4.0 Environmental Assessment - US EPA

1 4 - AssessmentWaterways and receiving waters near urban and suburban areas are often adverselyaffected by urban storm water runoff. The degree and type of impact varies from location tolocation, but it is often significant relative to other sources of pollution and environmentaldegradation. Urban storm water runoff affects water quality , water quantity, habitat andbiological resources, public health, and the aesthetic appearance of urban waterways. As reportedin the national water quality inventory 1996 Report to Congress (US EPA, 1998d), urban runoffwas the leading source of pollutants causing water quality impairment related to human activitiesin ocean shoreline waters and the second leading cause in estuaries across the nation. Urbanrunoff was also a significant source of impairment in rivers and lakes.

2 The percent of totalimpairment attributed to urban runoff is substantial. This impairment constitutes approximately5,000 square miles of estuaries, million acres of lakes, and 30,000 miles of rivers. Sevenstates also reported in the inventory that urban runoff contributes to wetland impacts on receiving waters associated with storm water discharges have beendiscussed by EPA (1995b) in terms of three general classes. These are: Short-term changes in water quality during and after storm events including temporaryincreases in the concentration of one or more pollutants, toxics or bacteria levels. Long-term water quality impacts caused by the cumulative effects associated withrepeated storm water discharges from a number of sources. Physical impacts due to erosion, scour, and deposition associated with increasedfrequency and volume of runoff that alters aquatic described in the Terrene Institute s Fundamentals of Urban Runoff Management(Horner et al, 1994), pollutants associated with urban runoff potentially harmful to receivingwaters fall into the categories listed below: Solids Oxygen-demanding substances Nitrogen and phosphorus Pathogens Petroleum hydrocarbons Metals Synthetic pollutants degrade water quality in receiving waters near urban areas, and oftencontribute to the impairment of use and exceedences of criteria included in State water qualitystandards.

3 The quantity of these pollutants per unit area delivered to receiving waters tends toincrease with the degree of development in urban - 2 While water quality impacts are often unobserved by the general public, other storm waterimpacts are more visible. Stream channel erosion and channel bank scour provide direct evidenceof water quantity impacts caused by urban storm water . Urban runoff increases directly withimperviousness and the degree of watershed development. As urban areas grow, urban streamsare forced to accommodate larger volumes of storm water runoff that recur on a more frequentbasis. This leads to stream channel instability. The change in watershed hydrology associated withurban development also causes channel widening and scour, and the introduction of largeramounts of sediment to urban streams.

4 Visible impacts include eroded and exposed stream banks,fallen trees, sedimentation, and recognizably turbid conditions. The increased frequency offlooding in urban areas also poses a threat to public safety and property. Both water quality and water quantity impacts associated with urban storm water combineto impact aquatic and riparian habitat in urban streams. Higher levels of pollutants, increased flowvelocities and erosion, alteration of riparian corridors, and sedimentation associated with stormwater runoff negatively impact the integrity of aquatic ecosystems. These impacts include thedegradation and loss of aquatic habitat, and reduction in the numbers and diversity of fish andmacroinvertebrates. Public health impacts are for the most part related to bacteria and disease causingorganisms carried by urban storm water runoff into waters used for water supplies, fishing andrecreation.

5 water supplies can potentially be contaminated by urban runoff, posing a public healththreat. Bathers and others coming in contact with contaminated water at beaches and otherrecreational sites can become seriously ill. Beach closures caused by urban runoff have a negativeimpact on the quality of life, and can impede economic development as well. Similarly, thebacterial contamination of shellfish beds poses a public health threat to consumers, and shellfishbed closures negatively impact the fishing industry and local impacts in the form of debris and litter floating in urban waterways andconcentrated on stream banks and beaches are quite visible to the general public. Storm water isa major source of floatables that include paper and plastic bags and packaging materials, bottles,cans, and wood.

6 The presence of floatables and other debris in receiving waters during andfollowing storm events reduces visual attractiveness of the waters and detracts from theirrecreational value. Nuisance algal conditions including surface scum and odor problems can alsobe attributed to urban storm water in many on available information and data, the following general statements can be madeabout urban storm water impacts. Impacts to water quality in terms of water column chemistry tend to be transient andelusive, particularly in rivers. Impacts to habitat and aquatic life are generally more profound, and are easier to seeand quantify than changes in water column - 3 Impacts are typically complex because urban storm water is often one of severalsources including municipal discharges and diffuse runoff from agricultural and ruralareas that affect urban waterways.

7 Impacts are often interrelated and cumulative. For example, both degraded waterquality and increased water quantity join to impact habitat and biological following sections describe the sources of urban storm water runoff, the pollutantscontained in urban runoff and the impacts attributable to urban storm water discharges. Examplessupported by field observation and data have been used extensively to show storm water impacts. The impacts described include water quality impacts, water quantity impacts, public healthimpacts, habitat impacts, and aesthetic impacts. of Storm water DischargesStorm water runoff from urbanized areas is generated from a number of sources includingresidential areas, commercial and industrial areas, roads, highways and bridges. Essentially, anysurface which does not have the capability to pond and infiltrate water will produce runoff duringstorm events.

8 When a land area is altered from a natural forested ecosystem to an urbanized landuse consisting of rooftops, streets and parking lots, the hydrology of the system is significantlyaltered. water which was previously ponded on the forest floor, infiltrated into the soil andconverted to groundwater, utilized by plants and evaporated or transpired into the atmosphere isnow converted directly into surface runoff. An important measure of the degree of urbanizationin a watershed is the level of impervious surfaces. As the level of imperviousness increases in awatershed, more rainfall is converted to runoff. Figure 4-1 illustrates this - 4 Natural Ground Cover10-20% Impervious30-50% Impervious75-100% Impervious25% DeepInfiltration25% ShallowInfiltration10% Runoff38% Evapotranspiration21% DeepInfiltration21% ShallowInfiltration20% Runoff5% DeepInfiltration10% ShallowInfiltration35% Evapotranspiration30% Runoff15% DeepInfiltration20 % ShallowInfiltration30% Evapotranspiration55% Runoff40% EvapotranspirationSource: Adapted from Arnold and Gibbons, 1996 Figure 4-1.

9 Effects of Imperviousness on Runoff and InfiltrationThe traditional means of managing storm water runoff in urban areas has been to constructa vast curb-and-gutter, catch basin, and storm drain network to transport this runoff volumequickly and efficiently away from the urbanized area and discharge the water to receiving streams. Two types of sewer systems are used to convey storm water runoff: separate storm sewers andcombined sewers. Separate storm sewer systems convey only storm water runoff. water conveyed inseparate storm sewers is frequently discharged directly to receiving streams withoutreceiving any intentional form of treatment. (In a municipality with a separate stormsewer system, sanitary sewer flows are conveyed in a distinct sanitary sewer system tomunicipal wastewater treatment plants.)

10 In a combined sewer system, storm water runoff is combined with sanitary sewer flowsfor conveyance. Flows from combined sewers are treated by municipal wastewater4 - 5treatment plants prior to discharge to receiving streams. During large rainfall eventshowever, the volume of water conveyed in combined sewers can exceed the storageand treatment capacity of the wastewater treatment system. As a result, discharges ofuntreated storm water and sanitary wastewater directly to receiving streams canfrequently occur in these systems. These types of discharges are known as combinedsewer overflows (CSOs).Historically, as urbanization occurred and storm drainage infrastructure systems weredeveloped in this country, the primary concern was to limit nuisance and potentially damagingflooding due to the large volumes of storm water runoff that are generated.