Urbanization increases the discharge of pollutants in stormwater runoff. Development introduces new sources of stormwater pollutants and provides impervious surfaces that accumulate pollutants between storms. Structural stormwater collection and conveyance systems allow stormwater pollutants to quickly wash off during storm or snowmelt events and discharge to downstream receiving waters. By contrast, in undeveloped areas, natural processes such as infiltration, interception, depression storage, filtration by vegetation, and evaporation can reduce the quantity of stormwater runoff and remove pollutants. Impervious areas decrease the natural stormwater purification functions of watersheds and increase the potential for water quality impacts in receiving waters.
Urban land uses and activities can also degrade groundwater quality if stormwater with high pollutant loads is directed into the soil without adequate treatment. Certain land uses and activities, sometimes referred to as stormwater “hotspots” (e.g., commercial parking lots, vehicle service and maintenance facilities, and industrial rooftops), are known to produce higher loads of pollutants such as metals and toxic chemicals. Soluble pollutants can migrate into groundwater and potentially contaminate wells in groundwater supply aquifer areas.
Table 2-3 lists the principal pollutants found in urban stormwater runoff, typical pollutant sources, related impacts to receiving waters, and factors that promote pollutant removal. Table 2-3 also identifies those pollutants that commonly occur in a dissolved or soluble form, which has important implications for the selection and design of stormwater management practices described later in this manual. Chapter Three contains additional information on pollutant removal mechanisms for various stormwater pollutants.
| Table 2‑3. Summary of Urban Stormwater Pollutants
|
Stormwater Pollutant
|
Potential Sources |
Receiving Water Impacts |
Removal
Promoted By1 |
| Excess Nutrients
Nitrogen, Phosphorus
(soluble) |
Animal waste, fertilizers, failing septic systems, landfills, atmospheric deposition, erosion and sedimentation, illicit sanitary connections |
Algal growth, nuisance plants, ammonia toxicity, reduced clarity, oxygen deficit (hypoxia), pollutant recycling from sediments, decrease in submerged aquatic vegetation (SAV) |
Phosphorus:
High soil exchangeable aluminum and/or iron content, vegetation and aquatic plants
Nitrogen:
Alternating aerobic and anaerobic conditions, low levels of toxicants, near neutral pH (7) |
| Sediments
Suspended, Dissolved, Deposited, Sorbed Pollutants |
Construction sites, streambank erosion, washoff from impervious surfaces |
Increased turbidity, lower dissolved oxygen, deposition of sediments, aquatic habitat alteration, sediment and benthic toxicity |
Low turbulence, increased residence time |
| Pathogens
Bacteria, Viruses |
Animal waste, failing septic systems, illicit sanitary connections |
Human health risk via drinking water supplies, contaminated swimming beaches, and contaminated shellfish consumption |
High light (ultraviolet radiation), increased residence time, media/soil filtration, disinfection |
| Organic Materials
Biochemical Oxygen Demand, Chemical Oxygen Demand |
Leaves, grass clippings, brush, failing septic systems |
Lower dissolved oxygen, odors, fish kills, algal growth, reduced clarity |
Aerobic conditions, high light, high soil organic content, low levels of toxicants, near neutral pH (7) |
| Hydrocarbons
Oil and Grease |
Industrial processes; commercial processes; automobile wear, emissions, and fluid leaks; improper oil disposal |
Toxicity of water column and sediments, bioaccumulation in food chain organisms |
Low turbulence, increased residence time, physical separation or capture techniques |
| Metals
Copper, Lead, Zinc, Mercury, Chromium, Aluminum
(soluble) |
Industrial processes, normal wear of automobile brake linings and tires, automobile emissions and fluid leaks, metal roofs |
Toxicity of water column and sediments, bioaccumulation in food chain organisms |
High soil organic content, high soil cation exchange capacity, near neutral pH (7) |
