FACTORS ASSOCIATED WITH SOURCES, TRANSPORT, AND FATE OF VOLATILE ORGANIC COMPOUNDS IN MAJOR AQUIFERS OF THE UNITED STATES

Factors associated with sources, transport, and fate of volatile organic compounds (VOCs) in major aquifer systems of the United States were evaluated using: (1) logistic regression, (2) quantile plots, (3) detection frequencies, (4) network analysis, and (5) mixture analysis. VOC data from 1,631 wells sampled between 1996 and 2002 as part of the National Water-Quality Assessment (NAWQA) Program of the U.S. Geological Survey were used in the analyses. Sampled wells were randomly distributed across 55 major aquifers used to supply drinking water in NAWQA regional study areas. Samples were analyzed for 53 VOCs from primarily domestic supply wells (1,184), followed by public-supply wells (216); the remaining wells (231) were from a variety of well types. The median well depth was 50 meters. Age-date analysis showed that more than one-half of the wells produced sampled water that was recharged after 1953. Chloroform, toluene, and perchloroethene were the three most frequently detected VOCs. Source factors, in order of importance, were general land-use activity (dispersed source), septic/sewer density (dispersed source), and sites where large concentrations of VOCs potentially are released (concentrated sources), such as leaking underground storage tanks. Mixture analysis showed that 11 percent of all samples had VOC mixtures that were associated with concentrated sources; 20 percent were associated with dispersed sources. Important transport factors included well depth, precipitation/recharge, air temperature, and amount of water removed from storage during withdrawal from the aquifer. Dissolved oxygen was the only explanatory factor that was strongly associated with the fate of VOCs; it proved crucial in explaining the detection and concentration of many VOCs. Well type (domestic or public supply) also was an important explanatory factor, but was classified as indeterminate because it was not clearly associated with the source, transport, or fate of VOCs. Results of multiple analyses showed the importance of (1) accounting for dispersed and concentrated sources of VOCs, (2) understanding the ground-water-flow system at different scales to help explain VOC detections, (3) measuring dissolved oxygen when sampling for VOCs, and (4) limiting the type of wells sampled in monitoring networks to avoid unnecessary variance in the data.