U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2004-3120

October 2004

Arsenic, Nitrate, and Chloride in Groundwater, Oakland 

County, Michigan

INTRODUCTION

In 1996, the U.S. Geological Survey (USGS), in cooperation with the Michigan Department of Environmental Quality 

(MDEQ) and nine southeast Michigan counties, began a study 

of the factors controlling arsenic concentrations in drinking 

water. The early results of this study raised broader concerns 

in Oakland County about the quality of groundwater in general 

and drinking water in particular. In response to these concerns, 

Oakland County worked with the USGS and the Center for 

Applied Environmental Research at the University of Michigan 

– Flint (CAER) to study distributions of arsenic, nitrate, and 

chloride in groundwater, with emphasis on sites where concentrations of these constituents exceeded the Maximum Contaminant Levels (MCL’s) and Secondary Maximum Contaminant 

Levels (SMCL’s) set by the U.S. Environmental Protection 

Agency (USEPA). The maps produced for this report are based 

on historical data compiled from MDEQ records. This fact 

sheet is a revision of USGS Fact Sheet 135-98, “Arsenic, nitrate, 

and chloride in groundwater in Oakland County in Oakland 

County, Michigan” (Aichele and others, 1998) to incorporate 

revisions to the USEPA MCL for arsenic. 

ARSENIC 

Arsenic is a naturally occurring element in the environment. 

Historically, arsenic compounds have been used in the manufacture of pesticides, metal products, pigments and dyes, and medicine. Concentrations of arsenic in water have been expressed 

in a variety of ways: milligrams per liter (mg/L), micrograms 

per liter (μg/L) or parts per billion (ppb). These units can be 

compared as follows: 1 mg/L = 1,000 μg/L = 1,000 ppb. The 

U.S. Environmental Protection Agency (2001) MCL for arsenic 

in drinking water is 0.01 mg/L, 10 μg/L or 10 ppb. This new 

standard was announced in January 2001 with full compliance 

required for all public water systems by January 2006. One part 

per billion, expressed as time, is 1 second in approximately 31 

and one half years.

What is the source of arsenic in groundwater?

Arsenic is the twentieth most common element in the Earth’s 

crust, and is present in the soil and rocks of Oakland County. 

Arsenic in groundwater is usually a result of arsenic minerals 

dissolving naturally over time.

Where does arsenic occur in Oakland County?

Low concentrations of arsenic can be found in wells all 

across Oakland County. Out of 1,988 wells sampled, 663 

Figure 1. A new well being drilled in Oakland County. Over 

3,000 new drinking-water wells are drilled each year in 

Oakland County.

(about 33 percent) exceeded the new MCL of 10 μg/L. These 

wells are distributed throughout the county, except in Royal 

Oak survey township (fig.2). Relatively little sampling has been 

done anywhere in the southeast part of the county. The highest 

arsenic concentration detected in Oakland County is 221 μg/L 

(221 ppb).

Arsenic is present in all of the major aquifers of Oakland 

County, including the glacial drift, the Marshall Sandstone, and 

the Coldwater Shale, although it does not appear to occur in 

very shallow (<40 ft) wells completed in glacial drift. Testing 

conducted as part of this study found arsenic concentrations in 

groundwater comparable to historical Oakland County Health 

Division (OCHD) and MDEQ measurements.

Is arsenic in drinking water a concern?

The health effects of arsenic depend on the amount consumed (measured in μg). This amount is determined by the 

concentration of arsenic in the water (measured in μg/L) and the 

quantity of water consumed (measured in L). A person who 
Important Points About 

Arsenic

•Arsenic occurs naturally 

in groundwater as arsenic 

minerals dissolve over 

time.

•Arsenic concentrations 

in 663 out of 1,988 wells 

sampled in Oakland 

County exceeded the new 

MCL of 0.01 mg/L (10 

μg/L or 10 ppb).

•Low concentrations of 

arsenic are present in all 

the major aquifers of Oakland County.

•Immediate reactions to 

arsenic in drinking water 

are unlikely at the concentrations found in Oakland 

County.

•The health effects of arsenic depend on the concentration of arsenic in drinking waterand the amount 

of water consumed.

•OCHD recommends that 

all private well owners test 

regularly for arsenic.

times higher than the highest concentration observed in Oakland 

County, and 150 times greater than the new MCL.

