HYDROLOGIC AND BIOGEOCHEMICAL CONTROLS ON NITRATE MASS TRANSFER FROM CONTAMINATED GROUNDWATER INTO A GROUNDWATER FLOW-THROUGH LAKE ON CAPE COD, MA

Groundwater discharge delivering anthropogenic N can have a profound impact on whole-lake nutrient budgets. A field study was conducted in a groundwater flow-through lake on Cape Cod which receives discharge of N-contaminated groundwater. Biogeochemical processes influencing the fate and transport of contaminant N species were examined seasonally with changes in temperature, lake level, and groundwater geochemical gradients. Sediment incubations were conducted to quantify potential rates of denitrification, anaerobic ammonium oxidation (anammox), and nitrate uptake on a relative basis at sites along transects where groundwater chemistry below the lake progressed from oxic to suboxic to anoxic with increasing nitrate concentrations (10-75 µM) and corresponding gradients in nitrite and nitrous oxide. Temporal shifts in lake level and groundwater geochemical gradients suggest that sediments from the same location experience many, if not all, of these chemical conditions throughout the year. Denitrification and nitrate uptake potential were 10-100 fold higher in surface sediments (0-5 cm) than in deeper sediments (5-30 cm) and correlated with sediment carbon content, but not necessarily with nitrate discharge. Functional genes coding for N₂O reductase (nosZ) in denitrification and hydrazine oxidoreductase (hzo) in anammox were present in sediments at all sites, though incubations spiked with ¹⁵N labeled tracer indicated that N₂ production was dominated by anammox in deeper sediments while denitrification dominated in shallow sediments. Porewater depth profiles of nitrate concentration and stable isotope fractionation indicated active nitrate consumption and dilution in the shallower sediments. Vertical porewater velocity, measured with seepage meters and conservative tracers injected into the sediment porewater, was 4-5 cm hr^-1. Thus, while nitrate removal rates in the more carbon-rich shallow sediments were relatively high, the short residence time allowed only a small fraction of the contaminant nitrate to be removed before discharge into the lake.