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4.0 Environmental Assessment

Waterways and receiving waters near urban and suburban areas are often adversely

affected by urban storm water runoff. The degree and type of impact varies from location to

location, but it is often significant relative to other sources of pollution and environmental

degradation. Urban storm water runoff affects water quality, water quantity, habitat and

biological resources, public health, and the aesthetic appearance of urban waterways. As reported

in the National Water Quality Inventory 1996 Report to Congress (US EPA, 1998d), urban runoff

was the leading source of pollutants causing water quality impairment related to human activities

in ocean shoreline waters and the second leading cause in estuaries across the nation. Urban

runoff was also a significant source of impairment in rivers and lakes. The percent of total

impairment attributed to urban runoff is substantial. This impairment constitutes approximately

5,000 square miles of estuaries, 1.4 million acres of lakes, and 30,000 miles of rivers. Seven

states also reported in the Inventory that urban runoff contributes to wetland degradation.

Adverse impacts on receiving waters associated with storm water discharges have been

discussed by EPA (1995b) in terms of three general classes. These are:

• Short-term changes in water quality during and after storm events including temporary

increases in the concentration of one or more pollutants, toxics or bacteria levels.

• Long-term water quality impacts caused by the cumulative effects associated with

repeated storm water discharges from a number of sources.

• Physical impacts due to erosion, scour, and deposition associated with increased

frequency and volume of runoff that alters aquatic habitat.

As described in the Terrene Institute’s Fundamentals of Urban Runoff Management

(Horner et al, 1994), pollutants associated with urban runoff potentially harmful to receiving

waters fall into the categories listed below: 

• Solids

• Oxygen-demanding substances

• Nitrogen and phosphorus

• Pathogens

• Petroleum hydrocarbons

• Metals

• Synthetic organics.

These pollutants degrade water quality in receiving waters near urban areas, and often

contribute to the impairment of use and exceedences of criteria included in State water quality

standards. The quantity of these pollutants per unit area delivered to receiving waters tends to

increase with the degree of development in urban areas.
4 - 2

While water quality impacts are often unobserved by the general public, other storm water

impacts are more visible. Stream channel erosion and channel bank scour provide direct evidence

of water quantity impacts caused by urban storm water. Urban runoff increases directly with

imperviousness and the degree of watershed development. As urban areas grow, urban streams

are forced to accommodate larger volumes of storm water runoff that recur on a more frequent

basis. This leads to stream channel instability. The change in watershed hydrology associated with

urban development also causes channel widening and scour, and the introduction of larger

amounts of sediment to urban streams. Visible impacts include eroded and exposed stream banks,

fallen trees, sedimentation, and recognizably turbid conditions. The increased frequency of

flooding in urban areas also poses a threat to public safety and property. 

Both water quality and water quantity impacts associated with urban storm water combine

to impact aquatic and riparian habitat in urban streams. Higher levels of pollutants, increased flow

velocities and erosion, alteration of riparian corridors, and sedimentation associated with storm

water runoff negatively impact the integrity of aquatic ecosystems. These impacts include the

degradation and loss of aquatic habitat, and reduction in the numbers and diversity of fish and

macroinvertebrates. 

Public health impacts are for the most part related to bacteria and disease causing

organisms carried by urban storm water runoff into waters used for water supplies, fishing and

recreation. Water supplies can potentially be contaminated by urban runoff, posing a public health

threat. Bathers and others coming in contact with contaminated water at beaches and other

recreational sites can become seriously ill. Beach closures caused by urban runoff have a negative

impact on the quality of life, and can impede economic development as well. Similarly, the

bacterial contamination of shellfish beds poses a public health threat to consumers, and shellfish

bed closures negatively impact the fishing industry and local economies.

Aesthetic impacts in the form of debris and litter floating in urban waterways and

concentrated on stream banks and beaches are quite visible to the general public. Storm water is

a major source of floatables that include paper and plastic bags and packaging materials, bottles,

cans, and wood. The presence of floatables and other debris in receiving waters during and

following storm events reduces visual attractiveness of the waters and detracts from their

recreational value. Nuisance algal conditions including surface scum and odor problems can also

be attributed to urban storm water in many instances.

Based on available information and data, the following general statements can be made

about urban storm water impacts.

• Impacts to water quality in terms of water column chemistry tend to be transient and

elusive, particularly in rivers.

• Impacts to habitat and aquatic life are generally more profound, and are easier to see

and quantify than changes in water column chemistry.
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• Impacts are typically complex because urban storm water is often one of several

sources including municipal discharges and diffuse runoff from agricultural and rural

areas that affect urban waterways.

• Impacts are often interrelated and cumulative. For example, both degraded water

quality and increased water quantity join to impact habitat and biological resources.

The following sections describe the sources of urban storm water runoff, the pollutants

contained in urban runoff and the impacts attributable to urban storm water discharges. Examples

supported 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 health

impacts, habitat impacts, and aesthetic impacts. 

4.1 Overview of Storm Water Discharges

Storm water runoff from urbanized areas is generated from a number of sources including

residential areas, commercial and industrial areas, roads, highways and bridges. Essentially, any

surface which does not have the capability to pond and infiltrate water will produce runoff during

storm events. When a land area is altered from a natural forested ecosystem to an urbanized land

use consisting of rooftops, streets and parking lots, the hydrology of the system is significantly

altered. Water which was previously ponded on the forest floor, infiltrated into the soil and

converted to groundwater, utilized by plants and evaporated or transpired into the atmosphere is

now converted directly into surface runoff. An important measure of the degree of urbanization

in a watershed is the level of impervious surfaces. As the level of imperviousness increases in a

watershed, more rainfall is converted to runoff. Figure 4-1 illustrates this transformation.
4 - 4

Natural Ground Cover 10-20% Impervious

30-50% Impervious 75-100% Impervious

25% Deep

Infiltration

25% Shallow

Infiltration

10% Runoff

38% Evapotranspiration

21% Deep

Infiltration

21% Shallow

Infiltration

20% Runoff

5% Deep

Infiltration

10% Shallow

Infiltration

35% Evapotranspiration

30% Runoff

15% Deep

Infiltration

20 % Shallow

Infiltration

30% Evapotranspiration

55% Runoff

40% Evapotranspiration

Source: Adapted from Arnold and Gibbons, 1996

Figure 4-1. Effects of Imperviousness on Runoff and Infiltration

The traditional means of managing storm water runoff in urban areas has been to construct

a vast curb-and-gutter, catch basin, and storm drain network to transport this runoff volume

quickly 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 and

combined sewers.

• Separate storm sewer systems convey only storm water runoff. Water conveyed in

separate storm sewers is frequently discharged directly to receiving streams without

receiving any intentional form of treatment. (In a municipality with a separate storm

sewer system, sanitary sewer flows are conveyed in a distinct sanitary sewer system to

municipal wastewater treatment plants.)

• In a combined sewer system, storm water runoff is combined with sanitary sewer flows

for conveyance. Flows from combined sewers are treated by municipal wastewater
4 - 5

treatment plants prior to discharge to receiving streams. During large rainfall events

however, the volume of water conveyed in combined sewers can exceed the storage

and treatment capacity of the wastewater treatment system. As a result, discharges of

untreated storm water and sanitary wastewater directly to receiving streams can

frequently occur in these systems. These types of discharges are known as combined

sewer overflows (CSOs).

Historically, as urbanization occurred and storm drainage infrastructure systems were

developed in this country, the primary concern was to limit nuisance and potentially damaging

flooding due to the large volumes of storm water runoff that are generated. Little, if any, thought

was given to the environmental impacts of such practices. As a result, streams that receive storm

water runoff frequently cannot convey the large volumes of water generated during runoff events

without significant degradation of the receiving stream. In addition to the problems associated

with excess water volume, the levels of toxic or otherwise harmful pollutants in storm water

runoff and CSOs can cause significant water quality problems in receiving streams.

