Seasonal and spatial variation in the location and reactivity of a nitrate-contaminated groundwater discharge zone in a lakebed

The U.S. Geological Survey Toxic Substances Hydrology Program at Cape Cod has been investigating the fate and transport of a treated-wastewater, groundwater contaminant plume. A portion of the contaminated groundwater discharges into Ashumet Pond, a kettle hole, freshwater lake. A study was conducted from June 2013 to June 2015 to document transport, transformation, and discharge of dissolved inorganic nitrogen species (DIN; nitrate, nitrite, and nitrous oxide) from the contaminant plume to the lake, across the groundwater-surface water interface. Groundwater, lakewater, lake sediment porewater, and sediment cores were collected and analyzed for DIN constituents as well as functional gene abundance for key N-cycle processes, and potential activity assays for denitrification, anammox, nitrate uptake, and nitrification. Changes in nitrate and oxygen concentrations at spatial locations were monitored with time and correlated with changes in lake stage.