater treatment engineering, may provide a structure and outline for the numerous pollutant-removal mechanisms and systems currently used for stormwater treatment (Minton 2005). In association with unit operations generally found in wastewater treatment technologies, BMPs may be generally classified by single or multiple FPCs (Metcalf and Eddy 2003) as outlined in Table 2-1. In many cases, the pri- mary FPC utilized is not well determined, and thus the effi- ciency of any of the unit processes may depend upon static and state variables (Quigley et al. 2002). Some static variables

Fundamental Process Category (FPC) Unit Operation or Process (UOP) Target Pollutants BMPs Flow and Volume Attenuation Extended detention basins Retention/detention ponds Wetlands Tanks/vaults Equalization basins Hydrologic Operations Volume Reduction Infiltration/exfiltration trenches and basins Permeable or porous pavement Bioretention cells Dry swales Dry well Extended detention basins Particle Size Alteration Coarse sediment Comminutors (not common for stormwater) Mixers (not common for stormwater) Physical Sorption Nutrients, metals, petroleum compounds Engineered media, granular activated carbon, and sand/gravel (at a lower capacity) Size Separation and Exclusion (screening and filtration) Coarse sediment, trash, debris Screens/bars/trash racks Biofilters Permeable or porous pavement Infiltration/exfiltration trenches and basins Manufactured bioretention systems Engineered media/granular/sand/compost filters Hydrodynamic separators Catch basin inserts (i.e., surficial filters) Density, Gravity, Inertial Separation (grit separation, sedimentation, flotation and skimming, and clarification) Sediment, trash, debris, oil and grease Extended detention basins Retention/detention ponds Wetlands Settling basins Tanks/vaults Swales with check dams Oil-water separators Hydrodynamic separators Aeration and Volatilization Oxygen demand, polycyclic aromatic hydrocarbons (PAHs), volatile organic carbons (VOCs) Sprinklers Aerators Mixers (not common for stormwater) Physical Treatment Operations Physical Agent Disinfection Pathogens Shallow detention ponds Ultraviolet systems Microbiotically Mediated Transformation (can include oxidation, reduction, or facultative processes) Metals, nutrients, organic pollutants Wetlands Bioretention systems Biofilters (and engineered biomedia filters) Retention ponds Media/sand/compost filters Biological Processes Uptake and Storage Metals, nutrients, organic pollutants Wetlands/wetland channels Bioretention systems Biofilters Retention ponds Chemical Sorption Processes Metals, nutrients, organic pollutants Subsurface wetlands Engineered media/sand/compost filters Infiltration/exfiltration trenches and basins Coagulation/Flocculation Fine sediment, nutrients Detention/retention ponds Coagulant/flocculent injection systems Ion Exchange Metals, nutrients Engineered media, zeolites, peats, surface complexation media Chemical Processes Chemical Disinfection Pathogens Custom devices for mixing chlorine or aerating with ozone Advanced treatment systems Table 2-1. Structural BMPs listed by fundamental process category and unit operation. 9

10 include the system design parameters (e.g., volumes, dimen- sions, and bypass systems), watershed location, size, slope, imperviousness, vegetative canopy, and soil type and com- paction (Huber et al. 2006). State variables include rainfall volume and intensity, detention time, season, vegetation, and maintenance. As implied within Table 2-1, some means of hydrologic control is commonly included in LID installations, such as swales and infiltration facilities. Also, some of the advanced treatment processes listed are unlikely to be routinely encoun- tered as stormwater BMPs and are more likely to be encoun- tered in the context of CSO control, for example. Additional information on all methods, including those not discussed herein, such as volatilization, aeration, and natural disinfec- tion, is included in Chapter 4 of the Guidelines Manual. 2.3.2 Settling/Sedimentation Settling/sedimentation is a physical process associated with the separation of particles downward because of a dif- ference in density between water and solids (Minton 2005). Generally, sedimentation is a two-phase process in which settling occurs during storm runoff under turbulent condi- tions, followed by intermittent settling between storm peri- ods under quiescent conditions (Urbonas 1995). Total suspended solids (TSS) and larger sediments, as well as adsorbed constituents such as heavy metals, are the primary pollutants associated with this removal mechanism. The rel- ative efficiency of BMPs utilizing sedimentation as an FPC depends upon numerous outside influences. Dynamic removal (under turbulent conditions) is generally depend- ent upon surface hydraulic loading, TSS particle-settling velocities, and shear stress, while removal under quiescent conditions is generally a function of particle density, parti- cle size, and fluid viscosity (which is affected by tempera- ture) (Urbonas 1995). Typically, sedimentation is a highly effective removal mechanism when higher pollutant con- centrations (>400 mg/L) and larger particle sizes (>50 μ) are encountered (Urbonas 1995, Minton 2005). Sedimentation of adsorbed or complexed constituents is often the most effective means for removal of these pollutants in their par- ticulate form. Generally all BMP systems use sedimentation as one of the fundamental unit processes prompting removal, especially retention/detention facilities such as ponds and wetlands, but also others, such as swales, hydrodynamic devices, and filters. Efficiency of any settling system is generally a function of res- idence time, which in turn is dependent upon the design of the sedimentation system itself (Huber et al. 2006). The influ- ent water characteristics (stormwater characterization or “treatability”) are also highly influential when determining the projected removal efficiency. 2.3.3 Filtration/Sorption Filtration is a process identified by the physical straining of particles through a porous medium, whereas sorption refers to the individual unit processes of both absorption and adsorption. Absorption is a physical process whereby a sub- stance of one state is incorporated into another substance of a different state (e.g., liquids being absorbed by a solid or gases being absorbed by water). Adsorption is the physio- chemical adherence or bonding of ions and molecules (ion exchange) onto the surface of another molecule. In stormwa- ter treatment applications, particularly for highway runoff, the primary pollutant types targeted with absorption unit processes are petroleum hydrocarbons, while adsorption processes typically target dissolved metals, nutrients, and organic toxicants such as pesticides and polycyclic aromatic hydrocarbons (PAHs). Adsorption of pollutants to a particu- lar media is based upon the characteristics of the sorption media and constituents present (Minton 2005). While filtra- tion and sorption are two distinct treatment processes, they are typically inseparable in media filter systems because both processes are a function of the water-media interface. Filtration and sorption media for BMPs vary