SAN DIEGO, Calif. — Solvents and other constituents - arsenic, uranium, and nitrate - were detected at high concentrations in up to four percent of untreated groundwater used for public water supply in the coastal region of Los Angeles and Orange Counties. Elsewhere in California, high concentrations of solvents are generally found in less than one percent of the aquifer system used for public supply. High concentrations of trace elements, such as arsenic and uranium, are found in 15 to 20 percent, and high concentrations of nitrate are found in 10 to 15 percent. High concentrations of solvents and nitrate are typically associated with human activity, whereas high concentrations of arsenic and uranium are naturally occurring. The Coastal Region includes the Hollywood, Santa Monica, West Coast, Central, and Orange County Coastal Plain groundwater basins.
As part of a statewide study assessing groundwater quality, U.S. Geological Survey scientists analyzed untreated groundwater from wells, looking for over 300 possible con'tituents. USGS did not analyze treated tap water. Groundwater is typically treated by water distributors prior to delivering it to customers to ensure compliance with water quality standards.
"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 Coastal Los Angeles Basin show where, what, and how much contamination is in the groundwater, focusing attention on improving water quality where it is needed."
Solvents, primarily tetrachloroethene (PCE), trichloroethene (TCE), and carbon tetrachloride, were detected at high concentrations, above the Environmental Protection Agency's established Maximum Contaminant Levels, in about four percent of the aquifer system. Moderate concentrations, found at greater than one-tenth of MCLs, were detected in an additional 11 percent of the aquifer system. The detected solvents are used for many purposes including manufacturing and cleaning, and were found most frequently in groundwater in the central and eastern parts of the Central Basin in Los Angeles County.
High concentrations of arsenic and uranium, above EPA MCLs, were detected in about 2 percent and one percent of the aquifer system, respectively. Moderate concentrations, found at greater than one-half of MCLs, of arsenic and uranium were detected in about four percent and 10 percent, respectively. Arsenic and uranium are naturally present in rocks and soil, and in the water that comes into contact with those materials. High concentrations in the Coastal Los Angeles Basin can be attributed to natural processes.
High concentrations of nitrate, above the EPA MCL, were detected in about two percent of the aquifer system, and moderate concentrations, found at greater than one-half of the MCL, in about two percent. Sources of nitrate include agriculture, effluent from wastewater treatment plants, and septic systems.
"Local water distributors, regional agencies, as well as the US EPA, are aware of the presence of solvents, arsenic, uranium, and nitrate in groundwater in both Los Angeles and Orange Counties, and are actively working to manage local groundwater resources and assure that water delivered to consumers meets water-quality standards," said Dr. Kenneth Belitz, chief of the USGS Groundwater Ambient Monitoring and Assessment program.
"The geographic distribution of solvents in the groundwater indicates that the solvents we found are not new in the aquifer system, but are from historic uses. It has taken many years for these solvents to be drawn from areas of natural and engineered recharge to water supply wells in the basin by pumping." said Dr. Miranda Fram, a USGS chemist 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.