E AP Project Summary

United States

Environmental Protection

Agency

Potential Groundwater

Contamination from Intentional

and Nonintentional Stormwater

Infiltration

Robert Pitt, Shirley Clark, and Keith Parmer

Risk Reduction

Engineering Laboratory

Cincinnati, OH 45268

Research and Development

The research summarized here was

conducted during the first year of a 3-

yr cooperative agreement to identify

and control stormwater toxicants, especially those adversely affecting

groundwater. The purpose of this research effort was to review the groundwater contamination literature as it

relates to stormwater. Potential problem pollutants were identified, based

on their mobility through the unsaturated soil zone above groundwater, their

abundance in stormwater, and their

treatability before discharge. This information was used with earlier EPA

research results to identify the possible sources of these potential problem pollutants. Recommendations were

also made for stormwater infiltration

guidelines in different areas and monitoring that should be conducted to

evaluate a specific stormwater for its

potential to contaminate groundwater.

This Project Summary was developed

by EPA’s Risk Reduction Engineering

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

Before urbanization, groundwater was

recharged by precipitation infiltrating

through pervious surfaces, including grasslands and woods. This infiltrating water

was relatively uncontaminated. Urbanization, however, reduced the permeable soil

surface area through which recharge by

infiltration could occur. This resulted in

much less groundwater recharge and

greatly increased surface runoff. In addition, the waters available for recharge generally carried increased quantities of

pollutants. With urbanization, waters having elevated contaminant concentrations

also recharge groundwater, including effluent from domestic septic tanks, wastewater from percolation basins and

industrial waste injection wells, infiltrating

stormwater, and infiltrating water from agricultural irrigation. This report addresses

potential groundwater problems associated

with stormwater toxicants and describes

how conventional stormwater control practices can reduce these problems.

Sources of Pollutants

High bacteria populations have been

found in sheetflow samples from sidewalks,

roads, and some bare ground (collected

from locations where dogs would most

likely be “walked”). Tables 1 and 2 summarize toxicant concentrations and likely

sources or locations having some of the

highest concentrations found during an

earlier phase of this EPA-funded research.

The detection frequencies for the heavy

metals are all close to 100% for all source

areas, and the detection frequencies for

the organics listed on these tables ranged

from about 10% to 25%. Vehicle service

areas had the greatest abundance of observed organics.

EPA/600/SR-94/051 May 1994
2

diazinon, dicamba, and malathion) on irrigated and sandy soils will likely contaminate groundwater. Fungicides and

nematocides must be mobile to reach the

target pest, and hence, they generally have

the highest groundwater contamination

potential. Pesticide leaching depends on

patterns of use, soil texture, total organic

carbon content of the soil, pesticide persistence, and depth to the water table.

The greatest pesticide mobility occurs

in areas with coarse-grained or sandy soils

without a hardpan layer, and with soils

that have low clay and organic matter

content and high permeability. Structural

voids, generally found in the surface layer

of finer-textured soils rich in clay, can transmit pesticides rapidly when the voids are

filled with water and the adsorbing surfaces of the soil matrix are bypassed. In

general, pesticides with low water solubilities, high octanol-water partitioning coefficients, and high carbon partitioning

coefficients are less mobile. The slower

moving pesticides that may better sorb to

soils have been recommended for use in

areas of groundwater contamination concern. These include the fungicides

iprodione and triadimefon, the insecticides

isofenphos and chlorpyrifos, and the herbicide glyphosate.

Pesticides decompose in soil and water, but the total decomposition time can

range from days to years. Literature halflives for pesticides generally apply to surface soils and do not account for the

reduced microbial activity found deep in

the vadose zone. Pesticides with a 30-

day half life can show considerable leaching. An order-of-magnitude difference in

half-life results in a five- to ten-fold difference in percolation loss. Organophosphate

pesticides are less persistent than organochlorine pesticides, but they also are

not strongly adsorbed by the sediment

and are likely to leach into the vadose

zone and the groundwater.