greatly, rang- ing from vegetation, sand, perlite, and other inert medias to compost, zeolite, activated carbon (used in drinking water treatment facilities), and numerous other organic and man- ufactured materials. The physical straining process observed with filtration systems, used specifically with inert filtration media, provides removal for particulate pollutants by lodging the solids between gaps in the media. This straining process is thus influenced by factors such as media size, type, and poros- ity and influent characteristics such as pollutant particle sizes (Minton 2005). The chemical removal process, observed when a sorptive medium is used, generally involves the processes of ion exchange, adsorption, and absorption between the water and the medium at a molecular level to remove dissolved con- stituents such as metals, hydrocarbons, nitrogen, and phos- phorus (Minton 2005). These processes are highly influenced by a number of factors, including medium surface area (for sorption), medium size and porosity, ionic conductivity (preference for specific ions of constituents in the water), and the operating capacity of the medium (determines how fre- quently the medium should be replaced) (Minton 2005). Filtration and sorption are common unit processes observed in a number of BMP systems, especially in swales, sand filters, and wet ponds, and within wetlands. Generally, the residence time of stormwater flowing through a system, in combination with the overall media contact area over which the filtration and/or sorption process(es) may occur, is the primary factor influencing the efficiency of the system. Influent characteristics such as constituent particle sizes and

water chemistry are also important when attempting to achieve a projected effluent quality or removal fraction. In order to function well, devices (such as some proprietary BMPs) that rely upon both filtration and sorption must remain in an aerobic state. If anaerobic conditions occur, the oxidation-reduction (redox) state will change, and sorbed metals will be released (J. Sansalone, personal communica- tion, 2003). In addition, higher-particulate metals concentra- tions (e.g., a higher proportion of metals sorbed to sediment) will occur when solids concentrations are greater. Because particulate-bound constituents are easier to remove than dis- solved constituents, it is possible that removal of total (dis- solved plus particulate) metals will be higher when runoff has more solids (and organic) content, than when suspended sed- iment concentrations are relatively low. 2.3.4 Flotation Flotation is a physical treatment process, essentially the reverse of sedimentation, in which particles are separated upwardly because of a density differential between the water and the pollutant. Flotation is generally encountered in the removal of petroleum hydrocarbons and trash and debris (bottles, papers, etc.)—pollutants that are of specific concern in highway environments. When the specific gravity of a pol- lutant is less then 1.0, as in the case of petroleum products and some plastics and paper, a negative settling velocity exists. Similar to removal via sedimentation, the negative settling velocity, or rise rate, of a substance indicates the rate at which pollutants may be removed from the water (Minton 2005). Oil/water separators are the primary BMP systems that utilize flotation (and subsequent skimming) as the fundamental unit operation. However, a number of hydrodynamic devices that incorporate centrifugal forces created by circular motion also rely on flotation (in addition to sedimentation) for removing pollutants. 2.3.5 Biological Treatment Biological processes use living organisms (plants, algae, and microbes) to transform or remove organic and inorganic pollutants. Relevant processes for stormwater treatment have been divided into two broad categories: microbially mediated transformations and uptake and storage. 2.3.5.1 Microbially Mediated Transformations Definitions. Microbially mediated transformations are the unit processes of microbial activity that promote or catalyze redox reactions and transformations. These processes include the degradation of organic pollutants as well as the oxidation or reduction of inorganic pollutants. Microbially mediated transformations are chemical transformations performed primarily by bacteria, algae, and fungi that exist in the water column, soil, root zone of plants, and on wetted surfaces, such as leaves (Kadlec and Knight 1996; Karthikeyan and Kulakow 2003; Minton 2005). Most microbes are concentrated in the upper layers (0.3 m) of soil and in the plant root zone. Of all transformation processes, conversion of nitrogen species (e.g., ammonia and nitrate) is probably the most significant in stormwater treatment systems. Metabolism. Microbially mediated transformations occur as a result of respiration, which is a redox reaction. Redox reactions are chemical transformations involving the transfer of protons and electrons. Terminal electron accep- tors are oxidizers, and electron donors are reducers. Respi- ration is the process that releases the energy and nutrients from food sources so that they can be assimilated by organ- isms. The process occurs in both aerobic (e.g., well-aerated terrestrial soil) and anaerobic (e.g., wetlands) environ- ments. Oxygen is used as the electron acceptor during aero- bic respiration, while other chemicals (e.g., nitrate or sulfate) function as electron acceptors during anaerobic res- piration. Facultative microbes undergo both aerobic and anaerobic respiration. Therefore, microbial transformations that occur in stormwater treatment systems are largely influ- enced by the oxidation-reduction (redox) potential of the system. Organic material decomposition and mineralization. When microbes aerobically oxidize simple organic com- pounds, the process releases, or mineralizes, organically bound elements. Mineralization refers to the release of ele- ments from organic matter to produce inorganic (mineral) forms. Most of the inorganic elements released by mineral- ization are in forms more available as nutrients to higher plants and microbes. Once released through mineralization, the elements can be further transformed by specific microbes. Alternatively, they can be sequestered by binding to other inorganic constituents or by sorbing to nondegradable organic matter (humus). Less-desirable products, such as methane and hydrogen sulfide, may form in anaerobic decomposition. Mineralization is an important source of nitrogen, sulfur, phosphorus, and other nutrients for plants and microbes. Rates of decomposition and mineralization depend on various chemical factors such as pH (near neutral is best), moisture, temperature (25oC to 35oC is best), oxygen, and quality of food sources for microbes. Inorganic transformations and the nitrogen cycle. Some microbes can enzymatically oxidize or reduce metals during respiration, affecting metal solubility and reactivity. Such inorganic transformations are used to treat metals in the 11

