THE FATE AND TRANSPORT OF DISSOLVED OXYGEN, INORGANIC NITROGEN AND ORGANIC CARBON DURING RECHARGE TO GROUNDWATER FROM A KETTLE LAKE ON CAPE COD, MA

Ashumet Pond is a groundwater flow-through kettle lake, heavily influenced by nutrient inputs from wastewater disposal and fertilizer use. Algal blooms occur in early summer at the inflow of the lake owing to discharge from groundwater elevated in nitrogen and carbon. At the downgradient end of the lake, lake water recharges the aquifer after reacting with lake bottom sediments, potentially leading to the release and transport of contaminants. This study focuses on understanding how lake-bottom biogeochemical processing affects redox conditions and transport of dissolved oxygen (DO), nitrogen and carbon during lake-water recharge. Biweekly and seasonal samples were collected from the lake, vertical porewater samplers (5-100 cm depth below lake bottom) located in the recharge zone, and two multi-level groundwater samplers (MLSs) 7 and 23 m downgradient from shore. Seasonally lake water ranges from 0 to 29°C and dissolved organic carbon concentrations (DOC) from 1.5 to 3.0 mg L^-1. DO varies on a diel cycle from 90 to 120 % saturation and pH from 6.5 to 8 with less dramatic cycles during the winter months. Based on seepage measurements, lake water flows downward at the recharge site at a flux rate of -0.18 to -0.35 m d^-1. Temperatures within sediment porewater profiles reflected that of the lake water at the time of sampling. Nitrate (NO₃^-) concentrations in the porewater were elevated relative to lake water, increased with depth during winter and early spring but decreased at the 100 cm depth in early summer, from a peak of 19 µM to 1 µM. Porewater DO concentrations decreased with depth throughout the year but anoxic conditions were present mainly in the warmer months. NO₃^- concentrations and isotopic compositions indicated nitrification in shallower porewater commonly followed by NO₃^- reduction in deeper porewater. DOC concentrations increased slightly in shallower porewater (0-5 cm) and then decreased in deeper porewater to ~1 mg L^-1. Differential vertical and temporal patterns in temperature, DO and NO₃^- in downgradient MLSs showed strong indications of lake water arrival throughout the year. Seasonal and biweekly results from lake sediment porewater and downgradient MLS sampling allow for a better understanding of how lake water, as it recharges back to the aquifer, impacts water quality of groundwater downgradient.