Other Organics

The most commonly occurring organic

compounds found in urban groundwaters

include phthalate esters (especially bis(2-

ethylhexyl)phthalate) and phenolic compounds. Other, more rarely found, organics

include the volatiles: benzene, chloroform,

methylene chloride, trichloroethylene,

tetrachloroethylene, toluene, and xylene.

Polycyclic aromatic hydrocarbons (PAHs)

(especially benzo(a)anthracene, chrysene,

anthracene, and benzo(b)fluoroanthenene)

have also been found in groundwaters

near industrial sites.

Groundwater contamination from organics, like that from other pollutants, occurs

Table 1. Concentrations of Heavy Metals in Observed Areas (μg/L)

Toxicant Highest Median Highest Observed

Cadmium Vehicle service area runoff 8 Street runoff 220

Chromium Landscaped area runoff 100 Roof runoff 510

Copper Urban receiving water 160 Street runoff 1250

Lead CSO 75 Storage area runoff 330

Nickel Parking area runoff 40 Landscaped area runoff 130

Zinc Roof runoff 100 Roof runoff 1580

Maximum, Detection

Toxicant μg/L Frequency, % Significant Sources

Benzo (a) anthracene 60 12 Gasoline, wood preservative

Benzo (b) fluoranthene 226 17 Gasoline, motor oils

Benzo (k) fluoranthene 221 17 Gasoline, bitumen, oils

Benzo (a) pyrene 300 17 Asphalt, gasoline, oils

Fluoranthene 128 23 Oils, gasoline, wood preservative

Naphthalene 296 13 Coal tar, gasoline, insecticides

Phenanthrene 69 10 Oils, gasoline, coal tar

Pyrene 102 19 Oils, gasoline, bitumen, coal tar,

wood preservative

Chlordane 2.2 13 Insecticide

Butyl benzyl phthalate 128 12 Plasticizer

Bis (2-chloroethyl) ether 204 14 Fumigant, solvents, insecticides,

paints, lacquers, varnishes

Bis (2-chloroisopropyl) ether 217 14 Pesticide manufacturing

1,3-Dichlorobenzene 120 23 Pesticide manufacturing

Stormwater Constituents

Having High Potential to

Contaminate Groundwater

Nutrients

Nitrates are one of the most frequently

encountered contaminants in groundwater. Phosphorus contamination of groundwater has not been as widespread, or as

severe, as that of nitrogen compounds.

Whenever nitrogen-containing compounds

come into contact with soil, a potential

exists for nitrate leaching into groundwater, especially in rapid-infiltration wastewater basins, stormwater infiltration

devices, and agricultural areas. Nitrate has

leached from fertilizers and affected

groundwaters under various turf grasses

in urban areas, including golf courses,

parks, and home lawns. Significant leaching of nitrates occurs during the cool, wet

seasons. Cool temperatures reduce denitrification and ammonia volatilization and

limit microbial nitrogen immobilization and

plant uptake. The use of slow-release fertilizers (including composted organic

mulches, urea formaldehyde (UF), methylene urea, isobutylidene diurea (IBDU),

and sulfur-coated urea) is recommended

in areas having potential groundwater nitrate problems.

Residual concentrations of nitrate in soil

vary greatly and depend on the soil texture, mineralization, rainfall and irrigation

patterns, organic matter content, crop yield,

nitrogen fertilizer/sludge application rate,

denitrification, and soil compaction. Nitrate

is highly soluble (>1 kg/L) and will stay in

solution in the percolation water. If it leaves

the root zone without being taken-up by

plants, it will readily reach the groundwater.

