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Chromium(VI) concentrations in excess of the California Maximum Contaminant Level (MCL) of 50 mu g/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 mu g/L was measured in aquifers 53 eroded from granitic rock. Chromium(VI) concentrations did not exceed 5 mu g/L at pH < 7.5 regardless of geology. delta Cr-53 values in native ground-water ranged from 0.7 to 5.1 parts per thousand and values were fractionated relative to the average 653 Cr composition of 0 parts per thousand in the earth’s crust. Positive delta Cr-53 values of 1.2 and 2.3 parts per thousand 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. delta Cr-53 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 delta Cr-53 values were observed as dissolved O-2 concentrations decreased, and Cr(VI) was reduced to Cr(III), and subsequently removed from solution. As a result, the highest delta Cr-53 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. delta Cr-53 values at an industrial site overlying mafic alluvium having high natural background Cr(VI) concentrations ranged from - 0.1 to 3.2 parts per thousand. Near zero delta Cr-53 values at the site were the result of anthropogenic Cr. However, mixing with native ground-water and fractionation of Cr within the plume increased delta Cr-53 values at the site. Although delta C-53 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 853 Cr values in conjunction with major-ion data, and the delta O-18 and delta D composition of water from wells. Published by Elsevier Ltd.