12 practice of bioremediation. Example reactions include the following: • Oxidation of ferrous to ferric ions precipitates ferric hydroxides or phosphates. • Reduction of sulfate to sulfide causes formation of insolu- ble metal sulfides, for instance, pyrite and mercuric sulfide. Hydrogen sulfide may form when concentrations are sig- nificantly greater than metals concentrations. • Reduction of hexavalent (Cr6+) to trivalent (Cr3+) chromium precipitates chromium oxides, sulfides, or phosphates. • Reduction of manganese from Mn4+ to Mn2+ releases solu- ble cations. The nitrogen cycle includes nitrogen transformations facil- itated by microbes (primarily bacteria) in addition to uptake and release of nitrogen from multicellular organisms and abiological processes. The microbial transformations of ammonification, nitrification, denitrification, and fixation are of interest for improvements in runoff water quality. Ammonification is the mineralization of organic nitrogen to ammonium by chemoheterotrophic bacteria and may occur aerobically or anaerobically. This is the main process supplying nitrogen to wetland plants, with about 1.5 to 3.5% of the organic nitrogen in soil mineralizing annually (Brady and Weil 2000). The rate of ammonification is typically high- est in the aerobic zone of wetland soils and decreases with depth because of the reduced efficiency of decomposition in anaerobic environments. However, because wetland soils have mostly anaerobic microbes, the overall mass of ammonium generated is greater in anaerobic conditions. Therefore, ammonification is significantly reduced under nonflooded conditions (Minton 2005; Kadlec and Knight 1996). Warm seasons, good moisture, or high organic matter content will increase the mineralization rate. Nitrification is the oxidation of ammonium to nitrate by chemoautotrophic bacteria in aerobic conditions (e.g., the water column, well-drained soils or the aerobic layer of flooded soils, and the plant root zone). It is a two-stage reac- tion, in which ammonium is oxidized to nitrite in the first stage (by Nitrosomonas primarily), and nitrite is oxidized to nitrate in the second stage (by Nitrobacter primarily). Typi- cally the second stage occurs quickly enough to prevent accu- mulation of nitrite. Denitrification is the reduction of nitrate to gaseous forms of nitrogen (nitric oxide, nitrous oxide, and dinitrogen gas) under anaerobic conditions, such as flooded soils. It may occur in wetland soils and in anaerobic pockets in terrestrial soils. Mechanisms for denitrification vary depending on the conditions and organisms involved. Nitric oxide and nitrous oxide are often formed under fluctuating oxygen levels, and, generally, when oxygen levels are very low, the end product is dinitrogen gas. The amount formed of each gas depends on pH, temperature, degree of oxygen depletion, and concentration of nitrate and nitrite. Nitrogen fixation is the process by which nitrogen gas in the atmosphere (or nitrogen gas generated during denitrifi- cation) is reduced to ammonia by bacteria, algae, and higher plants. Nitrogen-fixing bacteria form symbiotic relationships with certain plants, forming nodules in the roots. The plants provide the bacteria with carbohydrates for energy and a sta- ble environment for growth, while the bacteria give the plants usable nitrogen and other essential nutrients. Non-symbiotic fixation can also occur. Nitrogen can also be lost to the system as it volatilizes as ammonia gas at alkaline pH. This chemical process occurs frequently in wetlands. Degradation of xenobiotic compounds. In addition to simple organics, various microbes (primarily heterotrophic bacteria) are able to use more complex organics (such as xenobiotic compounds, that is, compounds foreign to the biological system) as energy sources during metabolism, which often results in microbial decomposition to less-toxic compounds. In some cases, xenobiotic compounds undergo incomplete degradation, and the products may be as toxic, or more toxic, than the parent compound. For example, trichloroethene (TCE) is degraded to vinyl chloride rather easily. However, subsequent degradation of vinyl chloride, a carcinogen, usually occurs slowly. Degradation can occur aerobically or anaerobically, although both processes occur relatively slowly (thus requir- ing long residence times). Significant degradation is possible for phenols, phthalate esters, naphthalenes, chlorinated ben- zenes, and nitroaromatics in aerobic conditions. Some com- pounds degrade more rapidly in anaerobic conditions, including carbon tetrachloride, chloroform, lindane, phenol, and methylene chloride (Minton 2005). Complete degrada- tion of some constituents may require alternating aerobic and anaerobic conditions (Knapp and Bromley-Challenor 2002). Under the right conditions, some microbes can transform xenobiotic compounds even when the chemical is not the primary energy source (cometabolism). Cometabolism is important for the breakdown of chlorinated solvents, poly- chlorinated biphenyls, and many PAHs, and, therefore, come- tabolism is the basis for bioremediation of many organic pollutants. Applicability to stormwater treatment. Stormwater treat- ment that incorporates vegetation and/or permanent water bodies usually has a diverse microbial population. While it is not possible to optimize conditions for all beneficial species, basic habitat requirements for all microbes include a sub- strate to colonize (e.g., soil, plant roots, or leaf surfaces), appropriate nutrients (including carbon sources), absence of

toxics, and sufficient moisture. The pH also affects microbe populations because different species have different limits of pH tolerance. Many microbes form symbiotic relations with plants and plant roots; therefore, increasing vegetation den- sity (and using the right plants) may increase microbial pop- ulations. Degradation that occurs in the plant root zone is referred to as rhizodegradation and occurs, for example, in the presence of deep-rooted turf grasses (e.g., swales). Adding organic amendments can also increase populations. Oxygen requirements are another important factor. Depending on the microbe, it may require the presence of oxygen (aerobic) or other electron-donating substances (facultative and anaero- bic) for metabolism. Various factors determine available oxy- gen, including soil characteristics and inundation patterns. Microbially mediated transformations can remove dis- solved nitrogen species (e.g., nitrate), metals, and simple and complex organic compounds. Many transformations only occur in the presence of specific microbes. Soils may be inoculated with desirable microbes to promote specific reactions or to boost a low initial microbial population. Transformations occur relatively slowly and require long residence times. Temperature affects microbial growth and transformation rates. Generally, increasing the temperature increases transformation kinetics. The optimum tempera- ture range for much microbial activity is between 15oC and 45oC (Tate 1995). Stormwater BMPs (wetlands, swales, and retention ponds) that facilitate these processes can require relevantly large land areas and therefore may not be suitable in highly urbanized areas. They may also have limited applications in arid cli- mates, areas with long dry seasons, and cold climates. Some microbially mediated processes have the potential for stream warming and should not be used where effluents discharge to temperature-sensitive water bodies, such as cold-water habi- tats. Nitrification may result in leaching of nitrate from the system, which is of particular concern in areas with water- quality impairment that is due to nutrient enrichment. 