Chromate generation by chromate depleted subsurface materials

Soluble chromate concentrations as high as 200 mug Cr L-1 have been reporte d in water samples from monitoring wells tapping alluvial deposits allegedl y contaminated by laboratory waste as well as control wells off site and up gradient (shallow aquifer) near Davis California, U.S.A. In this report we present evidence that these Cr(VI) levels could have been generated by geog enic processes rather than by anthropogenic inputs. We tested the hypothesi s that native Cr(III) has been and can be oxidized to chromate by native ma nganese oxides. Twenty-three drill core samples (all unsaturated) were retr ieved from depths varying from 1.5 to 22.5 m in 6 different wells. Visible nodules of MnO2 were dispersed throughout many of the samples and carbonate s were also present. Sample pH values averaged about 8.0 and organic C was mostly less than 1.0 g kg(-1). Total Mn and Cr averaged 835 and 191 mg kg(- )1 respectively. All samples had the capability to oxidize added Cr(III) to Cr(VI). To determine the inherent capability of the samples to produce Cr( VI) from native Cr(III), subsamples were extracted with 5 mM CaSO4 plus 5 m M MgSO4 until Cr(VI) was no longer detected. After freeze-drying, deionized -distilled water was added to the leached samples to approximately field ca pacity (0.03 MPa). Freeze drying did not generate Cr(VI). These samples wer e incubated in polyethylene film bags at room temperature in the dark. Afte r 1 week incubation, water in the samples was extracted by centrifugation a nd the extracts were analyzed for Cr(VI). All of the samples generated Cr(V I), and the concentrations in the extracts ranged from 20 to 100 mug Cr L-1 . Total chromium, endemic chromium VI and chromium VI generated in leached samples were not statistically different between samples from onsite and co ntrol samples taken offsite and upgradient in respect to the shallowest aqu ifer.