United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/SR-94/051 May 1994
EPA Project Summary
Potential Groundwater
Contamination from Intentional
and Nonintentional Stormwater
Infiltration
Robert Pitt, Shirley Clark, and Keith Farmer
The research summarized here was
conducted during the first year of a 3-
yr cooperative agreement to identify
and control stormwater toxicants, es-
pecially those adversely affecting
groundwater. The purpose of this re-
search effort was to review the ground-
water contamination literature as it
relates to stormwater. Potential prob-
lem pollutants were identified, based
on their mobility through the unsatur-
ated soil zone above groundwater, their
abundance in stormwater, and their
treatability before discharge. This in-
formation was used with earlier EPA
research results to identify the pos-
sible sources of these potential prob-
lem pollutants. Recommendations were
also made for stormwater infiltration
guidelines in different areas and moni-
toring 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 grass-
lands and woods. This infiltrating water
was relatively uncontaminated. Urbaniza-
tion, 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 addi-
tion, the waters available for recharge gen-
erally carried increased quantities of
pollutants. With urbanization, waters hav-
ing elevated contaminant concentrations
also recharge groundwater, including ef-
fluent from domestic septic tanks, waste-
water from percolation basins and
industrial waste injection wells, infiltrating
stormwater, and infiltrating water from ag-
ricultural irrigation. This report addresses
potential groundwater problems associated
with stormwater toxicants and describes
how conventional stormwater control prac-
tices 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 sum-
marize 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 ob-
served organics.
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Table 1. Concentrations of Heavy Metals in Observed Areas
Toxicant
Highest Median
Highest Observed
Cadmium
Chromium
Copper
Lead
Nickel
Zinc
Vehicle service area runoff
Landscaped area runoff
Urban receiving water
CSO
Parking area runoff
Roof runoff
8
100
160
75
40
100
Street runoff
Roof runoff
Street runoff
Storage area runoff
Landscaped area runoff
Roof runoff
220
510
1250
330
130
1580
Table 2. Maximum Concentrations of Toxic Organics from Observed Sources
Toxicant
Benzo (a) anthracene
Benzo (b) fluoranthene
Benzo (k) fluoranthene
Benzo (a) pyrene
Fluorantnene
Naphthalene
Phenanthrene
Pyrene
Chlordane
Butyl benzyl phthalate
Bis (2-chloroethyl) ether
Bis (2-chloroisopropyl) ether
1 ,3-Dichlorobenzene
Maximum,
W/L
60
226
221
300
128
296
69
102
2.2
128
204
217
120
Detection
Frequency, %
12
17
17
17
23
13
10
19
13
12
14
14
23
Significant Sources
Gasoline, wood preservative
Gasoline, motor oils
Gasoline, bitumen, oils
Asphalt, gasoline, oils
Oils, gasoline, wood preservative
Coal tar, gasoline, insecticides
Oils, gasoline, coal tar
Oils, gasoline, bitumen, coal tar,
wood preservative
Insecticide
Plasticizer
Fumigant, solvents, insecticides,
paints, lacquers, varnishes
Pesticide manufacturing
Pesticide manufacturing
Stormwater Constituents
Having High Potential to
Contaminate Groundwater
Nutrients
Nitrates are one of the most frequently
encountered contaminants in groundwa-
ter. Phosphorus contamination of ground-
water 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 groundwa-
ter, especially in rapid-infiltration waste-
water 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 leach-
ing of nitrates occurs during the cool, wet
seasons. Cool temperatures reduce deni-
trification and ammonia volatilization and
limit microbial nitrogen immobilization and
plant uptake. The use of slow-release fer-
tilizers (including composted organic
mulches, urea formaldehyde (UF), meth-
ylene urea, isobutylidene diurea (IBDU),
and sulfur-coated urea) is recommended
in areas having potential groundwater ni-
trate problems.
Residual concentrations of nitrate in soil
vary greatly and depend on the soil tex-
ture, 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 groundwa-
ter.
