|USGS South Dakota Water Science Center Publication|
By Paul J. Squillace and Michael J. Moran
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2005–5269
National Water-Quality Assessment Program
National Synthesis on Volatile Organic Compounds
Factors associated with sources, transport, and fate of volatile organic compounds (VOCs) in aquifer systems of the United States were evaluated using various statistical methods. 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 selected from aquifers used to supply drinking water in the regional study areas. Samples were analyzed for more than 50 VOCs from primarily domestic water supplies (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 information, available from 44 percent of the sampled wells, shows that about 60 percent of the sampled water was recharged after 1953.
Ten VOCs frequently detected in aquifer samples were selected for statistical explanatory analyses and included the solvents chloromethane, methylene chloride, 1,1,1-trichloroethane, trichlorethene, and perchloroethene; the trihalomethanes bromodichloromethane and chloroform; and the gasoline compounds toluene, 1,2,4-trimethylbenzene and methyl tert-butyl ether. Concentrations of VOCs generally were less than 1 µg/L. Source factors, in order of decreasing importance, were general land-use activity (dispersed source), septic/sewer density (dispersed source), and sites where large concentrations of VOCs are potentially 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, screen depth, precipitation/recharge, air temperature, various soil characteristics, and amount of water removed from storage during withdrawal from the aquifer. Dissolved oxygen was the explanatory factor that was strongly associated with the fate of VOCs; it proved crucial in explaining the detection and concentration of many VOCs. Chloroform, for example, is more stable in water that contains oxygen. This increased stability explained the larger detection frequencies and concentrations of chloroform in water containing oxygen compared with water having little or no oxygen. Well type (domestic or public supply) was also an important explanatory factor, but was classified as indeterminate because it was not clearly associated with the sources, transport, or fate of the VOC.
Results of multiple analyses show 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.
Purpose and Scope
Background and Previous Studies
Selected Aquifer Studies by NAWQA
Sampling and Analytical Methods
Analysis of the Source, Transport, and Fate of VOCs
Logistic Regression to Determine Significant Associations
Quantile Plots to Determine Significant Associations
Detection Frequency to Determine Significant Associations
Network Analysis to Determine Significant Associations
Use of Mixtures to Determine Significant Associations
Synopsis of Results
Implications for Ground-Water Management and Assessments
Squillace, P.J., and Moran, M.J., 2006, Factors associated with sources, transport, and fate of volatile organic compounds in aquifers of the United States and implications for ground-water management and assessments: U.S. Geological Survey Scientific Investigations Report 2005–5269, 40 p.
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Send questions or comments about this report to the author, P.J. Squillace (605) 394-3239.
For more information about USGS activities in South Dakota, visit the USGS South Dakota Water Science Center home page.
For more information about USGS National Water-Quality Assessment Program, visit the NAWQA Program home page or more information about the USGS National Water-Quality Assessment Program Volatile Organic Compound National Syntheses, visit the VOC National Synthesis home page.
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