| Synthetic Organic Chemicals
Pesticides, VOCs, SVOCs, PCBs, PAHs
(soluble) |
Residential, commercial, and industrial application of herbicides, insecticides, fungicides, rodenticides; industrial processes; commercial processes |
Toxicity of water column and sediments, bioaccumulation in food chain organisms |
Aerobic conditions, high light, high soil organic content, low levels of toxicants, near neutral pH (7), high temperature and air movement for volatilization of VOCs |
| Deicing Constituents
Sodium, Calcium, Potassium
Chloride
Ethylene Glycol
Other Pollutants
(soluble) |
Road salting and uncovered salt storage. Snowmelt runoff from snow piles in parking lots and roads during the spring snowmelt season or during winter rain on snow events. |
Toxicity of water column and sediments, contamination of drinking water, harmful to salt intolerant plants. Concentrated loadings of other pollutants as a result of snowmelt. |
Aerobic conditions, high light, high soil organic content, low levels of toxicants, near neutral pH (7) |
| Trash and Debris |
Litter washed through storm drain network |
Degradation of aesthetics, threat to wildlife, potential clogging of storm drainage system |
Low turbulence, physical straining/capture |
| Freshwater Impacts |
Stormwater discharges to tidal wetlands and estuarine environments |
Dilution of the high marsh salinity and encouragement of the invasion of brackish or upland wetland species such as Phragmites |
Stormwater retention and volume reduction |
| Thermal Impacts |
Runoff with elevated temperatures from contact with impervious surfaces (asphalt) |
Adverse impacts to aquatic organisms that require cold and cool water conditions |
Use of wetland plants and trees for shading, increased pool depths |
| 1Factors that promote removal of most stormwater pollutants include:
· Increasing hydraulic residence time
· Low turbulence
· Fine, dense herbaceous plants
· Medium-fine textured soil
|
Source: Adapted from DEP, 1995; Metropolitan Council, 2001; Watershed Management Institute, Inc., 1997.
Excess Nutrients
Urban stormwater runoff typically contains elevated concentrations of nitrogen and phosphorus that are most commonly derived from lawn fertilizer, detergents, animal waste, atmospheric deposition, organic matter, and improperly installed or failing septic systems. Nutrient concentrations in urban runoff are similar to those found in secondary wastewater effluents (American Public Works Association and Texas Natural Resource Conservation Commission). Elevated nutrient concentrations in stormwater runoff can result in excessive growth of vegetation or algae in streams, lakes, reservoirs, and estuaries, a process known as accelerated eutrophication. Phosphorus is typically the growth-limiting nutrient in freshwater systems, while nitrogen is growth-limiting in estuarine and marine systems. This means that, in marine waters algal growth usually responds to the level of nitrogen in the water, and in fresh waters algal growth is usually stimulated by the level of available or soluble phosphorus (DEP, 1995).
Nutrients are a major source of degradation in many of Connecticut’s water bodies. Excessive nitrogen loadings have led to hypoxia, a condition of low dissolved oxygen, in Long Island Sound. A Total Maximum Daily Load (TMDL) for nitrogen has been developed for Long Island Sound, which will restrict nitrogen loadings from point and non-point sources throughout Connecticut. Phosphorus in runoff has impacted the quality of many of Connecticut’s lakes and ponds, which are susceptible to eutrophication from phosphorus loadings. Nutrients are also detrimental to submerged aquatic vegetation (SAV). Nutrient enrichment can favor the growth of epiphytes (small plants that grow attached to other things, such as blades of eelgrass) and increase amounts of phytoplankton and zooplankton in the water column, thereby decreasing available light. Excess nutrients can also favor the growth of macroalgae, which can dominate and displace eelgrass beds and dramatically change the food web (Deegan et al., 2002).
Sediments
Sediment loading to water bodies occurs from washoff of particles that are deposited on impervious surfaces such as roads and parking lots, soil erosion associated with construction activities, and streambank erosion. Although some erosion and sedimentation is natural, excessive sediment loads can be detrimental to aquatic life (phytoplankton, algae, benthic invertebrates, and fish) by interfering with photosynthesis, respiration, growth, and reproduction. Solids can either remain in suspension or settle to the bottom of the water body. Suspended solids can make the water cloudy or turbid, detract from the aesthetic and recreational value of a water body, and harm SAV, finfish, and shellfish. Sediment transported in stormwater runoff can be deposited in a stream or other water body or wetland and can adversely impact fish and wildlife habitat by smothering bottom dwelling aquatic life and changing the bottom substrate. Sediment deposition in water bodies can result in the loss of deep-water habitat and can affect navigation, often necessitating dredging. Sediment transported in stormwater runoff can also carry other pollutants such as nutrients, metals, pathogens, and hydrocarbons.
Pathogens
Pathogens are bacteria, protozoa, and viruses that can cause disease in humans. The presence of bacteria such as fecal coliform or enterococci is used as an indicator of pathogens and of potential risk to human health (DEP, 1995). Pathogen concentrations in urban runoff routinely exceed public health standards for water contact recreation and shellfishing. Sources of pathogens in stormwater runoff include animal waste from pets, wildlife, and waterfowl; combined sewers; failing septic systems; and illegal sanitary sewer cross-connections. High levels of indicator bacteria in stormwater have commonly led to the closure of beaches and shellfishing beds along coastal areas of Connecticut.
Organic Materials
Oxygen-demanding organic substances such as grass clippings, leaves, animal waste, and street litter, are commonly found in stormwater. The decomposition of such substances in water bodies can deplete oxygen from the water, thereby causing similar effects to those caused by nutrient loading. Organic matter is of primary concern in water bodies where oxygen is not easily replenished such as slower moving streams, lakes, and estuaries. An additional concern for unfiltered water supplies is the formation of trihalomethane (THM), a carcinogenic disinfection byproduct generated by the mixing of chlorine with water high in organic carbon (NYDEC, 2001).
Hydrocarbons
Urban stormwater runoff contains a wide array of hydrocarbon compounds, some of which are toxic to aquatic organisms at low concentrations (Woodward-Clyde, 1990). The primary sources of hydrocarbons in urban runoff are automotive. Source areas with higher concentrations of hydrocarbons in stormwater runoff include roads, parking lots, gas stations, vehicle service stations, residential parking areas, and bulk petroleum storage facilities.
Metals
Metals such as copper, lead, zinc, mercury, and cadmium are commonly found in urban stormwater runoff. Chromium and nickel are also frequently present (USEPA, 1983). The primary sources of these metals in stormwater runoff are vehicular exhaust residue, fossil fuel combustion, corrosion of galvanized and chrome-plated products, roof runoff, stormwater runoff from industrial sites, and the application of deicing agents. Architectural copper associated with building roofs, flashing, gutters, and downspouts has been shown to be a source of copper in stormwater runoff in Connecticut and other areas of the country (University of Connecticut; Barron, 2000; Tobiason, 2001). Marinas have also been identified as a source of copper and aquatic toxicity to inland and marine waters (Sailer Environmental, Inc. 2000). Washing or sandblasting of boat hulls to remove salt and barnacles also removes some of the bottom paint, which contains copper and zinc additives to protect hulls from deterioration.
In Connecticut, discharge of metals to surface waters is of particular concern. Metals can be toxic to aquatic organisms, can bioaccumulate, and have the potential to contaminate drinking water supplies. Many major rivers in Connecticut have copper levels that exceed Connecticut’s Copper Water Quality Criteria. Although metals generally attach themselves to the solids in stormwater runoff or receiving waters, recent studies have demonstrated that dissolved metals, particularly copper and zinc, are the primary toxicants in stormwater runoff from industrial facilities throughout Connecticut (Mas et al., 2001; New England Bioassay, Inc., 2001). Additionally, stormwater runoff can contribute to elevated metals in aquatic sediments. The metals can become bioavailable where the bottom sediment is anaerobic (without oxygen) such as in a lake or estuary. Metal accumulation in sediments has resulted in impaired aquatic habitat and more difficult maintenance dredging operations in estuaries because of the special handling requirements for contaminated sediments.