Pesticides

Urban pesticide contamination of

groundwater can result from municipal and

homeowner use for pest control and the

subsequent collection of the pesticide in

stormwater runoff. Pesticides that have

been found in urban groundwaters include:

2,4-D, 2,4,5-T, atrazine, chlordane,

diazinon, ethion, malathion, methyl trithion,

silvex, and simazine. Heavy repetitive use

of mobile pesticides (those that are not

likely to be retained by various processes

in the soil before they reach the groundwater, such as 2,4-D, acenaphthylene,

alachlor, atrazine, cyanazine, dacthal,

Table 2. Maximum Concentrations of Toxic Organics from Observed Sources
3

more readily in areas with sandy soils and

where the water table is near the land

surface. Organics can be removed from

the soil and recharge water by volatilization, sorption, and degradation. Volatilization can significantly reduce the

concentrations of the most volatile compounds in groundwater, but the rate of

gas transfer from the soil to the air is

usually limited by the presence of soil

water. Hydrophobic sorption onto soil organic matter limits the mobility of less

soluble base/neutral and acid extractable

compounds through organic soils and the

vadose zone. Sorption is not always a

permanent removal mechanism, however.

Organic resolubilization can occur during

wet periods following dry periods. Many

organics can be degraded by microorganisms, at least partially, but others cannot.

Temperature, pH, moisture content, ion

exchange capacity of the soil, and air availability may limit the microbial degradation

potential for even the most degradable

organic compound.

Microorganisms

Viruses have been detected in groundwater where stormwater recharge basins

were located short distances above the

aquifer. Enteric viruses are more resistant

to environmental factors than are enteric

bacteria, and they exhibit longer survival

times in natural waters. They can occur in

potable and marine waters in the absence

of fecal coliforms. Enteroviruses are also

more resistant to commonly used disinfectants than are indicator bacteria (such

as fecal coliforms), and they can occur in

groundwater in the absence of indicator

bacteria.

The factors that affect the survival of

enteric bacteria and viruses in the soil

include pH, antagonism from soil microflora, moisture content, temperature, sunlight, and organic matter. The two most

important attributes of viruses that permit

their long-term survival in the environment

are their structure and very small size.

These characteristics permit virus occlusion and protection within colloid-size particles. Viral adsorption is promoted by

increasing cation concentration, decreasing pH, and decreasing soluble organics.

Since the movement of viruses through

soil to groundwater occurs in the liquid

phase and involves water movement and

associated suspended virus particles, the

distribution of viruses between the

adsorbed and liquid phases determines

the viral mass available for movement.

Once the virus reaches the groundwater,

it can travel laterally through the aquifer

until it is either adsorbed or inactivated.

The major bacterial removal mechanisms in soil are straining at the soil surface and at intergrain contacts,

sedimentation, sorption by soil particles,

and inactivation. Because their size is

larger than viruses, most bacteria are retained near the soil surface because of

this straining effect. In general, enteric

bacteria survive in soil between 2 and 3

mo, although survival times up to 5 yr

have been documented.

Metals

From a groundwater pollution standpoint,

the metals in stormwater presenting the

most environmental concern are aluminum, arsenic, cadmium, chromium, copper, iron, lead, mercury, nickel, and zinc.

The majority of these metals (with the

common exception of zinc) are, however,

mostly associated with the particulate fractions and can be mostly removed by either sedimentation or filtration processes.

In general, studies of recharge basins

receiving large metal loads found that most

of the heavy metals are removed either in

the basin sediment or in the vadose zone.

Dissolved metal ions are removed from

stormwater during infiltration mostly by

adsorption onto the near-surface particles

in the vadose zone, and the particulate

metals are filtered out at the soil surface.

Studies at recharge basins found that lead,

zinc, cadmium, and copper accumulated

at the soil surface with little downward

movement over many years. At a commercial site, however, nickel, chromium,

and zinc concentrations have exceeded

regulatory limits in the soils below a recharge area. Allowing percolation ponds

to go dry between storms can be counterproductive to the removal of lead from the

water during recharge. Apparently, the

adsorption bonds between the sediments

and the metals can be weakened during

the drying period.

Similarities in water quality between runoff water and groundwater have shown

that there is significant downward movement of copper and iron in sandy and

loamy soils. Arsenic, nickel, and lead, however, did not significantly move downward

through the soil to the groundwater. The

exception to this was some downward

movement of lead with the percolation

water in sandy soils beneath stormwater

recharge basins. Zinc, which is more

soluble than iron, has been found in higher

concentrations in groundwater than iron.