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 ground-
water, such as 2,4-D, acenaphthylene,
alachlor, atrazine, cyanazine, dacthal,
diazinon, dicamba, and malathion) on irri-
gated and sandy soils will likely contami-
nate 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 per-
sistence, 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 trans-
mit pesticides rapidly when the voids are
filled with water and the adsorbing sur-
faces of the soil matrix are bypassed. In
general, pesticides with low water solubili-
ties, high octanol-water partitioning coeffi-
cients, 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 con-
cern. These include the fungicides
iprodione and triadimefon, the insecticides
isofenphos and chlorpyrifos, and the her-
bicide glyphosate.
Pesticides decompose in soil and wa-
ter, but the total decomposition time can
range from days to years. Literature half-
lives for pesticides generally apply to sur-
face 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 leach-
ing. An order-of-magnitude difference in
half-life results in a five- to ten-fold differ-
ence in percolation loss. Organophosphate
pesticides are less persistent than orga-
nochlorine 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 com-
pounds. 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 organ-
ics, like that from other pollutants, occurs
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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 volatiliza-
tion, sorption, and degradation. Volatiliza-
tion can significantly reduce the
concentrations of the most volatile com-
pounds 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 or-
ganic 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 microorgan-
isms, at least partially, but others cannot.
Temperature, pH, moisture content, ion
exchange capacity of the soil, and air avail-
ability may limit the microbial degradation
potential for even the most degradable
organic compound.
Microorganisms
Viruses have been detected in ground-
water 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 disin-
fectants 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 microf-
lora, moisture content, temperature, sun-
light, 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 occlu-
sion and protection within colloid-size par-
ticles. Viral adsorption is promoted by
increasing cation concentration, decreas-
ing 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 mecha-
nisms in soil are straining at the soil sur-
face and at intergrain contacts,
sedimentation, sorption by soil particles,
and inactivation. Because their size is
larger than viruses, most bacteria are re-
tained 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 alumi-
num, arsenic, cadmium, chromium, cop-
per, iron, lead, mercury, nickel, and zinc.
The majority of these metals (with the
common exception of zinc) are, however,
mostly associated with the particulate frac-
tions and can be mostly removed by ei-
ther 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 com-
mercial site, however, nickel, chromium,
and zinc concentrations have exceeded
regulatory limits in the soils below a re-
charge area. Allowing percolation ponds
to go dry between storms can be counter-
productive 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 run-
off water and groundwater have shown
that there is significant downward move-
ment of copper and iron in sandy and
loamy soils. Arsenic, nickel, and lead, how-
ever, 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 > man-
ganese > 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 ground-
water 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 con-
taminate 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 ground-
water.
Studies of depth of pollutant penetra-
tion in soil have shown that sulfate and
potassium concentrations decrease with
depth, whereas sodium, calcium, bicar-
bonate, and chloride concentrations in-
crease 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 frac-
tion 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 con-
trol with the use of conventional stormwater
control practices which mostly rely on sedi-
mentation and filtration principles. Luckily,
most of the toxic organics and metals are
associated with the nonfilterable (sus-
pended solids) fraction of the wastewa-
ters 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 frac-
tions and would not be as readily con-
trolled with the use of sedimentation.
Sedimentation is the most common fate
and control mechanism for particulate-re-
lated pollutants. This would be common
for most stormwater pollutants, as noted
above. Particulate removal can occur in
many conventional stormwater control pro-
cesses, 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 en-
courages more efficient bonding of the
pollutants in soils to prevent their leaching
-------
Table 3. Reported Filterable Fractions of Stormwater Toxicants from Source Areas
Constituent
Cadmium
Chromium
Copper
Iron
Lead
Nickel
Zinc
Benzo (a) anthracene
Fluoranthene
Naphthalene
Phenanthrene
Pyrene
Chlordane
Butyl benzyl phthalate
Bis (2-chloroethyl) ether
Bis (2-chlrorisopropyl) ether
1 ,3-Dichlorobenzene
Filterable Fraction (%)
20 to 50
<10
<20
Small amount
<20
Small amount
>50
None found in
65
25
None found in
95
None found in
Irregular
Irregular
None found in
75
filtered fraction
filtered fraction
filtered fraction
filtered fraction
to groundwaters. Detention ponds are
probably the most common management
practice for the control of stormwater run-
off. 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 sedi-
mentation devices. Adequate cleaning can
help reduce the total solids and lead ur-
ban runoff yields by between 10% and
25%, and COD, total Kjeldahl nitrogen,
total phosphorus, and zinc by between
5% and 10%. Other important fate mecha-
nisms available in wet detention ponds,
but which are probably not important in
small enclosed sump devices such as
catchbasins, include volatilization and pho-
tolysis. Biodegradation, biotransformation,
and bioaccumulation (into plants and ani-
mals) may also occur in larger and open
ponds.
