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New Study Looks at Groundwater Quality in the North San Francisco Bay Region
Released: 9/3/2010 2:15:40 PM

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About 14 percent of groundwater basins have high concentrations of inorganic constituents; only 1 percent have high levels of man-made organic compounds.

SACRAMENTO, CA.High concentrations of naturally occurring inorganic constituents – including arsenic, boron and lead – are found in about 14 percent of the primary aquifers in Napa, Sonoma and Marin counties, according to a U.S. Geological Survey report prepared in cooperation with the California Water Resources Control Board. Primary aquifers are those that supply public-drinking water.

“High concentration” means that a level is above a health-based benchmark, some of which are regulatory. The USGS assessment analyzed untreated groundwater from wells, not water delivered to consumers through the tap. Regulatory benchmarks are enforced only for water delivered to consumers.

Arsenic was detected above the U.S. Maximum Contaminant Level of 10 parts per billion in about 10 percent of the primary aquifers. Boron was detected above the California Notification Level, a non-regulatory benchmark of one part per million, in about 4 percent of the primary aquifers. And lead was detected at high concentrations in about 2 percent of the primary aquifers. The U.S. Action Level for lead is 15 parts per billion.

High concentrations of arsenic were attributed to the dissolution of sediments that naturally contain arsenic. These processes are controlled by the amount of oxygen dissolved in the groundwater.

High concentrations of boron were primarily associated with groundwater mixing with hydrothermal waters or high-salinity waters in the Napa-Sonoma lowlands.

Concentrations of organic constituents – generally man-made compounds such as solvents and pesticides – were above health-based standards in about 1 percent of the groundwater.

The report, “Status and Understanding of Groundwater Quality in the North San Francisco Bay Groundwater Basins, 2004: California GAMA Priority Basin Project,” was conducted under the State of California’s Groundwater Ambient Monitoring and Assessment Program.

While the initial USGS well-sampling was conducted in 2004, and a data report released in 2006, this new interpretive report provides an assessment of the drinking water aquifers, based on the USGS data and more than 300,000 records in a California Department of Public Health database.

The report can be found on online. An accompanying fact sheet, or non-technical summary, may also be found online

The USGS has been coordinating closely with water purveyors in the region. The owners of wells sampled – cities, water agencies, private well owners – have received the results of the study.

Of the 255 organic and special-interest constituents tested for, twenty-six were detected. Two organic constituents were detected in 10 percent or more of the samples: chloroform, a byproduct from the disinfection of water, and the herbicide simazine. However, both were detected at very low concentrations -- below one-tenth of a health-based threshold. Nitrate was high in less than 1 percent of the primary aquifers.

Chloroform and simazine were generally detected in samples collected from shallow wells located in urban or agricultural areas.   In addition, age-dating of these samples indicated that all or some of the groundwater from these wells is less than 50 years old.  These results suggest that chloroform and simazine in groundwater are the result of human activities in the last 50 years.

“This aquifer assessment reveals that the major threats to groundwater quality are naturally occurring trace elements, rather than man-made compounds associated with human activities,” said Dr. Justin Kulongoski a research hydrologist and senior author of the USGS report.

“The work done by the Priority Basin Project in the North San Francisco Bay Area is important  because we are providing, for the first time, a quantitative assessment of the extent to which deep groundwater may have high concentrations of both  natural and man-made constituents,” said co-author Dr. Kenneth Belitz, chief of USGS’ GAMA program.  “This information can be used by managers to insure that our drinking water supply remains safe.”

The State Water Resources Control Board’s GAMA Program is collaborating with the USGS to monitor and assess water quality in 120 groundwater basins across California over a 10-year period. The main goals of GAMA are to improve comprehensive statewide groundwater monitoring and to increase the availability of groundwater-quality information to the public.

“Groundwater samples collected for this study contain fewer man-made compounds, at lower concentrations, than many groundwater basins studied by the USGS,” Kulongoski said. “The concentrations of these constituents were typically one-half to less than one-forty-thousandth of the levels utilized by the State of California’s Department of Public Health for regulatory purposes.”

“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,” Belitz said. “Our goal is to understand how these compounds are transported from the landscape and into the aquifer system.”

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Map showing the groundwater basins that were part of the study. (Click on image for a higher resolution image.) Map showing arsenic concentrations in wells tested by the USGS and in the California Department of Public Health database. (Click on image for a higher resolution image.)
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Map showing boron concentrations in wells tested by the USGS and in the California Department of Public Health database. (Click on image for a higher resolution image.) Map showing lead concentrations found in the study area, in wells tested by the USGS and the California Department of Public Health database. (Click on image for a higher resolution image.)

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