2.3.5.2 Uptake and Storage Definitions. Uptake and storage refer to the removal of organic and inorganic constituents by plants and microbes through nutrient uptake and bioaccumulation. Nutrient uptake converts required micronutrients and macronutrients (summarized at the end of this section) into living tissue, whereas bioaccumulation incorporates compounds (e.g., pollutants) into an organism, regardless of, or in excess of, what is immediately needed. Uptake and storage processes are generally not major pollutant-removal processes in stormwa- ter treatment systems because of the extended retention times required for such processes. Plants and microbes require essential nutrients to sustain growth (see Table 2-2), which may be assimilated from the water column or from soil solution through metabolic processes. In wetlands, free-floating plants take up nutri- ents from the water column; emergent plants take up nutri- ents from soil pore water; submerged plants may obtain nutrients from both the water column and soil pore water. The specific forms in which nutrients exist are determined largely by pH and redox potential. Micronutrient cations are most available for uptake under acidic conditions. The pres- ence of constituents such as silicate clays and organic matter also affects nutrient speciation. Organic matter, along with organic residues excreted by plant roots and microbes, may react with cationic micronutrients to form organometallic complexes (chelates), which are generally more available for uptake than nonchelated metal species in the aqueous phase. The strength of metal chelate formation is approximately of the order (Miller and Gardiner 1998): Fe3 > Al3 > Cu2 > Co2 > Zn2 > Fe2 > Mn2 > Ca2 = Mg2 Removal of phosphorus is the most significant uptake mechanism in stormwater treatment systems. Phosphorus uptake by plants and microbes may improve the capacity of the soil to sorb other constituents. However, phosphorus assimilated for metabolism is released back into the system upon death (or dormancy) of the microbe or plant. In addi- tion to nutrients, various algae, and wetland and terrestrial plants accumulate organic and inorganic constituents in excess of their immediate needs (bioaccumulation). Bioaccu- mulation is an evolutionary response to scarcity in the natu- ral environment and is the basis of phytoremediation. The organic compounds can remain in the water column or be metabolized in the root tissue, become assimilated into the cell wall, or become translocated to plant leaves and volatilized. These processes contribute to the effectiveness of constructed wetlands for wastewater treatment. The ability to remove chlorinated solvents, petroleum hydrocarbons, her- bicides, insecticides, and phenolic compounds has been investigated for wetland and terrestrial plants. Mechanisms of organic compound degradation by plants are not well under- stood (Scragg 1999). Degradation by plants is probably assisted by microbes, particularly in the root zone. The term hyperaccumulator applies to plants that can accumulate metals at concentrations 100-fold greater than concentrations found in the tissue of nonhyperaccumulators. Thus, hyperaccumulators can accumulate more than 10 ppm mercury, 100 ppm cadmium, 1,000 ppm cobalt, chromium, copper, and lead, and 10,000 ppm nickel and zinc. Metal tol- erance is the primary characteristic of these plants. These plants are also capable of translocating the metal from the root to plant stems and leaves. There are about 400 plants from 45 plant families capable of hyperaccumulation (USEPA 13

14 Nutrient Category Chemical Species Assimilated Function Primary Nutrients Nitrogen NO3–, NH4+ Constituent of amino acids, proteins, enzymes, and chlorophyll. Important in photosynthesis, metabolism, and protoplasm reactions. Component of DNA. Important for many growth and development processes. Stimulates uptake of other nutrients. Ammonium uptake is favored over nitrate. Phosphorus H2PO4– , HPO42– , PO43, organic phosphorus Constituent of proteins, phospholipids, enzyme systems, and nucleic acids. Essential component of Adenosine Triphosphate (ATP), which drives most energy-requiring biochemical processes, including nutrient uptake. Stimulates early growth and root formation. Important in photosynthesis. Comments: Organic phosphorus is a major nutrient source. Phosphorus concentrations in soil water are typically low because phosphorus tends to form insoluble compounds in soil. Potassium + Principal inorganic cation in cells. Cofactor of some enzymes. Affects cell division, formation of carbohydrates, translocation of sugars, various enzyme actions, disease resistance, stomata opening/closing, cell membrane permeability, and H+ relationships. Comments: Abundant in soils, but often bound to soil minerals, making it unavailable for assimilation. Secondary Nutrients Calcium 2+ Cofactor of enzymes. Regulates membrane permeability, cell integrity, and acidity. Essential component of plant cell walls and membranes. Magnesium Mg2+ Cofactor of many enzymes (for reactions such as denitrification and sulfate reduction). Present in cell walls, membranes, and phosphate esters. Constituent of chlorophyll. Aids mobility and efficiency of phosphorus. Sulfur SO42– , HS– , S0, S2O22– Sulfur oxidizing bacteria use FeS and FeS2 Essential for production of protein, constituent in amino acids. Promotes activity and development of enzymes and vitamins. Helps in chlorophyll formation. Improves root growth. Comments: Organic sulfur is a major nutrient source. Micronutrients K Ca Boron BO33– , B4O72– Required by higher plants and some microbes for growth of new cells. Chlorine Most likely Cl– Coenzyme for photosynthesis. Influences cell membrane permeability. Prevents desiccation. Required by halophilic bacteria (which also need sodium). Copper Cu+, Cu2+ Important in photosynthesis and vitamin A synthesis, protein and carbohydrate metabolism, and probably nitrogen fixation (cofactor for several enzymes). Iron Fe2+, Fe3+ Essential for chlorophyll synthesis. Catalyst in respiration. Important in cell division. Important for nitrogen fixation. Manganese Mn MoO 2+ Enzyme cofactor in many metabolic reactions. Catalyst with iron in chlorophyll synthesis. Role in chloroplast structure. Promotes pigment and vitamin C synthesis. Molybdenum 42– Required for nitrogen use. Needed for conversion of nitrate into amino acids and for nitrogen fixation. Role in plant hormones. Nickel Most likely Ni2+ Enzyme component. Important in nitrogen metabolism. Required for growth of some bacteria. Selenium SeO32– Present in some proteins. May be more important for microbes than plants. Zinc Zn2+ Enzyme component, including enzymes involved in zinc synthesis of hormones that regulate growth and development. Role in chlorophyll synthesis. Sources: Pittenger 2002; Miller and Gardiner 1998; Portier and Palmer 1989. Table 2-2. Characteristics of essential nutrients for plants and microbes.

