United States
Environmental Protection
Agency
Municipal Environmental Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-83-125 Jan. 1984
Project Summary
Development of Methods to
Define Water Quality Effects of
Urban Runoff
John L Mancini
Because the costs of treating com-
bined sewer overflows and urban runoff
nationwide are extremely high, methods
are needed for quantitatively evaluating
the impacts of these discharges on the
receiving water. This report summarizes
an investigation of methods for devel-
oping wet weather water quality criteria
that could form part of the basis for wet
weather standards. The wet weather
criteria could ultimately be used for
measuring the benefits of treating com-
bined sewer overflows and urban runoff.
This project considers short-term
water quality impacts that occur during
or shortly after a storm event. Examples
of short-term impacts are dissolved
oxygen depressions resulting from rapid
oxidation of contaminants, or a fish kill
caused by short-term increases in toxic
substances in the receiving water. The
nature and degree of the impacts de-
pend on characteristics of the event
such as the runoff volume, duration,
and concentration of contaminants,
and the dilution provided by the receiv-
ing water. This dilution in turn depends
on the scale of the total river width, the
joint occurrence of storm discharges
from urban areas, and the stream flow
in the receiving waters.
This project describes methods for
defining the effects of time-variable
concentrations on organism mortality
and considers the carryover effects
between storms as a result of varying
instream contaminant concentrations
during dry weather. The methods pre-
sented for evaluating the effects of
exposure to time-variable concentra-
tions can be used to define wet weather
water quality criteria.
This Project Summary was developed
by EPA's Municipal Environmental Re-
search Laboratory. Cincinnati. OH. to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
Nationwide programs to control dis-
charges from combined sewers and the
runoff from separately sewered urban
areas can require capital costs ranging
from tens to hundreds of billions of dol-
lars, with associated operating costs of
hundreds of millions of dollars annually.
Methods must therefore be available for
quantitatively evaluating the impacts of
these discharges on receiving waters so
that benefits can be estimated and com-
pared to the very substantial costs in-
volved. One method that indirectly
assesses receiving water benefits uses
local water quality standards. These
standards consider economic and social
impacts along with water quality criteria
defined by the beneficial water use to be
protected. This report describes methods
that can be used to develop wet weather
water criteria that could be the basis for
wet weather standards. The wet weather
criteria could ultimately be used to meas-
ure the benefits of treating combined
sewer overflow (CSO) and urban runoff.
Two types of water quality problems
are normally associated with the dis-
charges from combined sewers and urban
runoff: short-term and long-term effects.
The first problem involves the rapid,
short-term changes in water quality that
occur during and shortly after a storm
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event. The nature and degree of these
impacts depend on the characteristics of
the storm event such as runoff volume,
duration, and contaminant concentration,
and the dilution available in the receiving
water. Examples of the short-term im-
pacts are dissolved oxygen depressions
resulting from rapid oxidation of contam-
inants or a fish kill caused by short-term
increases in a toxic substance in the
receiving water.
The magnitude of water quality impacts
from urban stormwater discharges is
defined partly by the dilution available in
the receiving water. This dilution depends
on the scale of the total river width, by the
joint occurrence of storm discharges from
urban areas, and the stream flow in the
receiving waters. The project describes
methods for defining the effects of time-
variable concentrations on organism mor-
tality and considers the carryover effects
between storms as a result of varying
instream contaminant concentrations
during dry weather.
In addition to having short-term nega-
tive impacts, combined sewer oveflows
and urban runoff can also contribute to
longer-term water quality degradation.
The long-term impacts are caused by the
contaminants associated with the sus-
pended solids in the receiving water. In
this context, long-term impacts may also
be associated with both dissolved and
particulate nutrients. The long-term im-
pacts can include mass loading to receiv-
ing waters accumulated over long periods
of time. Examples of long-term impacts
are bottom oxygen demand of accumu-
lated sediments or the biological accumu-
lation of toxic substances as a result of
leaching from sediments or uptake by
benthic organisms. In the former example
for dissolved oxygen, the critical water
quality impact is usually associated with
the normal low-flow, high-temperature
summer periods. For both of these
examples and for all other long-term
effects, the effectiveness of control
actions can be measured by comparing
mass loadings from the various sources.
