Factors Associated with Sources, Transport, and Fate of Chloroform

Jul 16, 2012 - Janet M. Carter*†, Michael J. Moran‡, John S. Zogorski§, and Curtis V. .... Jinhua Li , Birget Moe , Rongfu Huang , Hongquan Zhang...
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Factors Associated with Sources, Transport, and Fate of Chloroform and Three Other Trihalomethanes in Untreated Groundwater Used for Drinking Water Janet M. Carter,*,† Michael J. Moran,‡ John S. Zogorski,§ and Curtis V. Price† †

U.S. Geological Survey, 1608 Mt. View Road, Rapid City, South Dakota 57702, United States U.S. Geological Survey, 160 N Stephanie Street, Henderson, Nevada 89074, United States § U.S. Geological Survey, P.O. Box 25585, Denver, Colorado 80225, United States ‡

S Supporting Information *

ABSTRACT: Multiple lines of evidence for indicating factors associated with the sources, transport, and fate of chloroform and three other trihalomethanes (THMs) in untreated groundwater were revealed by evaluating low-level analytical results and logistic regression results for THMs. Samples of untreated groundwater from wells used for drinking water were collected from 1996−2007 from 2492 wells across the United States and analyzed for chloroform, bromodichloromethane, dibromochloromethane, and bromoform by a low-level analytical method implemented in April 1996. Using an assessment level of 0.02 μg/L, chloroform was detected in 36.5% of public-well samples and 17.6% of domestic-well samples, with most concentrations less than 1 μg/L. Brominated THMs occurred less frequently than chloroform but more frequently in public-well samples than domestic-well samples. For both public and domestic wells, THMs occurred most frequently in urban areas. Logistic regression analyses showed that the occurrence of THMs was related to nonpoint sources such as urban land use and to point sources like septic systems. The frequent occurrence and concentration distribution pattern of THMs, as well as their frequent co-occurrence with other organic compounds and nitrate, all known to have anthropogenic sources, and the positive associations between THM occurrence and dissolved oxygen and recharge indicate the recycling of water that contains THMs and other anthropogenic contaminants.

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than 0.2 μg/L, which was the historical minimum reporting level used by USGS for analysis of VOCs in samples collected prior to April 1996. Although the samples collected between 1996 and 2001 for this study overlap with samples used in a previous USGS study (1986−2001),6 the low-level concentration data associated with the overlapping samples were censored at 0.2 μg/L in the previous study. Especially noteworthy, this article presents an evaluation of these lowlevel THM data by using analyses of land use, mixtures, and logistic regression to reveal important and new information on factors associated with sources, transport, and fate of THMs in groundwater used for drinking water.

INTRODUCTION Chloroform and three other trihalomethanes (THMs) bromodichloromethane, dibromochloromethane, and bromoformare disinfection byproducts (DBPs) commonly produced during the chlorination of water and wastewater. Chloroform also is produced and used in industrial and commercial activities. Although disinfectants such as chlorine are effective in controlling many microorganisms and their use has benefitted public health, DBPs formed from the use of chlorine, such as THMs, may pose health risks, such as bladder cancer and adverse reproductive and developmental effects, when they are present in drinking water.1 More than 260 million people in the United States are exposed to DBPs in drinking water.2 Chloroform was the most frequently detected volatile organic compound (VOC) in regional and national assessments of untreated groundwater quality conducted by the U.S. Geological Survey (USGS) (e.g., refs 3 and 4). The other three THMs (bromodichloromethane, dibromochloromethane, and bromoform) considered in this article were among the 15 most frequently detected VOCs in untreated groundwater in the United States.5 This article presents occurrence information for THMs in samples collected during 1996−2007 and analyzed using a new, low-level method that can detect THMs at concentrations less This article not subject to U.S. Copyright. Published 2012 by the American Chemical Society

