VOLATILE ORGANIC COMPOUNDS IN THE NATION'S GROUND WATER AND DRINKING-WATER SUPPLY WELLS

Personnel from the National Water-Quality Assessment (NAWQA) Program of the United States Geological Survey (USGS) completed a national assessment of the occurrence of 55 volatile organic compounds (VOCs) in 98 aquifers across the United States used as important supplies of drinking water. The occurrence of VOCs in samples of ground water collected prior to treatment or blending from domestic and public wells also was included.

VOCs were detected in aquifers across the Nation indicating the importance of monitoring the occurrence and trends of VOCs in ground-water resources. Analyses of VOCs was performed at low analytical levels in order to fully characterize VOC occurrence. Twenty-four VOCs warrant further monitoring in ground water resources because they were detected relatively frequently or because they had concentrations greater than, or approaching, Maximum Contaminant Levels (MCLs). In-depth studies on VOC occurrence in aquifers ultimately could focus on these compounds. Thirteen VOCs were not detected in any aquifer samples and may not warrant further monitoring in ground water resources.

The detection frequencies of most VOCs were higher in public well samples than domestic well samples. Less than two percent of samples from domestic or public wells had a VOC concentration of potential human-health concern. Further studies should investigate the source and transport of VOCs to wells that have VOC concentrations exceeding, or close to, concentrations of potential human-health concern.

Chloroform was the most frequently detected VOC in aquifers and chlorination of drinking water may be an important source of chloroform in ground water. Methyl tert-butyl ether (MTBE), a gasoline additive, also was one of the most frequently detected VOCs despite its relatively short and recent period of use. Solvents like PCE and TCE also were frequently detected in ground water and were among the VOCs that most frequently had concentrations greater than MCLs.

Some factors important to the occurrence of VOCs in aquifers included urban land use, septic systems, wet and dry climates, and dissolved oxygen. The natural and anthropogenic factors that explain the occurrence of a specific VOC in a particular aquifer need to be understood to effectively manage and protect aquifers.