Figure 2. Human-health benchmarks are available for 35 of the 

44 volatile organic compounds (VOCs) detected in domestic and (or) 

public well samples. Human-health benchmarks such as Maximum 

Contaminant Levels (MCLs) and Health-Based Screening Levels 

(HBSLs) provide a basis for evaluating VOC concentrations in the 

context of human health.

27 regulated VOCs

with MCLs

8 unregulated VOCs

with HBSLs

9 unregulated VOCs

without benchmarks

55 VOCs analyzed

44 VOCs detected

Figure 1. Volatile organic compounds (VOCs) were not detected 

in samples from 82 percent of the 3,497 domestic or public wells 

at a threshold of 0.2 part per billion. VOC concentrations were 

greater than human-health benchmarks in 45 well samples (about 

1 percent of all well samples).

Well samples with VOC detections but 

no benchmarks to compare to concentrations (2 percent)

Well samples with VOC detections less 

than or equal to benchmarks 

(15 percent)

Well samples with VOC detections 

greater than benchmarks (1 percent)

Well samples without VOC detections

at a threshold of 0.2 part per billion

(82 percent)

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2006–3043

Revised, June 2006

Volatile Organic Compounds in the Nation’s Drinking-Water 

Supply Wells—What Findings May Mean to Human Health

Printed on recycled paper

When volatile organic compounds (VOCs) are detected in 

samples from drinking-water supply wells, it is important to understand what these results may mean to human health. As a first step 

toward understanding VOC occurrence in the context of human 

health, a screening-level assessment was conducted by comparing 

VOC concentrations to human-health benchmarks. One sample 

from each of 3,497 domestic and public wells was analyzed for 55 

VOCs; samples were collected prior to treatment or blending. At 

least one VOC was detected in 623 well samples (about 18 percent 

of all well samples) at a threshold of 0.2 part per billion. Eight of 

the 55 VOCs had concentrations greater than human-health benchmarks in 45 well samples (about 1 percent of all well samples); 

these concentrations may be of potential human-health concern if 

the water were to be ingested without treatment for many years. 

VOC concentrations were less than human-health benchmarks in 

most well samples with VOC detections (fig. 1), indicating that 

adverse effects are unlikely to occur, even if water with such concentrations were to be ingested over a lifetime. Seventeen VOCs 

may warrant further investigation because their concentrations 

were greater than, or approached, human-health benchmarks.

Background

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

Assessment (NAWQA) Program recently completed a national 

assessment of volatile organic compounds (VOCs) in ground 

water (Zogorski and others, 2006; Moran and others, 2006). This 

Fact Sheet supplements Chapter 4 of Zogorski and others (2006), 

and focuses on the occurrence of 44 VOCs detected in samples 

collected prior to any treatment or blending from 2,401 domestic 

wells and 1,096 public wells (fig. 2). Because domestic and public 

wells serve as drinking-water supplies, it is important to understand 

what the VOCs detected in these well samples may mean to human 

health.

Maximum Contaminant Levels (MCLs)

MCLs are legally enforceable standards that set the 

maximum permissible level of a contaminant in water that is 

delivered to any user of a public water system. MCLs are set 

as close as feasible to the maximum level of a contaminant at 

which no known or anticipated adverse effects on human health 

would occur, taking into account the best available technology, 

treatment techniques, cost considerations, expert judgment, 

and public comments (U.S. Environmental Protection Agency, 

2006). MCLs were used as benchmarks in this screening-level 

assessment to evaluate water-quality data in a human-health 

context. Human exposure from tap water and other pathways 

was not quantified in this assessment.

VOC concentrations were compared to human-health 

benchmarks in a screening-level assessment to provide an initial 

national-scale perspective on the potential relevance of VOC concentrations to human health. The human-health benchmarks used in 

the screening-level assessment are the U.S. Environmental Protection Agency’s (USEPA) Maximum Contaminant Levels (MCLs) 

and USGS’s Health-Based Screening Levels (HBSLs) (Toccalino 

and others, 2003). Concentrations of regulated VOCs (those with 

MCLs) were compared to their MCLs, and concentrations of 

unregulated VOCs (those without MCLs) were compared to their 

HBSLs, when available (fig. 2).
VOC concentrations were seldom greater than 

human-health benchmarks in domestic and 

public well samples.

How are VOC concentrations compared with humanhealth benchmarks?

VOC concentrations in each water sample were compared to 

MCLs or HBSLs and were expressed as ratios to provide perspective on what VOC occurrence may mean to human health and to 

help prioritize further investigations. A concentration that is 10-

fold greater than an MCL or HBSL produces a ratio of 10, and a 

concentration that is 10-fold less than an MCL or HBSL produces 

a ratio of 0.1. A ratio of 1 represents a concentration equal to a 

human-health benchmark.