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BACKGROUND Few of the more than 600 DBPs identified in the literature have been assessed in occurrence or health-effects studies.1 Only the four THMs studied here plus five haloacetic acids, bromate, and chlorite are regulated DBPs. THMs are associated with acute and chronic health problems in humans and with adverse effects on unborn children (e.g., refs 1 and 7). Chloroform, Received: Revised: Accepted: Published: 8189

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Chloroform and other THMs also could originate from industrial sources and atmospheric deposition. In 1998, the most recent year with production data, about 46 million gallons of chloroform were produced by industry.6 In contrast to chloroform, the production of the other three THMs has been relatively small.6 Chloroform is a common contaminant found at many Superfund sites19 and has been detected in urban air20 and in rainwater.21 However, in studies of chloroform occurrence in shallow groundwater in Indiana22 and New Jersey,23 the concentrations of chloroform in the atmosphere and rainwater were not high enough to account for all the mass of chloroform measured in shallow groundwater. The sources of the chloroform in the New Jersey study were attributed to use of chlorinated drinking water and the associated recharge from septic tanks or from the irrigation of crops and lawns.23 In studies on the formation of multiple THMs during water chlorination, it has been shown that the relative concentrations usually decrease with increasing brominationchloroform (CHCl3) > bromodichloromethane (CHBrCl2) > dibromochloromethane (CHBr2Cl) > bromoform (CHBr3) (e.g., ref 24). Detection frequencies of THMs from national and regional groundwater studies commonly show the same trend as noted above (e.g., refs 3,4, and 25). Such observations, along with the sparse industrial production of brominated THMs, indicate that the presence of brominated THMs might be used as a valuable criterion for distinguishing chlorination-treated waters from other potential sources of THMs in groundwater.6

bromodichloromethane, and bromoform are suspected human carcinogens, whereas insufficient data exist to assess the human carcinogenic potential of dibromochloromethane.8,9 Because of potential human-health concerns, the total concentration of THMs in drinking water has been regulated by the U.S. Environmental Protection Agency (USEPA) since 1979, and the maximum contaminant level (MCL) for drinking water currently is 80 μg/L.8 Chloroform and other THMs commonly are generated by the haloform reaction during water chlorination when chlorine interacts with organic material dissolved in the water.10 If bromide is present (at concentrations of about 100 μg/L or greater) in source waters, one or more of the three brominated THMs may be generated by the haloform reaction.11 Sodium hypochlorite, gaseous chlorine, and chloramines (commonly used in public systems as disinfectants) and many organic chemicals contained in household cleaning products may react with organic matter in water to generate halogenated VOCs such as chloroform and carbon tetrachloride.1,12 Overall, an estimated 0.4 million gallons of chloroform are generated per year through the chlorination of drinking water in the United States.13 Concentrations of the four THMs, especially chloroform, can be especially high in treated drinking water as revealed by national data collected by the USEPA for the Information Collection Rule from 500 large drinking-water plants for which mean concentrations were 38 μg/L for the sum of the four THMs and 23 μg/L for chloroform.14 A primary source of THMs in untreated water from public wells is chlorinated water or wastewater that recharged the aquifer.6 In urban areas, it is presumed that released treated drinking water or wastewater, such as through leaking drinkingwater distribution and sewer pipes or from irrigation of athletic fields, lawns, crops, and golf courses, moves through the unsaturated zone to deeper groundwater supplying public wells. The loss of treated drinking water through leaking distribution pipes and other unknown routes may be as high as 15% in some systems.15 Although most homeowners who obtain their water from private wells do not continuously chlorinate their water prior to use, chloroform was the most frequently detected VOC at concentrations ranging from 0.002 μg/L to nearly 80 μg/L in groundwater from domestic wells sampled during 1985−2002 across the United States.16 Potential sources of chloroform to domestic wells include (1) shock chlorination, which is a periodic disinfection process in which a dilute solution of bleach (sodium hypochlorite solution) is added directly to a well to eliminate microbial contamination and whereby free chlorine reacts with organic matter in the groundwater in the well to form THMs, and (2) laundry wastewater containing bleach released to septic systems, which may then be a source of free chlorine or THMs to the domestic well. Septic systems may be an especially important source of chloroform to groundwater. Previous studies have documented the presence of chloroform in septic-system effluent (e.g., ref 17), and several States have identified septic systems as a major source of groundwater contamination, second only to underground storage tanks.18 In addition, improperly designed, maintained, or operated septic systems can result in groundwater contamination near the system, especially if the degradation of the organic matter is incomplete.18 It is presumed that domestic wells near faulty septic systems may capture wastewater containing THMs.

