905R91107
                     United States
                     Environmental Protection
                     Agency
                     Watershed Management Unit
                     Water Division, Region V
                     Chicago, IL
February 1991
vvEPA
Urban  Runoff Impacts to
Receiving  Waters
           Introduction
                    he EPA-sponsored National Urban Runoff Program (NURP)
                    identified the potential of stormwater to adversely affect
                    receiving waters and aquatic biota through increased frequency
                    and duration of peak flow rates, erosion/sedimentation,
                    eutrophication, or toxic impact. Assessments completed under
                    state clean lakes and nonpoint source programs have identified
                    the extent to which urban runoff is impairing water use.
     Stormwater Contaminants
               he state assessments identified
               several categories of potential urban
               stormwater pollutants: suspended
               solids, nutrients, bacteria,
               oils/grease, toxic organics, and toxic
               inorganics (heavy metals). Critical
               pollutants were identified by: (1)
     frequency of occurrence within the stormwater
     database, and (2) high concentrations relative to
     the EPA water quality criteria. An additional
     consideration was the degree to which urbanized
     stormflow hydrology alone impacted biota in
     natural stream courses. Potential impacts resulting
     from the presence of the above pollutants include:
     (1) physical impairment or habitat disruption to
     biota, (2) enrichment and subsequent
     eutrophication of receiving waters, and (3)
     exposure and physiological response to toxic
     substances by aquatic biota. The presence of such
     impacts are considered an impairment of the
     receiving water resource.

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   Automobile traffic Is a major source of pollutants resulting from urban stormwater runoff.
High Flows/Erosion/
Suspended Solids

Various urban runoff studies have effectively
demonstrated the impacts of high flows, erosion,
and deposition on urban streams and other
sensitive receiving waters. Increased frequency
and duration of high flows result in increased
erosion-related impacts to (1) eroded sites,
(2) conveyance systems including streams, and
(3) sites of deposition. In-stream impacts are
related to increased streambank erosion during
high flows, increased turbidity and suspended
solids concentrations, scouring habitat, and
downstream depositional impacts that degrade
habitat and reduce  hydraulic channel capacities.
Increased suspended solids concentrations and
turbidity in streams can be detrimental to aquatic
life (primary producers, benthic invertebrates, and
fish) by interfering with photosynthesis, respiration,
growth, and reproduction. The deposition of
relatively fine-grained sediments in stream beds
can dramatically reduce their value for insect
production and fish spawning. Erosion/
sedimentation impacts can be costly, requiring
removal of deposited materials to restore water
supply storage, flood control, habitat, and
recreational benefits of impacted resources.

Nutrients
Increased nutrient (phosphorus, nitrogen)
concentrations in stormwater have been shown to
result in greater nutrient enrichment and
associated algal productivity in lakes,
embayments, and other quiescent receiving
waters, often creating undesirable excessive
growth conditions. Phosphorus is often
emphasized as the nutrient controlling algal
growth; phosphorus loading rates from urban
areas have been determined to be three to seven
times greater than undeveloped woodland.
However, a preponderance of stormwater inflow
has been demonstrated to inhibit algal growth as a
result of the presence of toxic substances, despite
elevated nutrient concentrations.

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Toxic Organics
Toxic Metals
Toxic organic pollutants that are prevalent in urban
runoff include pesticides, phenols, phthalates, and
polynuclear aromatic hydrocarbons (PAHs). While
some exceedances of EPA freshwater chronic
water quality criteria have been reported,
concentrations in general  are sufficiently low to
preclude significant impacts to aquatic biota.
However, their potential for bioaccumulation and
status as human carcinogens warrants continued
consideration as pollutants of concern.
Toxic metals are the pollutants of greatest concern
in urban runoff. Lead, zinc, copper, and cadmium
have both a high frequency of occurrence and high
absolute concentrations in stormwater; numerous
exceedances of water quality criteria for these
metals have been reported. Metals have the
potential to bioaccumulate and persist in the
environment.
   EPA water quality criteria have been developed
for both acute and chronic toxicity values from
bioassays on representative biota; criteria are
designed to protect 95 percent of aquatic species.
Physiological effects of metals exposure include
algal growth inhibition and zooplankton/fish
mortality through gill adsorption and respiratory
impairment.
Uncertainties of Toxic Impacts
             hile stormwater impacts related to
             aesthetics, hydrologic changes to
             stream habitat, elevated fecal
             coliform counts, and eutrophication
             have been adequately
             demonstrated, the adverse effects of
             toxicants have been more difficult to
establish. Although some water quality
degradation may be occurring, such degradation
has generally not been perceived to result in
significant impairment to aquatic biota. For
example, of over 10,000 fish kills investigated by
EPA during the period 1970-1979, less than 150
were attributable to urban runoff. If present,
potential toxic impacts have been more subtle and
more easily overshadowed by larger, definite
impacts associated with scour and  sedimentation.
Other sources of uncertainty regarding toxic
pollutant impacts include the following:

   1.   Water quality metals criteria and most
       stormwater analyses are based on total
       concentrations, whereas only the smaller
       dissolved fraction is directly  related to
       toxicity. Criteria are therefore conservative
       as they also assume dissolution of the inert
       paniculate fraction.
   2.   Criteria are based on continuous bioassays
       for the defined exposure period. In reality,
       stormwater toxic exposure is intermittent
       and of short duration, whereupon receiving
       waters recover to relatively acceptable
       quality. These are recognized by EPA under
       the term "Estimated Effect Levels for
       Intermittent Exposure."

   3.   Bioassays and water quality criteria are
       based upon end-of-pipe stormwater
       concentrations. Criteria therefore do not
       assume dilution capability by the receiving
       water.

   4.   Pollutant forms and concentrations are
       dynamic relative to product and use trends.
       For example, lead concentrations in
       stormwater have declined in recent years
       with the progressive conversion of the motor
       vehicle fleet to lead-free combustion
       engines. Similarly, some pesticide products
       are being retired in favor of new products
       being introduced to the market.

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    Summary Assessment of Urban Runoff Impacts
              he majority of pollutant loading
              attributed to urban stormwater
              originates from endemic sources
              such as motor vehicle traffic and
              atmospheric fallout. While toxic
              metal criteria established from
              continuous exposure bioassays are
    regularly exceeded in stormwater, receiving water
    resources and local perception often do not reflect
a corresponding beneficial use impairment of the
resource. Possible reasons for this disparity are
due to the conservative nature of water quality
criteria designations and the complexity of
biochemical cause and effect relationships.
Instead, perceived or documented impairments
focus on aesthetics from oil and floatable debris,
species displacement from erosion/sedimentation
in conveyance streams, and enhanced
eutrophication potential from nutrient enrichment.
                              TERRENE
                                  INSTITUTE
This project was funded by the U.S. Environmental Protection Agency Office of Water Enforcement
and Permits-Water Permits Division and managed by Region  V Watershed Management Unit-
Water Division. Prepared by Entranco Engineers, Inc. and JT&A, Inc.   For copies of this publica-
tion, contact  The Terrene Institute, 1000 Connecticut Avenue,  NW, Suite 300, Washington, DC
20036, (202)  833-3380.
                                    Printed on Recycled Paper

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