Conventional analysis techniques such
as steady-state modeling can then be
used to estimate water quality improve-
ment and benefits. This report addresses
the latter situation and does not consider
water quality problems associated with
bottom scour of sediment.
Approach
A technique was developed to calculate
for each event the probability distribution
of the mean instream contaminant con-
centration. Example calculations are pre-
sented in the report for various regions of
the country. These calculations used
pollutant loading information from the
EPA Urban Rainfall-Runoff-Quality Data
Base and appropriate information on
regional stream flows and rainfall statis-
tics. The calculation results consider a
number of ratios of urban drainage area
to stream drainage area.
Techniques were developed for calcu-
lating distributions of event mean stream
concentrations for lead, copper, and ulti-
mate oxygen demand. These three con-
taminants were used to illustrate one
method of determining locations where
water quality problems could occur from
CSO or urban runoff discharges. Results
were determined for stream-to-urban-
drainage-area ratios ranging from 1 to
1,000. The lower drainage area ratio (1)
represents undiluted CSO and/or urban
runoff. The calculated instream concen-
trations were plotted against drainage
area ratio for a constant probability. This
transformation was used to determine
the median, expected mean, and 90%
probability levels.
Graphs were prepared to determine (1)
the percentage of rainfall events that
would produce instream contaminant
concentrations exceeding water quality
criteria, and (2) the drainage area ratio for
which water quality criteria will not be
violated for a given percentage of time
during periods of rainfall.
Findings
The findings of the project can be
summarized as follows:
• A method has been developed that
uses existing data from classical bio-
assay tests to calculate the effects
exposing aquatic organisms such as
fish to time-variable concentrations of
toxicants. The procedure calculates
the equivalent dosage and can define
the equivalent mortality dosage. This
procedure can be used to define water
quality criteria for time-variable, event-
related phenomena such as those
associated with urban discharges.
• The procedure for calculating equiva-
lent dosage has been tested using
Several sets of data collected inde-
pendently.
• A method has also been developed to
define how fish respond to the low
dissolved oxygen levels that occur on
the short-time scales associated with
stormwater discharges.
• A calculation procedure has been
developed to calculate the statistical
distribution of instream concentrations
of contaminants. The calculation com-
bines statistics on rainfall and runoff,
stream flow, concentrations in
streams, and concentrations in CSO
and/or urban runoff.
• The methods for calculating wet
weather criteria and the statistics of
instream concentrations can be com-
bined to define locations where water
quality problems are likely to occur as
a result of storm-related discharges
from urban areas.
• Judgments on the extent of water
quality problems associated with CSO
and urban runoff discharges depend to
a large extent on the water quality
criteria that are used. If generalized
water quality criteria (usually employed
for continuous discharges) are con-
sidered, CSO and urban runoff can
produce water quality problems under
most circumstances. If criteria are
developed that consider the short
duration of storm events and the rela-
tively large interval between storms,
water quality problems associated with
CSO and urban runoff appear to be
significantly reduced.
The full report was submitted in fulfill-
ment of Grant No. 806828 by Manhattan
College under the sponsorship of the U.S.
Environmental Protection Agency.
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John L Mancini is with Manhattan College, Bronx, NY 10471.
Douglas Ammon is the EPA Project Officer (see below).
The complete report, entitled "Development of Methods to Define Water Quality
Effects of Urban Runoff," (Order No. PB 84-122 928; Cost: $16.0O. subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
ft US GOVERNMENT PRINTING OFFICE 1984-759-015/7268
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Environmental Protection
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Information
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