CHARACTERIZATION OF THE POTENTIAL FOR OXIDATION AND MOBILIZATION OF GEOGENIC AND FERTILIZER-DERIVED CHROMIUM IN SOILS OF THE SACRAMENTO VALLEY, CALIFORNIA

Chromium occurs naturally as Cr(III), which is relatively immobile at circumneutral pH, and Cr(VI), which is highly soluble and potentially toxic. The refractory spinel mineral chromite [FeCr(III)₂O₄] is abundant in ultramafic rocks that outcrop in the Coast Range and Sierra Nevada foothills bordering the Sacramento Valley (California, USA). Data from a geochemical survey of surface soils in the Sacramento Valley indicate that they are enriched in total Cr (30-1420 mg kg^-1) relative to the U.S. geometric mean (37 mg kg^-1) due to transport of ultramafic material. Much of the Cr in valley soils appears to be bound in chromite but some oxidation of Cr is suggested by elevated Cr concentrations (up to 50 μg l^-1) in some valley ground waters. We investigated controls on oxidation of geogenic Cr in the valley by analyzing material from several cores (up to 30 m depth). This material contains up to 2200 mg kg^-1 total Cr. Correlation between borehole magnetic susceptibility surveys and total Cr concentrations suggests much of the Cr is present in spinels. The material also has large concentrations of easily reducible Mn oxides (up to 1000 mg Mn per kg soil), the major oxidant of Cr(III) in natural systems. However, only a small fraction of total Cr is present as Cr(VI) (up to 42 μg per kg soil) likely because the pH of the core material (7 to 9) inhibits the transport of Cr(III) to Mn oxide surfaces.

We performed incubation experiments with valley subsoil to determine if protons generated by microbial nitrification could increase the mobility of Cr(III) and its subsequent oxidation on soil Mn oxides. Chromium(VI) production was more rapid in soils amended with ammonium sulfate than in control soils until NH₄⁺ was consumed. Chromium(VI) then decreased in amended soils to concentrations below that in control soils. The results of microbial phospholipid fatty acid analyses suggest Cr(VI) was reduced by organic carbon released upon the lysis of nitrifier cells. Incubations with NH₄⁺ polyphosphate fertilizer indicated that the oxidation of Cr(III) present in phosphate fertilizers could supply more Cr(VI) to soils than oxidation of Cr(III) from ultramafic sources. These results suggest that geogenic Cr oxidation pathways exist in valley soils and may be enhanced by certain land uses, but that anthropogenic sources of Cr may also be important.