MOJAVE, Calif. (AP) — More aquifers in the Southern California desert contained high levels of arsenic, boron, fluoride and other naturally occurring elements compared with the rest of the state, a study released Wednesday found.

Federal scientists only looked at the presence of contaminants in raw, untreated groundwater and did not analyze tap water. Water agencies typically treat groundwater supplies to make drinking water and to comply with health standards.

Trace elements were found in high concentrations in 35 percent of groundwater used for public supply in the desert, compared with 10 to 25 percent elsewhere in the state.

One reason is that groundwater pumped from the desert tends to be older than groundwater pumped from other parts of the state, allowing more time for it to mix with elements found naturally in rocks and soil.

The study was conducted by the U.S. Geological Survey. Scientists tested groundwater samples from wells in the desert — including the Antelope Valley, Coachella Valley, Indian Wells Valley, Owens Valley, Mojave area, and the Colorado River basin — and around the state between 2004 and last year.

State and federal regulators and water agencies have long known about the existence of these elements in desert aquifers, but this is the most comprehensive assessment of groundwater quality in the desert.

Groups "are actively working to manage local groundwater resources and assure that water delivered to consumers meets water-quality standards," Miranda Fram, who heads the USGS groundwater monitoring program, said in a statement.

Besides studying the presence of naturally occurring contaminants, researchers looked at the role of human activity and found little impact on groundwater quality: High levels of solvents, pesticides and nitrates, typically associated with runoff from industries, agriculture and homes, were found in less than 1 percent of desert aquifers.

The USGS continues to monitor water quality in more than 100 groundwater basins around the state.