In addition to point sources such as municipal separate storm sewers and combined sewer

overflows, storm water runoff can enter receiving streams as a non-point source. Storm water

runoff from a variety of sources such as parking lots, highways, open land, rangeland, residential

areas and commercial areas can enter waterways directly as sheet flow or as a series of diffuse,

discrete flows. Due to the diffuse nature of many storm water discharges, it is difficult to quantify

the range of pollutant loadings to receiving streams that are attributable to storm water

discharges. It is much easier, however, to measure the increased stream flows during rainfall

events that occur in urbanized areas and to document impacts to streams that receive storm water

runoff.

Awareness of the damaging effects storm water runoff is causing to the water quality and

aquatic life of receiving streams is a relatively recent development. Storm water management

traditionally was, and still is in many cases, a flood control rather than a quality control program. 

Local governments intending to improve the quality of their runoff-impacted streams are

incorporating best management practices (BMPs) into their drainage programs. BMPs which

reduce the volume of runoff discharged to receiving streams, such as minimizing directly

connected impervious surfaces, providing on-site storage and infiltration and implementing stream

buffers and restoring riparian cover along urban streams can help to prevent further degradation

and even result in improvements of streams which receive storm water discharges. However, in

many existing urbanized areas, the cost of infrastructure changes necessary to retrofit existing

storm water drainage systems with structural BMPs--to provide for storm water quality as well as

quantity control--can be prohibitively expensive. In these cases, non-structural BMPs can be

implemented to reduce pollutant sources and to reduce the transfer of urban pollutants to runoff,

before more expensive, structural controls are instituted.

The climate of a region can have a significant impact on the quantity and quality of storm

water runoff. Factors such as the length of the antecedent dry periods between storms, the
4 - 6

average rainfall intensity, the storm duration and the amount of snowmelt present can have

significant impacts on the characteristics of runoff from an area. In areas where there is a

significant amount of atmospheric deposition of particulates, storm water runoff can contain high

concentrations of suspended solids, metals and nutrients. Areas that have infrequent rainfall such

as the southwest U.S. can have runoff with significant concentrations of pollutants, especially

from “hot spots” such as roads, parking lots and industrial areas. These areas, which typically

have high-intensity, short-duration rainfall events, can generate significant loadings of suspended

solids in storm water runoff. Many specific geographic factors can influence the nature and

constituents contained in storm water runoff. Factors such as the soil types, slopes, land use

patterns and the amount of imperviousness of a watershed can greatly affect the quality and

quantity of runoff that is produced from an area.

4.2 Pollutants in Urban Storm Water

Storm water runoff from urban areas can contain significant concentrations of harmful

pollutants that can contribute to adverse water quality impacts in receiving streams. Effects can

include such things as beach closures, shellfish bed closures, limits on fishing and limits on

recreational contact in waters that receive storm water discharges. Contaminants enter storm

water from a variety of sources in the urban landscape.

Urban storm water runoff has been the subject of intensive research since the inception of

the Water Quality Act of 1965. There have been numerous studies conducted to characterize the

nature of urban storm water runoff and the performance of storm water BMPs. Data sources

include the "208 Studies," the area-wide waste treatment management plans conducted by states

under section 208 of the 1972 CWA; EPA's Nationwide Urban Runoff Program (NURP); the

U.S. Geological Survey (USGS) Urban Stormwater Database; and the Federal Highway

Administration (FHWA) study of storm water runoff loadings from highways. In addition to

these federal sources, there is a great deal of information in the technical literature, as well as data

collected by states, counties and municipalities. A recent data source is storm water monitoring

data collected by municipalities regulated by the Phase I NPDES storm water regulations. As part

of the Phase I permit application, regulated municipalities were required to collect data from five

representative sites during a minimum of three storm events.

The most comprehensive study of urban runoff was NURP, conducted by EPA between

1978 and 1983. NURP was conducted in order to examine the characteristics of urban runoff

and similarities or differences between urban land uses, the extent to which urban runoff is a

significant contributor to water quality problems nationwide, and the performance characteristics

and effectiveness of management practices to control pollution loads from urban runoff (US EPA,

1983). Sampling was conducted for 28 NURP projects which included 81 specific sites and more

than 2,300 separate storm events. NURP focused on the following ten constituents:

• Total Suspended Solids (TSS)
4 - 7

• Biochemical Oxygen Demand (BOD)

• Chemical Oxygen Demand (COD)

• Total Phosphorus (TP)

• Soluble Phosphorus (SP)

• Total Kjeldahl Nitrogen (TKN)

• Nitrate + Nitrite (N)

• Total Copper (Cu)

• Total Lead (Pb)

• Total Zinc (Zn).

NURP examined both the soluble and the particulate fraction of pollutants, since the water

quality impacts can depend greatly on the form that the contaminant is present. NURP also

examined coliform bacteria and priority pollutants at a subset of sites. Median event mean

concentrations (EMCs) for the ten general NURP pollutants for various urban land use categories

are presented in Table 4-1.
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Table 4-1. Median Event Mean Concentrations for Urban Land Uses

Pollutant Units

Residential Mixed Commercial Open/

Non-Urban

Median COV Median COV Median COV Median COV

BOD mg/l 10 0.41 7.8 0.52 9.3 0.31 -- --

COD mg/l 73 0.55 65 0.58 57 0.39 40 0.78

TSS mg/l 101 0.96 67 1.14 69 0.85 70 2.92

Total Lead μg/l 144 0.75 114 1.35 104 0.68 30 1.52

Total Copper μg/l 33 0.99 27 1.32 29 0.81 -- --

Total Zinc μg/l 135 0.84 154 0.78 226 1.07 195 0.66

Total Kjeldahl

Nitrogen

μg/l 1900 0.73 1288 0.50 1179 0.43 965 1.00

Nitrate +

Nitrite

μg/l 736 0.83 558 0.67 572 0.48 543 0.91

Total

Phosphorus

μg/l 383 0.69 263 0.75 201 0.67 121 1.66

Soluble

Phosphorus

μg/l 143 0.46 56 0.75 80 0.71 26 2.11

COV: Coefficient of variation

Source: Nationwide Urban Runoff Program (US EPA 1983)

Results from NURP indicate that there is not a significant difference in pollutant

concentrations in runoff from different urban land use categories. There is a significant difference,

however, in pollutant concentrations in runoff from urban sources than that produced from nonurban areas.

The pollutants that are found in urban storm water runoff originate from a variety of

sources. The major sources include contaminants from residential and commercial areas, industrial

activities, construction, streets and parking lots, and atmospheric deposition. Contaminants

commonly found in storm water runoff and their likely sources are summarized in Table 4-2.
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Table 4-2. Sources of Contaminants in Urban Storm Water Runoff

Contaminant Contaminant Sources

Sediment and Floatables Streets, lawns, driveways, roads, construction

activities, atmospheric deposition, drainage

channel erosion

Pesticides and Herbicides Residential lawns and gardens, roadsides,

utility right-of-ways, commercial and

industrial landscaped areas, soil wash-off

Organic Materials Residential lawns and gardens, commercial

landscaping, animal wastes

Metals Automobiles, bridges, atmospheric deposition,

industrial areas, soil erosion, corroding metal

surfaces, combustion processes

Oil and Grease/

Hydrocarbons

Roads, driveways, parking lots, vehicle

maintenance areas, gas stations, illicit

dumping to storm drains

Bacteria and Viruses Lawns, roads, leaky sanitary sewer lines,

sanitary sewer cross-connections, animal

waste, septic systems

Nitrogen and Phosphorus Lawn fertilizers, atmospheric deposition,

automobile exhaust, soil erosion, animal

waste, detergents

The concentrations of pollutants found in urban runoff are directly related to degree of

development within the watershed. This trend is shown in Table 4-3, a compilation of typical

pollutant loadings from different urban land uses.
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Table 4-3. Typical Pollutant Loadings from Runoff by Urban Land Use (lbs/acre-yr) 

