Variability in Nitrate and Ammonium Distributions and Associated Processes at the Groundwater/Surface-water Interface in a Groundwater Flow-through Pond

We are studying transformations of nitrate and ammonium (N-compounds) at the groundwater/surface-water interface in Ashumet Pond on Cape Cod, Massachusetts, USA. The objective is to understand hydrologic and biogeochemical controls on nitrate and ammonium transformations such as nitrification, anammox, denitrification, and dissimilatory nitrate reduction to ammonium (DNRA) at the groundwater/surface-water interface. Ashumet Pond is fed by groundwater discharge on the north and northwest portions and drained by recharging the aquifer on the south and east portions of the pond. A wastewater plume with distinct zones containing nitrate and ammonium discharges into the pond. Sampling groundwater 0.15 m below the pond bottom along transects running from the shore out into the pond revealed sharp horizontal gradients in dissolved oxygen and N-compounds that correspond to sharp vertical gradients through the wastewater plume in the aquifer up-gradient of the pond. Sampling groundwater below the pond bottom at approximately 10-m intervals revealed distinct regions with up to 100 μM nitrate (nitrate 'hot spot') or 300 μM ammonium (ammonium 'hot spot'). Sampling at the 0.1-1-m scale vertically and horizontally within the nitrate hot spot revealed uniform nitrate concentrations and N₂O concentrations up to 2 μM, indicating active denitrification. However, no ammonium was detected in the nitrate hot spot. Anaerobic incubations using shallow (<0.2 m below the pond bottom) sediments from the nitrate hot spot amended with 100 μM nitrate exhibited denitrification activity (4.3 nmol N/g sediment/hr) and, possibly, DNRA activity. Aerobic incubations revealed minimal nitrification activity. Sampling at the 0.1-1-m scale within the ammonium hot spot revealed variable (150-300 μM) ammonium concentrations, no N₂O, and no nitrate. Anaerobic incubations of shallow sediments from the ammonium hot spot amended with 100 μM nitrate revealed denitrification activity, with a rate approximately half that observed in the sediments from the nitrate hot spot. Aerobic incubations showed significant nitrification activity (2.2 nmol N/g sediment/hr). On the recharge side of the pond, groundwater 0.15 m below the pond bottom was oxic with 5 μM nitrate and no ammonium. The groundwater had approximately 70 μM lower dissolved oxygen and 70 μM higher total dissolved carbon dioxide concentrations than pond water collected just above the pond bottom, suggesting active aerobic respiration during recharge across the pond bottom. Anaerobic incubations using sediments from this location amended with 100 μM nitrate revealed denitrification activity, with a rate similar to that from the nitrate hot spot. Aerobic incubations amended with 100 μM ammonium showed the fastest nitrification rate. Our results suggest significant microbial activity and processing of N-compounds near the groundwater-surface-water interface.