High concentrations of arsenic found in 9 percent of groundwater in the northern San Joaquin Valley
Naturally occurring compound enters groundwater from Sierra Nevada erosion
For Immediate Release
Sept. 28, 2010
SACRAMENTO, CA -- High concentrations of naturally occurring inorganic constituents – including arsenic and boron – are found in about 13 percent of the primary aquifers in the northern San Joaquin Valley, 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 above a health-based benchmark. Some benchmarks are enforced through state or federal regulations, and some are merely advisory. The USGS assessment analyzed untreated groundwater from wells, not water delivered to consumers through the tap. Enforceable benchmarks apply only to tap water.
Arsenic was detected above the U.S. Maximum Contaminant Level of 10 parts per billion in about 9 percent of the primary aquifers. Boron was detected above the California Notification Level, a non-regulatory benchmark of 1 part per million, in about 8 percent of the primary aquifers.
Samples were collected from 61 wells in parts of Alameda, Amador, Calaveras, Contra Costa, San Joaquin and Stanislaus counties.
USGS scientists say the arsenic in the groundwater comes from minerals eroded from the volcanic and granitic rocks of the Sierra Nevada. Nearly all samples with high arsenic concentrations were collected from wells near the margin of or within the Sacramento-San Joaquin River Delta.
High concentrations of boron were primarily found in the western portions of the San Joaquin Valley, in the aquifer that receives its water from precipitation runoff in the Coast Ranges.
Concentrations of organic constituents – generally man-made compounds such as solvents and pesticides – were above health-based standards in about 3 percent of the groundwater. The discontinued soil fumigants 1,2-dibromo-3-chloropropane (DBCP) and ethylene dibromide (EDB) were the only organic constituents detected above health-based thresholds. DBCP was detected in about 3 percent of the groundwater, often in areas associated with orchards and vineyards.
The report, “Status and Understanding of Groundwater Quality in the Northern San Joaquin Basin, 2005: 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 2005, and a data report released in 2006, this 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 the web at http://pubs.usgs.gov/sir/2010/5175. An accompanying fact sheet may be found at http://pubs.usgs.gov/fs/2010/3079/.
Of the 290 organic and special-interest constituents tested for, 84 were detected. Three organic constituents were detected in 10 percent or more of the samples: chloroform, a byproduct from the disinfection of water; the solvent tetrachloroethene (PCE), and the herbicide simazine. Chloroform and simazine were detected at very low concentrations -- below one-tenth of a health-based threshold. Detections of PCE were generally low; however it was detected in one sample at two-tenths of the health-based threshold.
“This aquifer assessment reveals that groundwater quality is currently affected more by naturally occurring trace elements than by man-made compounds associated with human activities,” said George Bennett, a hydrologist and author of the USGS report.
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.
Links and contacts within this release are valid at the time of publication.
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