Water containing VOCs with ratios greater than 1 may be of 

potential human-health concern if the water were to be ingested for 

many years as the primary drinking-water source without treatment. 

The likelihood for adverse effects generally increases as the VOC 

concentrations (and ratios) increase. Ingestion of water containing 

VOCs with ratios less than 1 is unlikely to result in adverse humanhealth effects. VOCs with ratios greater than or equal to 0.1 were 

identified as those that may warrant further investigation. A variety 

of ratios may be selected as threshold values to identify contaminants that may warrant further monitoring; using a threshold ratio 

of 0.1 is consistent with various State and Federal practices (for 

example, see U.S. Environmental Protection Agency, 2003a).

Assessing the potential relevance of VOC concentrations 

to human health is more complicated, however, than calculating 

ratios. NAWQA studies were not designed to evaluate specific 

effects of VOCs on human health, and the screening-level assessment is not a substitute for a comprehensive risk assessment, which 

could include additional factors such as the following:

Length of time a person is exposed;

Volume of water ingested per day;

Vulnerability of individuals (for example, children and immunocompromised persons may be more vulnerable);

Number of contaminants in the water supply (that is, 

contaminant mixtures);

Exposure routes such as ingestion and inhalation; and

Water treatment method, if any.

•

•

•

•

•

•

Health-Based Screening Levels (HBSLs)

HBSLs are benchmark concentrations of contaminants in 

water that may be of potential concern for human health. HBSLs 

are nonenforceable guidelines that were developed by the USGS 

in collaboration with USEPA and others using USEPA methodologies and the most current, USEPA peer-reviewed, publicly 

available human-health toxicity information (Toccalino and others, 2003). The USGS and its cooperators are continuing to refine 

the HBSL methodology.

HBSLs are based on health effects alone and do not consider 

cost or technical limitations. For noncarcinogens, the HBSL 

represents the contaminant concentration in drinking water that 

is not expected to cause any adverse effects over a lifetime of 

exposure. For carcinogens, the HBSL range represents the contaminant concentration in drinking water that corresponds to an 

excess estimated lifetime cancer risk of 1 chance in 1 million to 

1 chance in 10 thousand. HBSL calculations adopt USEPA 

assumptions for establishing drinking-water guidelines, namely, 

lifetime ingestion of 2 liters of water per day by a 70-kilogram 

adult. For noncarcinogens, it also is assumed that 20 percent 

of the total contaminant exposure comes from drinking-water 

sources and that 80 percent comes from other sources (for example, food and air). If data are available to quantify the percentage 

of contaminant exposure that comes from water, then a dataderived percentage is used instead of the default of 20 percent.

What does it mean when VOC concentrations are 

greater than human-health benchmarks?

In this national screening-level assessment, VOC concentrations of potential human-health concern were defined as concentrations greater than MCLs or HBSLs (ratios greater than 1) in 

samples from drinking-water supply wells. VOC concentrations 

were greater than MCLs in 45 of the 623 well samples with VOC 

detections (about 1 percent of all well samples) (fig. 1).

Eight regulated VOCs had concentrations greater than MCLs 

in one or more well samples (table 1), with maximum concentrations producing ratios of 2 to 30 (fig. 3). These eight VOCs warrant 

additional monitoring to characterize concentrations in drinking 

water and to establish trends in their occurrence. The fumigant 

dibromochloropropane (DBCP) and the solvents trichloroethene 

(TCE) and perchloroethene (PCE) accounted for the majority of 

VOC concentrations greater than MCLs in domestic and (or) public 

well samples (table 1). The potential health effects that may result 

from exposure to these VOCs at concentrations greater than the 

MCLs are summarized in table 1. None of the detected unregulated 

VOCs had concentrations greater than HBSLs.

If water containing VOC concentrations greater than MCLs 

or HBSLs is ingested, it does not necessarily indicate that adverse 

human-health effects will occur because there are mitigating factors 

such as the following:

Human-health benchmarks are intentionally conservative 

(protective) and incorporate safety factors and conservative 

assumptions to account for uncertainty in toxicity information.

Models used to develop MCLs and HBSLs assume lifetime exposure, whereas actual exposure may be less than a 

lifetime. In this screening-level assessment, one sample from 

each well was analyzed for VOCs, and it is unknown whether 

measured concentrations will remain the same over time.

All domestic and public well samples were collected at the 

wellhead (the point at which the water exits the ground) 

before any treatment or blending that potentially could reduce 

VOC concentrations. The sampled water may not, therefore, 

represent the quality of water ingested by the people served 

by these wells.