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MATERIALS AND METHODS Samples of untreated groundwater were collected during 1996−2007 from 2492 public and domestic wells across the United States and analyzed for four THMs; 631 samples were from public wells and 1861 samples were from domestic wells. Each well was sampled one time during this period to provide a decadal snapshot of groundwater quality in aquifers without regard to temporal variations. All samples were collected using nationally consistent USGS protocols and generally analyzed within 14 days by the USGS National Water Quality Laboratory in Denver, Colorado, as described in Supporting Information (SI) Text S1. The low-level VOC method used in this study incorporates purge and trap, capillary column, gas chromatography/mass spectrometry; however, the method differs from regulatory VOC methods in that all detections that fully meet qualification and laboratory blank criteria are reported with an associated concentration. This reporting strategy provides an enhanced understanding of a compound’s occurrence in water resources, and it is especially beneficial to facilitate logistic regression analyses to deduce factors associated with a compound’s occurrence. An assessment level of 0.02 μg/L was used for some analyses in this article (SI Text S2). An assessment level is a uniform level of censoring applied to all concentration data and is used in USGS studies when laboratory reporting levels vary over time and between compounds and when comparisons between different compounds and well types are sought. THM occurrence was examined by land use using land-use classifications (SI Text S3). Differences in THM concentrations from various land-use settings were evaluated using the Wilcoxon rank sum test.26 A statistical significance (α) level of 0.05 was used to determine significant associations. Logistic regression analyses (SI Text S4) were used to identify 8190

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Figure 1. Concentrations of trihalomethanes using no assessment level in (A) public-well samples, and (B) domestic-well samples, 1996−2007.

samples compared to domestic-well samples may be due, in part, to public wells having more sources of THMs in their capture zone relative to domestic wells. This is because public wells generally are located in more urbanized areas and these areas have more sources of anthropogenic contaminants. In addition, public wells generally have larger pumping rates compared to domestic wells and therefore have larger areas of the aquifer contributing water to the well that enables the capture of more potential sources and have faster movement of water from the top of the water table to the well screen. The locations of the sampled wells and samples in which individual THMs were detected are shown in SI Figures S1 and S2 for public wells and domestic wells, respectively. Chloroform was widely distributed throughout the United States.

significant associations between the presence of THMs and select anthropogenic and hydrogeologic variables associated with the sources, transport, and fate of THMs.

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OCCURRENCE OF TRIHALOMETHANES IN DRINKING-WATER SUPPLY WELLS Using an assessment level of 0.02 μg/L, chloroform was detected about twice as frequently in public-well samples (36.5%) as in domestic-well samples (17.6%), whereas other THMs occurred in public-well samples 6 to 11 times more frequently than in domestic-well samples (Figure 1). Using no assessment level, chloroform was detected in 44.4% of publicwell samples and 24.2% of domestic-well samples (SI Table S1). The greater detection frequency of THMs in public-well 8191

