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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