Groundwater-surface water interfaces can impact the geochemistry of both contributing and receiving waterbodies. Many freshwater lakes are groundwater flow-through systems, receiving up-gradient groundwater solutes and in turn imprinting a lake-derived signature, or lake shadow, on groundwater chemistry downgradient of the lake. Differences in flow direction (e.g., lake water recharge vs. discharge) can cause substantial variation in C and N cycling within lake-bed sediments, particularly when groundwater inputs contain anthropogenic C and N. To explore these effects, a field study was conducted at two locations in a groundwater flow-through lake on Cape Cod: one site (GWin) receives discharge of wastewater-contaminated groundwater and a second site (LWout) where lakewater is discharging to the subsurface. Sediment porewater at GWin contains elevated, relatively recalcitrant, dissolved organic matter (DOM) and inorganic nitrogen (DIN; nitrate, nitrite, and nitrous oxide) indicating up-gradient groundwater denitrification in the aquifer. Depth profiles indicated relatively little change in dissolved oxygen (DO), DOM, or DIN concentrations with upward transport, except in the top 5-10 cm of the sediment. Tracer tests indicated an active zone of denitrification in that depth interval, but a porewater residence time of about 1 hour. Diel variations were minimal. At LWout sediment porewater concentrations of DO, DOM, and DIN varied greatly by depth in shallow sediments (0-100 cm). In winter, porewater DIN and DOM concentrations were low and relatively uniform with depth, while DO had decreased ~50% at 100 cm. During the remainder of the year, porewater DO and DOM decreased markedly with sediment depth, exhibiting diel concentration fluctuations, even at 100 cm. DIN increased from below detection in lakewater to as much as 20 µM, including nitrous oxide at 50-100 cm. DIN also exhibited diel changes in concentration. Rapid degradation of DOM accompanied by mineralization and nitrification of organic-N were evident at LWout. Lake shadow effects on groundwater DO, DOM and DIN were detected in wells located downgradient from the lake shore. These results illustrate the importance of considering temporal and spatial variation of local hydraulic gradients when interpreting biogeochemical data from groundwater and surface water in the vicinity of lakes.

Location matters: Groundwater flow direction (recharge vs. discharge) controls DOM and DIN gradients and cycling in the bed sediments of a groundwater flow-through lake

253rd National Meeting (2017)

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