Arsenic is classified as a carcinogen by the USEPA. USEPA 

MCLs are set at 10 percent of the exposure dose that produces 

a response in 1 percent of the population (National Research 

Council,2000). For example, the National Research Council 

(2001) estimates that consumption of 1L per day of water with 

an arsenic concentration of 10 μg/L over a lifetime will produce 

bladder cancer in 12 out of 10,000 women and 23 out of 10,000 

men. 

NITRATE

Nitrate is formed when oxygen in air or dissolved in water 

combines with nitrogen. The U.S. Environmental Protection 

Agency (1996a) has set a MCL of 10 mg-N/L (milligrams of 

nitrogen per liter) for nitrate in drinking water. Nitrate can 

occur naturally in precipitation, at concentrations typically 

Figure 2. Oakland County arsenic distribution. 

drinks 1L (approximately four 8-ounce glasses) of water per 

day containing arsenic at a concentration of 10 μg/L, which 

is the MCL, would consume about 10 μg of arsenic per day 

from drinking water. Certain foods, such as shellfish, can also 

contain significant levels of arsenic. Individuals consuming 

up to 500 μg of arsenic per day from all sources are unlikely 

to experience any symptoms of illness, but may increase their 

long-term risk of cancer (Kosnett, 1997). Persons consuming 

more than 500 μg of arsenic per day from all sources for several 

years may exhibit certain chronic health problems, including a 

characteristic pattern of skin abnormalities, a tingling sensation 

in the extremities, or anemia (Kosnett, 1997). 

Persons regularly consuming more than 1,500 μg of arsenic 

per day may experience fatigue, gastrointestinal disturbances, 

and certain types of heart problems (Kosnett, 1997). To have 

this highest level of exposure from drinking water alone, a 

person would have to regularly consume 1L of water containing 

1500 μg/L of arsenic. This concentration is more than six 

42°30’

42°50’

83°30’

83°10’

NOVI

R OS E

TROY

LYON

AVON

OR ION

HOLLY

OXFOR D ADDIS ON

MILFOR D

OAKLAND

BR ANDON

HIGHLAND WHITE LAKE

GR OVE LAND

S PR INGF IE LD

WATE R FOR D

FAR MINGTON

COMME R CE

INDE P ENDE NCE

PONTIAC

S OUTHF IE LD

WE S T BLOOMF IE LD

BLOOMF IE LD

Wixom

Auburn Hills

R OYAL OAK

Holly

Oak P ark Fern

dale

Madison

Heights

Birmingham

R ochester

Milford

Berkley

F ranklin

Bloomfield

Hills

S outh

Lyon

Orchard

Lake

Clawson

Hazel

P ark

F armington

Oxford

Lake Angelus

Leonard

Lake Orion

Wolverine

Lake

Northville

Ortonville

Lathrup

Village

Beverly

Hills

Huntington

Woods

S ylvan

Lake

Clarkston

Keego

Harbor

Novi

P l R idge

R ochester Hills

Section where sample of arsenic was unavailable

Section where sample of arsenic had a concentration

greater than or equal to 0.01 mg/L

Section where sample of arsenic had a concentration

less than 0.01 mg/L

Oakland County, MI

E XP LANATION

0 1 6 MILE S

0 1 6 KILOME TE R S

SOURCES: Michigan Department of Environmental Quality 1983-1997 (excluding 1991-92)

Michigan Department of Natural Resources, Michigan Resource Information System (MIRIS), 1978
around 1 mg-N/L. Elevated concentrations, particularly those 

greater than 3 mg-N/L, are usually the result of human activity 

(U.S. Environmental Protection Agency, 1996b).

What is the source of nitrate in groundwater?

In Oakland County, the likely sources of elevated nitrate in 

drinking water are septic tank leachate and fertilizers, both of 

which are rich in nitrogen. Nitrate dissolves readily in water 

and moves freely with water through soil and rock where it is 

commonly consumed by microorganisms. Nitrate at concentrations greater than 3 mg-N/L indicates a fairly direct connection 

between one of the sources listed above and the drinking-water 

supply.

Where does nitrate occur in Oakland County?

Nitrate concentrations in excess of the 10 mg-N/L MCL 

have been found in groundwater in most townships in Oakland 

County (fig.3). The exceedances often occur in high-density 

lakeshore communities using on-lot wells and septic systems. 