Upland infiltration devices (such as infil-
tration trenches, porous pavements, per-
colation 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 sew-
ers, 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 con-
ditions that can make stormwater infiltra-
tion inappropriate include steep slopes,
slowly percolating soils, shallow ground-
water, and nearby groundwater uses.
Grass filter strips may be quite effective in
removing particulate pollutants from over-
land 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 pos-
sibly 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 (cop-
per, lead, zinc, and cadmium) concentra-
tions can be reduced by at least 50%,
COD, nitrate nitrogen, and ammonia nitro-
gen concentrations can be reduced by
about 25%, but only inconsistent concen-
tration reductions can be expected for or-
ganic nitrogen, phosphorus, and bacteria.
Sorption of pollutants to soils is prob-
ably 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. Incorpo-
ration of the pollutants onto soil with sub-
sequent biodegradation and minimal
leaching to the groundwater is desired.
Volatilization, photolysis, biotransformation,
and bioconcentration may also be signifi-
cant 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 groundwa-
ter quality. Much of the information will
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 con-
sider the risk associated with using ground-
water 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 conven-
tional stormwater sedimentation controls)
in the stormwater. The contamination po-
tential is the lowest rating of the influenc-
ing 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 groundwa-
ter 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 pre-
treatment 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 mo-
bility, 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/injec-
tion is used instead of surface percola-
tion, the compounds would most likely be
more mobile, making the abundance cri-
teria the most important, with some re-
gard given to the filterable fraction
information for operational considerations.
This table is only appropriate for initial
estimates of contamination potential be-
cause of the simplifying assumptions
made, such as the worst case mobility
conditions assumed (for sandy soils hav-
ing 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 frac-
tion information is generally applicable for
warm weather stormwater runoff in resi-
dential and commercial areas. The pollut-
ant concentrations and detection
-------
frequencies, however, would be greater
for critical source areas (especially ve-
hicle service areas) and critical land uses
(especially manufacturing industrial areas).
The stormwater pollutants of most con-
cern (those that may have the greatest
adverse impacts on groundwaters) include:
• Nutrients: nitrate has a low to moder-
ate potential for contaminating ground-
water when both surface percolation
and subsurface infiltration/injection are
used because of its relatively low con-
centrations in most stormwaters.
When the stormwater nitrate concen-
tration is high, then the groundwater
contamination potential would likely
also be high.
• Pesticides: lindane and chlordane
have moderate potentials for contami-
nating groundwater when surface per-
colation (with no pretreatment) or
when subsurface injection (with mini-
mal pretreatment) are used. The
groundwater contamination potentials
for both of these compounds would
very likely be substantially reduced
with adequate sedimentation pretreat-
ment.
• Other organics: 1,3-dichlorobenzene
may have a high potential for con-
taminating groundwater when subsur-
face infiltration/injection (with minimal
pretreatment) is used. It would, how-
ever, probably have a lower ground-
water contamination potential for most
surface percolation practices because
of its relatively strong sorption to va-
dose zone soils. Both pyrene and
fluoranthene would also very likely
have high groundwater contamination
potentials for subsurface infiltration/
injection practices, but lower contami-
nation potentials for surface percola-
tion practices because of their more
limited mobility through the unsatur-
ated zone (vadose zone). Others (in-
cluding benzo(a)anthracene, bis
(2-ethylhexyl) phthalate, pentachlo-
rophenol, and phenanthrene) may
also have moderate groundwater con-
tamination potentials when surface
percolation with no pretreatment, or
subsurface injection/infiltration, is
used. These compounds would have
low groundwater contamination poten-
tials when surface infiltration is used
with sedimentation pretreatment.