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Brominated THMs also were widely distributed across the United States but were detected much less frequently. The widespread geographic occurrence of the THMs indicates the ubiquitous nature of the sources of these compounds to both public and domestic wells. The spatial distribution of THM occurrence and nonoccurrence corresponds well with the redox conditions of aquifers. In general, THMs were less commonly detected in the central part of the United States, which corresponds with aquifers with higher percentages of anoxic waters.27 THMs are persistent under oxic conditions and biodegrade under anoxic conditions.5 Most of the quantified chloroform concentrations (i.e., samples in which the presence of chloroform met all qualification criteria) were less than 1 μg/L (89% in public wells and 93% in domestic wells), and many concentrations were less than 0.02 μg/L (18% in public wells and 27% in domestic wells (Figure 1). Median quantified concentrations for individual THMs ranged from 0.06 μg/L (chloroform) to 0.36 μg/L (bromoform) in public-well samples and from 0.04 μg/L (chloroform) to 0.08 μg/L (dibromochloromethane) in domestic-well samples. The larger median concentrations for the more brominated THMs (dibromochloromethane and bromoform) probably are due to the lower sensitivity of the analytical method for these compounds. No concentrations of any individual THM or total THMs in samples from public wells were greater than the MCL of 80 μg/L for total THMs, and only one sample from a domestic well had a chloroform concentration and a total THM concentration greater than this MCL (SI Figure S3).

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Figure 2. Detection frequencies of trihalomethanes by land-use category in (A) public-well samples, and (B) domestic-well samples using an assessment level of 0.02 μg/L, 1996−2007. [n, number of samples].

FACTORS ASSOCIATED WITH SOURCES, TRANSPORT, AND FATE OF TRIHALOMETHANES IN GROUNDWATER USED FOR DRINKING WATER Various analyses were performed that provide insight about the factors associated with sources, transport, and fate of THMs in groundwater supplying drinking water. Chloroform was detected in groundwater beneath a broad range of land-use settings, including agricultural, mixed, undeveloped, and urban (Figure 2). However, in nearly all cases, THMs were detected most frequently in urban land-use areas. Chloroform was detected more frequently in urban areas than in agricultural and undeveloped areas: about twice as frequently for public-well samples (Figure 2A) and about two to four times more frequently for domestic-well samples (Figure 2B). Similar to chloroform, brominated THMs in public-wells samples were detected most frequently in urban areas. Furthermore, brominated THMs were not detected in public-well samples in agricultural areas and were infrequently detected in domestic-well samples in agricultural and undeveloped areas probably due to a general lack of sources of chlorine-treated drinking water and/or wastewater in these areas. Median quantified concentrations of total THMs were highest in urban land-use areas for both public and domestic wells (SI Figure S3). Concentrations of total THMs were significantly higher in urban land-use settings than in mixed settings for both public and domestic wells (p-values of 0.006 and 0.0086, respectively) and in urban settings than in undeveloped settings for public wells (p-value of 0.0379). Differences in THM concentrations between urban and other settings were not statistically significant (p-values >0.05). For the 280 public wells and 457 domestic wells in which one or more THMs was detected using no assessment level, all four THMs were detected in 9.6% of public-well samples and in

1.5% of domestic-well samples. A quantile plot of THM concentrations in public-well samples shows that concentrations follow the general pattern CHCl3 > CHBrCl2 > CHBr2Cl > CHBr3 (Figure 3). In addition, detection frequencies of THMs using an assessment level of 0.02 μg/L show trends of decreasing detection frequency with increasing THM bromine content; that is, in the following order for public wells: CHCl3 (36.5%) > CHBrCl2 (10.5%) > CHBr2Cl (5.9%)

Figure 3. Quantile plot of trihalomethane concentrations in publicwell samples, 1996−2007. 8192

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with increasing number of detected THMs (SI Figure S5). In samples where no THMs were detected, pesticides were detected in 22% of samples and nitrate (>1 mg/L) was detected in 24% of samples. In samples in which at least one THM was detected, pesticides and nitrate were detected in 52% and 71% of samples, respectively. In samples in which two or more THMs were detected, pesticides and nitrate were detected in 71% and 87% of samples, respectively. The increasing detection of pesticides and nitrate with increasing number of THM detections could indicate the recycling of water that contains THMs and other anthropogenic contaminants. The statistically significant results of the logistic regression analyses (Table 1) are separated by public and domestic wells

> CHBr3 (5.4%), and in the following order for domestic wells: CHCl3 (17.6%) > CHBrCl2 (1.8%) > CHBr2Cl (0.7%) > CHBr3 (0.4%) (Figure 1 and SI Table S1). This interpretation concurs with studies that have found that relative concentrations and detection frequencies of THMs originating from chlorination usually decrease with increasing bromine content (e.g., refs 6 and 24). Mixtures of two or more THMs occurred in 13.2% of all public-well samples and 2.3% of all domestic-well samples using no assessment level (SI Table S2). Especially noteworthy is that these percentages increase to 29.6% for public wells and 9.2% for domestic wells when one considers only those samples that have one or more THMs. This also shows that mixtures occur much more frequently in public wells than domestic wells, which possibly indicates a more potent source of THMs in public wells and points to the urban areas in which the wells typically are located. In public-well samples evaluated using no assessment level, the brominated THMs commonly (92% of the samples) occurred in a mixture that contained chloroform (SI Figure S4A). In domestic-well samples, the brominated THMs always (100% of the samples) occurred in a mixture that contained chloroform (SI Figure S4B). In samples with THM detections, the most common mode of occurrence was chloroform detection alone (about 70% for public wells and about 90% for domestic wells). Bromoform was the only brominated THM to occur alone (without chloroform or other THMs) in any of the samples, but it only occurred alone in about 0.8% of the public-well samples with THM detections (SI Figure S4A). The brominated THMs did not occur alone in any of the domestic-well samples (SI Figure S4B). Chloroform and brominated THMs also commonly cooccurred with other VOCs, such as the solvents perchloroethene (PCE) and trichloroethene (TCE), the gasoline oxygenate methyl tert-butyl ether (MTBE), the gasoline hydrocarbon toluene, and the refrigerant dichlorodifluoromethane (SI Table S3). Mixtures of THMs and other VOCs were more common in public-well samples than in domesticwell samples (SI Table S3). The probability of detecting one or more VOCs in groundwater has been shown to increase with increasing population density,3 where population density is used as a surrogate for sources of VOCs (e.g., point-source releases and recycling of chlorinated water). Spatial analysis of 500 m radius buffers around the public and domestic wells using the 2000 Census data28 indicates that the buffers for public wells have an average population density about 11 times greater than buffers for domestic wells. Thus, the greater occurrence of mixtures in public-well samples compared to domestic-well samples likely is a result of the higher population density around public wells and more sources of VOCs. Mixtures of THMs with pesticides and nitrate also were common in samples from public and domestic wells. The cooccurrence of THMs with the five most frequently detected pesticides29 and concentrations of nitrate greater than 1 mg per liter (mg/L)considered indicative of human activity, such as fertilizer application, animal production, and septic systems, in many parts of the United States30was examined for publicwell samples from urban areas because public wells in these areas are most likely to capture water that may contain contaminants from a variety of sources. Of 412 samples from public wells in urban areas that were analyzed for THMs, 302 of these samples also were analyzed for pesticides and nutrients. The detection frequency of pesticides and nitrate increased

Table 1. Logistic Regression Analyses Results for Significant Relations for Chloroform and Brominated THMs in PublicAnd Domestic-Well Samples variable

type of variable

Chloroform in Public Wells dissolved-oxygen content fate annual air temperature transport recharge transport seasonally high water table depth transport/ fate number of households on septic source systemsa Chloroform in Domestic Wells fate dissolved-oxygen content (binary)b annual precipitation transport percentage of urban land usea source annual air temperature transport number of households on septic source systemsa agricultural land usea source soil available water capacity transport Brominated THMs in Public Wells dissolved-oxygen content fate percentage of urban land usea source soil thickness transport number of households on septic source systemsa Brominated THMs in Domestic Wells percentage of households on public source supplya soil percent sand transport source percentage of urban land usea

standardized coefficient 0.28 0.14 0.13 0.11 0.11

0.09 0.09 0.03 0.02 0.02 −0.02 −0.02 0.13 0.11 −0.06 0.04

0.02 −0.02 0.02

a

Within a 500-m-radius buffer around the well. bBinary coding: 0 < 0.5 mg/L; 1 ≥ 0.5 mg/L.

