USGS releases comprehensive groundwater quality data for southern Sierra Nevada

Compounds of concern detected, but far below health-based thresholds

Man-made constituents were detected in untreated groundwater in 24 of 50 wells sampled in the southern Sierra Nevada, but these compounds were all at concentrations far below health-based regulatory standards, the U.S. Geological Survey says in a study released today.

Under the State Water Board’s Groundwater Ambient Monitoring and Assessment (GAMA) Program, the USGS California Water Science Center is project lead for the Priority Basin Assessment Project, which is testing groundwater-quality throughout California. The GAMA program’s main objectives are to improve statewide ambient groundwater quality monitoring and assessment and to increase the availability of information about groundwater quality to the public. 

In June 2006, USGS scientists collected water samples from 50 public-supply, irrigation, and domestic wells in the Sierra portions of Kern and Tulare counties to test for hundreds of man-made and naturally-occurring constituents. Although man-made compounds were detected, the concentrations were below health-based standards. The detected concentrations of most naturally-occurring constituents were also below heath-based standards.

GAMA does not evaluate the quality of water delivered to consumers. After withdrawal from the ground, water for public systems is typically treated or mixed to maintain water quality before consumers receive it.
 
“The ability to detect the presence of man-made compounds in public-supply wells at ultra-low concentrations is important for the protection of our water resources,” said Dr. Kenneth Belitz, GAMA Program Chief Scientist. “An important goal for GAMA is to understand the current condition of water quality in our aquifers and to establish a basis for comparison in the future.”

The most frequently detected man-made compound was trichloromethane (chloroform), a byproduct formed during disinfection of drinking water. Detections of chloroform were all less than 0.15 micrograms per liter, which is well below the Environmental Protection Agency maximum contaminant level (MCL) for chloroform of 80 micrograms per liter.

A microgram per liter is equivalent to 1 part per billion – the same as one minute in 2,000 years or one cent in $10 million. The “maximum contaminant level” is the highest amount of a compound allowed in drinking water, under state or federal guidelines. 

The ultra-low-level analyses conducted by the GAMA Priority Basin Assessment Project is intended to help water suppliers better understand and manage groundwater basins, one of California’s primary sources of water. By providing an early warning system of very low detections, GAMA scientists are shedding light on the factors that affect groundwater quality.

A subset of the groundwater samples were analyzed for 46 naturally-occurring elements and nutrients, of which 28 have health-based standards. Only six of these naturally-occurring, regulated compounds were detected at concentrations above a drinking water standard.
 
Two constituents were detected at concentrations greater than an MCL or proposed MCL. Arsenic was detected above the MCL in four samples, although only one was from an active public supply well. Radon-222 was detected above the proposed alternative MCL in one sample from an irrigation well.

Groundwater samples were analyzed for major and minor ions, trace elements, nutrients, volatile organic compounds, pesticides and pesticide breakdown products, pharmaceutical compounds, radiochemical constituents, and constituents of special interest. Naturally occurring environmental tracers, including noble gases (in collaboration with Lawrence Livermore National Laboratory), tritium, carbon-14, and stable isotopes of water also were measured in these samples to help identify the source and age of the sampled ground water.

The USGS report is entitled, “Ground-Water Quality Data in the Southern Sierra Study Unit, California, 2006: Results from the California GAMA Program,” by Miranda S. Fram and Kenneth Belitz. U.S. Geological Survey Data Series Report 301 may be found on the Web.

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The U.S. Geological Survey's California Water Science Center operates project offices in Sacramento and San Diego where more than 130 scientists bring a broad range of disciplines to modern water-management issues. The center also has nine field offices where scientists and technicians gather hydrologic data on California's surface-water and ground-water resources.