The U.S. Geological Survey released the results of models it ran indicating that higher than normal levels of nitrates and arsenic could be beneath aquifers in the Southwest.

According to the modeling, up to 2.4 percent of the area may have levels of nitrates that equal or exceed drinking-water standards set by the EPA, and up to 43 percent of the area may levels that equal or exceed the standard for arsenic. About 40 percent of the water used in this region comes from aquifers, providing an important resource for people living in that area.

Nitrates can include fertilizers and runoff from sewers and septic tanks. Arsenic is a natural element found in water supplies in Southwestern states. High nitrate levels have already been found in the groundwater around Phoenix, Los Angeles and California’s Central Valley.

“The alluvial basins of the American Southwest can provide a valuable water resource to growing populations who often lack other sources of fresh water,” said USGS Director Marcia McNutt. “However, the results of this modeling study raise a cautionary flag for private well owners of the need to test water to ensure its safety and to take action to remediate any contamination that is found.”

Areas where nitrate concentrations are predicted to equal or exceed the EPA drinking-water standard (10 milligrams per liter as nitrogen) occur in several basins in central Arizona near Phoenix; the southern part of California’s Central Valley; as well as several basins near Los Angeles along the southern coast; and the San Luis Valley of south-central Colorado.

Much of the area where arsenic concentrations are predicted to equal or exceed the drinking-water standard (10 micrograms per liter) is within several basins in parts of southwestern Arizona, southeastern California, western Nevada, and western Utah. Most of the area with predicted high arsenic concentrations is in sparsely populated rangeland, whereas most of the area with predicted high nitrate concentrations occurs where agricultural or urban communities are located.