Land Use TSS TP TKN NH3-N NO2+NO3-N BOD COD Pb Zn Cu

Commercial 1000 1.5 6.7 1.9 3.1 62 420 2.7 2.1 0.4

Parking Lot 400 0.7 5.1 2 2.9 47 270 0.8 0.8 0.04

HDR 420 1 4.2 0.8 2 27 170 0.8 0.7 0.03

MDR 190 0.5 2.5 0.5 1.4 13 72 0.2 0.2 0.14

LDR 10 0.04 0.03 0.02 0.1 NA NA 0.01 0.04 0.01

Freeway 880 0.9 7.9 1.5 4.2 NA NA 4.5 2.1 0.37

Industrial 860 1.3 3.8 0.2 1.3 NA NA 2.4 7.3 0.5

Park 3 0.03 1.5 NA 0.3 NA 2 0 NA NA

Construction 6000 80 NA NA NA NA NA NA NA NA

HDR: High Density Residential, MDR: Medium Density Residential, LDR: Low Density Residential

NA: Not available; insufficient data to characterize loadings

Source: Horner et al, 1994

As indicated in Table 4-3, urban storm water runoff can contain significant concentrations

of solids, nutrients, organics and metals. A comparison of the concentration of water quality

parameters in urban runoff with the concentrations in domestic wastewater is shown in Table 4-4.
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Table 4-4. Comparison of Water Quality Parameters in Urban Runoff with Domestic

Wastewater (mg/l)

Constituent

Urban Runoff Domestic Wastewater

Separate Sewers Before Treatment After Secondary

Range Typical Range Typical Typical

COD 200-275 75 250-1,000 500 80

TSS 20-2,890 150 100-350 200 20

Total P 0.02-4.30 0.36 4-15 8 2

Total N 0.4-20.0 2 20-85 40 30

Lead 0.01-1.20 0.18 0.02-0.94 0.10 0.05

Copper 0.01-0.40 0.05 0.03-1.19 0.22 0.03

Zinc 0.01-2.90 0.02 0.02-7.68 0.28 0.08

Fecal Coliform

per 100 ml

400-50,000 106-108200

Source: Bastian, 1997

As indicated in Table 4-4, the concentrations of select water quality parameters in urban

runoff is comparable to that found in untreated domestic wastewater. When untreated urban

runoff is discharged directly to receiving streams, the loadings of pollutants can be much higher

than the loadings attributable to treated domestic wastewater.

The following paragraphs summarize the major pollutants which are commonly found in

urban storm water runoff.

4.2.1 Solids, Sediment and Floatables

Solids are one of the most common contaminants found in urban storm water. Solids

originate from many sources including the erosion of pervious surfaces and dust, litter and other

particles deposited on impervious surfaces from human activities and the atmosphere. Stream

bank erosion and erosion at construction sites are also major sources of solids. Solids contribute

to many water quality, habitat and aesthetic problems in urban waterways. Elevated levels of

solids increase turbidity, reduce the penetration of light at depth within the water column, and

limit the growth of desirable aquatic plants. Solids that settle out as bottom deposits contribute to

sedimentation and can alter and eventually destroy habitat for fish and bottom-dwelling organisms
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(see Figure 4-2). Solids also provide a medium for the accumulation, transport and storage of

other pollutants including nutrients and metals. Sediment bound pollutants often have a long

history of interaction with the water column through cycles of deposition, re-suspension, and redeposition. Impaired navigation due to sedimentation represents another impact affecting

recreation and commerce. The relative contribution of TSS in urban storm water from different

land uses is presented in Table 4-3. As shown in Table 4-4, the typical concentration of TSS in

urban runoff is substantially higher than that in treated wastewater (Bastian, 1997). Construction

produces the highest loading of TSS over other urban land use categories evaluated.

Figure 4-2. Effects of Siltation on Rivers and Streams

Source: US EPA, 1998d.

4.2.2 Oxygen-Demanding Substances and Dissolved Oxygen

The oxygen-demanding substances found in urban storm water can be measured by

Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), and Total Organic

Carbon (TOC). Maintaining appropriate levels of dissolved oxygen in receiving waters is one of

the most important considerations for the protection of fish and aquatic life. The amount of

dissolved oxygen in urban runoff is typically 5.0 mg/l or greater, and it rarely poses a direct threat

to in-stream conditions. As shown in Table 4-4, the level of COD associated with urban runoff is

comparable to treated wastewater. The direct impact of urban storm water runoff on dissolved

oxygen conditions in receiving waters is not thought to be substantial. However, the secondary

impacts on the dissolved oxygen balance in receiving waters due to nutrient enrichment,

eutrophication, and resulting sediment oxygen demand may be important.
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4.2.3 Nitrogen and Phosphorus

Nitrogen and phosphorus are the principal nutrients of concern in urban storm water. The

major sources of nutrients in urban storm water are urban landscape runoff (fertilizers, detergents,

plant debris), atmospheric deposition, and improperly functioning septic systems (Terrene

Institute, 1996). Animal waste can also be an important source. There are a number of

parameters used to measure the various forms of nitrogen and phosphorus found in runoff. 

Ammonia (NH3) nitrogen is the nitrogen form that is usually the most readily toxic to aquatic life. 

Nitrate (NO3) and nitrite (NO2) are the inorganic fractions of nitrogen. Very little nitrite is usually

found in storm water. Total Kjeldahl nitrogen (TKN) measures the organic and ammonia

nitrogen forms. By subtraction, the organic fraction can be determined. Total phosphorus

measures the total amount of phosphorus in both the organic and inorganic forms. Orthophosphate measures phosphorus that is most immediately biologically available. Most of the

soluble phosphorus in storm water is usually present in the ortho-phosphate form.

The degree to which nitrogen and phosphorus are present in a river, lake or estuary can

determine the trophic status and amount of algal biomass produced. Excess nutrients tend to

increase primary biological productivity. The major impact associated with nutrient overenrichment is excessive growth of algae that leads to nuisance algal blooms and eutrophic

conditions. A secondary impact is the residual negative effect of decomposing algae in the form

of sediment oxygen demand that depletes dissolved oxygen concentrations, particularly in bottom

waters. The NURP study reported that nutrient levels in urban runoff appear not to be high in

comparison with other possible discharges. However, more recent studies and programs have

recognized that the amount of nitrogen and phosphorus present in urban storm water can be

substantial, and becomes increasingly important as other point sources of nutrients are brought

under control. Walker (1987) reported that “cause-effect relationships linking urban development

to lake and reservoir eutrophication are well established,” and that “urban watersheds typically

export 5 to 20 times as much phosphorus per unit per year, as compared to undeveloped

watersheds in a given region.” The nutrient loadings from different urban and suburban land uses

are presented in Table 4-3. As shown in Table 4-4, the total phosphorus and total nitrogen

concentrations in urban runoff are substantially less than treated wastewater concentrations, but

storm water volumes can be greater during wet weather events.

4.2.4 Pathogens

Pathogens are disease-producing organisms that present a potential public health threat

when they are present in contact waters. Since storm water runoff typically does not come into

contact with domestic wastewaters, and direct exposure to runoff is usually limited, there is

generally little threat of pathogens in storm water runoff causing a public health risk. However,

where runoff is discharged to recreational waters such as beaches and lakes, or where runoff

comes into contact with shellfish beds, there is a potential public health risk associated with

pathogen contamination.
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There are a number of indicator organisms that have been used to evaluate the presence of

harmful pathogens in storm water runoff. Several strains of bacteria are present naturally in the

soil and can be transported by runoff. In addition, BMPs with standing water can be breeding

grounds for naturally occurring bacteria. Therefore, interpretation of bacteriological sampling

results can be difficult. Nevertheless, indicator organisms can provide useful insight into the

public health risk associated with runoff. Fecal coliform has been widely used as an indicator for

the presence of harmful pathogens in domestic wastewaters, and therefore studies characterizing

storm water runoff have frequently used this indicator as well. Other bacterial indicators that

have been used to evaluate the presence of harmful pathogens in storm water runoff include

Escherichia coli, streptococci and enterococci. The presence of enteric viruses has also been

evaluated in storm water runoff, as well as protozoans such as Giardia lamblia and

cryptosporidium.