Synthetic Organic Chemicals
Synthetic organic chemicals can also be present at low concentrations in urban stormwater. Pesticides, phenols, polychlorinated biphenyls (PCBs), and polynuclear or polycyclic aromatic hydrocarbons (PAHs) are the compounds most frequently found in stormwater runoff. Such chemicals can exert varying degrees of toxicity on aquatic organisms and can bioaccumulate in fish and shellfish. Toxic organic pollutants are most commonly found in stormwater runoff from industrial areas. Pesticides are commonly found in runoff from urban lawns and rights-of-way (NYDEC, 2001). A review of monitoring data on stormwater runoff quality from industrial facilities has shown that PAHs are the most common organic toxicants found in roof runoff, parking area runoff, and vehicle service area runoff (Pitt et al., 1995).
Deicing Constituents
Salting of roads, parking lots, driveways, and sidewalks during winter months and snowmelt during the early spring result in the discharge of sodium, chloride, and other deicing compounds to surface waters via stormwater runoff. Excessive amounts of sodium and chloride may have deleterious effects on water, soil, and vegetation, and can also accelerate corrosion of metal surfaces. Drinking water supplies, particularly groundwater wells, may be contaminated by runoff from roadways where deicing compounds have been applied or from highway facilities where salt mixes are improperly stored. In addition, sufficient concentrations of chlorides may prove toxic to certain aquatic species. Excess sodium in drinking water lead to health problems in individuals on low sodium diets. Other deicing compounds may contain nitrogen, phosphorus, and oxygen demanding substances. Antifreeze from automobiles is a source of phosphates, chromium, copper, nickel, and cadmium.
Other pollutants such as sediment, nutrients, and hydrocarbons are released from the snowpack during the spring snowmelt season or during winter rain on snow events. The pollutant loading during snowmelt can be significant and can vary considerably during the course of the melt event (NYDEC, 2001). For example, a majority of the hydrocarbon load from snowmelt occurs during the last 10 percent of the event and towards the end of the snowmelt season (Oberts, 1994). Similarly, PAHs, which are hydrophobic materials, remain in the snowpack until the end of the snowmelt season, resulting in highly concentrated loadings (Metropolitan Council, 2001).
Trash and Debris
Trash and debris are washed off of the land surface by stormwater runoff and can accumulate in storm drainage systems and receiving waters. Litter detracts from the aesthetic value of water bodies and can harm aquatic life either directly (by being mistaken for food) or indirectly (by habitat modification). Sources of trash and debris in urban stormwater runoff include residential yard waste, commercial parking lots, street refuse, combined sewers, illegal dumping, and industrial refuse.
Freshwater Impacts
Discharge of freshwater, including stormwater, into brackish and tidal wetlands can alter the salinity and hydroperiod of these environments, which can encourage the invasion of brackish or freshwater wetland species such as Phragmites.
Thermal Impacts
Impervious surfaces may increase temperatures of stormwater runoff and receiving waters. Roads and other impervious surfaces heated by sunlight may transport thermal energy to a stream during storm events. Direct exposure of sunlight to shallow ponds and impoundments as well as unshaded streams may further elevate water temperatures. Elevated water temperatures can exceed fish and invertebrate tolerance limits, reducing survival and lowering resistance to disease. Coldwater fish such as trout may be eliminated, or the habitat may become marginally supportive of coldwater species. Elevated water temperatures also contribute to decreased oxygen levels in water bodies and dissolution of solutes.
Concentrations of pollutants in stormwater runoff vary considerably between sites and storm events. Typical average pollutant concentrations in urban stormwater runoff in the Northeast United States are summarized in Table 2-4.
