Environ. Sci. Technol. 2007, 41, 2123-2130
Factors Associated with Sources, Transport, and Fate of Volatile Organic Compounds and Their Mixtures in Aquifers of the United States PAUL J. SQUILLACE AND MICHAEL J. MORAN* U.S. Geological Survey, 1608 Mountain View Road, Rapid City, South Dakota 57702
Factors associated with sources, transport, and fate of volatile organic compounds (VOCs) in groundwater from aquifers throughout the United States were evaluated using statistical methods. Samples were collected from 1631 wells throughout the conterminous United States between 1996 and 2002 as part of the National Water-Quality Assessment (NAWQA) Program of the U.S. Geological Survey. Water samples from wells completed in aquifers used to supply drinking water were analyzed for more than 50 VOCs. Wells were primarily rural domestic water supplies (1184), followed by public water supplies (216); the remaining wells (231) supplied a variety of uses. The median well depth was 50 meters. Age-date information shows that about 60% of the samples had a fraction of water recharged after 1953. Chloroform, toluene, 1,2,4-trimethylbenzene, and perchloroethene were some of the frequently detected VOCs. Concentrations generally were less than 1 µg/L. Source factors include, in order of importance, general land-use activity, septic/sewer density, and sites where large concentrations of VOCs are potentially released, such as leaking underground storage tanks. About 10% of all samples had VOC mixtures that were associated with concentrated sources; 20% were associated with dispersed sources. Important transport factors included well/screen depth, precipitation/groundwater recharge, air temperature, and various soil characteristics. Dissolved oxygen was strongly associated with VOCs and represents the fate of many VOCs in groundwater. Well type (domestic or public water supply) was also an important explanatory factor. Results of multiple analyses show the importance of (1) accounting for both dispersed and concentrated sources of VOCs, (2) measuring dissolved oxygen when sampling wells to help explain the fate of VOCs, and (3) limiting the type of wells sampled in monitoring networks to avoid unnecessary variance in the data, or controlling for this variance during data analysis.
Introduction Groundwater is used by about one-half of the population of the United States as a source of potable water, including nearly all of the 40 million or more people served by domestic * Corresponding author phone: (605) 394-3244; fax: (605 3554523; e-mail:
[email protected]. 10.1021/es061079w Not subject to U.S. Copyright. Publ. 2007 Am. Chem. Soc. Published on Web 03/06/2007
water supplies (1). Concern about the quality of this heavily used resource has led to many small-scale investigations that define the risk and remediation of concentrated sources where contaminants are released at one location. A complementary interest in dispersed sourcesscontaminants released over large areas from activity prevalent in a given settingshas led to large-scale water-quality investigations of aquifers. As part of the National Water-Quality Assessment (NAWQA) Program of the U.S. Geological Survey (USGS), the source, transport, and fate of VOCs in shallow groundwater beneath new residential/commercial areas was previously investigated and reported (2). These results could be helpful for well head protection strategies in new development surrounding major metropolitan areas. Also as part of the NAWQA program, VOCs in deeper major aquifers were investigated (3) by sampling existing water-supply wells. These large-scale studies can provide an indication of aquifer vulnerability and the quality of the water in the aquifer as a whole, as well as identify contaminants that present the greatest risk to aquifers. The purpose of this paper is to summarize factors associated with the sources, transport, and fate of 10 frequently detected VOCs in these aquifers using various statistical techniques. These factors were derived from ancillary data and were categorized as being related to sources, transport, and fate processes. Water samples for this analysis were collected between 1996 and 2002 as part of 55 NAWQA aquifer studies across the United States.