2000a). Various constructed wetland plants, such as duck- weed (Lemna minor) and water hyacinth (Eichhornia cras- sipes) can hyperaccumulate metals (Zhu et al. 1999; Zayed et al. 1998; Qian et al. 1999). Other plants keep metals sorbed in the root zone and excrete matter that causes metal precipitation. This is a defen- sive strategy to prevent toxicity by inhibiting translocation from the roots to other parts of the plant and is referred to as phytostabilization in phytoremediation. Phytostabilizing plants exhibit low levels of metal accumulation in their shoots. Plants with this characteristic are also effective for ero- sion control because of their extensive and deep root systems. Uptake of organics. Plant uptake of organics is a function of the organic compound’s solubility, hydrophobicity (octanol-water distribution coefficient, Kow), and polarity. Generally, moderately hydrophobic compounds with log Kow between 0.5 and 3.0 are most readily taken up by and translo- cated within plants. More hydrophobic compounds may be sorbed by roots, but not translocated (USEPA 2000a). Non- polar molecules with molecular weights less than 500 will sorb to root surfaces, while polar molecules will enter the root and be translocated. Soil conditions (e.g., pH, acid ionization constant [pKa], organic and moisture content, and texture) affect the solubility of the organic compound. Plant physiol- ogy also influences uptake of organics (Salt et al. 1998). Plant ET rates are important because of the movement of organics through the plant. Seasonal and diurnal shifts in transpiration rates are relevant. In order for uptake mechanisms to occur, plants with appropriate characteristics must be selected. Dif- ferences in uptake of organics among plant species are well recognized (Salt et al. 1998). Uptake of metals. Hyperaccumulating plants have affini- ties for specific metals, and metal affinity may vary within different species within the same genus. Consequently, sig- nificant metal uptake by plants will not occur unless the appropriate species are selected. The number of plant species that hyperaccumulate specific metals is shown in Table 2-3. Uptake of metals depends on metal bioavailability. Low bioavailability may explain why there are so few hyperaccu- mulators of lead, as lead tends to form insoluble precipitates. Organic matter excreted by roots can increase metal bioavail- ability by lowering the pH or by forming metal chelates. Uptake and storage can be used to remove dissolved met- als, nutrients (phosphorus and nitrogen), and organic com- pounds. The process is suitable where soil properties and water quality are adequate to support organism growth. As a general rule, readily bioavailable metals for plant uptake include cadmium, nickel, zinc, arsenic, selenium, and copper. Moderately bioavailable metals are cobalt, manganese, and iron. Lead, chromium, and uranium are not very bioavailable. Lead can be made much more bioavailable by the addition of chelating agents to soils. The efficiency of uptake processes may be reduced in cold or arid climates. Many systems require a large area of land. Some uptake and storage processes have the potential for stream warming and should not be used where effluents discharge to temperature-sensitive water bod- ies, such as cold-water habitats. Concentrations in stormwater treatment systems may not be high enough for processes such as metal hyperaccumulation or organic compound reduction to occur. Uptake varies by season, latitude, and plant species. Uptake only occurs during the growing season. Establishment and growth of plants and microbes is affected by various soil char- acteristics including texture, pH, nutrient levels, salinity and toxicity, soil moisture, and drainage (oxygen). Various soil amendments can be used to make the substrate more suitable for plant and microbial growth.Plants should be suitable for the climate and hydrologic regime, be tolerant of concentrations in stormwater, and have appropriate growth characteristics. Uptake processes are enhanced in warm climates because of the extended growing season. Symbiotic microbes also enhance nutrient uptake by plants. Soils can be inoculated with the desired beneficial microbe (such as nitrogen-fixing bacteria). 2.3.6 Chemical Processes The chemical characteristics of stormwater (e.g.,pH,alkalin- ity, hardness, redox conditions, organic carbon, and ionic concentrations) affect the partitioning and speciation of stormwater pollutants, which in turn dictate the type of UOPs necessary to treat those pollutants. Three common chemical UOPs applied in the field of stormwater treatment include sorp- tion, coagulation/flocculation, and chemical agent disinfection. 2.3.6.1 Sorption While often inseparable from filtration unit operations, sorption refers to the individual unit processes of absorption and adsorption. Absorption is a physical process whereby a Metal Number of Hyperaccumulating Plant Species Nickel > 300 Cobalt 26 Copper 24 Zinc 18 Manganese 8 Lead 5 Cadmium 1 Source: U.S.EPA 2000a. Table 2-3. Number of hyper- accumulating plant families. 15

16 substance in one state is incorporated into another substance in a different state (e.g., liquids being absorbed by a solid or gases being absorbed by water). Adsorption is the physio- chemical adherence or bonding of ions and molecules (ion exchange) onto the surface of another molecule. In stormwa- ter treatment application, particularly for highway runoff, the primary pollutant types targeted with absorption unit processes are petroleum hydrocarbons, while adsorption processes typically target dissolved metals, nutrients, and organic toxicants such as pesticides and PAHs. Different types of filter media may provide either or both of these unit processes: these filter media include the use of activated car- bon to improve adsorption and synthetic polymers to improve absorption. Media can be engineered so that the chemistry of the media promotes chemical processes that result in more permanent chemical bonds between media and adsorbed solute (Liu et al. 2005b). Sorptive unit processes are specific mechanisms that range from surface complexation to precipitation, and such processes are generally designed for solute mass transfer onto materials with high surface area, generally engineered media. In stormwater, solutes of interest include phosphorus and metals. In combination with sorptive processes, unit opera- tions such as filtration can be an effective treatment control for dissolved and particulate-bound metal and phosphorus species. Mass transfer of dissolved species can occur to either engineered media or to stormwater runoff particles (parti- tioning), and then the dissolved species can be separated as particulate-bound constituents through filtration. Mass transfer for solutes occurs through different mechanisms and at different rates in stormwater. For example, phosphorus mass transfer to particles is generally through a combination of sorption and precipitation, depending on pH, and the rate of reaction can be very rapid, on the order of minutes to sev- eral hours. In contrast, mass transfer for different metals occurs differently and also has differing kinetics. For exam- ple, mechanisms of lead mass transfer to particles (depend- ing on the solid phase and pH) generally range from precipitation to surface complexation, with relatively rapid kinetics, while zinc mass-transfer mechanisms generally range from surface complexation to hydrolysis, with relatively slow kinetics. However, it must be recognized that the sorp- tion phenomena rates are dependent on the