As an example, wells within 1 mile of Union Lake, Wolverine 

Lake, or Commerce Lake account for more than one third of all 

nitrate concentrations greater than 10 mg-N/L.

Is nitrate in drinking water a concern?

A nitrate standard is required by the USEPA because nitrite, 

which is produced when nitrate is broken down in the body, 

can impair the ability of the blood to carry oxygen. Elevated 

nitrate is not typically a concern for healthy people, but fetuses, 

infants (under 6 months), and the elderly are populations at risk. 

“Blue-baby syndrome” (infantile methemoglobinemia) resulting from high nitrate consumption can be lethal in infants (U.S. 

Environmental Protection Agency, 1996b).

42°30’

42°50’

83°30’

83°10’

NOVI

R OS E

TROY

LYON

AVON

OR ION

HOLLY

OXFOR D ADDIS ON

MILFOR D

OAKLAND

BR ANDON

HIGHLAND WHITE LAKE

GR OVE LAND

S PR INGF IE LD

WATE R FOR D

FAR MINGTON

COMME R CE

INDE P ENDE NCE

PONTIAC

S OUTHF IE LD

WE S T BLOOMF IE LD

BLOOMF IE LD

Wixom

Auburn Hills

R OYAL OAK

Holly

Oak P ark Fern

dale

Madison

Heights

Birmingham

R ochester

Milford

Berkley

F ranklin

Bloomfield

Hills

S outh

Lyon

Orchard

Lake

Clawson

Hazel

P ark

F armington

Oxford

Lake Angelus

Leonard

Lake Orion

Wolverine

Lake

Northville

Ortonville

Lathrup

Village

Beverly

Hills

Huntington

Woods

S ylvan

Lake

Clarkston

Keego

Harbor

Novi

P l R idge

R ochester Hills

Section where sample of nitrate was unavailable

Section where sample of nitrate had a concentration

greater than or equal to 10.0 mg-N/L

Section where sample of nitrate had a concentration

less than 10.0 mg-N/L

Oakland County, MI

E XP LANATION

0 1 6 MILE S

0 1 6 KILOME TE R S

SOURCES: Michigan Department of Environmental Quality 1983-1997 (excluding 1991-92)

Michigan Department of Natural Resources, Michigan Resource Information System (MIRIS), 1978

Important Points about 

Nitrate

·Nitrate concentrations that 

exceed the 10 mg-N/L MCL 

have been found in most 

Oakland County townships.

·In Oakland County, the 

likely sources of elevated 

nitrate concentrations in 

groundwater are septic tank 

leachate and fertilizers.

·Nitrate concentrations 

in 84 out of 6,197 wells 

sampled in Oakland County 

exceeded the MCL of 10 

mg-N/L.

·Nitrate concentrations of 

10 mg-N/L or greater are 

often found in high-density 

lakeshore communities.

·Elevated concentrations 

are not typically a problem for healthy people, but 

they pose a risk to fetuses, 

infants and the elderly.

·OCHD recommends that 

all private well owners test 

regularly for nitrate.

Figure 3. Oakland County nitrate distribution. 
CHLORIDE

Chloride is an element found in most common salts, such 

as road salt, table salt, and water-softener salt. The USEPA 

(1996a) has set a SMCL of 250 mg/L for chloride. This limit 

was established because water with chloride concentrations 

greater than 250 mg/L tastes salty to most people. Chloride is 

naturally present in groundwater, particularly in deep bedrock 

aquifers. Background concentrations of chloride in drinkingwater wells are highly variable, ranging from less than 

10 mg/L in some glacial aquifers to more than 500 mg/L in 

deeper bedrock wells.

What is the source of chloride in drinking water?

Chloride occurs naturally in some sedimentary bedrock layers, particularly shales. This chloride is a remnant of the seawater present at the time the rocks were formed. Chloride is 

very soluble in water and moves freely with water through soil 

and rock. Chloride is not readily consumed by

microorganisms, so it is more persistent than nitrate. Road 

salt, applied for deicing or dust control, may infiltrate the soil 

and contaminate groundwater. Chloride also may originate 

from septic tank leachate, as a result of water softening or other 

activities in the household.

Where does chloride occur in Oakland County?

