Quantifying Cr(VI) Production and Export from Serpentine Soil of the California Coast Range

Abstract

Hexavalent chromium (Cr(VI)) is generated in serpentine soils and exported to surface and groundwaters at levels above health-based drinking water standards. Although Cr(VI) concentrations are elevated in serpentine soil pore water, few studies have reported field evidence documenting Cr(VI) production rates and fluxes that govern Cr(VI) transport from soil to water sources. We report Cr speciation (i) in four serpentine soil depth profiles derived from the California Coast Range serpentinite belt and (ii) in local surface waters. Within soils, we detected Cr(VI) in the same horizons where Cr(III)-minerals are colocated with biogenic Mn(III/IV)-oxides, suggesting Cr(VI) generation through oxidation by Mn-oxides. Water-extractable Cr(VI) concentrations increase with depth constituting a 7.8 to 12 kg/km reservoir of Cr(VI) in soil. Here, Cr(VI) is produced at a rate of 0.3 to 4.8 kg Cr(VI)/km/yr and subsequently flushed from soil during water infiltration, exporting 0.01 to 3.9 kg Cr(VI)/km/yr at concentrations ranging from 25 to 172 μg/L. Although soil-derived Cr(VI) is leached from soil at concentrations exceeding 10 μg/L, due to reduction and dilution during transport to streams, Cr(VI) levels measured in local surface waters largely remain below California's drinking water limit.

Local and Global Sensitivity Analysis of Cr (VI) Geogenic Leakage Under Uncertain Environmental Conditions

“…These parameters are considered as proxies of the environmental conditions under which Cr oxidation takes place. Our choice to rest on a simplified geochemical model stems from the observation that a comprehensive mechanistic model of Cr dynamics would require considering both kinetic and equilibrium (bio) geochemical processes coupled to local flow and transport dynamics (McClain et al, , and references therein). While considering such a modeling approach is fully compatible with our theoretical framework for uncertainty assessment, focusing on a setting associated with a limited number of uncertain parameters enables us to clearly illustrate and compare the salient points of the sensitivity analyses techniques we consider.…”

“…According to thermodynamics arguments, spontaneous oxidation of Cr (III) is characterized by slow kinetics (Apte et al, 2005) and needs to be catalyzed to take place in a natural system at a significant rate. A variety of possible catalysts have been suggested to act to this end, including manganese oxides (Ivarsson et al, 2011;Kaprara et al, 2015;McClain et al, 2017), microbial activity (Fendorf et al, 2000;Ivarsson et al, 2011), hydrogen peroxide, or simply free oxygen (Fantoni et al, 2002;Lin, 2002). While these processes are not explicitly included in our simplified approach, we treat oxygen fugacity as an effective indicator of the redox state of the system.…”

Governing Constraints of Chromium(VI) Formation from Chromium(III)-Bearing Minerals in Soils and Sediments

“…Weathering of ultramafic rocks in Mediterranean climates (e.g., California Coast Range and Central Valley) yields soils with modest iron oxide content and alkaline pH. With an increase in pH above 8 (e.g., California Sacramento Valley) [21,30], secondary weathering products become more soluble than primary minerals like chromite, especially in the presence of high bicarbonate concentrations [31]. If chromite remains a dominate source of Cr(III), increasing pH values from pH 7.6 to pH 9 diminishes Cr(III) concentrations and concomitant Cr(VI) concentrations by an order of magnitude.…”

“…4.5 mm of the advective flow channel. In aquifers impacted by groundwater pumping, water level fluctuations can occur on the order of days and weeks and are of large magnitude (9-25 m) [30,32,36]. If Cr(VI) generation sites are within a few centimeters of advective flow channels, steady-state Cr(VI) concentrations should be reached within several days (assuming porewater velocity around 0.1 m d −1 ); if Cr(VI) generation sites lie further away from advective flow channels, however, groundwater flow conditions would likely change before steady-state Cr(VI) concentrations were reached in aquifers with high hydraulic conductivity or in systems impacted by land runoff or groundwater pumping.…”

Evaluating Ferrous Chloride for Removal of Chromium From Ion‐Exchange Waste Brines

“…Chromium (Cr) has gained considerable attention in the last several decades as a contaminant of concern in drinking water. Chromium can enter groundwater through anthropogenic activities or through the dissolution of natural mineral formations that contain Cr (McClain et al , Vengosh et al , Kimbrough et al ). Chromium occurs primarily as trivalent chromium (Cr(III)) or hexavalent chromium (Cr(VI)) in aqueous systems.…”