Contaminants in US Groundwater

The USGS study (Contaminants in Groundwater used for Public Supply) focuses primarily on source (untreated) water collected from public wells before treatment or blending rather than the finished (treated) drinking water that water utilities deliver to their customers. "By focusing primarily on source-water quality, and by testing for many contaminants that are not regulated in drinking water, this USGS study complements the extensive monitoring of public water systems that is routinely conducted for regulatory and compliance purposes by federal, state and local drinking-water programs," said Matthew C. Larsen, USGS Associate Director for Water. "Findings assist water utility managers and regulators in making decisions about future monitoring needs and drinking-water issues."

Findings showed that naturally occurring contaminants, such as radon and arsenic, accounted for about three-quarters of contaminant concentrations greater than human-health benchmarks in untreated source water. Naturally occurring contaminants are mostly derived from the natural geologic materials that make up the aquifers from which well water is withdrawn. Man-made contaminants were also found in untreated water sampled from the public wells, including herbicides, insecticides, solvents, disinfection by-products, nitrate and gasoline chemicals. Man-made contaminants accounted for about one-quarter of contaminant concentrations greater than human-health benchmarks, but were detected in 64% of the samples, predominantly in samples from unconfined aquifers.

"Detections of contaminants do not necessarily indicate a concern for human health because USGS analytical methods can detect many contaminants at concentrations that are 100-fold to 1,000-fold lower than human-health benchmarks", said lead scientist Patricia Toccalino. "Assessing contaminants in these small amounts helps to track emerging issues in our water resources and to identify contaminants that may warrant inclusion in future monitoring."

Scientists tested water samples for 337 properties and chemical contaminants, including nutrients, radionuclides, trace elements, pesticides, solvents, gasoline hydrocarbons, disinfection by-products and manufacturing additives. This study did not assess pharmaceuticals or hormones. Most (279) of the contaminants analysed in this study are not federally regulated in finished drinking water under the Safe Drinking Water Act.

The USGS also sampled paired source and finished (treated) water from a smaller subset of 94 public wells. Findings showed that many man-made organic contaminants detected in source water were generally detected in finished water at similar concentrations. Organic contaminants detected in both treated and source water were typically detected at concentrations well below human-health benchmarks, however.

Additionally, the study shows that contaminants found in public wells usually co-occurred with other contaminants as mixtures. Mixtures can be a concern because the total combined toxicity of contaminants in water may be greater than that of any single contaminant. Mixtures of contaminants with concentrations approaching benchmarks were found in 84% of wells, but mixtures of contaminants above health benchmarks were found less frequently (in 4% of wells).

This USGS study identifies which contaminant mixtures may be of most concern in groundwater used for public-water supply and can help human-health researchers to target and prioritise toxicity assessments of contaminant mixtures. The USGS report identifies the need for continued research because relatively little is known about the potential health effects of most mixtures of contaminants.

Wells included in this study are located in 41 states and withdraw water from parts of 30 regionally extensive aquifers, which constitute about one-half of the principal aquifers used for water supply in the United States. Human-health benchmarks used in this study include US Environmental Protection Agency Maximum Contaminant Levels for regulated contaminants and USGS Health-Based Screening Levels for unregulated contaminants, which are non-enforceable guidelines developed by the USGS in collaboration with the EPA and other water partners.