Hydrologic, Erosional and Biogeochemical Factors Controlling Cr Cycling in Serpentine Soils.

Quantifying Cr weathering rates and chemical transformations, from eroding upland ultramafic terrains to depositional environments, is a key first step in unraveling the processes governing the distribution of naturally occuring Cr(VI) contamination in aquifers. Cr is a redox sensitive element, abundant in rocks as Cr(III). Ultramafic rocks are highly enriched in Cr. Following bedrock weathering, Cr is subsequently reduced and sequestered in more reactive secondary phases, such as Fe-oxyhydroxides and possibly organic matter. When these secondary phases are transported to and deposited in alluvial basins and floodplains, they provides a weakly sequestered reservoir of Cr that may be more readily oxidized, depending on biogeochemical and hydrologic conditions. Here we present a mass balance model that quantatively tracks Cr cycling during pedogenesis by modeling soil production, erosion, Cr oxidation and reduction, and fluid flow. We apply this model to data for solid and aqueous speciation, and physical and chemical gradients from a serpentine mollisol developed on ultramafic bedrock in the Putah Creek watershed of the northern California Coast Range. In serpentine soils, chemical, mineralogical and physical properties are controlled by the relative rates of biogeochemical reactions, fluid flow and erosion. These observed gradients in solids reflect a long-term average over hundreds to thousands of years of weathering. Coupled solute concentrations reflect contemporary conditions and processes arising from the modern state of the system. Constrained by field measurements, our model results suggest that long-term and contemporary Cr weathering rates calculated at the pedon scale are comparable. At the catchment scale, based on river discharge and concentration measurements, only ~1% of dissolved Cr weathering from the soil profile exits the watershed in dissolved form. This indicates the importance of sediment transport and Cr reduction and immobilization along flowpaths. The relative rates of erosion and flow are key model parameters at the pedon scale, while at smaller length scales, microbial reactions and secondary mineral precipitation drive observed Cr gradients. Thus, for Cr cycling the relative importance of several physical and biogeochemical processes is critically dependent on the scale of observation.