Hydrologic and biogeochemical controls on hexavalent chromium in soils, rivers, and groundwaters [electronic resource]
Keywords: Geological Sciences.

Chemical weathering of ultramafic rocks, soils and sediment releases chromium (Cr) to surface and groundwater resources. Identifying the hydrologic and biogeochemical factors that control the release of Cr is critical for assessing (i) risk to human health from geogenic Cr(VI) contamination, and (ii) past and present global Cr and biogeochemical cycling. Here, I present a study of Cr cycling during the weathering of ultramafic rocks using a source to sink approach that starts in serpentine soils within an ultramafic watershed in the Coast Range mountains of California, and follows Cr during transport to downstream depositional environments, such as alluvial plains that host drinking water aquifers, and ultimately the ocean. This research combines field studies, with micron-scale synchrotron radiation imaging, reactive transport modeling, and meta-analysis to assess the spatiotemporal distribution of Cr, and how this distribution may have changed over human and geologic timescales. In California alone, consumption of water containing > 10 μg/L Cr(VI) may adversely impact more than one million people, as Cr(VI) is a probable carcinogen when ingested. Chromium(VI) occurs naturally in serpentine soils, generated through redox reactions between Cr(III) minerals and Mn(III/IV)-oxides, and is flushed from serpentine soil to streams at concentrations above California's drinking water limit (10 μg/L). Chromium(VI) is partially removed by reduction in streambed sediments and transported downstream, loading depositional environments such as alluvial plains with Cr(III)-phases susceptible to re-oxidation that may ultimately contribute to Cr(VI) groundwater contamination. In alluvial plains of the southwestern Sacramento Valley of California, Cr(VI) is produced in unsaturated, sandy, oxic, ultramafic, Cr-rich, alluvial sediments, by Mn(IV)-oxide minerals co-located with Cr(III) minerals, and is partially reduced under moderate reducing conditions in a redoxcline within fine-grained sediments below the historic water table (5 m). In shallow groundwater near the City of Davis, 90% of wells sampled exceed California's Cr(VI) drinking water limit. Chromium(VI) concentrations decrease with depth and are increasing over time, similar to trends observed for nitrate. These spatiotemporal patterns in regional groundwater Cr(VI) concentrations can be explained by locally variable lithology, irrigation and groundwater pumping. Thus, geogenic Cr(VI) groundwater contamination may be exacerbated by human-induced changes in land and water use, posing an even greater threat to human health as population growth and climate change increase demand on groundwater resources. Globally, present-day riverine dissolved Cr fluxes to oceans are spatially variable and estimated to be 1.7 × 10^9 mol/yr, three times higher than previously reported. Riverine Cr and major ion fluxes increase together suggesting silicate weathering is a key control on Cr fluxes, and that major element weathering fluxes may be a useful proxy for estimating Cr and other trace element fluxes. Compared to large rivers worldwide, ultramafic watersheds have high Cr and major ion fluxes and may thus have a disproportional impact on global Cr-budgets, and moderation of the Earth's carbon cycle over million-year timescales. As the global distribution of ultramafic rocks, Cr concentrations in continental rocks, and climactic conditions (e.g. atmospheric CO2 levels or runoff) have varied throughout Earth's history, weathering fluxes may have also varied. Thus, I identify a critical uncertainty that has implications for how Cr delivery to oceans, Cr oceanic residence time, Cr records in marine rocks, and long-term carbon cycling may have changed over geologic time.