PROCESSES INFLUENCING THE FATE AND TRANSPORT OF NITRATE AND AMMONIUM DURING DISCHARGE FROM GROUNDWATER TO SURFACE WATER PONDS ON WESTERN CAPE COD, MASSACHUSETTS, USA
Small-scale (<2 m) variability in nitrate concentrations was minimal in the nitrate hot spot with no ammonium detected. N2O was detected at concentrations up to 2 µM, suggesting active denitrification. Anaerobic incubations, conducted with sediment collected 0-0.2 m below the pond bottom in the nitrate hot spot showed a denitrification rate of 4.3 nmol N/g sediment/hr when amended with 100 µM nitrate. Nitrification activity, measured in aerobic incubations, was minimal. Small-scale concentrations of ammonium in the ammonium hot spot varied by a factor of two due, in part, to changes in chemical conditions that impact sorption. Nitrate and N2O were not detected in the ammonium hot spot, though the denitrification rate was approximately half that measured on sediments from the nitrate hot spot when amended with nitrate (100 µM). Significant nitrification activity (2.2 nmol N/g sediment/hr) was observed for sediment collected from the ammonium hot spot when amended with ammonium (100 µM). The rate of denitrification for sediments collected from the groundwater recharge area of Ashumet Pond (low nutrient concentrations) was similar to that from the nitrate hot spot while the nitrification rate was similar to that measured in the ammonium hot spot. This work illustrates the importance of nitrogen cycling processes at the groundwater-surface water interface.