NON-ANTHROPOGENIC HEXAVALENT CHROMIUM GENESIS IN SOIL AND GROUNDWATER

Hexavalent chromium is a mutagen, teratogen, and carcinogen typically originating from anthropogenic activity. However, naturally occurring sources of Cr(VI) at concentrations up to 73 µg L^-1 have been reported in ground and surface waters from New Caledonia, California, Italy, Zimbabwe, Pakistan and Mexico, values exceeding the World Health Organization's limit for drinking water of 50 µg Cr(VI) L^-1. Ultramafic rocks or their metamorphic equivalents (serpentinites) are univalently-rich in the Cr(III)-bearing mineral chromite and universally found in these areas. Processes for Cr(III) release to solution from chromite are perplexing as this mineral is known to be resistant to weathering, diagenesis, and low-grade metamorphic reactions. Here, we report experiments demonstrating accelerated dissolution of chromite and subsequent oxidation of Cr(III) to aqueous Cr(VI) in the presence of birnessite, a common manganese mineral, explaining the generation of Cr(VI) by a Cr(III)-bearing mineral considered geochemically inert. Resulting dissolution/oxidation rates provide conservative estimates of aqueous Cr(VI) formation within soils derived from serpentinite or ultramafic rocks. Our results demonstrate that Cr(III) within ultramafic- and serpentinite-derived soils/sediments can be oxidized and dissolved through natural processes, leading to elevated levels of aqueous Cr(VI) in surface and groundwater.