Nitrate was detected at high concentrations in 10 percent of the aquifer system used for public water supply in coastal areas of Santa Barbara and San Luis Obispo counties, according to a new U.S. Geological Survey report.
Trace elements, such as naturally occurring arsenic and molybdenum, were found at high concentrations in 27 percent of the aquifer system. In comparison, elsewhere in California high concentrations of nitrate have generally been found in less than 1 percent to 8 percent of the groundwater used for public supply, and trace elements in 6 percent to 28 percent.
As part of a statewide study assessing groundwater quality, scientists analyzed untreated groundwater from wells — not treated tap water. Groundwater is typically treated by water distributors prior to delivering it to customers to ensure compliance with water quality standards.
For this study, “high” concentrations are defined as being above the U.S. Environmental Protection Agency’s or California Department of Public Health’s established Maximum Contaminant Levels (MCLs), or above other nonregulatory health-based levels for chemicals without MCLs.
“High nitrate concentrations have been detected in 8 percent to 15 percent of the aquifer system used for public supply in other groundwater basins located in the South Coast and Transverse Ranges, so it was not surprising that high nitrate was prevalent in this area,” said Carmen Burton, a USGS hydrologist and author of the report prepared in collaboration with the California State Water Resources Control Board.
The trace elements most commonly detected at high concentrations were arsenic and molybdenum. Arsenic was detected at concentrations greater than the MCL in 7 percent of the aquifer system. Molybdenum was detected at concentrations greater than the nonregulatory EPA health advisory level in 25 percent.
Elevated concentrations of nitrate generally occur as a result of human activities, such as applying fertilizer to crops or landscaping. Septic systems, as well as livestock in concentrated numbers, also produce nitrogenous waste that can leach into groundwater. Arsenic and molybdenum are naturally present in rocks and soils and in the water that comes in contact with those materials.
The Groundwater Ambient Monitoring and Assessment Priority Basin Project has found that high molybdenum concentrations are more prevalent in the southern part of the South Coast ranges than in most other parts of the state.
“The South Coast Range–Coastal study is important because we are providing a quantitative assessment of the type and amount of natural and human-made constituents that occur in the deeper groundwater that is used for public-drinking water supplies,” said Miranda Fram, chief of the USGS Groundwater Ambient Monitoring and Assessment Program. “This information can be used by managers to ensure that our drinking water supply remains safe.”
The study was part of the statewide GAMA Program’s Priority Basin Project, which was designed to assess groundwater quality in aquifers that may be used for public water supply, and to better understand the natural and human factors affecting groundwater quality. USGS scientists drew samples from wells in 2008, looking for as many as 289 chemical constituents in areas including the Santa Ynez River Valley, San Antonio Creek Valley, Santa Maria River Valley, Los Osos Valley, and San Luis Obispo Valley groundwater basins, and also in parts of the surrounding upland areas in Santa Barbara and San Luis Obispo counties.
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 10-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.