Fecal coliform concentrations in urban runoff were evaluated by NURP at 17 sites for 156

storm events. NURP reported that coliform bacteria are present at high levels in urban runoff and

can be expected to exceed EPA water quality criteria during and immediately after storm events in

many surface waters, even those providing high degrees of dilution. Concentrations of fecal

coliform found by NURP exhibited a large degree of variability, and did not indicate any

distinctions based on land use. Data from different sites did show a dramatic seasonal effect on

coliform concentrations. Coliform counts in urban runoff during warmer periods of the year were

found to be approximately 20 times greater that those found during colder periods. Based on this

data, NURP concluded that coliform sources unrelated to those traditionally associated with

human health risk may be significant. 

The Terrene Institute (1996) reported that the primary sources of pathogens in urban

storm water drains are animal wastes (including pets and birds), failing septic systems, illicit

sewage connections, and boats and marinas. Field et al (1993) reported pathogens levels from

storm water runoff and urban streams as shown in Table 4-5. Pathogens enumerated included

bacteria (total and fecal coliform, fecal streptococci, enterococci, Pseudomonas aeruginosa,

Staphylococcus aureus, and Salmonella) and enteroviruses (poliovirus, Coxsackie virus, and

Echovirus).
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Table 4-5. Densities of Selected Pathogens and Indicator Microorganisms in Storm Water

in Baltimore, Maryland Area

Geometric Mean Densities

Sampling

Station

Enterovirus

Salmon

sp.

Pseudomon.

aeruginosa

Staph.

aureus

Total

Coliform

Fecal

Coliform

Fecal

Strep.

Enterococci

PFU/

10 L

MPN/

10 L

MPN/

10 L

MPN/

100 mL

MPN/

100 mL

(10^4)

MPN/

100 mL

(10^3)

No./

100 mL

(10^4)

No./100 mL

(10^4)

Bush St. 6.9 30 2000 120 38 83 56 12

Northwood 170 5.7 590 12 3.8 6.9 5 2.1

PFU: Plaque-forming units

MPN: Most Probable Number

Source: Field et al, 1993

As shown earlier in Table 4-4, typical fecal coliform concentrations for separate urban

storm sewers varied widely, ranging between 400-50,000 mpn/100 ml. An example of fecal

coliform concentrations measured in sheet flow associated with different impervious surfaces is

presented in Table 4-6. The broad range in concentrations illustrates the highly variable nature of

fecal coliform concentrations in storm water.

Table 4-6. Fecal Coliform Concentrations Collected in Sheetflow from Urban Land Uses

Land Use

Median

(MPN/100 ml)

Range

(MPN/100 ml)

Unpaved driveways and storage areas 26 0.02-300

Roof runoff 1.6 0.56-2.6

Sidewalks 55 19-90

Paved parking and driveways 2.8 0.03-66

Paved roads 19 1.8-430

MPN: Most Probable Number

Source: Field et al, 1993.

4.2.5 Petroleum Hydrocarbons

Petroleum hydrocarbons include oil and grease; the “BTEX” compounds: benzene,

toluene, ethyl benzene, and xylene; and a variety of polynuclear aromatic hydrocarbons (PAHs).

Sources of petroleum hydrocarbons include parking lots and roadways, leaking storage tanks,
4 - 16

auto emissions, and improper disposal of waste oil. Petroleum hydrocarbons are typically

concentrated along transportation corridors.

Petroleum hydrocarbons are known for their acute toxicity at low concentrations

(Schueler, 1987). A study by Shepp (1996) measured the petroleum hydrocarbon concentrations

in urban runoff from a variety of impervious areas in the District of Columbia and suburban

Maryland. The amount of car traffic affects the concentration of hydrocarbons in runoff, with

median concentrations ranging from 0.7 to 6.6 mg/l. Concentrations at these levels exceed the

maximum concentrations recommended for the protection of drinking water supplies and fisheries

protection. As pointed out by Shepp, the maximum concentration of petroleum hydrocarbons for

protection of fisheries is 0.01 to 0.1 mg/l. 

4.2.6 Metals

The primary sources of metals in urban storm water are industry and automobiles. 

Atmospheric deposition (both wet and dry) can make a substantial contribution in some parts of

the country. A major finding of the NURP study is as follows: 

Heavy metals (especially copper, lead and zinc) are by far the most prevalent priority pollutant

constituents found in urban runoff. End-of-pipe concentrations exceed EPA ambient water

quality criteria and drinking water standards in many instances. Some of the metals are present

often enough and in high enough concentrations to be potential threats to beneficial uses.

Metals in urban storm water have the potential to impact water supply and cause acute or

chronic toxic impacts for aquatic life. Typical pollutant loading rates and urban runoff

concentrations for lead, zinc and copper are presented in Tables 4-3 and 4-4. The frequency with

which metals were detected as priority pollutants in the NURP study is presented in Table 4-7.
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Table 4-7. Most Frequently Detected Priority Pollutants in Nationwide Urban Runoff

Program Samples (1978-83)

Inorganics Organics

Detected in 75% or more

94% Lead

94% Zinc

91% Copper

None

Detected in 50-74%

58% Chromium

52% Arsenic

None

Detected in 20-49%

48% Cadmium

43% Nickel

23% Cyanides

22% Bis(2-ethylhexyl)phthalate

20% "-Hexachloro-cyclohexane

Detected in 10-19%

13% Antimony

12% Beryllium

11% Selenium

19% "-Endosulfan

19% Pentachlorophenol*

17% Chlordane*

15% Lindane*

15% Pyrene**

14% Phenol

12% Phenanthrene**

11% Dichloromethane

10% 4-Nitrophenol

10% Chrysene**

10% Fluoranthene**

* Chlorinated hydrocarbon

** Polynuclear aromatic hydrocarbon

Source: US EPA, 1983

A major study of the quality of Wisconsin storm water (Bannerman et al, 1996) found that

the probability of event mean concentrations for some metals (particularly copper and zinc)

exceeding Wisconsin water quality criteria for cold water fish communities was high (Table 4-8).

A study in Coyote Creek, California reported lead and zinc levels from urban runoff of 100 to 500

times the concentration in the ambient water column (Pitt, 1995).
4 - 18

Table 4-8. Probability of Event Mean Concentration of Constituents in Wisconsin Storm

Water Exceeding Wisconsin Surface Water and Ground Water Quality Standards: Metals

Constituent

Probability of exceeding

acute toxicity criteria for

cold water fish

communities (percent)

Storm Sewers Streams

Cadmium, total recoverable 11 0

Copper, total recoverable 87 9

Lead, total recoverable 18 0

Silver, total recoverable 20 -

Zinc, total recoverable 91 7

Source: Bannerman et al, 1996.

4.2.7 Synthetic Organic Compounds

Synthetic organic compounds include a variety of manufactured compounds covering

pesticides, solvents and household and industrial chemicals. The frequency that synthetic

inorganics were detected as priority pollutants in the NURP study is presented in Table 4-7. In

general, organic contaminants were found in less than 20 percent of samples. Nevertheless,

synthetic organics do represent a threat. Even low concentrations of some synthetic organics

over a long period of time have the potential to pose a severe health risks to humans and aquatic

life though direct ingestion or bioaccumulation in the food chain. There is also some evidence

that pesticides are found in higher concentrations in urban areas than agricultural areas (US EPA,

1995b). Further, Bannerman et al found that the probability for storm water and urban stream

samples to exceed human cancer criteria for public water supply, and toxicity criteria for

coldwater fish communities equaled or approached 100 percent for 10 compounds (Table 4-9).
4 - 19

Table 4-9. Probability of Event Mean Concentration of Constituents in Wisconsin Storm

Water Exceeding Wisconsin Surface Water and Ground Water Quality Standards:

Synthetic Organic Compounds

Constituent

(Human cancer criteria

for public water

supply/ coldwater fish

communities)

Probability of exceedance

(percent)

Storm Sewers Streams

Benzo[a]anthracene 98 100

Benzo[a]pyrene 99 100

Benzo[b]fluoranthene 100 100

Benzo[ghi]perylene 99 100

Benzo[k]fluoranthene 99 99

Chrysene 100 100

Indeno pyrene 100 99

Phenanthrene 100 99

Pyrene 100 100

DDT 98 100

Source: Bannerman et al, 1996

4.2.8 Temperature

Water temperature is an important measure of water quality. As described by Malina

(1996), “the temperature of water affects some of the important physical properties and

characteristics of water, such as... specific conductivity and conductance, salinity, and the

solubility of dissolved gases (e.g., oxygen and carbon dioxide).” Specifically, water holds less

oxygen as it becomes warmer, resulting in less oxygen being available for respiration by aquatic

organisms. Furthermore, elevated temperatures increase the metabolism, respiration, and oxygen

demand of fish and other aquatic life, approximately doubling the respiration for a 10EC (18EF)

temperature rise; hence the demand for oxygen is increased under conditions where supply is

lowered (California SWRCB, 1963). 