The order of attenuation in the vadose

zone from infiltrating stormwater is: zinc

(most mobile) > lead > cadmium > manganese > copper > iron > chromium >

nickel > aluminum (least mobile).

Salts

Salt applications for winter traffic safety

is a common practice in many northern

areas, and the sodium and chloride, which

are collected in the snowmelt, travel down

through the vadose zone to the groundwater with little attenuation. Soil is not

very effective at removing salts. Salts that

are still in the percolation water after it

travels through the vadose zone will contaminate the groundwater. Infiltrating

stormwater has increased sodium and

chloride concentrations above background

concentrations. Fertilizer and pesticide

salts also accumulate in urban areas and

can leach through the soil to the groundwater.

Studies of depth of pollutant penetration in soil have shown that sulfate and

potassium concentrations decrease with

depth, whereas sodium, calcium, bicarbonate, and chloride concentrations increase with depth. Once contamination

with salts begins, the movement of salts

into the groundwater can be rapid. The

salt concentration may not lessen until the

source of the salts is removed.

Treatment of Stormwater

Table 3 summarizes the filterable fraction of toxicants found in runoff sheet flows

from many urban areas found during an

earlier phase of this EPA-funded research.

Pollutants that are mostly in filterable forms

have a greater potential of affecting

groundwater and are more difficult to control with the use of conventional stormwater

control practices which mostly rely on sedimentation and filtration principles. Luckily,

most of the toxic organics and metals are

associated with the nonfilterable (suspended solids) fraction of the wastewaters during wet weather. Possible

exceptions include zinc, fluoranthene,

pyrene, and 1,3-dichlorobenzene, which

may be mostly found in the filtered sample

portions. Pollutants in dry-weather storm

drainage flows, however, tend to be much

more associated with filtered sample fractions and would not be as readily controlled with the use of sedimentation.

Sedimentation is the most common fate

and control mechanism for particulate-related pollutants. This would be common

for most stormwater pollutants, as noted

above. Particulate removal can occur in

many conventional stormwater control processes, including catchbasins, screens,

drainage systems, and detention ponds.

Sorption of pollutants onto solids and metal

precipitation increases the sedimentation

potential of these pollutants and also encourages more efficient bonding of the

pollutants in soils to prevent their leaching
4

also be useful for analyzing stormwater

problems and needed controls for surface

water discharges.

Table 4 is a summary of the pollutants

found in stormwater that may cause

groundwater contamination problems for

various reasons. This table does not consider the risk associated with using groundwater contaminated with these pollutants.

Causes of concern include high mobility

(low sorption potential) in the vadose zone,

high abundance (high concentrations and

high detection frequencies) in stormwater,

and high soluble fractions (small fraction

associated with particulates that would

have little removal potential using conventional stormwater sedimentation controls)

in the stormwater. The contamination potential is the lowest rating of the influencing factors. As an example, when no

pretreatment is used before percolation

through surface soils, the mobility and

abundance criteria are most important.

When a compound is mobile but in low

abundance (such as for volatile organic

compounds, VOCs), then the groundwater contamination potential would be low.

When the compound is mobile, however,

and also in high abundance (such as for

sodium chloride, in certain conditions), then

the groundwater contamination potential

would be high. When sedimentation pretreatment is to be used before infiltration,

then some of the pollutants will likely be

removed before infiltration. In this case,

all three influencing factors (pollutant mobility, pollutant abundance in stormwater,

and fraction of the pollutant associated

with the filtered sample fraction) would be

considered. As an example, chlordane

would have a low contamination potential

with sedimentation pretreatment, whereas

it would have a moderate contamination

potential when no pretreatment is used. In

addition, when subsurface infiltration/injection is used instead of surface percolation, the compounds would most likely be

more mobile, making the abundance criteria the most important, with some regard given to the filterable fraction

information for operational considerations.

