Understanding Chemical and Microbial Contaminants in Public Drinking Water

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Collaborative joint agency study provides nationally consistent and rigorously quality-assured datasets on a wide range of chemical and microbial contaminants present in source and treated public drinking water supplies. Tap water was not analyzed in this study.

Buras water tank

U.S. Geological Survey (USGS) and Environmental Protection Agency (USEPA) scientists collected samples of source (untreated, raw) and treated drinking waters from 29 drinking-water treatment plant across the country. (Credit: Barb Sturner, Federal Emergency Management Agency. Public domain.)

Public drinking water in the United States is of high quality. Disinfection alone has greatly reduced acute health risks of drinking public water supplies. Protection of the Nation's drinking water resources is a priority for and the responsibility of the U.S. Environmental Protection Agency (USEPA) under the Safe Drinking Water Act in conjunction with State and Tribal agencies and water utilities. Water purveyors consistently monitor more than 100 chemical and microbial contaminants as part of compliance requirements under the Safe Drinking Water Act. The public has access to compliance monitoring results and water purveyors summarize this information annually in Consumer Confidence Reports issued to all customers. Beyond compliance monitoring, there currently are other sources of data on regulated and unregulated contaminants in untreated and treated drinking water as part of the USEPA's Unregulated Contaminant Monitoring Rule and from the independent efforts of public water purveyors themselves, academia, and other research and assessment programs.

Tens of thousands of organic chemicals such as pharmaceuticals, fragrances, plastic components, surfactants, fire retardants, hormones, and pesticides, as well as inorganic chemicals such as metals, are used in large quantities for a range of purposes by modern society. Many of these chemicals have been detected in a variety of environmental waters that serve as source waters for drinking water treatment plants (DWTPs) and in some cases have been detected in public drinking water.

The USEPA and the U.S. Geological Survey (USGS) joined efforts to broaden the scope of existing public drinking water information by providing a nationally consistent and rigorously quality-assured dataset on a wide range of chemical and microbial contaminants present in source and treated waters. The two agencies analyzed water samples for 233 chemical and 14 microbial contaminants in source (untreated, raw) and treated drinking waters from 29 DWTPs. Tap water was not sampled as part of this research. Based on previous studies, the concentrations measured in this study were thought to be close to laboratory detection limit capabilities. Therefore, a rigorous quality-assurance plan was utilized to include quality-control samples for more than 70 percent of all samples collected.

Spigot sticking out of a pipe in a drinking-water treatment plant

Typical spigot near the wellhead inside of a drinking-water treatment plant that the scientists used to collect raw water samples prior to any treatment. (Credit: Laura Hubbard, USGS. Public domain.)

The results of this collaborative study were published in eight papers in the journal Science of the Total Environment. The highlights are summarized in this article and the links to the full articles are listed below. Similar to previous studies done with a more limited list of contaminants, scientists determined that samples from DWTPs serving a range of population sizes that use a variety of water sources and employ common treatment processes contain a wide range of regulated and unregulated chemical and microbial contaminants in source as well as treated water.

Wellhead inside of a drinking-water plant

This study included drinking-water treatment plants that used surface-water and groundwater sources. This photo shows a typical wellhead inside of a drinking-water plant. (Credit: Kymm Barnes, USGS. Public domain.)

Not surprisingly, many of the targeted organic chemical contaminants tended to be infrequently detected in spite of low detection capabilities, whereas a smaller subset of per- and poly-fluoroalkyl chemicals and inorganic constituents were more frequently detected even in finished drinking water samples. Inorganic contaminants are almost always detected in water because most are naturally occurring (often referred to as "geogenic"), whereas organic chemicals analyzed are largely associated with human-made sources (often referred to as "anthropogenic"). Fewer chemical and microbial contaminants were detected in treated drinking water samples (121) than source-water samples (148). Pathogenic microorganisms were widespread in source waters but treatment was generally effective in reducing these to below detection limits. The one exception was mycobacteria (previously documented to grow within DWTPs and known to be resistant to drinking water treatment), which were commonly detected in treated water when a combination of live and dead cells were measured.

These results indicate that although most of the chemical contaminants were below detection after treatment, many were present in water distributed for potable use. The concentrations of chemical contaminants were typically in the parts per trillion range. Determinations of the potential health implications of drinking water at those concentrations was beyond the scope of this effort however comparisons to existing health benchmarks revealed no immediate public health concerns. The unique datasets developed by this research will be used for a range of potential activities including USEPA's drinking water standard determination process and other research on drinking water exposures and health being explored collaboratively by USGS and public-health experts.

The USGS Environmental Health Program (Toxic Substances Hydrology and Contaminant Biology) and the USEPA Office of Research and Development funded this study through Interagency Agreement DW14922330.