California desert aquifers used for drinking water have high levels of inorganic elements, including arsenic and boron


     The U.S. Geological Survey reports that inorganic elements - arsenic, boron, fluoride, and five other inorganic elements - have been detected at high concentrations in 35 percent of untreated groundwater used for public water supply in the deserts of southern California. In contrast, human-made organic chemical constituents and nitrate were found at high concentrations in less than 1 percent of the desert region’s aquifers.

     This study's findings are important, in part, because elsewhere in California, high concentrations of inorganic elements generally are found in 10 to 25 percent of the aquifer system used for public supply, nitrate in 1 to 8 percent, and human-made organic chemical constituents in up to 2 percent.  "High" concentrations are defined as above the Environmental Protection Agency's or California Department of Public Health's established Maximum Contaminant Levels or other non-regulatory health-based levels for chemical constituents or elements not having MCLs. The U.S. Geological Survey did not analyze treated tap water.  Water distributors typically treat water supplies prior to delivering it to customers to ensure compliance with water quality standards for human health.

     As part of a statewide study assessing groundwater quality, USGS scientists analyzed untreated groundwater from wells in the desert region between 2006 and 2008, looking for as many as 207 chemical constituents. California's desert region includes Antelope Valley, Coachella Valley, Indian Wells Valley, Owens Valley, Mojave River area, and the Colorado River basin.

    "Over a ten-year period, the USGS is characterizing groundwater quality in 120 groundwater basins and other areas that supply about 95 percent of public groundwater supplies," explained USGS director Marcia McNutt. "The new results for the desert region show where, what, and how much contamination is in the groundwater, focusing attention on improving water quality where it is needed."

       Naturally occurring inorganic elements were found in high concentrations in 22% of Owens Valley, 30% of Antelope Valley, 28% of the Mojave area, 42% of Coachella Valley, 45% of Colorado River basins, and 62% of Indian Wells Valley. In these areas with high concentrations, one or more of the following eight inorganic elements was found at high concentrations: arsenic, boron, fluoride, gross-alpha radioactivity, molybdenum, strontium, vanadium, and uranium. These elements are naturally present in rocks and soils, and the water that comes in contact with those materials. High concentrations generally are the result of natural processes, but human activities may have some influence. An additional 28 percent of the aquifer systems had moderate concentrations, greater than one-half the comparison level, of at least one inorganic constituent.

      In contrast, high concentrations of organic constituents (such as solvents, gasoline components, and pesticides) and nitrate were found in less than 1 percent of the desert region's aquifers. Perchlorate was found at high concentrations in 10% of Coachella Valley aquifers and was not found at high concentrations in any of the other areas. High concentrations of organic constituents, nitrate, and perchlorate are typically associated with human activity.

      "Local water distributors and regional, state, and federal agencies, as well as the US EPA, are aware of the presence of arsenic, fluoride, uranium, and other inorganic constituents in groundwater in the desert region, and are actively working to manage local groundwater resources and assure that water delivered to consumers meets water-quality standards," said Dr. Miranda Fram, chief of the USGS Groundwater Ambient Monitoring and Assessment program.

   "One of the most interesting results of this study was the contrast between the relatively large percentage of groundwater with high concentrations of inorganic elements and the small percentage with high concentrations of human-made organic constituents; this contrast is much larger in the desert region than in other parts of California," said Barbara Dawson, a USGS hydrologist and author of the report prepared in collaboration with the California State Water Resources Control Board.


The USGS California Water Science Center is the technical lead for the State Water Resources Control Board GAMA Program's Priority Basin Project. The USGS is monitoring and assessing water quality in 120 priority groundwater basins, and groundwater outside of basins, across California over a ten-year period. The main goals of the State Water Board’s GAMA Program Priority Basin Project are to improve comprehensive statewide groundwater monitoring and to increase the availability of groundwater-quality information to the public.


The full report and six accompanying non-technical Fact Sheets are available online: