Processes Controlling the Mobility of Naturally Occurring Arsenic In a Hydrogeologically and Biogeochemically Well-Characterized Aquifer

Processes influencing the mobility of arsenic in groundwater are being investigated using laboratory and field experiments and reactive transport modeling at the USGS research site on Cape Cod, Massachusetts, USA. Sixty years of land disposal of wastewater to an oxic, quartzitic, sand and gravel aquifer caused the development of a distinct set of biogeochemical zones varying in dominant microbial terminal electron accepting processes (TEAPs), which strongly influence contaminant mobility. Adsorbed As(V) accounts for greater than 25 percent of the naturally occurring arsenic in the sediments. In uncontaminated groundwater and the denitrifying zone of the contaminant plume, only As(V) is observed. Arsenic(III) injected into either zone is oxidized over short transport distances by manganese oxides associated with surface coatings on mineral grains. Arsenic(V) is nearly immobile in uncontaminated groundwater owing to extensive adsorption onto the sediments but exhibits limited mobility in the wastewater-contaminated zone. Observed mobility of As(V) is consistent with models accounting for competitive adsorption between As(V) and phosphate, which is observed throughout the wastewater-contaminated zone. Microbial reduction of As(V) accounts for the co-occurrence of As(V) and As(III) in the anoxic zone, where dissimilatory Fe(III) reduction is the dominant TEAP. Anaerobic injection of nitrate into the Fe-reducing zone resulted in the occurrence of extensive Fe-coupled denitrification over short transport distances. Concentrations of wastewater-derived phosphate and naturally occurring As(V) and As(III) decreased significantly as a result of Fe-coupled denitrification in a manner consistent with adsorption onto freshly precipitated Fe oxyhydroxides. Phosphate, As(V), and As(III) concentrations recovered slowly but steadily upon re-establishment of Fe-reducing conditions once nitrate had disappeared. The dominant mechanisms controlling arsenic mobility in this aquifer are As(V) reduction coincident with dissimilatory Fe reduction; depletion of Mn oxides, inhibition of As(V) and As(III) adsorption by phosphate; and oxidation of Fe(II) to hydrous Fe oxide by nitrate pulses that pass through.