and by chloroform and brominated THMs. Significantly associated independent variables are sorted from highest to lowest standardized coefficients. The occurrence of THMs in groundwater from public and domestic wells was positively associated with the dissolved-oxygen content of groundwater for all THMs with the exception of brominated THMs in domestic wells, for which the relation was not significant (Table 1). The dissolved-oxygen content of an aquifer is associated with the fate of THMs in the subsurface. The association of THMs in groundwater from public and domestic wells with urban land use near the well was significantly positive for all THMs except for the occurrence of chloroform in public wells, for which the relation was not 8193

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occurrence of THMs in mixtures of other contaminants with known urban sources, such as MTBE, TCE, PCE, and toluene. Although the source of much of the chloroform detected in this study is urban in nature, the sources do not appear to be point-source contamination sites. The logistic regression analyses performed for this study did not reveal any significant associations between the occurrence of chloroform in public or domestic wells and specific industrial contaminant sources, such as Superfund and RCRA (Resource Conservations and Recovery Act) sites, even though chloroform is known to occur frequently at Superfund sites.19 Other recent studies have confirmed the lack of association between the occurrence of chloroform and bromodichloromethane in public wells and industrial contamination sources (e.g., ref 32). The results of this assessment point to urban nonpoint sources of THMs to groundwater used for drinking water such as recharge of chlorine-treated drinking water or wastewater (e.g., leaking distribution lines). However, it was very difficult to identify a variable that could be used in relational analyses to specifically represent a nonpoint, chlorine-treated, drinkingwater or wastewater source for THMs. Some variables potentially could be good indicators of this source such as the percentage of households on public water systems or recharge. However, these variables were considered, at best, to be only surrogates for actual inputs, which have not been monitored, of chlorine-treated drinking water or wastewater to hydrologic systems. For public wells, the occurrence of chloroform was found to be positively associated with recharge, which is a factor associated with the transport of chloroform from sources such as leaking drinking-water distribution lines through the unsaturated zone to deeper groundwater supplying public wells. The occurrence of brominated THMs was found to be positively associated with percentage of households on public water supply, which points to chlorine-treated drinking water as a nonpoint source of THMs to groundwater. Other sources of THMs to groundwater used for drinking water are likely. For example, the main source of THMs to domestic wells probably is not chlorine-treated wastewater because most domestic wells are located in relatively rural areas where large-scale use of chlorine-treated water would not occur. Instead, shock chlorination and septic systems likely are the main sources of THMs to domestic wells.33 In support of this hypothesis, the logistic regression results from this study indicate a positive association between the occurrence of chloroform in domestic-well samples and the number of households on septic systems. The logistic regression results also indicate associations that run counter to the above theories on sources of THMs to public and domestic wells. For example, a positive association was revealed between the occurrence of chloroform in public wells and the number of households on septic systems. Also, a positive association was revealed between the occurrence of brominated THMs in domestic wells and urban land use. In cases where public wells are located in rural settings (e.g., smaller municipal systems and rural water systems), septic systems could be an important source of THMs. Likewise, domestic wells located in urban areas could be affected by a nonpoint source of THMs from chlorine-treated drinking water or wastewater. These findings indicate the sources of THMs to drinking water are complex and that the underlying processes may not be well described by the variables included in the regression analysis. Information on groundwater ages and

significant (Table 1). The occurrence of THMs in groundwater from public and domestic wells also was significantly positively associated with the number of households on septic systems for all THMs except the brominated THMs in domestic wells, for which the relation was not significant. In addition, the positive association was significant between the occurrence of brominated THMs in domestic wells and the number of household systems on public water supply. All of these independent variables are factors associated with sources of THMs to groundwater. The occurrence of chloroform in public wells was significantly positively associated with recharge, whereas the occurrence of chloroform in domestic wells was significantly positively associated with annual precipitation (Table 1). The occurrence of THMs also was significantly associated with several variables including annual air temperature, seasonally high water table, soil available water capacity, soil thickness, and soil percent sand. All these independent variables are associated with the transport of THMs to groundwater because these factors either facilitate or impede the transport of THMs from sources located at or near the ground surface to the aquifer.

