Historical trends in occurrence and atmospheric inputs of halogenated volatile organic compounds in untreated ground water used as a source of drinking water
Introduction
Approximately 50% (percent) of the US population and 97% of the US population in rural areas relies on ground water as a source of domestic supply (Mlay, 1990). Ground water, however, is subject to many potential sources of contamination in urban, suburban and rural areas. Contamination of ground water with volatile organic compounds (VOCs) is of particular concern because many VOCs are known or suspected carcinogens. Federal drinking-water standards exist for only a subset of VOCs (US Environmental Protection Agency (USEPA) primary and secondary drinking water regulations). Other VOCs that are known to occur in drinking water are unregulated but are included in the USEPA's Contaminant Candidate List (CCL) for potential regulation under the Safe Drinking Water Act (SDWA), and still other VOCs have yet to be included in either category.
VOCs are used extensively in industrial applications and consumer products, and are released into the environment during their production, distribution, storage, handling and use. In many ground-water flow systems, VOCs are mobile and do not easily degrade (Dinicola et al., 2000, Harte et al., 1991, Jeffers et al., 1989, Stackelberg et al., 2000). In some highly contaminated ground-water flow systems, degradation of VOC parent compounds to numerous daughter compounds is common (Davis et al., 2003, Dinicola et al., 2000, Barrio-Lage et al., 1986, Bouwer and McCarty, 1983a, Bouwer and McCarty, 1983b, Gupta et al., 1996, Parsons et al., 1984). Consequently, it is not uncommon to observe detectable concentrations of multiple VOCs in ground water (Westrick, 1990, Bender et al., 1999, Moran et al., 1999, Squillace et al., 1999, Lapham et al., 2000, Rowe et al., 2001, Zogorski et al., 2001, Shapiro et al., 2002, Squillace et al., 2002, Stackelberg et al., 2000). The co-occurrence of VOCs in ground water used for domestic supply, even at concentrations below the maximum contaminant level (MCL), may have a negative long-term cumulative and synergistic effect on human health (e.g. Carpenter et al., 1998, Carpenter et al., 2002, Colburn et al., 1996, Lappé, 1991, Squillace et al., 2002, Thomas, 1990).
Recent surveys have focused on characterizing the extent of VOC contamination in ground water (Westrick, 1990, Bender et al., 1999, Moran et al., 1999, Squillace et al., 1999, Lapham et al., 2000, Rowe et al., 2001, Zogorski et al., 2001, Shapiro et al., 2002, Moran et al., 2002, Squillace et al., 2002, Stackelberg et al., 2000). In a survey of drinking-water wells, including domestic and public supply wells, primarily in rural areas, Squillace et al. (2002) detected at least one VOC or pesticide or anthropogenic nitrate in 70% of wells, with a minimum detection limit of 10−3 micrograms per liter (μg/l), that is parts per billion (ppb), for concentrations of VOCs and pesticides. With a detection limit of 10−6 μg/l (parts per quadrillion), Shapiro et al. (2002) reported that 98% of the samples from the untreated-drinking-water sites surveyed contained at least one VOC. With a lower detection limit, Shapiro et al. (2002) showed that the percentage of the samples from these sites containing at least trace concentrations of one VOC was substantially larger than previously recognized (see, e.g. Mlay, 1990, Squillace et al., 1999, Squillace et al., 2002). The inverse relation between analytical reporting limits and frequency of pesticide detection reported by Kolpin et al. (1995) is apparently also applicable to the occurrence of VOCs at drinking-water sites. Lapham et al. (2000) also noted this relation for VOCs at much higher concentrations, where detection frequency (for wells in which one or more VOCs were detected) increased notably (from 22 to 54%) as the reporting limit decreased (from 2 to 0.2 μg/l).
Surveys of trace concentrations of VOCs are potentially important to ground-water resource managers because they offer an uncensored assessment of VOC occurrence. Detection of trace concentrations of VOCs in ground water indicates the source of water is susceptible to contamination (e.g. Shelton et al., 2001), and the potential may exist for more severe deterioration in ground-water quality. In addition the detection of VOCs at trace concentrations provides recognition of the types of VOCs that may need to be monitored with greater frequency.
