Collaborative Research: Surface water-groundwater connectivity in the tidal freshwater zone and the fate of nitrogen in tidal rivers

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Humans have dramatically altered the nitrogen cycle through food and energy production activities. Increased nitrogen loading to landscapes often results in nitrogen export to the coast, leading to algal blooms, dead zones, and declines in fisheries. Nitrate removal in riparian aquifers and riverbeds provides an important ecosystem service that mitigates nitrogen loads to coasts. However a vast majority of nitrogen never reaches the coast, rather it is retained within the watershed or transformed by microbes. Riverbanks and bottoms are prime locations for this retention and transformation, providing a valuable ecosystem service. In tidal freshwater zones (TFZs) where tides pump river water in and out of the riverbanks and riverbed, nitrate removal may be particularly effective. This study will determine the nitrogen removal efficiency of TFZs. If TFZs play a disproportionately large role in nitrogen removal within watersheds, management strategies should seek to protect TFZs by maintaining riparian buffer zones and limiting sediment sources that could clog riverbeds, reducing their removal efficiency. The research should improve our ability to manage nitrate in freshwater and better value the ecosystems services of tidal freshwater zones and estuaries. The results will be shared with the public in two ways: (1) Creek Fest, an outreach event that promotes watershed stewardship to over 1000 attendees, the PIs will will discuss implications for TFZ management with local stakeholders; and (2) the PIs will also develop a virtual field trip to educate high school and college students in land-locked classrooms on ecosystem services in tidal environments. Rarely monitored for discharge or nutrient fluxes, TFZs are dynamic transition zones between rivers and estuaries. In TFZs, semidiurnal stage fluctuations should promote intense bank storage and release. Bank storage zones may be efficient sites of nitrate removal. However, associated water table fluctuations may also aerate shallow groundwater and enhance nitrification, adding nitrate to groundwater. The net effect of these two tidally induced, opposing processes could range from net nitrate production to removal within TFZs. This proposal asks how tidally induced hydrodynamic processes such as bank storage and water table fluctuations control nitrogen transformations within the riparian aquifers of TFZs and how these processes upscale to influence nitrogen fluxes from land to sea. It is hypothesized that: 1) TFZ hydrodynamics promote two hot spots of nitrogen transformation in the riparian aquifer: a nitrification hot spot at the soil-groundwater interface where semidiurnal water table fluctuations oxygenate the shallow groundwater, and a denitrification hot spot near the river-groundwater interface where surface water exchanges through oxygen depleted sediments; 2) as tidal range increases in the downstream direction within the TFZ, nitrate production via nitrification increases more than nitrate removal via denitrification. These hypotheses will be tested using a combination of field observations within a TFZ, laboratory experiments, and numerical models. The field component will characterize fluid and nitrogen fluxes across the river-aquifer interface and identify non-conservative nitrogen transport in the riparian aquifer of a TFZ within the Christina River Basin (Delaware). Sediment cores will be used in laboratory column experiments to explore relationships between water table fluctuations, groundwater redox conditions, and nitrogen transformation. Numerical models will be used to upscale nitrogen fate along the TFZ of the Christina River Basin.