Chromium, chromium isotopes and selected trace elements, western Mojave Desert, USA

Chromium(VI) concentrations in excess of the California Maximum Contaminant Level (MCL) of 50 μg/L occur naturally in alkaline, oxic ground-water in alluvial aquifers in the western Mojave Desert, southern California. The highest concentrations were measured in aquifers eroded from mafic rock, but Cr(VI) as high as 27 μg/L was measured in aquifers eroded from granitic rock. Chromium(VI) concentrations did not exceed 5 μg/L at pH < 7.5 regardless of geology. δ⁵³Cr values in native ground-water ranged from 0.7 to 5.1‰ and values were fractionated relative to the average δ⁵³Cr composition of 0‰ in the earth’s crust. Positive δ⁵³Cr values of 1.2 and 2.3‰ were measured in ground-water recharge areas having low Cr concentrations, consistent with the addition of Cr(VI) that was fractionated on mineral surfaces prior to entering solution. δ⁵³Cr values, although variable, did not consistently increase or decrease with increasing Cr concentrations as ground-water flowed down gradient through more oxic portions of the aquifer. However, increasing δ⁵³Cr values were observed as dissolved O₂ concentrations decreased, and Cr(VI) was reduced to Cr(III), and subsequently removed from solution. As a result, the highest δ⁵³Cr values were measured in water from deep wells, and wells in discharge areas near dry lakes at the downgradient end of long flow paths through alluvial aquifers. δ⁵³Cr values at an industrial site overlying mafic alluvium having high natural background Cr(VI) concentrations ranged from −0.1 to 3.2‰. Near zero δ⁵³Cr values at the site were the result of anthropogenic Cr. However, mixing with native ground-water and fractionation of Cr within the plume increased δ⁵³Cr values at the site. Although δ⁵³Cr was not necessarily diagnostic of anthropogenic Cr, it was possible to identify the extent of anthropogenic Cr at the site on the basis of the δ⁵³Cr values in conjunction with major-ion data, and the δ¹⁸O and δD composition of water from wells.