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The U.S. Geological Survey (USGS) released a report describing the occurrence of volatile organic compounds (VOCs) in groundwater and drinking water supply wells across the nation. Volatile organic compounds are produced in large volumes and are associated with a myriad of products such as plastics, adhesives, paints, gasoline, fumigants, refrigerants and dry-cleaning fluids.
The report concludes that VOCs were detected in aquifers across the nation, and not limited to just a few specific aquifers or regions. Despite the nationwide occurrence, VOCs were not detected in most of the sampled wells (about 80% had no detections above a threshold of 0.2 ppb). VOCs were detected in some domestic and public supply wells, but seldom at concentrations greater than U.S. EPA regulatory or USGS health-based guidelines.
“VOCs are an important group of environmental contaminants to monitor and manage in groundwater because of their widespread and long-term use. Once released, VOCs tend to persist in the environment and migrate in groundwater, potentially to drinking water supply wells. Some VOCs are of concern because of their potential carcinogenicity or other health effects, and because they can change the taste and odor of drinking water,” said Dr. Robert Hirsch, USGS associate director for water. “The USGS assessment provides the most comprehensive national-scale analysis to date of VOC occurrence in aquifers used as an important supply of drinking water.”
The report is based on analysis of groundwater samples from nearly 3,500 wells, which are distributed randomly across broad regions and represent 98 aquifer studies across the nation—from Florida to the Pacific Northwest—plus a regional study in the High Plains aquifer system.
Although the USGS study did not analyze drinking water after treatment, the results from drinking water supply wells were compared to federal drinking water standards and other human health-based benchmarks as an initial screening level assessment. According to senior author John Zogorski, “VOCs were detected in drinking water supply wells—specifically, in 14% of domestic wells and 26% of public wells, but only a small number of samples (less than 2%) had VOC concentrations that were greater than federal drinking water standards.”
Concentrations greater than standards were accounted for by eight compounds, in large part by the solvents perchloroethene (PCE) and trichloroethene (TCE), and the agricultural fumigant dibromochloropropane (DBCP). “VOCs were detected more frequently in public wells than in domestic wells. It is likely that the higher rate of detection of VOCs in public wells is a result of their larger withdrawal rates and their proximity to developed areas, Zogorski said.
USGS findings suggest strong relations between VOCs in groundwater and percentage of urban land use within a half-kilometer radius of sampled wells. Zogorski added, “It is likely that urban areas have more sources of VOCs compared to other land-use settings. Source-water protection programs are critical for the effective management of VOC contamination, particularly for urban wells.”
The report provides a detailed examination of which VOC compounds occur most frequently in groundwater, and general patterns and associations explaining where they may be found. VOCs were detected in 90 of 98 aquifer studies completed across the nation. In general, detections of most VOC compounds were distributed across the nation; a few VOCs, such as the gasoline oxygenate methyl tert-butyl ether (MTBE) and soil fumigants, were found in a few distinct regions.
Many of the aquifer samples had low concentrations of VOCs, defined in the report as less than 1 ppb. The prevalence of VOCs at low concentrations indicates the need for groundwater managers and policy makers to continue to manage and monitor the occurrence of these contaminants over the long term. Each VOC has a unique pattern of occurrence depending on many factors related to its sources and to its persistence and transport in aquifers. The most frequently detected VOCs were chloroform, the solvents PCE and TCE, and MTBE. Thirteen VOCs were not detected at all.