Certain species of fish, such as salmon and trout, are particularly sensitive and require

relatively low water temperatures. Even lower temperatures are required for spawning and egg
4 - 20

hatching (US EPA, 1976). If the temperature of a stream reach is raised by 5 to 10EC (9 to

18EF), it is probable that such cold-water game fish will avoid this reach and that they will be

replaced by “rougher,” more tolerant fish (California SWRCB, 1963). Thus, even without direct

mortality, the character of the fish life will change. Sudden changes in temperature directly stress

the aquatic ecosystem. The states have adopted varying criteria to protect fisheries from such

stresses. Typically, states limit in-stream temperature rises above natural ambient temperatures to

2.8EC (5EF). Allowable temperature rises in streams that support cold water fisheries may be

lower, with some states adopting values as low as 1EC (1.8EF) and 0.6EC (1EF) (US EPA, 1988).

The temperature of urban waters is often affected directly by urban runoff. Urban runoff

can be heated as it flows over rooftops, parking lots and roadways. When it reaches urban

waterways it can cause a temporary fluctuation in the in-stream water temperature. Other factors

that tend to increase summer water temperature in urban waters include the removal of vegetation

from stream banks, reduced ground water baseflow, and discharges from storm water facilities

with elevated water temperature. Frequent fluctuations in stream temperature stress the aquatic

ecosystem, and make it difficult for temperature-sensitive species to survive.

Galli (1990a) undertook a major study of thermal impacts associated with urbanization

and storm water management in Maryland. Temperature observations were taken at stream

stations representing different levels of development, with impervious cover ranging from 1

percent to 60 percent. Results were compared with Maryland Class III standards for natural trout

waters (68 EF) and Class IV standards for recreational trout waters (75 EF). As shown in Figure

4-3, streams in developed watersheds (Lower Whiteoak and Tanglewood Stations) have

significantly higher spring and summer temperatures than streams in less developed watersheds. 

Galli also found that “imperviousness together with local meteorological conditions had the

largest influence on urban stream temperatures.” As shown in Figure 4-4, the rate of increase in

baseflow water temperature in this study was determined to be 0.14 EF for each one percent

increase in watershed imperviousness.
4 - 21

Figure 4-3. Relationship Between Increasing Imperviousness and

Urban Stream Temperature
4 - 22

Figure 4-4. Relationship Between Watershed Imperviousness and Baseflow Water

Temperature

4.2.9 pH

As pointed out by Novotny and Olem (1994), “most aquatic biota are sensitive to pH

variations,” and “fish kills and reduction and change of other species result when the pH is altered

outside their tolerance limits.” Most pH impacts in urban waters are caused by runoff of

rainwater with low pH levels (acid precipitation). In fact, urban areas tend to have more acidic

rainfall than less developed areas. Some buffering of low pH rainwater occurs during contact

with buildings, parking lots, roads and collection systems, and during overland flow. This is often

very site specific. The alkalinity and thus the capacity of receiving waters to neutralize acidic

storm water can also be important, and again is very site specific. Examples of pH impacts on fish

populations are difficult to identify due to the cumulative, overlapping impacts from other factors. 

However, it is thought that the acidification problem in both the United States and Canada grows

in magnitude when “episodic acidification” (brief periods of low pH levels from snow melt or

heavy downpours) is taken into account (US EPA, 1992a). The spring snow melt can coincide

with fish spawning periods.
4 - 23

Estuaries and Ocean Shoreline

Lakes

Rivers and Streams

0 10 20 30 40 50

Percent of Impaired Waters

Major Impairment Moderate / Minor Not Specified

4.3 Reported Impacts of Urban Storm Water

Urban runoff, which includes runoff from impervious surfaces such as streets, parking lots,

buildings, lawns and other paved areas is one of the leading causes of water quality impairment in

the United States. Based on the 1996 state Water Quality Inventory reports, siltation (sediment

discharged from urban runoff, as well as construction sites, agriculture, mining and forests) is the

leading cause of impaired water quality in rivers and streams. In the portion of the inventory

identifying sources, urban runoff was listed as the leading source of pollutants causing water

quality impairment related to human activities in ocean shoreline waters and the second leading

cause in estuaries across the nation. Urban runoff was also a significant source of impairment in

rivers and lakes. Urban runoff accounts for 47 percent of impaired miles of surveyed ocean

shoreline, 46 percent of the impaired square miles of surveyed estuaries, 22 percent of the

impaired acres of surveyed lakes and 14 percent of the impaired miles of surveyed rivers. Figure

4-5 illustrates the level of impairment attributable to urban storm water runoff based on states’

Water Quality Inventory assessment reports.

Figure 4-5. Proportions of Impaired Water Bodies Attributed to Urban Runoff

 Source: EPA, 1998d.

4.3.1 Flow Impacts

The volume and flow rate of storm water discharges can have significant impacts on

receiving streams. In many cases, the impacts on receiving streams due to high storm water flow
4 - 24

rates or volumes can be more significant than those attributable to the contaminants found in

storm water discharges. While studies linking increased storm water flows due to urbanization to

stream degradation are generally lacking in quantitative data, there are a number of studies that

support this hypothesis. EPA summarized studies which contain documented evidence of impacts

on steams due to urbanization (US EPA, 1997a). Impacts of urbanization and increased storm

water discharges to receiving streams documented in this evaluation include:

• Increase in the number of bankfull events and increased peak flow rates

• Sedimentation and increased sediment transport

• Frequent flooding

• Stream bed scouring and habitat degradation

• Shoreline erosion and stream bank widening

• Decreased baseflow

• Loss of fish populations and loss of sensitive aquatic species

• Aesthetic degradation

• Changes in stream morphology

• Increased temperatures.

The amount of runoff generated within a watershed increases steadily with development.

The presence of impervious areas such as roofs, parking lots and highways limits the volume of

rain water infiltrated into the soil, and increases the amount of runoff generated. Urbanized areas

also tend to have reduced storage capacities for runoff because of regrading, paving, and the

removal of vegetative cover. Decreases in infiltration and evapotranspiration and an increase in

runoff are the result of urbanization, with runoff volume linked to the percent of impervious area. 

The relationship between runoff coefficient and percent impervious area is illustrated in Figure 4-

6.
4 - 25

0 

0.2 

0.4 

0.6 

0.8 

1 

Runoff Coefficient

0 20 40 60 80 100 

W atershed Imperviousness (%)

Source: Schueler, 1987

Figure 4-6. Relationship of Watershed Imperviousness to Runoff Coefficient Levels

As shown in Table 4-10, the physical impacts to streams associated with increased

imperviousness are substantial (US EPA, 1997a).
4 - 26

Table 4-10. Impacts from Increases in Impervious Surfaces

Increased

Imperviousness

Leads to:

Resulting Impacts

Flooding Habitat loss Erosion Channel

Widening

Stream bed

Alteration

Increased Volume U U U U U

Increased Peak

Flow

U U U U U

Increased Peak

Duration

U U U U U

Increased Stream

Temp.

U

Decreased Base

Flow

U

Changes in

Sediment Loading

U U U U U

Source: EPA, 1997

The Delaware Department of Natural Resources and Environmental Control also

identified a list of impacts on physical stream habitat attributed to urban storm water (DE

DNREC, 1997). This list is as follows:

• Accelerated bank erosion

• Accelerated bank undercutting

• Increased siltation (burial of stable habitats)

• Elimination of meanders (channelization)

• Channel widening

• Reduced depth

• Reduced baseflow

• Loss of shade

• Increased temperature.