Chloride concentrations in excess of 250 mg/L are widely 

distributed in Oakland County (fig.4). They are commonly 

associated with high-density lakeshore communities using 

on-lot wells and septic systems. The chloride found in shallow 

wells around lakes and in densely populated areas probably 

comes from salts used in the household and then discharged 

through septic systems. Elevated chloride levels also may be 

42°30’

42°50’

83°30’

83°10’

NOVI

R OS E

TROY

LYON

AVON

OR ION

HOLLY

OXFOR D ADDIS ON

MILFOR D

OAKLAND

BR ANDON

HIGHLAND WHITE LAKE

GR OVE LAND

S PR INGF IE LD

WATE R FOR D

FAR MINGTON

COMME R CE

INDE P ENDE NCE

PONTIAC

S OUTHF IE LD

WE S T BLOOMF IE LD

BLOOMF IE LD

Wixom

Auburn Hills

R OYAL OAK

Holly

Oak P ark Fern

dale

Madison

Heights

Birmingham

R ochester

Milford

Berkley

F ranklin

Bloomfield

Hills

S outh

Lyon

Orchard

Lake

Clawson

Hazel

P ark

F armington

Oxford

Lake Angelus

Leonard

Lake Orion

Wolverine

Lake

Northville

Ortonville

Lathrup

Village

Beverly

Hills

Huntington

Woods

S ylvan

Lake

Clarkston

Keego

Harbor

Novi

P l R idge

R ochester Hills

Section where sample of chloride was unavailable

Section where sample of chloride had a concentration

greater than or equal to 250.0 mg/L

Section where sample of chloride had a concentration

less than 250.0 mg/L

Oakland County, MI

E XP LANATION

0 1 6 MILE S

0 1 6 KILOME TE R S

SOURCES: Michigan Department of Environmental Quality 1983-1997 (excluding 1991-92)

Michigan Department of Natural Resources, Michigan Resource Information System (MIRIS), 1978

Important Points about 

Chloride

·Chloride occurs naturally 

in groundwater and is also 

in most common salts.

·Chloride concentrations 

in 366 out of 6,227 wells 

sampled in Oakland County 

exceeded the SMCL of 250 

mg/L.

·High concentrations are 

often associated with high 

density, lakeshore commu- nities.

·Elevated chloride concentrations may be associated 

with road salt applications 

or storage and septic tank 

leachate.

·Chloride is not generally 

considered to be a health 

concern.

·The SMCL of 250 mg/L is 

based on aesthetic considerations (taste) rather than 

health considerations.

Figure 4. Oakland County chloride distribution. 
associated with road salt application or storage. Several 

caseshave been documented in Oakland County of road salt 

storage facilities leaching chloride into groundwater.

Is chloride in drinking water a concern?

Chloride is not generally considered to be a health concern. 

However, some evidence indicates that high chloride intake 

may pose a hazard to persons with heart or kidney disease 

(Hutchinson, 1970).

HOW WERE THE MAPS PREPARED?

The MDEQ maintains a database of results from waterquality tests conducted for homeowners and businesses. In 

this study, maps were prepared from records of tests in Oakland County where arsenic, nitrate and chloride measurements 

were made. The data spanned the years from 1988 through 

1997 (excluding 1991-92).

Addresses for each of the records were matched to an Oakland County road file through an address-matching procedure 

in a Geographic Information System. The address-matching 

percentage for each constituent indicates how many records 

were finally mapped (fig. 5). All sites with arsenic concentrations greater than the MCL were mapped. For nitrate and 

chloride, the success rates were 94 percent and 81 percent, 

respectively. Data coverage for all the chemical constituents was extensive, although the distribution of test results 

for arsenic was least concentrated in the southeastern part of 

the county, where pulic-water supply is most available. The 

number of nitrate and chloride test results is much greater, and 

their locations more evenly distributed throughout the county 

(fig. 5).

To prepare the final maps, locations of wells with a measurement for each of the chemical constituents were plotted on 

an Oakland County base map. When wells had more than one 

measurement, the highest measured concentration was used. 

Sections containing a well with a measured concentration 

higher than the (S)MCL were identified with dark shading. 

Sections containing a well with a measured concentration 

less than the (S)MCL were identified with a lighter shading. 

Sections were left unshaded if no sample information was 

available.

HOW WERE THE SAMPLE DATA COLLECTED AND 

EVALUATED?

