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Decade-Long Study Shows Natural Contaminants More Prevalent than Human-Made Contaminants in California Groundwater

Released: 15 July 2015
Contact Information: Bonnie Dickson, USGS 916-278-3318
Kenneth Belitz, USGS 858-775-6309

In partnership with:

California State Water Resources Control Board

California State Water Resources Control Board

Graph illustrating groundwater quality constituent by conentration

Groundwater quality in California: A large area and a large number of people are affected by high concentrations of trace elements, nitrate, and organic compounds.

SACRAMENTO, Calif. - Natural contaminants are more prevalent than human-made contaminants in California groundwater used for public supply, is the conclusion of a decade-long study by U.S. Geological Survey scientists. The study evaluated the quality of nearly all of the groundwater used for public supply in California and is the most comprehensive assessment of groundwater in any state to date. Groundwater provides about one-third of California's drinking supply in a typical year, but more during drought conditions.

Unlike other regional assessments, the USGS incorporated area and population as indicators of groundwater quality, an approach that captures a more accurate regional picture and is more applicable at multiple scales. Measurements of contaminants were weighted according to the population depending on the groundwater, and the size of the affected area. The study used data from 11,000 public supply wells in California, including 2,400 sampled by USGS, to evaluate nearly 200 natural and human-made contaminants in untreated groundwater.

The study, conducted as part of the California State Water Board's Groundwater Ambient Monitoring and Assessment Program, found that natural contaminants, such as arsenic and uranium, occur at high concentrations in about 20 percent of the groundwater resources used for supply. Human-made contaminants, such as nitrate and solvents, occur at high concentrations in about 5 percent of the resource. "High" concentrations are defined as above the Environmental Protection Agency's or California's State Water Board's Maximum Contaminant Levels or other non-regulatory health-based levels for chemical constituents or elements not having MCLs.

The USGS evaluated the quality of groundwater that occurs in place; those supplies are typically blended or treated prior to delivery to consumers in order to ensure that the water is safe to drink.

"Other studies typically evaluate a portion of the resource and attempt to extrapolate the results to other areas," said Dr. Kenneth Belitz, the USGS scientist who led the study. "The GAMA study covers just about the entire resource used for public supply, and evaluates unregulated contaminants, such as manganese, as well as regulated contaminants." Manganese, an unregulated contaminant, occurs at concentrations above non-regulatory human health benchmarks in about 5 percent of the resource used for drinking water supply.

Human-made contaminants are not as prevalent at high concentrations as natural contaminants, but they do occur in locations where large numbers of people rely upon groundwater. For example, organic solvents are present in parts of the San Fernando, San Gabriel, and Santa Ana Basins, where millions of people depend on groundwater for part of their drinking supply. The presence of solvents at high concentrations in these basins has long been known, and the problem is actively managed by local and regional agencies.

High concentrations of nitrate in groundwater are generally associated with the application of fertilizers, animal manure, and septic systems. In California, the occurrence of high concentrations of nitrate in groundwater used for public supply is primarily due to activities that occurred in the past. Consequently, high concentrations of nitrate can be found in areas where the land use in the past was agricultural but currently is urban, and not just in areas that are currently agricultural. Examples include the Livermore Valley and parts of the Santa Ana Basin.

In addition, there are areas where the land use is currently agricultural, but high concentrations of nitrate are not currently prevalent at the depth zone used for public supply. Concentrations of nitrate might become high in the future as groundwater moves from the land surface downward toward the depth zone used for public supply. Examples include parts of the San Joaquin and Sacramento Valley.

The assessment of groundwater used for public supply is the culmination of a 10-year USGS study sponsored by the State Water Board, and helps to fulfill a mandate provided by the Groundwater Quality Monitoring Act of 2001.

"The GAMA studies are an invaluable resource for the public and water managers to improve our understanding of groundwater quality. This comprehensive effort allows for better knowledge and access to information, which leads to informed groundwater management at the local, regional, and statewide levels," said John Borkovich of the State Water Board.

The article, "Metrics for Assessing the Quality of Groundwater Used for Public Supply, CA, USA: Equivalent-Population and Area" by Kenneth Belitz, Miranda Fram, and Tyler Johnson was recently published in the journal "Environmental Science and Technology" and is available online.

More information on the GAMA Program, including data, fact sheets, and reports, is available on the USGS California Water Science Center GAMA Project page and on the State Water Board GAMA page.

Links and contacts within this release are valid at the time of publication.


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