Specific impacts in the areas of flooding, stream bank erosion, and ground water recharge

are described in the following subsections. 
4 - 27

Flooding

Urbanization increases the frequency and severity of flooding due to increased runoff. 

Because of the decreased availability of pervious, permeable surfaces, and the related decrease in

storage capacity, smaller more frequently occurring storms can create flooding problems. 

Hydrographs in urban streams peak higher and faster than streams in undeveloped areas. A

comparison of estimated runoff volume and peak discharge for developed and undeveloped areas

is presented in Table 4-11. As shown, both runoff volume and peak discharge are substantially

increased under developed conditions.

Table 4-11. Comparison of Estimated Runoff Volume and Peak Discharge for Developed

and Undeveloped Areas

Storm

Frequency 

(years)

Undeveloped Conditions 

(Woods in good condition)

Developed Conditions 

(Half-Acre Residential)

Estimated

Runoff (in)

Estimated Peak

Discharge (cfs)

Estimated

Runoff (in)

Estimated Peak

Discharge (cfs)

2 0.14 1.00 0.60 11.6

10 0.52 5.60 1.33 27.4

100 1.40 19.7 2.64 58.6

Source: Horner et al, 1994

The effects of urbanization on stream shape and the flood plain are illustrated in Figure 4-

7. Increased peak discharge raises the flood plain level, flooding areas which were previously not

at risk.
4 - 28

Floodplain Limit

Summer Low Flow Level

Floodplain Limit

Summer Low Flow Level

Undeveloped

Urbanized

Source: Adapted from Schueler, 1987

Figure 4-7. Effect of Urbanization on Stream Slope and Flooding

A comparison of hydrographs from an urbanized stream (Lincoln Creek) and a nonurbanized stream (Jackson Creek) in Wisconsin are presented in Figure 4-8 (Masterson and

Bannerman, 1994). As illustrated, the hydrograph for the urbanized stream exhibits a much

higher peak flow rate that would correspond to a higher flood level. 
4 - 29

0 

200 

400 

600 

800 

1000 

Discharge (cfs)

06/17/93 06/17/93 06/18/93 06/19/93 06/20/93 06/20/93 06/21/93

Jackson Creek (non-urban) Lincoln Creek (urban) 

Source: Masterson and Bannerman, 1994

Figure 4-8. Hydrographs for Urban and Non-Urban Streams

Stream Bank Erosion

Stream bank erosion is a natural phenomenon and source of both sediment and nutrients. 

However, urbanization can greatly accelerate the process of stream bank erosion. As the amount

of impervious area increases, a greater volume of storm water is discharged directly to receiving

waters, often at a much higher velocity. The increased volume and velocity of the runoff can

overwhelm the natural carrying capacity of the stream network. In addition, streams in urbanized

areas can experience an increase in bankfull flows. Since bankfull flows are highly erosive,

substantial alterations in stream channel morphology can result.

Excessive bank erosion occurs as streams become wider and straighter to accommodate

greater flows and an excess number of erosion-causing events. Signs of stream bank erosion

attributable to increased storm water include undercut and fallen stream banks, felled bushes and
4 - 30

trees along the banks, and exposed sewer and utility pipes. Sediments from eroding banks (and

upland construction) are deposited in areas where the water slows, causing buildup, destruction of

benthic habitat, and a decreased stream capacity for flood waters. This ultimately results in a

greater potential for further erosion.

Krug and Goddard (1986) documented these phenomena in their study of Pheasant

Branch, a developing watershed of 24.5 square miles near Middleton, Wisconsin. Local

population grew markedly between 1970 to 1980, from 8,246 to 11,851, and is projected to reach

18,000 by the year 2000. Problems of stream channel erosion and suspended sediment developed

in Pheasant Branch as a result of this growth. The increased erosion and sediment loadings have

decreased the mean stream bed elevation by almost 2 feet, and increased the mean channel width

by nearly 35 percent.

Table 4-12 shows the modeled percent increase at three sites for the volume of the 2-year

flood, bankfull width, and bankfull depth under two development scenarios. These are the

projected development levels in the year 2000 (projected urbanization), and complete urbanization

of the watershed. The projected results are shown relative to pre-development conditions.

Table 4-12. Percent Increase of Two-Year Flood, Bankfull Width, and Bankfull Depth

from Pre-Development Conditions to Urbanized Conditions (Based on Modeling Results)

Site

Projected Urbanization Complete Urbanization

2-year Width Depth 2-year Width Depth

(Percent Increase from Preurbanization)(Percent Increase from Preurbanization)

Site 1 99 40 30 140 60 40

Site 2 324 110 80 361 110 80

Site 3 32 10 10 224 80 60

Source: EPA, 1997a

An example of the impact of urbanization on increased sediment loadings in several small

streams in Wisconsin before, during and after development is illustrated in Figure 4-9 (Krug and

Goddard, 1986). Sediment loads are greatest during construction, but remain elevated after

construction relative to pre-development conditions.
4 - 31

Figure 4-9. Sediment Loadings on Small Streams in Wisconsin

0 

2000 

4000 

6000 

8000 

10000 

Cumulative Monthly Sediment Load (ton)

Pheasant Branch at Middleton

South Fork Pheasant Branch

Pheasant Branch at Cemtury

Source: Krug and Goddard, 1986

1978 1979 1980 1981

Water Year

Pheasant Branch at Mouth

Cumulative Monthly Sediment Load

Ground Water Recharge 

Urbanization can have a major impact on ground water recharge. As shown earlier in

Figure 4-1, both shallow and deep infiltration decrease as watersheds undergo development and

urbanization. Ground water recharge is reduced along with a lowering of the water table. This

change in watershed hydrology alters the baseflow contribution to stream flow, and it is most

pronounced during dry periods. Ferguson (1990) points out that “base flows are of critical

environmental and economic concern for several reasons. Base flows must be capable of

absorbing pollution from sewage treatment plants and non-point sources, supporting aquatic life

dependent on stream flow, and replenishing water-supply reservoirs for municipal use in the

seasons when [water] levels tend to be lowest and water demands highest.” 

Base flows on Long Island, New York were substantially impacted by the construction of

storm water conveyance systems during the period of rapid development between the 1940s and
4 - 32

1970s. As illustrated in Table 4-13, a steady decline in the average percent of baseflow was

observed for streams in urbanized sewered areas relative to streams in un-sewered or rural areas

(US EPA, 1997a).

Table 4-13. Average Percent Base Flow of Selected Streams on Long Island by Area

Years

Urbanized Sewered

Area (% Flow from

Base Flow)

Urbanized Un-sewered

Area (% Flow from

Base Flow)

Rural Un-sewered Area

(% Flow from Base

Flow)

Stream 1 Stream 2 Stream 1 Stream 2 Stream 1 Stream 2

1948-1953 (No data) 86 84 94 96 95

1953-1964 63 69 89 89 95 97

1964-1970 17 22 83 84 96 97

Source: US EPA, 1997a

4.3.2 Habitat Impacts

Natural ecosystems are a complex arrangement of interactions between the land, water,

plants, and animals. The relationship between storm water discharge and the biological integrity of

urban streams is illustrated in Figure 4-10 (Masterson and Bannerman, 1994). As shown, habitat

is impacted by changes in both water quality and quantity, and the volume and quality of

sediment. As reported by Schueler (1987), “no single factor is responsible for the progressive

degradation of urban stream ecosystems. Rather, it is probably the cumulative impacts of many

individual factors such as sedimentation, scouring, increased flooding, lower summer flows,

higher water temperatures, and pollution.”
4 - 33

Biological Integrity of Urban Streams

Stormwater Discharges

Toxicity and

bioaccumulation

insects

macroinvertebrates

benthic

organisms

fin fish

shellfish

Degraded

biological

integrity

Contaminated

Sediments

Excessively

high and low

flows

Poor physical

habitat

Poor water

quality

Increased

sediment

 volume

aquatic

plants

Source: Adapted from Masterson and Bannerman, 1994

Figure 4-10. Relationship Between Urban Storm Water and Aquatic Ecosystems

Schueler and Claytor (1995) also suggest a direct relationship between watershed

imperviousness and stream health (Figure 4-11), and found that stream health impacts tend to

begin in watersheds with only 10-20 percent imperviousness (the ten percent threshold). As

shown, sensitive streams can exist relatively unaffected by urban storm water with good levels of

stream quality where impervious cover is less than 10 percent although some sensitive streams

have been observed to experience water quality impacts at as low as 5 percent imperviousness. 

