Arsenic, Chromium, and Uranium Occurrence in California Groundwater at the Macro, Well-bore, and Micro Scale

Uranium (Nuclear Minerals) / Exploration, Mining, Marketing, & Acquisitions

Date posted: 02/20/15

Izbicki, et al., of the U.S.G.S., report that arsenic and uranium exceed their respective Maximum Contaminant Levels - MCLs - of 10 micrograms per liter -ug per L- and 30 u ­g per L - about 20 picoCuries per liter) - part of the GAMA program. Arsenic is less abundant in the northern part of the state, but widely present at concentrations in excess of its MCL in other areas. Uranium, although less common at concentrations above the MCL, occurs at concentrations greater than its MCL in agricultural areas in the eastern San Joaquin Valley and in bedrock wells in granitic terrain. Hexavalent chromium - Cr VI - exceeded the recently proposed California MCL of 10 ug-L in 7 percent of sampled public supply wells. Chromium, as Cr VI, occurs at concentrations greater than its MCL in alluvial aquifers eroded from mafic rocks such as those found in the Coast Ranges of California, and in some desert aquifers. Occurrence of these elements in groundwater is ultimately related to their abundance in rocks and minerals that erode to form aquifers. However, arsenic, uranium, and chromium have redox chemistry and pH-dependent sorptive properties that combine with geologic and hydrologic processes to influence the occurrence of these elements in groundwater.