Abstract

Source water, defined as groundwater collected from a community water system well prior to water treatment, was sampled from 221 wells during October 2002 to July 2005 and analyzed for 258 anthropogenic organic compounds. Most of these compounds are unregulated in drinking water and include pesticides and pesticide degradates, gasoline hydrocarbons, personal-care and domestic-use products, and solvents. The laboratory analytical methods used in the study have detection levels that commonly are 100 to 1,000 times lower than State and Federal standards and guidelines for protecting water quality. Detections of anthropogenic organic compounds do not necessarily indicate a concern to human health but rather help to identify emerging issues and track changes in occurrence and concentrations over time.

Less than one-half (120) of the 258 compounds were detected in at least one source-water sample. Chloroform, in 36 percent of samples, was the most commonly detected of the 12 compounds that were in about 10 percent or more of source-water samples. The herbicides atrazine, metolachlor, prometon, and simazine also were among the commonly detected compounds. The commonly detected degradates of atrazine—deethylatrazine and deisopropylatrazine—as well as degradates of acetochlor and alachlor, generally were detected at concentrations similar to or greater than concentrations of the parent herbicide. The compounds perchloroethene, trichloroethene, 1,1,1-trichloroethane, methyl tert-butyl ether, and cis-1,2-dichloroethene also were detected commonly. The most commonly detected compounds in source-water samples generally were among those detected commonly across the country and reported in previous studies by the U.S. Geological Survey’s National Water-Quality Assessment Program.

Relatively few compounds were detected at concentrations greater than human-health benchmarks, and 84 percent of the concentrations were two or more orders of magnitude less than benchmarks. Five compounds (perchloroethene, trichloroethene, 1,2-dibromoethane, acrylonitrile, and dieldrin) were detected at concentrations greater than their human-health benchmark. The human-health benchmarks used for comparison were U.S. Environmental Protection Agency Maximum Contaminant Levels (MCLs) for regulated compounds and Health-Based Screening Levels developed by the U.S. Geological Survey in collaboration with the U.S. Environmental Protection Agency and other agencies for unregulated compounds. About one-half of all detected compounds do not have human-health benchmarks or adequate toxicity information to evaluate results in a human-health context.

Ninety-four source-water and finished-water (water that has passed through all the treatment processes but prior to distribution) sites were sampled at selected community water systems during June 2004 to September 2005. Most of the samples were analyzed for compounds that were detected commonly or at relatively high concentrations during the initial source-water sampling. The majority of the finished-water samples represented water blended with water from one or more other wells. Thirty-four samples were from water systems that did not blend water from sampled wells with water from other wells prior to distribution.

The comparison of source- and finished-water samples represents an initial assessment of whether compounds present in source water also are present in finished water and is not intended as an evaluation of water-treatment efficacy. The treatment used at the majority of the community water systems sampled is disinfection, which, in general, is not designed to remove the compounds monitored in this study.

Concentrations of all compounds detected in finished water were less than their human-health benchmarks. Two detections of perchloroethene and one detection of trichloroethene in finished water had concentrations within an order of magnitude of the MCL. Concentrations of disinfection by-products were expected to increase in finished water relative to source water because of disinfection. The MCL for concentrations of disinfection by-products analyzed in this study is for total trihalomethanes, and concentrations were within an order of magnitude of the MCL in finished water from nine sites, but only three detections of chloroform and one detection of bromoform were within an order of magnitude of the MCL. Concentrations of all other compounds were more than an order of magnitude less than human-health benchmarks.

About one-half (57 percent) of the detections from the 34 community water systems where finished water was not blended with other source water were in both source and finished water, and concentrations were similar, with the exception of disinfection by-products. Most of the detections were gasoline-related compounds, herbicides and herbicide degradates, and solvents. Results for blended finished-water samples generally were similar to nonblended finished-water samples, and blending did not always reduce concentrations to less than the laboratory reporting level. Differences in the occurrence of compounds in source and finished water could be the result of water treatment, volatilization, blending, or analytical variability at concentrations near or less than the laboratory reporting level. Large changes in concentration from source to finished water of solvents in a few samples likely are attributable to additional water treatment steps used by the CWS to treat water known to contain elevated concentrations of organic compounds.

Mixtures of two or more compounds were detected in about 70 percent of source-water samples and in 82 percent of finished-water samples. Mixtures occur more commonly in finished water than source water because of the formation of disinfection by-products in finished water. The potential human-health significance of the frequent presence of mixtures of organic compounds in finished water remains largely unknown.

Revised April 9, 2010

First posted December 4, 2009