Impacted streams are threatened and exhibit physical habitat changes (erosion and channel

widening) and decreasing water quality where impervious cover is in the range of 10 to 25

percent. Streams in watersheds where the impervious cover exceeds 25 percent are typically

degraded, have a low level of stream quality, and do not support a rich aquatic community. 
4 - 34

0 

20 

40 

60 

Watershed Impervious Cover

Level of Stream Quality

good fair low

Non supporting (>25%)

Impacted (11to 25%)

Sensitive (0 to 10%)

Source: Schueler and Claytor, 1995

Figure 4-11. Relationship Between Impervious Cover and Stream Quality

A summary of water quality impacts on habitat is presented in Table 4-14. The alteration

of species distribution is the major impact, with pollutant tolerant and less sensitive species

replacing native species in storm water impacted receiving waters.
4 - 35

Table 4-14. Water Quality Parameters Affecting Habitat

Water Quality

Parameter Habitat Effect

Bacteria Contamination

Heavy metals Alteration of species distribution

Toxic organics Alteration of species distribution

Nutrients Eutrophication, algal blooms

Sediment Decreased spawning areas

BOD Reduced dissolved oxygen levels

Temperature Reduced dissolved oxygen levels

pH Alteration of species distribution

Figure 4-12 illustrates that the pH tolerance of various forms of aquatic life varies

substantially (US EPA, 1992b). The tolerance of aquatic life to changes in temperature, turbidity

and toxic substances is also very important. Contaminants like heavy metals, pesticides, and

hydrocarbons can alter the species distribution in receiving waters. Acute and chronic toxicity

impacts may also occur. The relative toxicity of storm water samples from a variety of loading

source areas is presented in Table 4-15. Some of the identified chronic toxicity effects are

decreased growth and respiration rates (US EPA, 1996a). Toxic loads can reduce the hatching

and survival rates of aquatic organisms, cause gross effects such as lesions or fin erosion in fish,

and can eventually destroy the entire population of some sensitive species (Novotny and Olem,

1994). Hydrocarbons can be especially detrimental to benthic organisms because they can

become bound to urban runoff sediments (Schueler, 1987).
4 - 36

pH

Trout

Bass

Perch

Frogs

Salamanders

Clams

Crayfish

Snails

Mayfly

7.0 6.5 6.0 5.5 5.0 4.5 4.0

Figure 4-12. Low pH Tolerance by Different Species

Source: EPA, 1992b
4 - 37

Table 4-15. Relative Toxicities of Samples Using Microtox® Measurement Method

Local Source Areas Highly Toxic (%) Moderately Toxic (%) Not Toxic (%)

Roofs 8 58 33

Parking areas 19 31 50

Storage areas 25 50 25

Streets 0 67 33

Loading docks 0 67 33

Vehicle service areas 0 40 60

Landscaped areas 17 17 66

Urban creeks 0 11 89

Detention ponds 8 8 84

All source areas 9 32 59

Note: Microtox® results are primarily for comparison purposes.

Source: Pitt et al, 1995.

The physical impacts to streams due to urbanization and changes in watershed hydrology

also cause many habitat changes. As illustrated in the comparison of healthy and eroding stream

banks in Figure 4-13, loss of depth, sediment deposition, loss of shoreline vegetation, and higher

temperatures combine to impact habitat.
4 - 38

Healthy Stream Bank

Eroding Stream Bank

cooler temperatures

ample depth

shoreline vegetation

channel widening

insufficient depth

increased sedimentation

higher

temperaures

shoreline

erosion

shade/cover

Source: Adapted from Corish, 1995

Figure 4-13. Comparison of a Healthy Stream Bank and an Eroding Bank

Schueler (1987) states that sediment pollution in the form of increased suspended solids

can cause the following harmful impacts to aquatic life:

• Increased turbidity

• Decreased light penetration

• Reduced prey capture for sight feeding predators

• Clogging of gills/filters of fish and aquatic invertebrates

• Reduced spawning and juvenile fish survival.

Sediment is also a carrier of metals and other pollutants, and a source of bioaccumulating

pollutants for bottom feeding organisms. The rate of bioaccumulation is widely variable based

upon site specific conditions including species, concentration, pH, temperature, and other factors.

Barron (1995) reports that the bioaccumulation of organic contaminants results primarily from

direct exposure to water and sediment rather than through the food chain.
4 - 39

Macroinvertebrate Impacts

The biological integrity of receiving waters impacted by urban storm water is typically

reduced from more pristine, undeveloped circumstances. Impacts include a reduction in total

numbers and diversity of macroinvertebrates, and the emergence of more pollutant-tolerant

species. In a study in Delaware, it was found that approximately 70 percent of the

macroinvertebrate community in streams in undeveloped, forested watersheds consisted of

pollution sensitive mayflies, stoneflies and caddisflies, as compared with 20 percent in urbanized

watersheds (Maxted and Shaver, 1997). As shown in Table 4-16, the relative abundance of

pollution tolerant organisms increased with urbanization, including worms, midges and beetles. 
4 - 40

Table 4-16. Delaware Insect Population Abundance by Degree of Urbanization

Population Description

Relative Abundance

by Degree of

Urbanization (%)

Class/ Order Genus species Common

Name

PT None Low High

Insecta/Trichoptera Diplectrona modesta caddisfly 0 14 2 1

Insecta/Ephemeroptera Ephemerella spp. mayfly 1 12 1 0

Insecta/Plecoptera Allocapnia spp. stonefly 3 10 18 3

Insecta/Ephemeroptera Eurylophella spp. mayfly 1 8 1 2

Insecta/Coleoptera Anchytarsus bicolor beetle 4 6 3 0

Insecta/Ephemeroptera Stenonema spp. mayfly 4 5 3 1

Insecta/Coleoptera Optiservus spp. beetle 4 4 2 8

Insecta/Coleoptera Oulimnius latiusculus beetle 2 4 3 5

Insecta/Trichoptera Cheumatopsyche spp. caddisfly 5 1 10 8

Insecta/Trichoptera Hydropsyche betteni caddisfly 6 1 4 5

Insecta/Diptera Simulium vittatum blackfly 7 0 8 1

Insecta/Diptera Parametriocnemus spp. midge 5 0 0 4

Oligochaeta unidentified (Tubificidae) worm 10 0 0 4

Note: rare organisms (fewer than 4 per 100 organisms) not included. Relative abundance (%) and pollution

tolerance (PT) of macroinvertebrate species commonly found in Piedmont streams of Delaware for three levels of

urbanization; none (0-2% impervious cover), low (6-13%), and high (15-50%); PT range from 0 (low tolerance) to

10 (high tolerance).

Source: Maxted and Shaver, 1997.

