Abstract

More than one-third of the Nation’s population receives their drinking water from public water systems that use groundwater as their source. The U.S. Geological Survey (USGS) sampled untreated source water from 932 public-supply wells, hereafter referred to as public wells, as part of multiple groundwater assessments conducted across the Nation during 1993–2007. The objectives of this study were to evaluate (1) contaminant occurrence in source water from public wells and the potential significance of contaminant concentrations to human health, (2) national and regional distributions of groundwater quality, and (3) the occurrence and characteristics of contaminant mixtures. Treated finished water was not sampled.

The 932 public wells are widely distributed nationally and include wells in selected parts of 41 states and withdraw water from parts of 30 regionally extensive aquifers used for public water supply. These wells are distributed among 629 unique public water systems—less than 1 percent of all groundwater-supplied public water systems in the United States—but the wells were randomly selected within the sampled hydrogeologic settings to represent typical aquifer conditions. Samples from the 629 systems represent source water used by one-quarter of the U.S. population served by groundwater-supplied public water systems, or about 9 percent of the entire U.S. population in 2008.

One groundwater sample was collected prior to treatment or blending from each of the 932 public wells and analyzed for as many as six water-quality properties and 215 contaminants. Consistent with the terminology used in the Safe Drinking Water Act (SDWA), all constituents analyzed in water samples in this study are referred to as “contaminants”. More contaminant groups were assessed in this study than in any previous national study of public wells and included major ions, nutrients, radionuclides, trace elements, pesticide compounds, volatile organic compounds (VOCs), and fecal-indicator microorganisms. Contaminant mixtures were assessed in subsets of samples in which most contaminants were analyzed.

Contaminant concentrations were compared to human-health benchmarks—regulatory U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Levels (MCLs) for contaminants regulated in drinking water under the SDWA or non-regulatory USGS Health-Based Screening Levels (HBSLs) for unregulated contaminants, when available. Nearly three-quarters of the contaminants assessed in this study are unregulated in drinking water, and the USEPA uses USGS data on the occurrence of unregulated contaminants in water resources to fulfill part of the SDWA requirements for determining whether specific contaminants should be regulated in drinking water in the future.

More than one in five (22 percent) source-water samples from public wells contained one or more naturally occurring or man-made contaminants at concentrations greater than human-health benchmarks, and 80 percent of samples contained one or more contaminants at concentrations greater than one-tenth of benchmarks. Most individual contaminant detections, however, were less than one-tenth of human-health benchmarks. Public wells yielding water with contaminant concentrations greater than benchmarks, as well as those with concentrations greater than one-tenth of benchmarks, were distributed throughout the United States and included wells that withdraw water from all principal aquifer rock types included in this study.

Ten contaminants individually were detected at concentrations greater than human-health benchmarks in at least 1 percent of source-water samples and collectively accounted for most concentrations greater than benchmarks. Seven of these 10 contaminants occur naturally, including three radionuclides (radon, radium, and gross alpha-particle radioactivity) and four trace elements (arsenic, manganese, strontium, and boron); three of these 10 contaminants (dieldrin, nitrate, and perchloroethene, or PCE) primarily have man-made sources. Radon activities were greater than the proposed MCL of 300 picocuries per liter (pCi/L) in 55 percent of samples and were greater than the proposed Alternative MCL of 4,000 pCi/L in 0.6 percent of samples. The remaining six contaminants from natural sources were detected at concentrations greater than benchmarks in 3 to 19 percent of samples. Dieldrin, PCE, and nitrate each were detected at concentrations greater than benchmarks in 1 to 3 percent of samples. Contaminants from natural sources accounted for three-quarters of contaminant concentrations greater than human-health benchmarks in source-water samples; contaminants from man-made sources accounted for one-quarter of such concentrations.

Trace elements and radionuclides were detected at concentrations greater than benchmarks in samples from both confined and unconfined aquifers, consistent with the fact that these contaminants originate primarily from aquifer materials. By contrast, man-made contaminants were detected at concentrations greater than benchmarks mainly in samples from unconfined aquifers, consistent with the fact that these contaminants originate from man-made sources at the land surface. For example, more than two-thirds of the samples with concentrations of pesticide compounds or VOCs greater than benchmarks were from public wells in the highly populated areas of states bordering the East Coast, primarily in semi-consolidated sand and gravel aquifers (mostly unconfined aquifers) that underlie the Coastal Plains.

Collectively, pesticide compounds or VOCs were detected in 64 percent of source-water samples, and concentrations of one or more of these contaminants were greater than benchmarks in 4.5 percent of samples. The most frequently detected organic contaminants were disinfection by-products, such as chloroform and bromodichloromethane, the herbicide atrazine and one of its degradates, deethylatrazine, the gasoline oxygenate methyl-tert-butyl ether (MTBE), and solvents, such as PCE, trichloroethene (TCE), and 1,1,1-trichloroethane. MCLs or HBSLs were not available for some frequently detected contaminants, such as MTBE and deethylatrazine, so detections of these contaminants could not be evaluated in the context of human health. One or more properties or contaminants were detected at concentrations outside of USEPA’s recommended ranges for the aesthetic quality of water in about one-half of the samples. Fecal-indicator microorganisms were detected in about 12 percent of source-water samples.

Contaminants detected in source-water samples usually co-occurred with other contaminants as mixtures. Few human-health benchmarks have been established for contaminant mixtures, but about 4 percent of samples contained mixtures of two or more contaminants at concentrations greater than individual benchmarks, and most samples (84 percent) contained mixtures of two or more contaminants at concentrations greater than one-tenth of individual benchmarks. Mixtures of two or more contaminants at concentrations greater than one-tenth of individual benchmarks were dominated by inorganic contaminants, and all of the most common unique mixtures (detected in 5 to 33 percent of samples) were composed of a trace element (arsenic, strontium, or uranium were most common), nitrate, and (or) radon (activities greater than 300 pCi/L). When mixtures of organic contaminants were assessed, regardless of the availability of human-health benchmarks, about three-quarters of the organic mixtures contained atrazine, simazine, or deethylatrazine, about two-thirds contained chloroform, and 43 percent contained PCE or TCE. The most complex mixtures in source water—those with the greatest number of contaminants—were most often detected in samples from unconfined aquifers, mostly from unconsolidated sand and gravel aquifers in the western United States.

Findings from this study indicate the ubiquitous nature of natural and man-made contaminant sources and that all principal aquifer rock types included in this study are vulnerable to contamination. Early attention to potential groundwater contamination is vital because groundwater contamination is difficult and costly to reverse once it occurs.

First posted May 21, 2010