PROCESSES INFLUENCING THE FATE AND TRANSPORT OF NITRATE AND AMMONIUM DURING DISCHARGE FROM GROUNDWATER TO SURFACE WATER PONDS ON WESTERN CAPE COD, MASSACHUSETTS, USA

Constituents of a plume of wastewater-contaminated groundwater on western Cape Cod, Massachusetts discharge into Ashumet Pond, a groundwater-flow-through kettle pond. Sharp vertical gradients, observed in the aquifer between pristine and contaminated zones, are expressed as sharp horizontal gradients in groundwater 0.15 m below the pond. Discharge to the pond and the effect on nitrogen cycling processes were assessed during a recent field campaign. Large-scale (≥10 m) spatial variability in nitrate and ammonium concentrations, in distinct groundwater “hot spots” below the pond, were consistent with the distributions observed in the wastewater plume. Ammonium concentrations below the pond decreased from 500 to 300 µM since the previous survey in 1999 with little change in the hot spot location. Nitrate concentrations below the pond decreased from 900 to 100 µM and the size of the nitrate hot spot diminished considerably, consistent with the wastewater plume.

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.