This report evaluates the occurrence of selected halogenated VOCs at a detection limit of 10−6 μg/l in water samples taken from community-, non-community- (such as restaurants), and domestic-supply wells throughout the US. A subset of the data compiled by Shapiro et al. (2002) is used for this report with additional interpretation of trace concentrations of VOCs from the chromatograms used in the compilation of those data on VOC occurrence. The data for this report can be found in Shapiro et al. (2003). This report also uses modeled ground-water residence times, determined from concentrations of dichlorodifluoromethane (CFC-12), to assess historical trends in the detection frequency and the magnitude of VOC concentrations for all the VOCs combined, as well as individual VOCs, such as carbon tetrachloride (CCl4), chloroform and tetrachloroethene (PCE). The VOC detections are assessed in terms of the percentage that is due to equilibrium with average atmospheric concentrations vs. the percentage that is due to additional enhancements resulting from local atmospheric enrichment or direct contaminant input to ground-water flow systems. The occurrence of VOCs is also evaluated with respect to factors that may affect ground-water vulnerability, such as the type of well (community, non-community, or domestic), well depth, hydrogeologic conditions, local land use and ground-water redox conditions as inferred from measured concentrations of dissolved oxygen and methane.
Section snippets
Data selection
The data compiled by Shapiro et al. (2002) were obtained from archived chromatograms that were originally used to measure concentrations of CFC-12, trichloromonofluoromethane (CFC-11) and 1,1,2-trichloro-1,2,2-trifluoroethane (CFC-113) in water samples by purge-and-trap gas chromatography with an electron-capture detector (GC-ECD) at the US Geological Survey (USGS) Chlorofluorocarbon (CFC) Laboratory in Reston, VA. The water samples were collected over the period from 1997 to 2000, using metal
Detection frequency of VOCs on archived chromatograms
All CFC and VOC data from archived chromatograms for the 413 samples are compiled in Shapiro et al. (2003). Details about the sites can also be found in Shapiro et al. (2003). The six most frequently detected VOCs, other than CFC-11, CFC-12 and CFC-113, on the CFC chromatograms include the chlorinated solvents 1,1-dichloroethylene (DCE), 1,1,1-trichloroethane (TCA), CCl4, chloroform, trichloroethylene (TCE) and PCE (Shapiro et al., 2002). In Shapiro et al. (2003), the retention time of a
Historical trends in VOC occurrence in ground water and in atmospheric sources of VOCs to ground water
Previous investigators have found a high frequency of detection of anthropogenic compounds in ground water recharged in the past 50 years (e.g. Kolpin et al., 1995, Böhlke and Denver, 1995, Busenberg and Plummer, 1992, Plummer and Friedman, 1999, Plummer and Busenberg, 2000; http://water.usgs.gov/lab/cfc accessed on July 29, 2003; Kolpin et al., 1995, Fogg et al., 1999, Shelton et al., 2001, Stackelberg et al., 2000). For example, Kolpin et al. (1995) found a correlation between tritium
Assessment of aquifer vulnerability using VOC data
Numerous investigators have considered the correlation between the occurrence of VOCs in ground water and factors such as the hydrogeologic and physical setting, local land use, well type and geochemical conditions (e.g. Lapham et al., 2000, Moran et al., 2001, Shelton et al., 2001, Squillace and Moran, 2000, Squillace et al., 1999, Squillace et al., 2002, Stackelberg et al., 2000). For example, Squillace et al. (2002) found a higher detection frequency for VOCs in unconfined aquifers than in
Summary and conclusions
Archived chromatograms from purge-and-trap GC-ECD analyses of drinking-water samples were used to determine the frequency of detection of halogenated VOCs in water samples from 413 community-, non-community- (such as restaurants), and domestic-supply wells throughout the US. A subset of the data on VOC occurrence compiled by Shapiro et al. (2002) is used in this report with additional interpretation of trace concentrations of VOCs from the chromatograms used in the compilation. When the
Acknowledgements
The authors gratefully acknowledge Jerry Casile (USGS, Reston, Va.) and Julian Wayland (USGS, Reston, Va.) for CFC analyses, and Allen M. Shapiro (USGS, Reston, Va.), Patricia Toccalino (Oregon Health and Science University, Beaverton, Ore.) and Michael Moran (USGS, Rapid City, S.D.) for their insightful reviews of the manuscript. This project was conducted in cooperation with the US Environmental Protection Agency Office of Ground Water and Drinking Water.
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