Arsenic contaminated groundwater is a global problem, negatively impacting the health of millions of people worldwide who rely on groundwater for drinking and irrigation purposes — including those who live in Washington State.  Given the prevalence and negative health consequences of arsenic-contaminated groundwater, it is important —at both the global and local scale— to develop robust and sustainable groundwater arsenic remediation strategies.  The objective of the proposed project was to advance understanding of the long term sustainability of arsenic removal from groundwater following field-scale application of induced microbial sulfate reduction with and without zerovalent iron (ZVI).

This project provided evidence of the sequestration mechanisms involved with the technique applied for field-scale remediation of arsenic in groundwater. In laboratory tests of induced sulfate reduction, arsenic has been removed from groundwater by adsorbing onto the surface of freshly formed, kinetically favored, amorphous iron monosulfide phases. Our field data, appears to be consistent with previous laboratory studies. At both of our locations, arsenic was associated with iron-sulfide phases, like mackinawite and amorphous FeS, but was not yet incorporated into pyrite.  At our site, treatment without ZVI was not effective at removing arsenic from groundwater, however, inclusion of ZVI successfully removed dissolved arsenic from groundwater over a multi-year period . Our data suggest that without ZVI, the aquifer did not have enough reducing power to stably sequester arsenic into the solid phase.

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