This table is only appropriate for initial

estimates of contamination potential because of the simplifying assumptions

made, such as the worst case mobility

conditions assumed (for sandy soils having low organic content). When the soil is

clayey and has a high organic content,

then most of the organic compounds would

be less mobile than that shown on this

table. The abundance and filterable fraction information is generally applicable for

warm weather stormwater runoff in residential and commercial areas. The pollutant concentrations and detection

to groundwaters. Detention ponds are

probably the most common management

practice for the control of stormwater runoff. If properly designed, constructed, and

maintained, wet detention ponds can be

very effective in controlling a wide range

of pollutants. The monitored performance

of wet detention ponds indicates more than

90% removal for suspended solids, 70%

for BOD5 and COD, about 60% to 70% for

nutrients, and about 60% to 95% for heavy

metals. Catchbasins are very small sedimentation devices. Adequate cleaning can

help reduce the total solids and lead urban runoff yields by between 10% and

25%, and COD, total Kjeldahl nitrogen,

total phosphorus, and zinc by between

5% and 10%. Other important fate mechanisms available in wet detention ponds,

but which are probably not important in

small enclosed sump devices such as

catchbasins, include volatilization and photolysis. Biodegradation, biotransformation,

and bioaccumulation (into plants and animals) may also occur in larger and open

ponds.

Upland infiltration devices (such as infiltration trenches, porous pavements, percolation ponds, and grass roadside

drainage swales) are located at urban

source areas. Infiltration (percolation)

ponds are usually located at stormwater

outfalls or at large paved areas. These

basins, along with perforated storm sewers, can infiltrate flows and pollutants from

all upland sources combined. Infiltration

devices can safely deliver large fractions

of the surface flows to groundwater, if

carefully designed and located. Local conditions that can make stormwater infiltration inappropriate include steep slopes,

slowly percolating soils, shallow groundwater, and nearby groundwater uses.

Grass filter strips may be quite effective in

removing particulate pollutants from overland flows. The filtering effects of grasses,

along with increased infiltration/recharge,

reduce the particulate sediment load from

urban landscaped areas. Grass swales

are another type of infiltration device and

can be used in place of curb and gutter

drainages in most land uses, except possibly strip commercial and high density

residential areas. Grass swales allow the

recharge of significant amounts of surface

flows. Swales can also reduce pollutant

concentrations because of filtration.

Soluble and particulate heavy metal (copper, lead, zinc, and cadmium) concentrations can be reduced by at least 50%,

COD, nitrate nitrogen, and ammonia nitrogen concentrations can be reduced by

about 25%, but only inconsistent concentration reductions can be expected for organic nitrogen, phosphorus, and bacteria.

Sorption of pollutants to soils is probably the most significant fate mechanism

of toxicants in biofiltration devices. Many

of the devices also use sedimentation and

filtration to remove the particulate forms

of the pollutants from the water. Incorporation of the pollutants onto soil with subsequent biodegradation and minimal

leaching to the groundwater is desired.

Volatilization, photolysis, biotransformation,

and bioconcentration may also be significant in grass filter strips and grass swales.

Underground seepage drains and porous

pavements offer little biological activity to

reduce toxicants.

Results and Conclusions

This entire research project will provide

guidance on critical source area treatment,

especially for the protection of groundwater quality. Much of the information will

Constituent Filterable Fraction (%)

Cadmium 20 to 50

Chromium <10

Copper <20

Iron Small amount

Lead <20

Nickel Small amount

Zinc >50

Benzo (a) anthracene None found in filtered fraction

Fluoranthene 65

Naphthalene 25

Phenanthrene None found in filtered fraction

Pyrene 95

Chlordane None found in filtered fraction

Butyl benzyl phthalate Irregular

Bis (2-chloroethyl) ether Irregular

Bis (2-chlrorisopropyl) ether None found in filtered fraction

1,3-Dichlorobenzene 75

Table 3. Reported Filterable Fractions of Stormwater Toxicants from Source Areas
5

frequencies, however, would be greater

for critical source areas (especially vehicle service areas) and critical land uses

(especially manufacturing industrial areas).