sorbent, hydro- dynamics, and water chemistry, and such phenomena are reversible (desorption). Thus, leaching of metals or phos- phorus from filter media is possible. Designs that allow draw- down of stormwater from a filter medium within several hours can help prevent leaching and other issues, such as bio- logical growth, and therefore reduce hydraulic conductivity (Liu et al. 2005b). Engineered media such as oxide-coated filter media with high surface area and amphoteric (pH-dependent) surface charge can be utilized to carry out the combined unit opera- tions of filtration and processes of surface complexation for a range of treatment configurations for in situ, decentralized treatment or centralized stormwater runoff treatment. Such treatment can be designed as a passive and integral part of existing urban infrastructure (e.g., urban and transportation infrastructure), or it can be designed as a centralized stormwater treatment component. For process design, con- trol, and optimization, it is important to know the quantita- tive metal species adsorption properties of the engineered media. Experimental studies and modeling of media metal species adsorption properties are required for a quantitative evaluation of stormwater media. One particular combined UOP providing in situ treatment for metal and phosphorus species removal in stormwater is an upflow sorptive buoyant media clarifier (SBMC). SBMCs are particularly well suited for stormwater discharges from elevated urban infrastructure such as an elevated roadway over water, where both solutes and particles in stormwater runoff are a concern. There are many examples of in situ treatments that combine sorptive processes and filtration operations either intentionally (by design) or inadvertently. Equilibrium isotherms are an important tool for describ- ing the equilibrium between aqueous and solid (media) phases for a known combination and concentration of solute(s), media, water chemistry, media/solution ratio, experimental geometry, and hydrodynamics. Isotherms indi- cate the adsorption capacity of a media or particulate solid under a prescribed or given set of conditions. Sorption isotherms are used to relate the concentration of solute adsorbed and/or absorbed to the soil or medium as a function of the solute concentration in solution. Three com- monly used isotherms are the linear, Freundlich, and Lang- muir isotherms widely discussed in contaminant hydrogeology literature (e.g., see Fetter 1999). Their general shapes and equations are shown in Figure 2-1, where Cs  mass of solute sorbed per dry weight of soil (mg/kg), C  solute concentration in solution at equilib- rium with sorbed mass of solute (mg/L), Kd  distribution coefficient (L/kg), K & N  constants found by regression, α  sorption constant (L/mg), and β  maximum sorbed concentration (mg/kg). Isotherm analysis can provide engineering parameters as well as a qualitative indication of more fundamental mecha- nisms. Isotherms generated for a given set of conditions can provide a quantitative indication of media capacity for indi- vidual solutes (i.e., mass of solute adsorbed per dry mass of media) even under the competitive conditions (multiple

competing solutes) of stormwater. For a basic analysis of media parameters required for design and analysis, the engi- neering behavior of any media or substrate utilized for sorp- tion requires knowledge of the equilibrium capacity of the media (how much pollutant will the media retain at equilib- rium) and the pace at which the pollutant can be transferred and retained by the media. The equilibrium capacity is sim- ply how much pollutant the media will hold when the rate of adsorption equals the rate of desorption. Sorption isotherms are derived experimentally by placing a known mass of soil (or media) in a known volume and con- centration of solute or solutes in solution (i.e., water), allow- ing the solute(s) to reach equilibrium, and then measuring the remaining solute(s) concentration in solution. The reduc- tion in solute concentration in solution is assumed to be the result of sorption, and the sorbed concentration can be cal- culated. This is repeated for a range of soil masses and solute volumes (and possibly concentrations) and to derive experi- mental data that can then be fit to the preferred or most rep- resentative sorption isotherm. The development of sorption isotherms can be used in part to evaluate the potential impacts from highway construction and repair materials (Nelson et al. 2001). Isotherms are an effective tool for describing solute species (e.g., aqueous metals and aqueous phosphorus) interaction and media capacity as a function of equilibrium conditions and concentration of solid and aqueous phases; however, isotherms will yield only indirect information on the role of water chemistry and adsorbent characteristics. There are additional tools to model complex and variable natural or engineered systems. In contrast to isotherm models, surface complexation (SC) models, based on double-layer theory, belong to a group of models that take a mechanistic and molecular-scale approach to surface interaction. As a result, SC models are versatile and have a fundamental physical- chemical basis for predicting metal species surface interac- tion phenomena over a wide range of experimental and natural conditions. SC models are particularly important when the species of interest are minor or trace ionic species and the solid phase substrate exhibits a pH-dependent (amphoteric) surface charge. It is important to recognize that most stormwater particles exhibit amphoteric behavior, and engineered media are generally amphoteric, in many cases by design. It cannot be overemphasized that an isotherm is a spe- cific relationship for adsorption capacity under a specific set of conditions. Isotherms should be generated for the specific conditions of the media application and range that are consistent with the variability anticipated. Often the media manufacturer will be able to assist with the develop- ment or predication of potential removal of pollutants of interest by the media. This information is central to sorp- tive filter designs. A review of media, isotherms, and kinet- ics can be found elsewhere (Liu et al. 2005a; Sansalone and Teng 2004; Teng and Sansalone 2004a, 2004b; Liu et al. 2001a, 2001b; Sansalone 1999; Masel 1996; Bar-Tal et al. 1990). SC models represent an additional tool to examine the complexity of sorption interactions. Such models are becoming more common in routine environmental chem- istry applications and are emerging as stormwater tools (Dean et al. 2005). Small pore spaces and large surface areas are desirable properties for media used to remove stormwater pollutants with sorption processes. Because of the propensity of small C Solution C s So il Linear Freundlich Langmuir Cs = Kd C Cs = K C N Cs = (α β C) / (1 + α C) Source: Adapted from Fetter 1999. Figure 2-1. Sorption isotherms. 17