Table 2‑4. Average Pollutant Concentrations in Urban Stormwater Runoff
Constituent
|
Units |
Concentration |
| Total Suspended Solids1 |
mg/l |
54.5 |
| Total Phosphorus1 |
mg/l |
0.26 |
| Soluble Phosphorus1 |
mg/l |
0.10 |
| Total Nitrogen1 |
mg/l |
2.00 |
| Total Kjeldahl Nitrogen1 |
mg/l |
1.47 |
| Nitrite and Nitrate1 |
mg/l |
0.53 |
| Copper1 |
mg/l |
11.1 |
| Lead1 |
mg/l |
50.7 |
| Zinc1 |
mg/l |
129 |
| BOD1 |
mg/l |
11.5 |
| COD1 |
mg/l |
44.7 |
| Organic Carbon2 |
mg/l |
11.9 |
| PAH3 |
mg/l |
3.5 |
| Oil and Grease4 |
mg/l |
3.0 |
| Fecal Coliform5 |
Colonies/100 ml |
15,000 |
| Fecal Strep5 |
Colonies/100 ml |
35,400 |
| Chloride (snowmelt)6 |
mg/l |
116 |
Source: Adapted from NYDEC, 2001; original sources are listed below.
1Pooled Nationwide Urban Runoff Program/USGS (Smullen and Cave, 1998)
2Derived from National Pollutant Removal Database (Winer, 2000)
3Rabanal and Grizzard, 1996
4Crunkilton et al., 1996
5Schueler, 1999
6Oberts, 1994
mg/l = milligrams per liter
mg/l= micrograms per liter
Stormwater runoff is a natural part of the hydrological cycle, which is the distribution and movement of water between the earth’s atmosphere, land, and water bodies. Rainfall, snowfall, and other frozen precipitation send water to the earth’s surfaces. Stormwater runoff is surface flow from precipitation that accumulates in and flows through natural or man-made conveyance systems during and immediately after a storm event or upon snowmelt. Stormwater runoff eventually travels to surface water bodies as diffuse overland flow, a point discharge, or as groundwater flow. Water that seeps into the ground eventually replenishes groundwater aquifers and surface waters such as lakes, streams, and the oceans. Groundwater recharge also helps maintain water flow in streams and wetland moisture levels during dry weather. Water is returned to the atmosphere through evaporation and transpiration to complete the cycle. A schematic of the hydrologic cycle is shown in Figure 2-1.
Urban stormwater runoff can be considered both a point source and a nonpoint source of pollution. Stormwater runoff that flows into a conveyance system and is discharged through a pipe, ditch, channel, or other structure is considered a point source discharge under EPA’s National Pollutant Discharge Elimination System (NPDES) permit program, as administered by DEP. Stormwater runoff that flows over the land surface and is not concentrated in a defined channel is considered nonpoint source pollution. In most cases stormwater runoff begins as a nonpoint source and becomes a point source discharge (MADEP, 1997). Both point and nonpoint sources of urban stormwater runoff have been shown to be significant causes of water quality impairment (EPA, 2000).
Impervious cover has emerged as a measurable, integrating concept used to describe the overall health of a watershed. Numerous studies have documented the cumulative effects of urbanization on stream and watershed ecology (Schueler et al., 1992; Schueler, 1994; Schueler, 1995; Booth and Reinelt, 1993, Arnold and Gibbons, 1996; Brant, 1999; Shaver and Maxted, 1996). Research has shown that when impervious cover in a watershed reaches between 10 and 25 percent, ecological stress becomes clearly apparent. Beyond 25 percent, stream stability is reduced, habitat is lost, water quality becomes degraded, and biological diversity decreases (NRDC, May 1999). Figure 2-3 illustrates this effect.
Table 2-2 indicates typical percentages of impervious cover for various land uses in Connecticut and the Northeast United States. It is important to note that these tabulated values reflect impervious coverage within individual land uses, but do not reflect overall watershed imperviousness, for which the ecological stress thresholds apply. However, in developed watersheds with significant residential, commercial, and industrial development, overall watershed imperviousness often exceeds the ecological stress thresholds.
Increased runoff volume