The stormwater pollutants of most concern (those that may have the greatest

adverse impacts on groundwaters) include:

• Nutrients: nitrate has a low to moderate potential for contaminating groundwater when both surface percolation

and subsurface infiltration/injection are

used because of its relatively low concentrations in most stormwaters.

When the stormwater nitrate concentration is high, then the groundwater

contamination potential would likely

also be high.

• Pesticides: lindane and chlordane

have moderate potentials for contaminating groundwater when surface percolation (with no pretreatment) or

when subsurface injection (with minimal pretreatment) are used. The

groundwater contamination potentials

for both of these compounds would

very likely be substantially reduced

with adequate sedimentation pretreatment.

• Other organics: 1,3-dichlorobenzene

may have a high potential for contaminating groundwater when subsurface infiltration/injection (with minimal

pretreatment) is used. It would, however, probably have a lower groundwater contamination potential for most

surface percolation practices because

of its relatively strong sorption to vadose zone soils. Both pyrene and

fluoranthene would also very likely

have high groundwater contamination

potentials for subsurface infiltration/

injection practices, but lower contamination potentials for surface percolation practices because of their more

limited mobility through the unsaturated zone (vadose zone). Others (including benzo(a)anthracene, bis

(2-ethylhexyl) phthalate, pentachlorophenol, and phenanthrene) may

also have moderate groundwater contamination potentials when surface

percolation with no pretreatment, or

subsurface injection/infiltration, is

used. These compounds would have

low groundwater contamination potentials when surface infiltration is used

with sedimentation pretreatment.

VOCs may also have high groundwater contamination potentials if present

in the stormwater (which is possible

for some industrial and commercial

facilities and vehicle service establishments).

• Pathogens: enteroviruses very likely

have high potentials for contaminating groundwater when any percolation or subsurface infiltration/injection

practice is used, depending on their

presence in stormwater (especially if

contaminated with sanitary sewage).

Other pathogens, including Shigella,

Pseudomonas aeruginosa, and various protozoa, would also have high

groundwater contamination potentials

when subsurface infiltration/injection

practices are used without disinfection. When disinfection (especially by

chlorine or ozone) is used, then disinfection by-products (such as

trihalomethanes or ozonated bromides) would have high groundwater

contamination potentials.

• Heavy Metals: nickel and zinc possibly have high potentials for contaminating groundwater when subsurface

infiltration/injection is used. Chromium

and lead would have moderate

groundwater contamination potentials

for subsurface infiltration/injection

practices. All metals would possibly

have low groundwater contamination

potentials when surface infiltration is

used with sedimentation pretreatment.

• Salts: chloride would very likely have

a high potential for contaminating

groundwater in northern areas where

road salts are used for traffic safety,

irrespective of the pretreatment, infiltration, or percolation practices used.

Pesticides have been mostly found in

urban runoff from residential areas, especially in dry weather flows associated with

landscaping irrigation runoff. The other organics, especially the volatiles, are mostly

found in industrial areas. The phthalates

are found in all areas. The PAHs are also

found in runoff from all areas, but they are

in higher concentrations and occur more

frequently in industrial areas. Pathogens

are most likely associated with sanitary

sewage contamination of storm drainage

systems, but several bacterial pathogens

are commonly found in surface runoff in

residential areas. Zinc is mostly found in

roof runoff and other areas where galvanized metal comes into contact with rainwater. Salts are at their greatest

concentrations in snowmelt and early

spring runoff in northern areas.

The control of these compounds requires various approaches, including

source area controls, end-of-pipe controls,

and pollution prevention. All dry weather

flows should be diverted from infiltration

devices because of their potentially high

concentrations of soluble heavy metals,

pesticides, and pathogens. Similarly, all

runoff from manufacturing industrial areas

should also be diverted from infiltration

devices because of their relatively high

concentrations of soluble toxicants. Combined sewer overflows should also be diverted because of sewage contamination.