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DISCUSSION Previous researchers theorized that chlorine-treated drinking water and/or wastewater may be sources of THMs to hydrologic systems and thereby groundwater that supplies drinking-water wells,6 which is supported by results from this study. The general downward trend of THM concentrations with increasing THM bromine content has been noted in previous studies as resulting from the haloform reaction during disinfection of drinking-water supplies.31 This same trend was observed in the relative concentrations of THMs in public wells in this study (Figure 3) indicating that the source could be treated drinking water. Decreasing concentrations of brominated THMs from the haloform reaction also likely results in a downward trend in detection frequencies of THMs with increasing bromination. This would occur because lower concentrations of brominated THMs would have a lower likelihood of analytical detection at a specified assessment level. Numerous other studies have noted a downward trend in THM detection frequencies with increasing bromination when more than one THM has been detected.31 In this study, THMs display a pattern of decreasing detection frequency with increasing bromination in both public and domestic wells (Figure 1). No other clear sources for THMs are evident. The occurrence of brominated THMs in public wells is almost certainly the result of formation by the haloform reaction. Bromodichloromethane and dibromochloromethane have no known historical or current industrial production, and bromoform has only a small historical production.6 Additionally, brominated THMs nearly always occur together indicating a common source. The occurrence of chloroform without the brominated THMs in some samples may indicate an industrial/ commercial source for those samples, but more likely is a consequence of low-level detections of chloroform with concentrations of the brominated THMs below the detection limit. An urban source was evident for the occurrence of THMs, especially for chloroform in public wells. In this study, and for both public and domestic wells, the detection frequency of chloroform was higher in urban areas than other land-use areas such as agricultural or undeveloped. In addition, an urban source for THMs is further supported by the common 8194

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logistic regression models of chloroform in public and domestic wells and brominated THMs in public wells. High nitrate concentrations also have been associated with oxygenated groundwater;37 thus, the common co-occurrence of THMs and nitrate (SI Figure S5) is not surprising. These results indicate that the dissolved-oxygen condition of an aquifer is very important in the occurrence of THMs and to aquifer vulnerability to contamination from anthropogenic sources. Understanding the dissolved-oxygen conditions can provide valuable insight into the vulnerability of aquifers to contamination by THMs, which is most critical for settings where both relatively high dissolved-oxygen conditions and sources of THMs exist. Recharge was a significant transport variable in the multivariate logistic regression model of chloroform occurrence in public wells. Other transport surrogates such as annual precipitation and soil properties were significant in the multivariate logistic regression models of THM occurrence. Local-scale research is needed to uncover specific controlling transport variables in various areas so that groundwater managers can accurately identify and reduce resource vulnerability. In addition, private well owners may find this information valuable so that septic systems can be located, operated, and maintained in a manner that will minimize potential contamination by THMs.

traveltimes probably would have improved insights, but was only available for a small subset of wells. The occurrence of chloroform in public and domestic wells and the occurrence of brominated THMs in public wells were significantly positively associated with dissolved-oxygen content of groundwater and this variable had the highest standardized coefficient in all multivariate models where it was significant. This indicates that the presence or absence of oxygen in an aquifer is an important factor in the occurrence of THMs. This interpretation is consistent with published data on the hypoxic biodegradation of THMs and indicates that biodegradation is important in controlling the fate of THMs in aquifers.34,35