A study by Kohlepp and Hellenthal (1992) quantified the effects of sediment deposits on

macroinvertebrates in Juday Creek, a tributary to the St. Joseph River in Indiana. The study

included data before and after upstream channel maintenance operations introduced a large

amount of sediment to the creek, similar to increased sediment yield from urban areas. A dramatic

change in the species distribution of macroinvertebrates in the river was observed, and this was

attributed to the changing sediment load and increased sedimentation. As shown in Figure 4-14,

“the result was a shift from a community dominated by filter-feeders in both numbers and

production rate in 1981-82, to a community in 1989-90 in which less desirable collector-gatherers

and shredders increased in importance in terms of relative contribution to both numbers and

production.”
4 - 41

Proportion by Functional Feeding Group (percent)

Figure 4-14. Effects of Sediment Deposits on Macroinvertebrates in Juday Creek, Indiana

 Source: Kohlepp and Hellenthal, 1992

Fish Impacts

The health of an ecosystem is often measured by the abundance and variety of fish species

present, and the presence of native species. A case study in California compared fish populations

in urbanized and non-urbanized sections of Coyote Creek (Pitt, 1995). The relative abundance of

different fish species in the different reaches is presented in Table 4-17. As shown, the native fish

are generally replaced by introduced fish in the urbanized section.
4 - 42

Table 4-17. Relative Abundance of Native and Introduced Fish in Urbanized and NonUrbanized Areas in Coyote Creek, California

Species

Relative Abundance (%)

Non-urbanized Reach Urbanized Reach

Native Fish

 Hitch 34.8 4.8

 Threespine stickleback 27.3 0.8

 Sacramento sucker 12.6 0.1

Introduced Fish

 Mosquitofish 5.6 66.9

 Fathead Minnow 0.6 20.6

 Threadfin shad - 2.4

Source: Pitt, 1995

An illustration of the abundance of fish eggs and larvae associated with different levels of

urban land use in New York is presented in Figure 4-15 (Limburg and Schmidt, 1990). This

graph supports the “10 percent rule” reported by Schueler and Claytor (1995): stream impacts

tend to begin in watersheds with only 10 to 20 percent imperviousness.
4 - 43

0 

1 

2 

3 

4 

Natural log of Mean Density (#/ m^3)

0 20 40 60 80 

Percent of Watershed in Urban Land Use

Source: Limburg and Schmidt, 1990

10% Threshold

Figure 4-15. Average Densities of Fish Eggs and Larvae in New York

The change in the resident fish community due to urbanization in Tuckahoe Creek in

Virginia was quantified by Weaver and Garman (1994). With urbanization increasing the percent

of urban land from 7 percent to 28 percent between 1958 and 1990, a dramatic change in the fish

assemblage was observed. As shown in Table 4-18, the total number of fish observed dropped

sharply along with the total number of species present and the number of common species

present.
4 - 44

Table 4-18. Effects of Urbanization on the Fish Community of Tuckahoe Creek, Virginia 

(Composite of 6 Sites)

Indicator

Fish Assemblage

Year

1958 1990 

% Urban (by land area) 7 28 

total abundance 2,056 412 

# species - total 31 23 

# species - common* 21 6 

% bluegill/shiner 28 67 

* more than 10 individuals

Source: Weaver and Garman, 1994

4.3.3 Public Health Impacts

Public health impacts associated with urban storm water occur when humans ingest or

come in contact with pathogens. While these impacts are not widely reported, they do occur, and

some impacts have been documented. Examples related to swimming and contact recreation

impacts and shellfish impacts are presented.

Contact Recreation Impacts

Beach closures are a common occurrence in many communities throughout the United

States. Beach closures are primarily due to high levels of bacteria in water samples. The

presence of medical waste and other dangerous floatable substances on beaches can also cause

beach closures to occur. Storm water runoff can be responsible for both bacteria and floatables. 

Elevated levels of bacteria and viruses represent the most common threat to public health. 

Diarrhea and infection of the ear, eye, nose, or throat are possible.

A study of epidemiological impacts associated with swimming in the vicinity of storm

water outfalls in Santa Monica Bay in California was conducted in 1995 (SMBRP, 1996). The

study focused on health effects, and not on possible sources of contamination to the storm drain
1

 Pilot studies conducted in the Bay prior to 1995 noted that some outfalls had regular dry

weather discharges; this is a common indicator of storm drain contamination (SMBRP, 1990;

SMBRP, 1992).

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system, such as illicit sewage connections and infiltration.1 While the effects observed may be

atypical of properly constructed and maintained storm drain outfalls, the findings indicate the

potential health risks associated with pathogens. Major findings of this study are as follows:

• There is an increased risk of illness associated with swimming near flowing storm drain

outlets in Santa Monica Bay.

• There is an increased risk of illness associated with swimming in areas with high 

densities of bacterial indicators.

• The total coliform to fecal coliform ratio was found to be one of the better indicators

for predicting health risks.

• Illnesses were reported more often on days when the samples were positive for enteric

viruses.

• High densities of bacterial indicators were measured on a significant number of survey

days, particularly in front of drains.

People who swim in areas adjacent to flowing storm drains were found to be 50 percent

more likely to get sick than people who swam in other areas. The sicknesses included fever,

nausea, gastroenteritis, and flu-like symptoms such as nasal congestion, sore throat, fever, or

coughing. As illustrated in Figure 4-16, swimmers who swam directly in front of storm drains

were much more likely to become ill than those who swam away from the storm drains at

distances of 100 to 400 meters. A comparative health outcome in terms of relative risk for

swimming in front of the storm drain vs. swimming 400 meters away is presented in Table 4-19.
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Figure 4-16. Health Effects Observed Relative to Distance

from Santa Monica Bay Storm Drains

Source: Santa Monica Bay Restoration Project, 1996
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Table 4-19. Comparative Health Outcomes for Swimming in Front of Drains in

Santa Monica Bay

Health Outcome

Relative Risk

for Swimming

in Front of

Drains*

Estimated No. Of Excess Cases

per 10,000 Persons

Fever 57% 259

Chills 58% 138

Ear Discharge 127% 88

Vomiting 61% 115

Coughing with phlegm 59% 175

Any of the above symptoms 44% 373

HCGI-2 111% 95

SRD 66% 303

HCGI-2 or SRD 53% 314

* Compared to swimming 400 meters or more away from drains

Source: Santa Monica Bay Restoration Project, 1996

Seafood Hazard

The consumption of contaminated seafood, particularly shellfish, is a major public health

problem. Shellfish are susceptible to bioaccumulating bacteria and viruses because they are filter

feeders. In waters polluted by urban runoff, bacteria and viruses can be concentrated in the

shellfish to much higher levels than those found in the surrounding waters. This becomes a public

health concern because many potentially harmful bacteria and viruses can be ingested when people

eat contaminated shellfish. As shown in Figure 4-17, the largest proportion of shellfish

harvesting restrictions are caused by urban runoff (US EPA, 1995a).
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Figure 4-17. Sources Associated with Shellfish Harvesting Restrictions, in Percent

Source: US EPA, 1995a

Fish can also be contaminated for a number of reasons. Recent fish sampling surveys in

regions of the U.S. have shown widespread mercury contamination in streams, wetlands,

reservoirs, and lakes. Based on 1997 data, 33 states have issued fish consumption advisories

because of mercury contamination (US EPA, 1998a). Mercury is an urban/industrial pollutant

that is released into the air and ends up in urban runoff by atmospheric deposition (Krabbenhoft

and Rickert, 1995). The effects of fish contamination go beyond health issues, and hurt the

recreational fishing industry as a whole.

4.3.4 Aesthetic Impacts

The aesthetic impacts associated with urban storm water are often difficult to quantify. 

However, aesthetic impacts are often very visible to the general public. EPA reports that “people

have a strong emotional attachment to water, arising from its aesthetic qualities--tranquillity,

coolness, and beauty” (US EPA, 1995c). The presence of floatables within urban waters and

deposited along the banks of waterways represents a common aesthetic impact in most urban

settings. Floatable wastes originate from street litter and improper solid waste disposal practices. 

The average total street debris loading rate in New York City was quantified at approximately

156 pounds per curb-mile per day, with a range from 3 to 2,700 pounds (HydroQual, 1995).
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Aesthetic impacts from the eutrophication of urban waterways is caused in part by

nutrients delivered in urban storm water. As reported by Schueler (1987), aesthetic impacts and

nuisance conditions associated with eutrophication can include: 

• Surface algal scum

• Water discoloration

• Strong odors

• Release of toxins.

The visual damage to urban streams from accelerated rates of storm water runoff also

contribute to aesthetic impacts. These include eroded stream banks, fallen trees, and

sedimentation. In summary, aesthetic impacts are often very visible in public areas where

shoreline recreation occurs. Aesthetic impacts are therefore the storm water impacts most

familiar to the general public.