In areas of snow and ice control, winter

snowmelt and runoff and early spring runoff should also be diverted from infiltration

devices.

All other runoff should include pretreatment using sedimentation processes before infiltration, to both minimize

groundwater contamination and to prolong

the life of the infiltration device (if needed).

This pretreatment can take the form of

grass filters, sediment sumps, wet detention ponds, etc., depending on the runoff

volume to be treated, treatment flow rate,

and other site specific factors. Pollution

prevention can also play an important role

in minimizing groundwater contamination

problems, including reducing the use of

galvanized metals, pesticides, and fertilizers in critical areas. The use of specialized treatment devices, such as those

being developed and tested during this

research, can also play an important role

in treating runoff from critical source areas before these more contaminated flows

commingle with cleaner runoff from other

areas. Sophisticated treatment schemes,

especially the use of chemical processes

or disinfection, may not be warranted, except in special cases, especially when the

potential of forming harmful treatment byproducts (such as THMs and soluble aluminum) is considered.

The use of surface percolation devices

(such as grass swales and percolation

ponds) that have a substantial depth of

underlying soils above the groundwater is

preferable to the use of subsurface infiltration devices (such as dry wells, trenches

or seepage drains, and especially injection wells), unless the runoff water is known

to be relatively free of pollutants. Surface

devices are able to take greater advantage of natural soil pollutant removal processes. Unless all percolation devices are

carefully designed and maintained, however, they may not function properly and

may lead to premature hydraulic failure or

contamination of the groundwater.

Recommendations

With a reasonable degree of site-specific design considerations to compensate

for soil characteristics, infiltration may be
6

Mobility Contamination Potential

(sandy/low Abundance Fraction Surface Infilt. and Surface Infilt. with Sub-surface Inj. with

Compounds organic soils) in Stormwater Filterable No Pretreatment Sedimentation Minimal Pretreatment

Nutrients nitrates mobile low/moderate high low/moderate low/moderate low/moderate

Pesticides 2,4-D mobile low likely low low low low

γ -BHC (lindane) intermediate moderate likely low moderate low moderate

malathion mobile low likely low low low low

atrazine mobile low likely low low low low

chlordane intermediate moderate very low moderate low moderate

diazinon mobile low likely low low low low

Other VOCs mobile low very high low low low

organics 1,3-dichloro-

 benzene low high high low low high

anthracene intermediate low moderate low low low

benzo(a)

 anthracene intermediate moderate very low moderate low moderate

bis (2-ethylhexyl)

 phthalate intermediate moderate likely low moderate low moderate

butyl benzyl

 phthalate low low/moderate moderate low low low/moderate

fluoranthene intermediate high high moderate moderate high

fluorene intermediate low likely low low low low

naphthalene low/inter. low moderate low low low

pentachlorophenol intermediate moderate likely low moderate low moderate

phenanthrene intermediate moderate very low moderate low moderate

pyrene intermediate high high moderate moderate high

Pathogens enteroviruses mobile likely present high high high high

Shigella low/inter. likely present moderate low/moderate low/moderate high

Pseudomonas

 aeruginosa low/inter. very high moderate low/moderate low/moderate high

protozoa low/inter. likely present moderate low/moderate low/moderate high

Heavy

metals nickel low high low low low high

cadmium low low moderate low low low

chromium inter./very low moderate very low low/moderate low moderate

lead very low moderate very low low low moderate

zinc low/very low high high low low high

Salts chloride mobile seasonally high high high high

high

very effective in controlling both urban runoff quality and quantity problems. This

strategy encourages infiltration of urban

runoff to replace the natural infiltration capacity lost through urbanization and to

use the natural filtering and sorption capacity of soils to remove pollutants; however, the potential for some types of urban

runoff to contaminate groundwater through

infiltration requires some restrictions. Infiltration of urban runoff having potentially

high concentrations of pollutants that may

pollute groundwater requires adequate pretreatment or the diversion of these waters

away from infiltration devices. The following general guidelines for the infiltration of

stormwater and other storm drainage effluent are recommended in the absence

of comprehensive site-specific evaluations:

• Dry weather storm drainage effluent

should be diverted from infiltration

devices because of their probable high

concentrations of soluble heavy metals, pesticides, and pathogenic microorganisms.

