New USGS report details groundwater quality in Central Eastside San Joaquin Valley
Report looks at groundwater basins in Stanislaus and Merced counties
Jim Nickles
United States Geological Survey
04/13/2010
Inorganic constituents such as arsenic, vanadium and nitrate were detected above health-based benchmarks in nearly one in five groundwater samples collected during a comprehensive assessment of water quality in aquifers in Stanislaus and Merced counties in California.
The samples were collected at wells, not household taps.
The findings are detailed in a new report from the U.S. Geological Survey (USGS) and in a related four-page fact sheet intended for the public. These documents are based on testing of 78 wells in 2006, in addition to a comprehensive review and analyses of existing data from the California Department of Public Health database. The study was conducted under the State of California’s Groundwater Ambient Monitoring and Assessment (GAMA) Program.
The water sampled was untreated and as it emerged from wells. Contrast that with water from public systems, which comes out of a tap, and which may be disinfected, filtered, mixed, and/or exposed to the atmosphere before it is delivered to consumers. Water in California’s public systems is regulated by the state’s Department of Public Health. This study was designed to characterize the quality of untreated groundwater, but for context it used benchmark standards -- State or Federal Maximum Contaminant Levels, where applicable -- established for drinking water.
The study focused on the Central Eastside San Joaquin Basin study unit, which consists of three groundwater subbasins that include the cities of Modesto, Turlock and Merced. While the vast majority of the region’s groundwater is used for agricultural irrigation, the basins still account for about 75 percent of the cities’ public drinking-water supplies.
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.
According to the USGS report, concentrations of inorganic constituents -- which are naturally occurring but can be influenced by natural processes and human activities -- were above human-health benchmarks in 18 percent of the samples. The inorganic constituents most commonly present above benchmarks were arsenic, vanadium and nitrate.
Concentrations of organic constituents, predominantly volatile organic compounds -- which are present in products used in the home, business, industry or agriculture -- were above human-health benchmarks in 1.2 percent of the samples. The organic constituents present above benchmarks were the discontinued soil fumigant 1,2-dibromo-3-chlororopane (DBCP) and the solvent tetrachloroethene (PCE).