The Safe Drinking Water Act (SDWA) applies to water distributed through a public water system, but in this assessment, public 

wells were sampled at the wellhead prior to distribution. Drinkingwater standards (MCLs) are not violated if VOC concentrations are 

greater than MCLs in the domestic or public well samples considered in this assessment because these samples are not subject to 

regulation by the SDWA.

•

•

•
Figure 3. Ratios were less than 1 for 27 of the 35 volatile 

organic compounds (VOCs) with human-health benchmarks 

detected in domestic and (or) public well samples. Ratios 

less than 1 indicate that maximum concentrations were 

less than Maximum Contaminant Levels (MCLs) or HealthBased Screening Levels (HBSLs). Eight VOCs had maximum 

concentrations of potential human-health concern (ratios 

were greater than 1) (see table 1). Seventeen VOCs had ratios 

greater than 0.1 and may warrant further investigation (see 

tables 1 and 2).

0.000001

0.00001

0.0001

0.001

0.01

0.1

1

10

100

VOCs with ratios greater than 1

 (8 VOCs with MCLs) 

VOCs with ratios between 0.1 and 1

 (9 VOCs with MCLs) 

VOCs with ratios less than 0.1

 (10 VOCs with MCLs and 8 VOCs 

 with HBSLs) 

Ratios of maximum concentrations to MCLs or HBSLs for the 

35 detected VOCs with benchmarks 

RATIOS

What does it mean when concentrations are less 

than human-health benchmarks?

VOCs were not detected in 82 percent of the samples from 

3,497 domestic or public wells in this screening-level assessment 

at a threshold of 0.2 part per billion. VOC concentrations were less 

than MCLs or HBSLs in most well samples with VOC detections 

(fig. 1). Twenty-seven VOCs (19 regulated and 8 unregulated 

VOCs) had concentrations less than MCLs or HBSLs in one or 

more well samples, with maximum concentrations producing ratios 

of 0.9 to 0.000006 (fig. 3). Ingesting water with concentrations 

less than human-health benchmarks is unlikely to result in adverse 

human-health effects, even if water with such concentrations were 

to be ingested over a lifetime.

Seventeen VOCs may warrant further investigation because 

their ratios were greater than 0.1; all 17 are regulated VOCs with 

MCLs. These 17 VOCs include the eight VOCs with ratios greater 

than 1 (table 1) and nine VOCs with ratios between 0.1 and 1 

(fig. 3, table 2). Additional monitoring for these 17 VOCs allows 

for analyzing trends in their occurrence and, for the nine VOCs 

with ratios between 0.1 and 1, may provide an early indication if 

VOC concentrations approach human-health benchmarks.

Identifying VOCs with concentrations less than or greater than 

human-health benchmarks provides useful information about what 

VOC occurrence may mean to human health, but screening-level 

assessments have limitations. For example, MCLs and HBSLs 

generally are developed using toxicity information for single 

chemicals. However, mixtures of two or more VOCs were found 

in about 4 and 13 percent of all domestic and public well samples, 

respectively. The long-term cumulative effects of low concentrations of multiple contaminants on human health currently are 

unknown. An additional limitation is that HBSLs have not been 

developed for nine detected unregulated VOCs because of a lack of 

toxicity information (fig. 2, table 3). The potential relevance of the 

occurrence of these nine VOCs to human health cannot, therefore, 

be evaluated at this time.

Table 1. Eight regulated volatile organic compounds (VOCs) were detected at concentrations greater than Maximum Contaminant Levels 

(MCLs) in domestic and (or) public well samples (ratios were greater than 1). The VOCs are listed in order of decreasing ratios, corresponding 

to the eight purple symbols on figure 3. Seven of the eight VOCs are regulated as carcinogens, and most may adversely affect the liver.

Compound name 

(abbreviation) 

Number of samples with 

concentrations greater 

than MCLs

Carcinogen1 Potential health effects from exposure to 

concentrations greater than the MCL2

Trichloroethene (TCE) 15 yes Liver problems; increased risk of cancer

Ethylene dibromide (EDB) 3 yes Problems with liver, stomach, reproductive system, or 

kidneys; increased risk of cancer

Methylene chloride 2 yes Liver problems; increased risk of cancer

Dibromochloropropane (DBCP) 14 yes Reproductive difficulties; increased risk of cancer

Perchloroethene (PCE) 14 yes Liver problems; increased risk of cancer

1,1-Dichloroethene 3 no Liver problems

1,2-Dichloropropane 3 yes Increased risk of cancer

Vinyl chloride 2 yes Increased risk of cancer

1

Probable or likely human carcinogen as defined by U.S. Environmental Protection Agency.