VOCs may also have high groundwa-
ter contamination potentials if present
in the stormwater (which is possible
for some industrial and commercial
facilities and vehicle service estab-
lishments).
• Pathogens: enteroviruses very likely
have high potentials for contaminat-
ing groundwater when any percola-
tion 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 vari-
ous protozoa, would also have high
groundwater contamination potentials
when subsurface infiltration/injection
practices are used without disinfec-
tion. When disinfection (especially by
chlorine or ozone) is used, then disin-
fection by-products (such as
trihalomethanes or ozonated bro-
mides) would have high groundwater
contamination potentials.
• Heavy Metals: nickel and zinc possi-
bly have high potentials for contami-
nating 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, infil-
tration, or percolation practices used.
Pesticides have been mostly found in
urban runoff from residential areas, espe-
cially in dry weather flows associated with
landscaping irrigation runoff. The other or-
ganics, 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 galva-
nized metal comes into contact with rain-
water. Salts are at their greatest
concentrations in snowmelt and early
spring runoff in northern areas.
The control of these compounds re-
quires 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. Com-
bined sewer overflows should also be di-
verted because of sewage contamination.
In areas of snow and ice control, winter
snowmelt and runoff and early spring run-
off should also be diverted from infiltration
devices.
All other runoff should include pretreat-
ment using sedimentation processes be-
fore 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 deten-
tion 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 fertiliz-
ers in critical areas. The use of special-
ized treatment devices, such as those
being developed and tested during this
research, can also play an important role
in treating runoff from critical source ar-
eas 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, ex-
cept in special cases, especially when the
potential of forming harmful treatment by-
products (such as THMs and soluble alu-
minum) 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 infil-
tration devices (such as dry wells, trenches
or seepage drains, and especially injec-
tion wells), unless the runoff water is known
to be relatively free of pollutants. Surface
devices are able to take greater advan-
tage of natural soil pollutant removal pro-
cesses. Unless all percolation devices are
carefully designed and maintained, how-
ever, they may not function properly and
may lead to premature hydraulic failure or
contamination of the groundwater.
Recommendations
With a reasonable degree of site-spe-
cific design considerations to compensate
for soil characteristics, infiltration may be
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Table 4. Potential of Stormwater Pollutants to Contaminate Groundwater
Compounds
Nutrients
Pesticides
Other
organics
Pathogens
Heavy
metals
Salts
nitrates
2,4-D
Y-BHC (lindane)
malathion
atrazine
chlordane
diazinon
VOCs
1 ,3-dichloro-
benzene
anthracene
benzo(a)
anthracene
bis (2-ethylhexyl)
phthalate
butyl benzyl
phthalate
fluoranthene
fluorene
naphthalene
pentachlorophenol
phenanthrene
pyrene
enteroviruses
Shigella
Pseudomonas
aeruginosa
protozoa
nickel
cadmium
chromium
lead
zinc
chloride
Mobility
(sandy/low
organic soils)
mobile
mobile
intermediate
mobile
mobile
intermediate
mobile
mobile
low
intermediate
intermediate
intermediate
low
intermediate
intermediate
low/inter.
intermediate
intermediate
intermediate
mobile
low/inter.
low/inter.
low/inter.
