Volatile Organic Compounds in the Nation's Ground Water and Drinking-Water Supply Wells: Supporting Information

Note to Accompany Fact Sheet 2006-3043
Volatile Organic Compounds in the Nation’s Drinking-Water Supply Wells—What Findings May Mean to Human Health

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).

The purpose of this documentation is to explain how the percentages were calculated for the pie chart on Figure 1 in the U.S. Geological Survey Fact Sheet 2006-3043 (Toccalino and others, 2006). One sample from each of 3,497 domestic and public wells was analyzed for 55 volatile organic compounds (VOCs) using the methodology described by Moran and others (2005).

Four criteria were used to count the 3,497 well samples according to the hierarchy listed below. For simplicity, each well sample was counted only once. For example, if a well sample met the first criterion, then it was not counted using any of the remaining criteria. If a well sample did not meet the first criterion, but met the second criterion, then it was not counted using either the third or fourth criteria. The first three criteria are based on VOC concentrations greater than or equal to 0.2 part per billion.


Criterion

Number of well samples meeting criterion

Percent of well samples meeting criterion

1. Well samples with one or more VOC detections at concentrations greater than human-health benchmarks. The human-health benchmarks used in this assessment are the U.S. Environmental Protection Agency’s Maximum Contaminant Levels and the U.S. Geological Survey’s Health-Based Screening Levels.

45

1

2. Well samples with one or more VOC detections at concentrations less than or equal to human-health benchmarks.

508

15

3. Well samples with one or more VOC detections, but human-health benchmarks are not available for the detected VOCs.

70

2

4. Well samples without VOC detections at concentrations greater than or equal to 0.2 part per billion.

2,874

82

Total

3,497

100

A limitation of the counting methodology described above is that a well sample may meet multiple criteria, though each well sample was counted only once. For example, a well sample with multiple VOC detections may contain VOCs with concentrations both greater than and less than human-health benchmarks. Because only 45 (or about 1 percent of) well samples had a VOC concentration greater than a human-health benchmark, however, the effect of the counting methodology on the number of well samples with one or more VOC detections at concentrations less than or equal to human-health benchmarks is not substantial. The counting methodology used for Figure 1 had the largest effect on the number of well samples with VOC detections but no human-health benchmarks to compare to concentrations. When all well samples are considered, 137 wells (about 4 percent of the 3,497 well samples) had one or more VOC detections (greater than or equal to 0.2 part per billion) for which human-health benchmarks are not available.

References

Moran, M.J., Zogorski, J.S., and Rowe, B.L., 2005, Approach to an assessment of volatile organic compounds in the Nation's ground-water and drinking-water supply wells: U.S. Geological Survey Open-File Report 2005-1452, 36 p. Available at http://pubs.usgs.gov/of/2005/1452/.

Toccalino, P.L., Rowe, B.L., and Norman, J.E., 2006, Volatile organic compounds in the Nation’s drinking-water supply wells—What findings may mean to human health: U.S. Geological Survey Fact Sheet 2006-3043, 4 p. Available at http://pubs.usgs.gov/fs/2006/3043/.

Contact for additional information

Patricia Toccalino
U.S. Geological Survey
6000 J Street, Placer Hall
Sacramento, CA 95816
Phone: (916) 278-3090 Email: ptocca@usgs.gov

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