0 2 4 6 8 10 12 14 16 18 20 22

trans-1,2-Dichloroethene (trans-1,2-DCE)

Methylene chloride

Dibromochloromethane

Bromoform

1,1-Dichloroethane (1,1-DCA)

Trichlorofluoromethane (CFC-11)

Bromodichloromethane

Chloromethane

1,1,1-Trichloroethane (TCA)

Dichlorodifluoromethane (CFC-12)

Toluene

Trichloroethene (TCE)

Methyl tert-butyl ether (MTBE)

Perchloroethene (PCE)

Chloroform

DETECTION FREQUENCY, IN PERCENT OF SAMPLES

Trihalomethanes (THMs)

Gasoline hydrocarbons

Gasoline oxygenates

Refrigerants

Solvents

Assessment level of 0.02 microgram per liter

Assessment level of 0.2 microgram per liter

VOC 

Groups

0 5 10 15 20 25

DETECTION FREQUENCY, IN PERCENT OF SAMPLES

Organic synthesis compounds

Fumigants

Refrigerants

Gasoline oxygenates

Solvents

Trihalomethanes (THMs)

Gasoline hydrocarbons

Assessment level of 0.2 microgram

 per liter (colored bars)

Assessment level of 0.02 microgram

 per liter

Compound name VOC group

Acrolein Organic synthesis compound

Acrylonitrile Organic synthesis compound

Hexachlorobutadiene Organic synthesis compound

1,2,3-Trichlorobenzene Organic synthesis compound

Vinyl bromide Organic synthesis compound

1,3-Dichlorobenzene Solvent

Hexachloroethane Solvent

1,2,4-Trichlorobenzene Solvent

1,1,2-Trichloroethane Solvent

cis-Dichloropropene Fumigant

trans-Dichloropropene Fumigant

Styrene Gasoline hydrocarbon

Ethyl tert-butyl ether Gasoline oxygenate

Compound name VOC group

1,2,4-Trimethylbenzene

Toluene

Chloromethane

Methylene chloride

Solvent

Chloroform

Solvent

Solvent

1,1,1-Trichloroethane

Bromodichloromethane Trihalomethane

Methyl tert-butyl ether 

Gasoline hydrocarbon

Gasoline hydrocarbon

Gasoline oxygenate

Solvent

Solvent

Trichloroethene

Perchloroethene

Trihalomethane

Compounds included in analysis Factors most commonly associated with

VOCs in aquifers 

Septic systems

Source factors

Transport factors

Fate factor

Indeterminate factor

•

• Urban land

Resource Conservation and Recovery Act

(RCRA) hazardous-waste facilities

•

• Gasoline storage and release sites

• Climatic conditions

• Hydric (anoxic) soils

• Dissolved oxygen in ground water

• Type of well

• Depth to top of well screen

Alaska

Hawaii

Oahu

EXPLANATION

No detection or less than 0.02

Total solvent concentration,

 in micrograms per liter

0.02 to less than 0.2

0.2 or greater

Alaska

Hawaii

Oahu

EXPLANATION

Total fumigant concentration, 

 in micrograms per liter

0.02 to less than 0.2

0.2 or greater

No detection or less than 0.02

Confined Aquifer Confined Aquifer

Confining Unit Confining Unit

Detection frequency at an assessment level of

 0.02 microgram per liter, in percent

Chloroform Ground-water flow

Methylene chloride

Chloromethane

Young Oxic Water

0

10

20

30

0

10

20

30

Old Anoxic Water

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Map 2931

The Occurrence of Volatile Organic Compounds in Aquifers of the United States

By Wayne W. Lapham, Janet M. Carter, John S. Zogorski, and Joshua F. Valder

Study basics

The U.S. Geological Survey’s National Water-Quality Assessment (NAWQA) 

Program recently completed a national assessment of volatile organic compounds 

(VOCs) in ground water (Zogorski and others, 2006). As part of this assessment, 

samples of ambient ground water collected from 3,498 wells during 1985–2002 

were selected for characterizing the occurrence of 55 VOCs in 98 aquifer studies. 

The 55 VOCs were assigned to the following groups on the basis of their primary 

usage (or origin): (1) fumigants, (2) gasoline hydrocarbons, (3) gasoline oxygenates, (4) organic synthesis compounds, (5) refrigerants, (6) solvents, and (7) trihalomethanes (chlorination by-products).