The USGS collected water samples across Oakland County 

at selected sites with previous analyses for arsenic, nitrate, 

or chloride. Each new sample was collected using clean 

sampling techniques to minimize the risk of contamination. 

Duplicate samples were sent to both the MDEQ lab and the 

USGS National Water Quality Lab (NWQL) in Denver, Colorado. Results from the USGS NWQL were compared against 

the results from MDEQ and against results from the previous 

samples. The results from the MDEQ lab agreed very closely 

with the results from the USGS NWQL. This sample testing 

confirmed the validity of using the MDEQ database in maps 

prepared for the study.

Figure 5. Locations of well records used to prepare the county 

maps.

Total arsenic samples: 3,509 Total unique samples: 2,373

Unique locations mapped: 1,988 (84%)

Total nitrate samples: 12,942 Total unique samples: 7,814

Unique locations mapped: 6,197 (79%)

Total arsenic samples: 12,960 Total unique samples: 7,809

Unique locations mapped: 6,227 (80%)
What does all this mean?

·The maps refer to all areas not serviced by 

public water. Public water supply wells are 

routinely monitored to ensure compliance 

with U.S. Environmental Protection Agency 

standards.

·Arsenic, nitrate, and chloride were the only 

constituents addressed in this study. Other 

potential water contaminants have been 

observed in Oakland county and may occur in 

your area. Please contact the OCHD for details 

specific to your area.

·All the maps support the OCHD 

recommendation that homeowners regularly test 

drinking water that comes from wells.

·Each of the chemical constituents evaluated 

is widely distributed and homeowners should 

carefully review the county maps to see if their 

homes are in areas where concentrations above 

the U.S. Environmental Protection Agency 

standard have been reported. Township maps, 

which are included on separate fact sheets, 

provide more specific details on particular 

locations.

·Some individuals may be more sensitive than 

others to contaminants in drinking water. If 

symptoms persist, OCHD recommends you 

consult a physician for assistance.

SUMMARY

The distributions of arsenic, nitrate and chloride concentrations in groundwater in Oakland County, Michigan were investigated on the basis of historical water-quality data obtained from 

MDEQ databases. Results indicate that of the wells sampled 

in Oakland County, about 33 percent exceeded the USEPA 

MCL for arsenic, about 1 percent exceeded the USEPA MCL 

for nitrate, and about 6 percent exceeded the USEPA SMCL for 

chloride. Maps provided in this report show the known distribution of arsenic, nitrate, and chloride in groundwater within 

Oakland County, Michigan.

−−By Steve Aichele (USGS)

References

Aichele, S.S., Hill-Rowley, R., and Malone, M., 1998, Arsenic, Nitrate, and Chloride in Groundwater in Oakland County, 

Michigan: U.S. Geological Survey Fact Sheet 135-98, 6 p.

Hutchinson, F.E., 1970, Environmental pollution from highway 

deicing compounds. Journal of Soil and Water Conservation, v. 

25, n.4, p. 144-46.

Kosnett, M., 1997, Clinical Guidance in the Evaluation of 

patients with Potential Exposure to Arsenic in Drinking Water. 

Michigan Department of Community Health, Lansing, Michigan, 10 p. 

Environmental Protection Agency, 2001, National Primary 

Drinking Water Regulations; Arsenic and Clarifications to 

compliance and New Source Contaminants Monitoring. Federal 

Register, v. 66, n. 14, p. 6976-7066.

National Research Council, 2000, Arsenic in Drinking Water. 

National Academy Press, Washington D.C., 308 p.

National Research Council, 2001, Arsenic in Drinking Water, 

2001 Update. National Academy Press, Washington D.C.,

223 p.

Pontius, F., Brown, K., and Chen, C., 1994, Of arsenic in drinking water. Journal of the AWWA, v. 86, n. 10, p. 52-63.

U.S. Environmental Protection Agency, 1996a, Drinking water 

regulations and health advisories. Washington, D.C., U.S. Environmental Protection Agency, Office of Water, 

EPA 822-B-96-002.

For more information, contact:

U.S. Geological Survey,

Water Resources Division

6520 Mercantile Way, Ste.5

Lansing, MI 48911

(517) 887-8903

http://www.usgs.gov

Oakland County Health Division

1200 N. Telegraph Road

Pontiac, MI 48341-0432

(248) 858-1312

http://www.co.oakland.mi.us