Experimental Section NAWQA’s Aquifer Studies. Groundwater samples from 55 aquifer studies were collected between 1996 and 2002 as part of the NAWQA Program of the USGS (Figure 1 and Table 1 in the Supporting Information). About 30 existing wells were sampled for each aquifer study. These studies have three unique characteristics. First, samples were collected before treatment to define source-water quality in the aquifer. Second, sampled wells were spatially distributed and randomly selected among existing wells within the targeted aquifer without respect to land use. However, sampled wells mostly were in rural areas of the nation because urban areas constitute a small fraction of the total landscape. Aquifers were selected for investigation because of their importance as a supply of potable water. Third, most of the samples were collected from domestic water supplies; 73% of the samples (1, 184) were collected from domestic water supplies, 13% (216) were from public water supplies, and the remaining 14% (231) were from a variety of other well types such as irrigation or monitoring wells (Table 1 in the Supporting Information). Domestic water supplies were sampled more frequently because they were most commonly available in many parts of the investigated aquifers. NAWQA aquifer studies are intended to be resource assessments of the selected aquifer; consequently, sampled wells are spatially distributed and not stratified on the basis of factors such as water use, land use, or population density (4). Sampled wells were in areas where population density is higher on average than for the conterminous United States as a whole; the median population density near wells sampled for aquifer studies (about 20 people/km2) is one order of magnitude larger than the median density for the conterminous United States (about two people/km2) but most of the sampled wells were still in rural areas (Figure 2 in the Supporting Information). These aquifer studies, consequently, are groundwater resource assessments of rural areas VOL. 41, NO. 7, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Statistical summary of detection frequencies of chloroform, methylene chloride, and chloromethane in young oxic water compared with old anoxic groundwater along a hypothetical groundwater flowpath. (Detection frequencies were calculated using no common assessment level.) where groundwater is used principally for domestic water supply. More information about the design of these studies has been published by Gilliom and others (4). Sampling and Analytical Methods. VOC analyses were done for more than 50 VOCs selected by the VOC National Synthesis Team of NAWQA (5). All analyses were done at the USGS National Water Quality Laboratory in Denver, Colo., using the method that provided the lowest concentration information currently in use (2006) at the USGS National Water Quality Laboratory (6). Analysis was done by use of purge-and-trap, capillary column gas chromatography/mass spectrometry. All concentrations reported by the laboratory were used in this analysis including the smallest concentrations with estimated values. Nondetect values were recorded as less than the long-term method detection level. A subset of 10 VOCs listed in the Supporting Information Table 2 were frequently detected VOCs and were selected for analysis in this paper. All VOC data are available at http://water.usgs.gov/ nawqa/vocs/national_assessment/ and have been discussed in more detail by Moran and others (7). Sampling protocols and quality-assurance/quality-control plans are described by Koterba and others (8) and Moran and others (7). Ancillary Data. A variety of ancillary data were used to represent hydrogeologic and anthropogenic factors that can affect the source, transport, and fate of VOCs in groundwater. Twenty-four hydrogeologic and 24 anthropogenic factors were tested (7). A 500 m and 1000 m radius buffer was used in calculating land-use characteristics around the well. The selection of a 500 m buffer is based on previous investigations (8-10). A 1000 m radius buffer was used when counting the number of potential concentrated sources of VOCs (for example, leaking underground storage tanks) associated with a sampled well because of uncertainty in locating these concentrated sources. Previous publications (2, 7, 11) discuss in more detail the limitations, assumptions, and sources of the ancillary data. 2124
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Well depths ranged from 4 to 825 m, with a median depth of 50 m. Domestic water supplies had a median well depth of 45 m, and public water supplies had a median depth of 120 m. There were 922 wells finished in unconfined aquifers and 381 wells in confined aquifers; for the remaining 328 wells, aquifer types were mixed or not identified. Dissolvedoxygen concentrations in groundwater were classified as oxic or anoxic. Water was classified as oxic if dissolved-oxygen concentrations were greater than 0.5 mg/L and anoxic if dissolved-oxygen concentrations were less than or equal to 0.5 mg/L. Ancillary information was classified as relating to source, transport, or fate of VOCs, with the exception of well type, which was classified as an indeterminate factor because the reason why it affects VOC detection is uncertain. For example, pumping rates, travel times in groundwater, and size of contributing areas are generally different for domestic and public water supplies, and all of these factors can affect VOC detection (12). Statistical Methods. Statistics used for this analysis are described in detail by Menard (13) and Helsel and Hirsch (14), and are summarized by Squillace and others (2). An alpha level of 0.05 was used for all statistical analyses to determine significant differences and associations. The concentration data used in this analysis were not censored to a common reporting level, for example 0.2 µg/L, because (1) the same laboratory and analytical methods were used for all samples, (2) censoring would reduce the ability to show associations between a particular VOC and explanatory factors, and (3) statistical methods such as logistic regression are done for each VOC individually. Logistic regression models were used to look for associations between groundwater quality at a particular sampled well and current land-use activity within a circular buffer of 500-m radius around the well. Various hydrogeologic, geochemical, and land-use conditions could adversely affect
the results (2); nevertheless, several previous studies have shown a relation between land use and water quality (9, 10, 15-19). For each VOC, various logistic regression models are possible, but information from a variety of statistical measures (2) was used to select the final model. Every final model for every VOC had a significant overall likelihood ratio statistic p-value (
