IN SITU DETERMINATION OF THE FATE AND FLUX OF NITRATE IN GROUNDWATER AS IT DISCHARGES THROUGH THE SURFACE SEDIMENTS INTO A GROUNDWATER FLOW-THROUGH LAKE ON WESTERN CAPE COD, MA

Anthropogenic N compounds entering lakes, streams, and rivers through discharging groundwater can have a negative impact on water quality by causing eutrophication. Tracer tests were conducted to examine in situ microbial processes controlling N compounds during groundwater discharge into a groundwater flow-through lake on Cape Cod, which receives discharge of N-contaminated groundwater. Approximately 100 liters of anoxic groundwater withdrawn near the injection point and amended with 75μM nitrate and 1600μM bromide were injected 100cm below the bottom of the lake at the location where anoxic, nitrate-contaminated groundwater was discharging. Breakthrough curves (BTCs) were constructed using nitrite, N₂O, and bromide concentrations determined in groundwater samples collected from 75, 40 and 15cm, and 5cm below the lake bottom. Reactive transport modeling of the tracer test results was performed. Values of the pore velocity, dispersivity, and size of the injection were adjusted to fit bromide BTC’s by minimizing the sum of squares of the difference between simulated and measured bromide concentrations. The first order rate constants for nitrite and N₂O were adjusted to minimize the sum of squares of the difference between the simulated and experimentally observed concentrations. No discernible loss of nitrite was evident in BTCs from samplers installed 15cm below the lake bottom or deeper, whereas a substantial loss of nitrite was evident from samplers 5cm below the bottom. The rate constant that provided the best fit to the observed loss of nitrite 5cm below the lake bottom was ~500 times the maximum rate constant that would result in no observable loss of nitrite or N₂O in deeper sediments. This observation is consistent with the potential denitrification rates determined in laboratory incubation experiments. An additional test which included acetate as a tracer suggested an electron donor limitation of denitrification in the deeper sediments. The results suggest that while nitrate removal rates were high in the more carbon-rich shallow sediments, the short residence time (~1h) limited the extent of denitrification, allowing only a small fraction of the nitrate to be removed by the microbes in the shallow sediments before discharge into the lake.