Since the Earth’s “critical zone” was first defined by Gail Ashley (NRC, 2001), Susan Brantley has led a successful effort to develop the new field of critical zone science, which seeks to integrate ecology, hydrology, soil science, geochemistry and geomorphology to better understand relations between structure and evolution of the Earth’s land surface (Brantley et al., 2006). Geochemical approaches to critical zone (CZ) science focus on the bedrock to vegetation canopy weathering profile and impacts of biology and hydrology on chemical transformations in both pristine and human-impacted environments. The largest changes to the CZ are now human influenced, with humans moving about 10 times more material as all natural processes combined, particularly through mining (Hooke, 2000). As a result of mining, new rock substrate is exposed to climatic forcing in CZ locations worldwide. This presentation focuses on legacy mine tailings, which represent a prevalent source of particulate and dissolved metal(loid) contamination throughout the southwestern US with significant potential for human health effects. Weathering of sulfide-ore derived mine tailings under semi-arid climate normally occurs in systems devoid of vegetation, creating sharp reaction fronts that alter both major element and contaminant metal(loid) forms over relatively short distances below the ground surface. We combined multi-scale experimental and spectroscopic studies to examine how the molecular speciation of metal(loid)s in sulfide-ore derived tailings is altered across these reaction fronts, and correlated that with the kinetic lability of these species in natural waters, as well as in lung and gastric biofluid simulants. We further investigated the change in biological weathering processes that results from the introduction of plants, which is used as a remediation strategy to stabilize the tailings against human exposures via wind and water erosion. We observed significant effects of reaction front location on metal(loid) speciation, lability and bioaccessibility.  Further, root-microbe-metal(loid) interactions influenced changes in molecular speciation that resulted from the infusion of water, organic matter and heterotrophic microbial biomass. Results provide insights into the coupled geochemical and biological processes that control toxic metal(loid) speciation during weathering in the CZ.

Biogeochemical transformation of metal(loid)s in a disturbed critical zone

253rd National Meeting (2017)

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