18 pore spaces to clog, pretreatment for particulates is essential for continued functionality of sorptive media. 2.3.6.2 Precipitation/Coagulation/Flocculation Precipitation, coagulation, and flocculation are three processes that occur simultaneously or in quick succession. Precipitation is the process by which a pollutant is trans- formed from a primarily dissolved state to a solid state. Coagulation is the process by which colloidal particles are destabilized so that particle growth can occur. Flocculation is the process by which fine particles collide to form larger particles that can be readily removed through filtration and settling. While these three processes will occur naturally, the addition of chemicals is usually necessary to accelerate the process. Engineered chemical and physical flocculation is begin- ning to be applied in specific applications for stormwater runoff. Depending on parameters such as mixing, pH, ionic strength, and particle properties, natural flocculation can begin within several hours to 12 hours of initial runoff. Nat- ural flocculation, while generally not accounted for, can have a significant impact on stormwater runoff clarification in unit operations such as sedimentation basins or detention/ retention facilities. When existing treatment technologies do not provide enough treatment to achieve water-quality goals, the use of chemicals may be necessary. The types of pollutants typi- cally targeted with precipitation/coagulation/flocculation processes include fine and colloidal particulates, dissolved metals, and phosphorus. The disadvantage of using these processes in stormwater treatment applications is the gener- ation of potentially significant quantities of sludge that must be properly handled and disposed. Depending on the partic- ular chemicals used, the effluent may not be suitable for discharge because of reduced or elevated pH, high dissolved- aluminum or iron concentrations, or the presence of other undesirable by-products. The conditions and factors that enhance precipitation and flocculation processes are highly dependent on the chemicals being used; the primary factors include pH, temperature, and hardness. Other factors such as the particle-size distribution, free-ion concentration, and electronegativity of colloidal par- ticles will also influence these processes. 2.3.6.3 Chemical Disinfection Chemical disinfection refers to the mitigation of stormwa- ter-borne pathogens through the use of chemical agents such as chlorine and its compounds and ozone. The California Department of Transportation (Caltrans) has recently exam- ined chemical disinfection as a potential new technology for application to highway runoff (Caltrans 2004). Chemical dis- infection is used more extensively in wastewater applications with a wider range of chemical agents, including chlorine and its compounds, bromine, iodine, ozone, phenol and phenolic compounds, alcohol and heavy metals and related com- pounds, dyes, soaps and synthetic detergents, quaternary ammonium compounds, hydrogen peroxide, paracetic acid, various alkalis, and various acids (Metcalf and Eddy 2003). Because wastewater technologies are often adopted for stormwater applications, the list of agents currently used for stormwater disinfection could expand in the future. Chemical disinfection immobilizes pathogens through a variety of mechanisms, including damage to pathogen cell walls, alteration of pathogen cell-wall permeability, alteration of the colloidal nature of the protoplasm of the pathogen, alteration of the DNA or the RNA of the pathogen, and the inhibition of pathogen enzyme activity (Metcalf and Eddy 2003). The factors that affect the chlorine disinfection process include initial mixing, chemical characteristics of the influ- ent, impact of particles in the influent, particles with coliform organisms, and the characteristics of the target organisms (Metcalf and Eddy 2003). The effectiveness of ozone disin- fection systems depends on the dose, mixing, and contact time. Tables 2-4 and 2-5 present the impact of stormwater constituents on chlorine disinfection and ozone disinfection, respectively. Projects that have identified pathogens as constituents of concern must select either chemical or natural disinfection. These are the only two unit processes discussed in this docu- ment that specifically target pathogens. Chemical-agent disinfection may be cheaper than natural disinfection and hence may be a more suitable choice for a tight budget. Chlorine disinfection leaves a residual in the effluent that may provide added benefits by preventing the regrowth of pathogens and improving downstream water quality. Projects that use upstream BMPs that significantly reduce organic content and suspended solid content will increase the efficiency of any downstream chemical facilities. 2.3.7 Targeted Pollutants As implied in the previous discussion, different FPCs are applied for removal or reduction of different pollutants. Tar- geted pollutants for each FPC are included in Table 2-1. The interrelationship of pollutants, FPCs, and structural BMPs is summarized in another way in Table 2-6. As indicated in both Table 2-1 and Table 2-6, the choice of a BMP should be based on the water-quality characteristics of the stormwater relative to the treatment goals. Chapter 4 provides a thorough dis- cussion of pollutant sources and the water-quality character- istics of stormwater runoff from various land uses, including highways.

2.4 Characterization by BMP Type 2.4.1 Introduction Another common initial categorization measure for BMP systems is based upon system design, whether structural (constructed on site), proprietary (pre-engineered), or non- structural (source control). Structural BMPs are generally above-ground systems that are constructed on site and are intended to provide passive treatment or flow control of the stormwater using a variety of FPCs, as described above. Non- structural BMPs are generally associated with source control measures aimed at reducing the volume of runoff and the amount of pollutants directly at the source (Urbonas and Stahre 1993). Finally, proprietary BMPs are pre-engineered and typically premanufactured devices that use one or more treatment mechanisms and unit processes. They are often installed underground to minimize the required land area and are often used in conjunction with other BMPs in a treat- ment train. 2.4.2 Structural BMP Systems Structural systems generally rely on more than one unit process to achieve removal, and they may be configured and installed above or below ground, in a series (treatment train), or stand alone, depending upon the BMP(s) chosen, site Constituent Effect BOD, COD, TOC etc. Organic compounds that comprise the BOD and COD can exert a chlorine demand. The degree of interference depends on their functional groups and their chemical structure. Humic Materials Reduce effectiveness of chlorine by forming chlorinated organic compounds that are measured as chlorine residual but are not effective for disinfection. Oil and Grease Can exert a chlorine demand. TSS Shield embedded bacteria. Alkalinity No effect or minor effect. Hardness No effect or minor effect. Ammonia Combines with chlorine to form chloramines. Nitrite Oxidized by chlorine, formation of NDMA. Nitrate Chlorine dose is reduced because chloramines are not formed. Complete nitrification may lead to the formation of NDMA because of the presence of free chlorine. Partial nitrification may lead to difficulties in establishing the proper chlorine dose. Iron Oxidized by chlorine. Manganese Oxidized by chlorine. pH Affects distribution between hypochlorous acid and hypochlorite ion. Note: BOD =biochemical oxygen demand, COD = chemical oxygen demand, TOC =total organic carbon, NDMA = N-nitrosodimethylamine. Source: Metcalf and Eddy 2003. Constituent Effect BOD, COD, TOC etc. Organic compounds that comprise BOD and COD can exert an ozone demand. The degree of interference depends on their functional groups and their chemical structure. Humic Materials Affects the rate of ozone decomposition and the ozone demand. Oil and Grease Can exert an ozone demand. TSS Increase ozone demand and shielding of embedded bacteria. Alkalinity No effect or minor effect. Hardness No effect or minor effect. Ammonia No effect or minor effect, can react at high pH. Nitrite Oxidized by ozone. Nitrate Can reduce effectiveness of ozone. Iron Oxidized by ozone. Manganese Oxidized by ozone. pH Affects the rate of ozone decomposition. Note: BOD = biochemical oxygen demand, COD = chemical oxygen demand, TOC = total organic carbon. Source: Metcalf and Eddy 2003. Table 2-4. Potential effects of selected constituents on the use of chlorine. Table 2-5. Potential effects of selected constituents on the use of ozone. 19

