Volatile organic compounds in untreated ambient groundwater of the United States, 1985-1995

As part of the National Water-Quality Assessment Program of the U.S. Geolog ical Survey, an assessment of 60 volatile organic compounds (VOCs) in untre ated, ambient groundwater of the conterminous United States was conducted b ased on samples collected from 2948 wells between 1985 and 1995. The sample s represent urban and rural areas and drinking-water and nondrinking-water wells. A reporting level of 0.2 mu g/L was used with the exception of 1,2-d ibromo-3-chloropropane, which had a reporting level of 1.0 mu g/L. Because ambient groundwater was targeted, areas of known point-source contamination were excluded from this assessment. VOC concentrations generally were low; 56% of the concentrations were less than 1 mu g/L In urban areas, 47% of t he sampled wells had at least one VOC, and 29% had two or more VOCs; furthe rmore, U.S. Environmental Protection Agency drinking-water criteria were ex ceeded in 6.4% of all sampled wells and in 2.5% of the sampled drinking-wat er wells. In rural areas, 14% of the sampled wells had at least one VOC; fu rthermore, drinking-water criteria were exceeded in 1.5% of all sampled wel ts and in 1.3% of the sampled drinking-water wells. Solvent compounds and t he fuel oxygenate methyl tert-butyt ether were among the most frequently de tected VOCs in urban and rural areas. It was determined that the probabilit y of finding VOCs in untreated groundwater can be estimated on the basis of a logistic regression model by using population density as an explanatory variable. Although there are limitations to this national scale model, it f it the data from 2354 wells used for model development and adequately estim ated the VOC presence in samples from 589 wells used for model validation. Model estimates indicate that 7% (6-9% on the basis of one standard error) of the ambient groundwater resources of the United States probably contain at least one VOC at a reporting level of 0.2 mu g/L. Groundwater is used in these areas by 42 million people (35-50 million based on one standard erro r); however, human exposure to VOCs from this ambient groundwater is uncert ain because the quality of the finished drinking water is generally unknown .