FACTORS IMPACTING THE FATE AND TRANSPORT OF WASTEWATER-DERIVED NITROGEN AND ARSENIC DURING GROUNDWATER DISCHARGE TO A FLOW-THROUGH LAKE

Groundwater flow-through lakes, such as Ashumet Pond in MA, are good environments to study the processes controlling contaminant fate and transport under uni-directional flow conditions. Our work examines coupling between groundwater flow and redox reactions influencing the fate of carbon (C), nitrogen (N), iron (Fe), sulfur (S), and trace elements like arsenic (As) in groundwater and shallow lake sediments. A wastewater-derived plume discharges below the western shore of the lake. Two sites, 25 m from the lake shore and 30 m apart in June 2015, experience contrasting groundwater flow conditions. Site 1 has no detectable vertical groundwater flow, sediments are highly reducing, with elevated, spatially variable concentrations of Fe(II), S(-II), methane, and ammonium. Aqueous As (1-5 nM) is dominated by As(III). Site 2 has anoxic ammonium-contaminated groundwater discharging, in which aqueous Fe(II) and methane concentrations are below detection, and aqueous S occurs as sulfate. Dissolved inorganic carbon concentrations at Site 2 reflect the composition of up-gradient groundwater. Aqueous As concentrations are spatially uniform (20 nM) down to 1 m below the lake bottom. Laboratory incubations suggest that the top layer (0-5 cm) of sediment possesses the greatest potential for denitrification, likely due to the increased supply of labile organic matter at the groundwater-surface water interface. Nitrogen cycling appears to be much slower in deeper sediments with the potential for anaerobic ammonium oxidation (anammox). Next generation sequencing analysis of 16S rRNA genes is conducted to compare microbial communities responsible for C, N, S, Fe, and As cycling at the two sites. Flow direction and electron donor supply are critical factors for redox speciation and N-cycling with significant variation evident over relatively small spatial scales.