Approximately 1.1 billion people lack access to safe drinking water due to an uneven distribution of water resources and chronic water quality issues. Both anthropogenic and natural sources of contamination threaten drinking water quality in groundwater supplies around the world. We seek to understand the biogeochemical and physical processes controlling the cycling of toxic metals/metalloids (e.g. uranium, chromium, arsenic) and their interaction with other species (e.g. nitrate, phosphate, organic carbon) in groundwater systems. The fate and transport of these contaminants is often governed by reduction-oxidation (redox) state, which is a function of both biotic and abiotic processes. In addition to biogeochemical processes, physical heterogeneity of soil and sediments can control the release of contaminants to the surrounding groundwater. To examine these processes, we use a combination of approaches including (1) simplified laboratory experiments (2) field sampling and observations, and (3) reactive transport modeling.
A few ongoing projects:
Hydrologic and biogeochemical controls on hexavalent chromium generation
As our understanding of known Cr(VI) sources expands from industrial point sources to include natural soils and sediments, it is critical to identify the processes which control Cr(VI) generation within native aquifers. Chromium(III) within minerals common to ultramafic rocks, their metamorphic derivatives, and weathering products, can be oxidized to Cr(VI) via reaction with Mn-oxides, chromium’s principal oxidant under environmental conditions. We use dynamic lab microcosms in order to explore the mechanism for Cr(VI) generation in pristine aquifers. In these controlled environments, we explore the control of microbial communities, along with abiotic processes, on both Cr(VI) oxidation and reductive immobilization. We also incorporate geospatial analysis and reactive transport modeling to further constrain the controls on Cr(VI) cycling within the environment.
Geochemical triggers of arsenic release during managed aquifer recharge
Managed aquifer recharge is an increasingly popular method to augment local groundwater supplies. However, artificial recharge can alter the native groundwater chemistry of an aquifer, resulting in the release of naturally-occurring contaminants including arsenic. We work with Orange County Water District, which is currently the largest potable reuse advanced treatment plant in the world, to understand how high purity recharge water can result in the release of arsenic to groundwater. We use soil columns packed with sediment samples collected near recharge basins to examine interactions between recharge water and native As within sediments. For more info: http://news.stanford.edu/2015/09/02/arsenic-mystery-solved-090215/