• Combined sewage overflows should

be diverted from infiltration devices

because of their poor water quality,

especially their high pathogenic microorganism concentrations and high

clogging potential.

• Snowmelt runoff should be diverted

from infiltration devices because of

its potential for having high concentrations of soluble salts.

• Runoff from manufacturing industrial

areas should be diverted from infiltration devices because of its potential

for having high concentrations of

soluble toxicants.

• Construction site runoff must be diverted from stormwater infiltration devices (especially subsurface devices)

because of its high suspended solids

concentrations, which would quickly

clog infiltration devices.

• Runoff from other critical source areas, such as vehicle service facilities

and large parking areas, should at

least receive adequate pretreatment

to eliminate their groundwater contamination potential before infiltration.

• Runoff from residential areas (the largest component of urban runoff in most

cities) is generally the least polluted

urban runoff flow and should be considered for infiltration. Very little treatTable 4. Potential of Stormwater Pollutants to Contaminate Groundwater
7

ment of residential area stormwater

runoff should be needed before infiltration, especially if surface infiltration

is through the use of grass swales.

When subsurface infiltration (seepage

drains, infiltration trenches, dry wells,

etc.) is used, then some pretreatment

may be needed, such as by using

grass filter strips, or other surface filtration devices.

Recommended Stormwater

Quality Monitoring to Evaluate

Potential Groundwater

Contamination

Most past stormwater quality monitoring efforts have not adequately evaluated

stormwater’s potential for contaminating

groundwater. The following list shows the

stormwater contaminants that are recommended for monitoring when stormwater

contamination potential needs to be considered, or when infiltration devices are to

be used. Other analyses are appropriate

for additional monitoring objectives (such

as evaluating surface water problems). In

addition, all phases of urban runoff should

be sampled, including stormwater runoff,

dry-weather flows, and snowmelts.

• Urban runoff contaminates with the

potential to adversely affect groundwater:

- Nutrients (especially nitrates)

- Salts (especially chloride)

- VOCs (if expected in the runoff,

such as runoff from manufacturing industrial or vehicle service areas, could

screen for VOCs with purgable organic carbon analyses)

- Pathogens (especially enteroviruses,

if possible, along with other pathogens such as Pseudomonas

aeruginosa, Shigella, and pathogenic

protozoa)

- Bromide and total organic carbon

(to estimate disinfection by-product

generation potential, if disinfection by

either chlorination or ozone is being

considered)

- Pesticides, in both filterable and total sample components (especially lindane and chlordane)

- Other organics, in both filterable and

total sample components (especially

1,3 dichlorobenzene, pyrene,

fluoranthene, benzo(a)anthracene, bis

(2-ethylhexyl) phthalate, pentachlorophenol, and phenanthrene)

- Heavy metals, in both filterable and

total sample components (especially

chromium, lead, nickel, and zinc)

• Urban runoff compounds with the potential to adversely affect infiltration

and injection operations:

- Sodium, calcium, and magnesium

(to calculate the sodium adsorption

ratio to predict clogging of clay soils)

- Suspended solids (to determine the

need for sedimentation pretreatment

to prevent clogging)
8

Robert Pitt, Shirley Clark and Keith Parmer are with the Department of Civil and

Environmental Engineering, the University of Alabama at Birmingham,

Birmingham, AL 35294

Richard Field is the EPA Project Officer (see below).

The complete report, entitled "Potential Groundwater Contamination from

Intentional and Nonintentional Stormwater Infiltration,” (Order No. PB94-

165354AS; Cost: $27.00, 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:

Risk Reduction Engineering Laboratory

U.S. Environmental Protection Agency

Edison, NJ 08837-3679

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