The samples were collected throughout the conterminous United States as well as 

Alaska and Hawaii. The sampled wells had a variety of uses including domestic supply 

(61 percent), public supply (15 percent), monitoring (10 percent), other (13 percent), and 

unknown (1 percent).

NAWQA aquifer studies are large-scale resource assessments of ground water that provide a general characterization of water-quality conditions in locally and regionally 

important aquifers or portions thereof. In general, the aquifers (or portions thereof) 

selected for study were some of the most intensively used aquifers for drinking water in 

Occurrence

Solvents were detected in aquifers throughout the Nation

• Although infrequent, total VOC concentrations of 10 micrograms per liter or greater per 

 sample were found in many States throughout the Nation.

• VOC contamination occurs in aquifers across the Nation, albeit over a large range of 

 concentrations.

Fumigant detections in aquifers generally are related to areas of known 

fumigant use, such as the Central Valley of California and Oahu

Implications

• Many of the Nation’s aquifers are vulnerable to low-level VOC contamination, indicating the need to include VOCs in ground-water 

 monitoring programs to track the trend of the low-level VOC contamination identified in this assessment.

• The natural and anthropogenic factors important to VOC occurrence in a particular aquifer need to be understood in order to effectively 

 manage and protect aquifers that are susceptible to VOC contamination.

• A careful review of the importance and feasibility of further reducing or eliminating VOC sources to aquifers also is needed to manage 

 and protect these aquifers.

• Some VOCs that are mobile and persistent (such as MTBE, perchloroethene, and chloroform) may reach especially susceptible aquifers

 within a decade or less of extensive use and potentially adversely affect ground-water quality.

• The environmental behavior and fate properties of anthropogenic compounds should be included in decision-making processes prior to 

 their approval for large-scale commercial, industrial, and other uses.

• Some of the 55 VOCs included in this assessment may not warrant continued inclusion in large-scale resource assessments, such as 

 aquifer studies completed in the NAWQA Program, if it is confirmed that their use, release, and behavior and fate characteristics pose 

 a small or negligible likelihood of ground-water contamination.

Reference

Zogorski, J.S., Carter, J.M., Ivahnenko, Tamara, Lapham, W.W., Moran, M.J., Rowe, B.L., Squillace, P.J., and Toccalino, 

P.L., 2006, The quality of our Nation’s waters—Volatile organic compounds in the Nation’s ground water and drinkingwater supply wells: U.S. Geological Survey Circular 1292, 101 p.

• Trihalomethanes (THMs) and solvents were the most frequently detected groups of VOCs in aquifers.

• The detection frequencies in aquifers vary between VOC groups.

• Forty–two of the 55 VOCs were detected in aquifers at an assessment level of 0.2 microgram per liter.

• The 15 most frequently detected VOCs in aquifers are shown below.

- Chloroform was the most frequently detected compound.

- Methyl tert-butyl ether (MTBE) was one of the most frequently detected VOCs despite the short

 period of its extensive use.

- The most frequently detected VOCs are from 5 of the 7 VOC groups.

• Of the 55 VOCs included in the national assessment, 13 VOCs (shown 

 below) were not detected in aquifer samples at a concentration of 

 0.2 microgram per liter or larger.

• At least one VOC in 5 of the 7 VOC groups was not detected.

EXPLANATION

Total VOC concentration, in 

 micrograms per liter

No detection or less than 0.02

0.02 to less than 0.2 

0.2 to less than 1

1 to less than 10

10 or greater

Hawaii

Oahu

Alaska

the Nation. The 98 aquifer studies collectively provide an important national perspective 

on the current (1985–2002) extent of VOC contamination and regional patterns of VOC 

occurrence in ground water. More information about this national assessment of VOCs is 

available at a supporting Web site (http://water.usgs.gov/nawqa/vocs/national_assessment).

• Ten frequently detected VOCs were statistically

 associated with factors that relate to their sources,

 transport, and fate in ground water.

• The source, transport, and fate properties shown below

 were associated with the occurrence of one or more of 

 the 10 frequently detected VOCs shown on the left.

• The concentration of dissolved oxygen was the most

 common explanatory factor.

• The detection frequencies of most VOCs were dependent on the dissolved-oxygen 

 conditions of ground water and the type of VOC.

• For example chloroform in young oxic ground water will tend to biodegrade to

 form methylene chloride and chloromethane in old anoxic ground water.