low
low
inter. /very low
very low
low/very low
mobile
Abundance
in Stormwater
low/moderate
low
moderate
low
low
moderate
low
low
high
low
moderate
moderate
low/moderate
high
low
low
moderate
moderate
high
likely present
likely present
very high
likely present
high
low
moderate
moderate
high
seasonally
high
Fraction
Filterable
high
likely low
likely low
likely low
likely low
very low
likely low
very high
high
moderate
very low
likely low
moderate
high
likely low
moderate
likely low
very low
high
high
moderate
moderate
moderate
low
moderate
very low
very low
high
high
Surface Infill, and
No Pretreatment
low/moderate
low
moderate
low
low
moderate
low
low
low
low
moderate
moderate
low
moderate
low
low
moderate
moderate
moderate
high
low/moderate
low/moderate
low/moderate
low
low
low/moderate
low
low
high
Contamination Potential
Surface Infill, with Sub-surface Inj. with
Sedimentation Minimal Pretreatment
low/moderate
low
low
low
low
low
low
low
low
low
low
low
low
moderate
low
low
low
low
moderate
high
low/moderate
low/moderate
low/moderate
low
low
low
low
low
high
low/moderate
low
moderate
low
low
moderate
low
low
high
low
moderate
moderate
low/moderate
high
low
low
moderate
moderate
high
high
high
high
high
high
low
moderate
moderate
high
high
very effective in controlling both urban run-
off quality and quantity problems. This
strategy encourages infiltration of urban
runoff to replace the natural infiltration ca-
pacity lost through urbanization and to
use the natural filtering and sorption ca-
pacity of soils to remove pollutants; how-
ever, the potential for some types of urban
runoff to contaminate groundwater through
infiltration requires some restrictions. Infil-
tration of urban runoff having potentially
high concentrations of pollutants that may
pollute groundwater requires adequate pre-
treatment or the diversion of these waters
away from infiltration devices. The follow-
ing general guidelines for the infiltration of
Stormwater and other storm drainage ef-
fluent 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 met-
als, pesticides, and pathogenic mi-
croorganisms.
Combined sewage overflows should
be diverted from infiltration devices
because of their poor water quality,
especially their high pathogenic mi-
croorganism concentrations and high
clogging potential.
Snowmelt runoff should be diverted
from infiltration devices because of
its potential for having high concen-
trations of soluble salts.
Runoff from manufacturing industrial
areas should be diverted from infiltra-
tion devices because of its potential
for having high concentrations of
soluble toxicants.
Construction site runoff must be di-
verted from Stormwater infiltration de-
vices (especially subsurface devices)
because of its high suspended solids
concentrations, which would quickly
clog infiltration devices.
Runoff from other critical source ar-
eas, such as vehicle service facilities
and large parking areas, should at
least receive adequate pretreatment
to eliminate their groundwater con-
tamination potential before infiltration.
Runoff from residential areas (the larg-
est component of urban runoff in most
cities) is generally the least polluted
urban runoff flow and should be con-
sidered for infiltration. Very little treat-
-------
ment of residential area stormwater
runoff should be needed before infil-
tration, 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 fil-
tration devices.
Recommended Stormwater
Quality Monitoring to Evaluate
Potential Groundwater
Contamination
Most past stormwater quality monitor-
ing efforts have not adequately evaluated
stormwater's potential for contaminating
groundwater. The following list shows the
stormwater contaminants that are recom-
mended for monitoring when stormwater
contamination potential needs to be con-
sidered, 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 ground-
water:
- Nutrients (especially nitrates)
- Salts (especially chloride)
- VOCs (if expected in the runoff,
such as runoff from manufacturing in-
dustrial or vehicle service areas, could
screen for VOCs with purgable or-
ganic carbon analyses)
- Pathogens (especially enteroviruses,
if possible, along with other patho-
gens 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 to-
tal sample components (especially lin-
dane and chlordane)
- Other organics, in both filterable and
total sample components (especially
1,3 dichlorobenzene, pyrene,
fluoranthene, benzo(a)anthracene, bis
(2-ethylhexyl) phthalate, pentachlo-
rophenol, and phenanthrene)
- Heavy metals, in both filterable and
total sample components (especially
chromium, lead, nickel, and zinc)
Urban runoff compounds with the po-
tential 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)
-------
Robert Pitt, Shirley Clark and Keith Farmer 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, VA22161
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
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
Penalty for Private Use
$300
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
EPA/600/SR-94/051
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