20 constraints, and removal desired. A fair amount of research, both publicly (e.g., USEPA 1983) and privately funded, has been conducted regarding common structural systems, focusing upon their design and projected efficiency. A variety of standard BMP systems has been identified, and extensive literature exists for a number of these systems: • Wet ponds—(MacDonald et al. 1999; Schueler et al. 1992; Urbonas and Stahre 1993; Yonge et al. 2002) • Dry ponds and retention ponds—(Schueler et al. 1992) • Infiltration trenches—(ASCE 2001; Hathhorn and Yonge 1996; Schueler et al. 1992) • Wetlands—(Kadlec and Knight 1996; Rushton et al. 2002; Schueler et al. 1992) • Bioswales and filter strips—(Barrett et al. 1995a, 1995b; Fletcher et al. 2002; Schueler et al. 1992; Walsh et al. 1997; Yu et al. 2001) • Oil/grit separators—(ASCE 2001; Schueler et al. 1992) • Sand filters—(Barrett 2003; Keblin et al. 1997; Schueler et al. 1992; Tenney et al. 1995) • LID facilities—(Prince George’s County 2000; Puget Sound Action Team 2003) There are also a number of web sites pertaining to standard BMP evaluations (e.g., the International BMP Database site [www.bmpdatabase.org] discussed in Section 8.3). One exam- ple of a useful compendium of web sites and BMP documents is available from the California State Water Resources Con- trol Board (www.swrcb.ca.gov/rwqcb2/news_items/Tobi%20 Reference%20Attachment%20sept%206.doc). In addition, a number of references are available that describe the design and performance of standard BMP systems along with descriptions of the surrounding land and hydrologic charac- teristics. These references include Sutherland 1991; Schueler et al. 1992; Christensen et al. 1995; Barrett et al. 1995a,1995b, 1998; WEF and ASCE 1998; ASCE 2001; Strecker et al. 2001; and Minton 2005. 2.4.3 Proprietary BMP Systems Like structural systems, proprietary BMPs also may rely on more than one unit process for removal. Generally, propri- etary systems are more compact than many standard systems and are installed underground; thus, in many urban and ultra-urban settings, proprietary systems are preferred and used. Ranging from filtration to hydrodynamic devices, these proprietary systems are unique in both their design and oper- ation. A variety of proprietary BMPs has been identified dur- ing the course of this project. A selection of these devices judged to be currently in use is given in Table 2-7. Informa- tion on most of these devices may be found on the Internet. Whereas much analysis has been conducted on the effec- tiveness of standard BMPs, there is limited independent, third- party literature regarding the proprietary BMP technology and its pollutant-removal effectiveness. Independent eval- uations are available for at least four of the devices listed: Stormcepter (Winkler 1997b), CDS (Herrera Environmental Consultants 2002), StormTreat (Winkler 1997a), and Vortechs BMPs Pollutants Gravity Settling/Flotation Filtration/ Sorption Infiltration (Inf.) Biological Chemical Others/ Proprietary BMPs Particulates Sediments Solids Heavy metals Organics Nutrients Retention ponds Detention basins Wetlands Tanks/Vaults Biofilters Media filters Compost filters Wetlands Inf. trenches Inf. basins Porous pavement Swales Biofilters/ Bioretention Biofilters/Compost filters Wetlands/Wetland channels Coagulation/ Flocculation Wet vaults Vortex separators Modular wetland systems Inert media filters Solubles Heavy metals Organics/ BOD Nutrients Media filters Compost filters Wetlands/Wetland channels Retention ponds Inf. trenches Inf. basins Porous pavement Biofilters/Compost filters Wetlands/Wetland channels Precipitation/ Flocculation Activated carbon Media filters (StormFilter) Trash/ Debris Trash/ Debris N/A Screening N/A* N/A N/A Vortex separators Skimmers Floatables Oil and Grease Retention ponds Wetlands Hooded catch basins Catch basin inserts Vault filters Compost filters N/A Biofilters/Compost filters Wetlands N/A Oil/Water separators Absorptive media filters * N/A = not applicable. Table 2-6. Summary of groups of pollutants and relevant BMPs listed based on FPCs.

(Taylor Associates 2002; Winkler and Guswa 2002). Each proprietary system web site contains valuable information regarding constructability, system operation, system performance, and cost. Public agencies are also becoming involved in identifying and assessing proprietary BMP systems, e.g., USEPA Technology Verification program (www.epa.gov/etv/verifications/vcenter9-9.html), Washing- ton State Department of Ecology (2002), City of Portland Bureau of Environmental Services (BES 2001a). In addition, a number of individuals have analyzed or assessed proprietary BMP systems within the realm of their individual fields of research (e.g., Minton 2005; ICBIC 1995; Lau et al. 2001; Alsaigh et al. 1999). 2.4.4 Nonstructural BMP Systems Nonstructural BMPs, listed in Table 2-8, are generally source controls undertaken by communities to promote good housekeeping measures and activities aimed at reducing and preventing pollution on a community or neighborhood basis. These range from BMP maintenance and source control efforts such as reducing vehicle use, sweeping streets, clean- ing catch basins, controlling and picking up litter, and con- trolling vegetation, to publishing informational brochures, making presentations at schools, stenciling storm drains, and regulating land use. Nonstructural BMPs are generally most effective with the full support and participation of the com- munity (WEF and ASCE 1998). However, the emphasis of this report is on structural and proprietary systems suitable for highways; thus, nonstructural systems will not be discussed any further. Proprietary BMP Trade Names StormCeptor BaySaver StormVault Wet Vaults ADS Retention/Detention System Constructed Wetlands StormTreat Vortechs Aquafilter V2B1 Downstream Defender Hydrodynamic/Vortex Separators Continuous Deflective Separation (CDS) Unit Inert/Sorptive Media Filters StormFilter High-flow Bypass (Flow Splitter) StormGate Modular Pavement Various Table 2-7. Example proprietary BMPs in current use by treatment type. Nonstructural BMP Type Street sweeping Catch basin cleaning Good housekeeping practices, e.g., covering of stockpiled materials, washing of construction vehicles before leaving construction sites Safer alternative products, e.g., highway construction materials, herbicides, road salts Material storage control Reduction in vehicle use Household hazardous waste collection* Used oil recycling Vehicle spill control Above-ground tank spill control Illegal dumping control Vegetation control Storm drain flushing Roadway and bridge maintenance Detention and infiltration device maintenance Litter control Source Control/Maintenance Litter pickup Education, e.g., newspapers, brochures, K–12 Land use planning and management Adopt-A-Highway Integrated pest management Public Education and Participation Storm drain system signs (stenciling) Curb elimination Other Reduction of runoff velocity *Entries in italics are unlikely to be applicable to highways. Table 2-8. Nonstructural BMPs. 21