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IMPLICATIONS Chlorine-treated drinking water or wastewater appears to be the most important nonpoint source of THMs to public wells. This interpretation is important because groundwater from public wells supplied drinking water to approximately 105 million people in the United States in 2007.36 Consequently, local-scale research is needed to reveal the specific sources of THMs to public wells so groundwater managers can accurately identify resource vulnerability and control these sources. For public wells, this likely involves understanding how to minimize the recharge of chlorine-treated drinking water or wastewater from sources such as irrigation or leaking drinking-water or wastewater distribution lines. For public wells located in rural areas or for public wells serving rural water systems, this also may involve understanding how septic systems may be a source of THMs and how to minimize potential contributions from this source. Likewise, local-scale research also is needed to understand the specific sources of THMs to domestic wells so that private well owners can take appropriate measures to protect their groundwater resource. For domestic wells, this likely involves better understanding of how to minimize potential THM contributions from shock chlorination and septic systems. Additional research also is needed to better understand how chlorine-treated drinking water or wastewater from public systems may be a source of THMs to domestic wells in or near urban areas, and how to minimize potential contributions from this source. Most THM concentrations in public or domestic wells were less than 1 μg/L and only one concentration of total THMs in one domestic well exceeded the current USEPA MCL of 80 μg/L. Clearly, most of the concentrations of THMs encountered in this study do not pose a substantial humanhealth hazard. However, if the occurrence of THMs in public wells is believed to have a source from the chlorine-treatment of drinking water, then it is possible that other drinking-water DBPs may be present in ambient groundwater supplying drinking water. Many hundreds of DBPs are known to occur in treated water,1 and all but nine DBPs are unregulated and therefore are not monitored in many public water systems. Many of these unregulated DBPs can be formed from disinfection processes other than chlorination and some are more genotoxic than some of the regulated DBPs.1 Likewise, if the THMs in domestic wells have a source from septic systems, other contaminants such as nitrate and microbes could be present. Monitoring of public and domestic wells for a variety of contaminants is necessary to identify contaminants that may pose human-health hazards. Dissolved oxygen was the most important factor (highest standardized coefficient value) of all variables in multivariate

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ASSOCIATED CONTENT

S Supporting Information *

Text S1: Sample collection and analytical methods; Text S2: Assessment level; Text S3: Land-use classification; Text S4: Logistic regression analyses; Figure S1: Geographical distribution of concentrations of (A) chloroform, (B) bromodichloromethane, (C) dibromochloromethane, and (D) bromoform, in 631 public-well samples throughout the United States, 1996− 2007; Figure S2: Geographical distribution of concentrations of (A) chloroform, (B) bromodichloromethane, (C) dibromochloromethane, and (D) bromoform, in 1861 domestic-well samples throughout the United States, 1996−2007; Figure S3: Concentrations of total trihalomethanes by land-use category in public- and domestic-well samples, 1996−2007; Figure S4: Pie charts showing the distribution of trihalomethane detections in (A) public-well samples and (B) domestic-well samples using no assessment level, 1996−2007; Figure S5: Detection frequencies of pesticides and nitrate (nitrate concentrations >1 mg/L) in samples from public wells in urban areas in comparison to the number of trihalomethanes detected in same samples using no assessment level, 1996−2007; Table S1: Detection frequencies of trihalomethanes in public- and domestic-well samples at an assessment level of 0.02 μg/L and no assessment level, 1996−2007; Table S2: Number of trihalomethanes detected using no assessment level and the percentage of samples with detections from public and domestic wells, 1996−2007; Table S3: Co-occurrence of trihalomethanes (THMs) with other VOCs using no assessment level, 1996−2007. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 8195

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ACKNOWLEDGMENTS We thank the many USGS personnel who collected and compiled the water-quality data used in this article. We thank Susan Richardson (USEPA), Sandra Eberts (USGS), Patricia Toccalino (USGS), and the anonymous journal reviewers for providing insightful reviews that substantially improved this paper. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

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