2

Data from U.S. Environmental Protection Agency (2003b).
Table 2. Nine regulated volatile organic compounds (VOCs) 

detected in domestic and (or) public well samples had 

concentrations approaching Maximum Contaminant Levels (MCLs) 

and may warrant inclusion in future ground-water monitoring 

efforts. The VOCs are listed in order of decreasing number of 

samples with concentrations producing ratios between 0.1 and 1.

Compound name 

(abbreviation)

Number of samples with ratios 

between 0.1 and 1

Chloroform 10

Carbon tetrachloride 8

Benzene 7

1,2-Dichloroethane 5

Dibromochloromethane 3

Bromoform 2

1,1,1-Trichloroethane (TCA) 1

Bromodichloromethane 1

cis-1,2-Dichloroethene 1

Contacts for additional information

About VOCs in the Nation’s ground water and drinkingwater supply wells:

John Zogorski 

VOC National Synthesis 

National Water-Quality Assessment Program 

U.S. Geological Survey 

1608 Mountain View Rd. 

Rapid City, SD 57702 

Phone: (605) 394-3214 Email: jszogors@usgs.gov 

http://water.usgs.gov/nawqa/vocs/national_assessment

About Health-Based Screening Levels:

Patricia Toccalino

U.S. Geological Survey 

6000 J Street, Placer Hall 

Sacramento, CA 95816 

Phone: (916) 278-3090 Email: ptocca@usgs.gov 

http://water.usgs.gov/nawqa/HBSL/

Table 3. Nine unregulated volatile organic compounds detected 

in domestic and (or) public well samples have no human-health 

benchmarks. Health-Based Screening Levels could not be 

calculated for these VOCs because of a lack of available toxicity 

information. The VOCs are listed in order of decreasing numbers of 

samples with detections.

Compound name 

(abbreviation)

Number of samples with 

detections

Methyl tert-butyl ether (MTBE) 105

1,1-Dichloroethane 29

1,2,4-Trimethylbenzene 9

Chloroethane 5

tert-Amyl methyl ether (TAME) 5

Diisopropylether (DIPE) 4

Ethyl tert-butyl ether (ETBE) 1

n-Butylbenzene 1

n-Propylbenzene 1

What actions may be warranted when concentrations 

are greater than human-health benchmarks?

If water from untreated domestic or public well samples 

contains VOC concentrations of potential human-health concern, it 

does not necessarily indicate that water from these wells should not 

be ingested, although various responses may be warranted. Prudent 

responses by water utilities could include limiting human exposure 

through water treatment such as filtration using granular activated 

carbon, blending with a less contaminated water source, or using an 

alternative water supply. Other responses could include identifying 

and reducing potential sources of contamination, evaluating methods for protecting source waters, and ongoing monitoring to see if 

concentrations remain at levels of potential concern.

Individuals who receive their water from a water utility can 

examine the Consumer Confidence Report published by their water 

utility to determine which, if any, contaminants are present in their 

water after it is treated, and if so, at what concentrations. Domestic 

(private) well owners may contact local and State health agencies 

for information about the characteristics of well water in their area 

and may have their water tested for contaminants that may be of 

local potential concern. Additionally, individuals may use in-home 

water treatment devices designed to remove particular contaminants.

References

Moran, M.J., Hamilton, P.A., and Zogorski, J.S., 2006, Volatile organic 

compounds in the Nation’s ground water and drinking-water supply 

wells—A summary: U.S. Geological Survey Fact Sheet 2006-3048, 

6 p.

Toccalino, P.L., Nowell, L.H., Wilber, W.G, Zogorski, J.S., Donohue, 

J.M., Eiden, C.A, Krietzman, S.J., and Post G.B., 2003, Development of health-based screening levels for use in state- or local-scale 

water-quality assessments: U.S. Geological Survey Water-Resources 

Investigations Report 03-4054, 22 p.

U.S. Environmental Protection Agency, 2003a, Code of Federal 

Regulations, 40CFR159, Subpart D—Reporting requirements for 

risk/benefit information: Washington, D.C., revised July 1, 2003, 

http://www.gpoaccess.gov/ecfr/index.html.

U.S. Environmental Protection Agency, 2003b, Water on tap—What 

you need to know: Office of Water, U.S. Environmental Protection 

Agency, EPA 816-K-03-007, http://www.epa.gov/safewater/wot/

pdfs/book_waterontap_full.pdf.

U.S. Environmental Protection Agency, 2006, Setting standards for 

safe drinking water: accessed March 27, 2006 at http://www.epa.

gov/safewater/standard/setting.html.

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 drinking-water supply wells: U.S. 

Geological Survey Circular 1292, 101 p.

By Patricia L. Toccalino, Barbara L. Rowe, and 

 Julia E. Norman