Collaborative Research: Quantifying Geologic and Temporal Controls on Water and Chemical Exchange between Groundwater and Surface Water in Coastal Estuarine Systems

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Collaborative Research: Quantifying Temporal and Geologic Controls on Water and Chemical Exchange between Groundwater and Surface Water in Coastal Estuarine SystemsH. Michael, J. Bratton, D. Krantz, L. Konikow, and A.S. AndresEstuarine ecosystems are increasingly threatened by excess nutrient loading. Groundwater is an important nutrient source, but the spatial and temporal heterogeneity of groundwater discharge makes quantifying fluxes and transformations of nutrient species prior to discharge challenging. A multi-disciplinary investigation in the well characterized and representative estuary of Indian River Bay in Delaware will improve understanding of the interaction of physical and chemical processes critical to evaluation of the role of groundwater discharge in nitrogen loading to coastal waters.The primary goal of the research is to characterize the controls of geologic heterogeneity and temporally-variable hydraulic forcing on fluid and nitrogen fluxes between aquifers and coastal surface waters. We hypothesize that: (1) mixing between fresh and saline water, driven by transient forcing and geologic heterogeneity, enhances denitrification of freshwater-derived nitrate before discharge, and (2) saline exchange flux, driven by hydraulic forcing and dispersion-induced subsurface density gradients, transports ammonium derived from decay of sedimentary organic matter into coastal waters. The project will include numerical modeling, hydrologic and geochemical monitoring, and geophysical investigations conducted within and surrounding Indian River Bay, DE. Variable-density numerical models will be developed over multiple spatial and temporal scales to quantify fluxes, evaluate system controls, synthesize data, and guide data collection. Direct measurements of groundwater discharge, hydraulic gradients, and salinities will be obtained over tidal to annual cycles. The complex subsurface salinity distribution will be mapped using continuous resistivity profiling, and high-resolution geochemical sampling will be conducted along flowpaths to determine both source and fate of solutes and to quantify denitrification. Analyses will include nitrogen species and isotopes, noble gases to enable calculations of excess N2, and organic matter in saline recharge zone sediments as a source of ammonium. Geologic features such as paleochannels will be imaged using CHIRP seismic surveys, and hydrogeologic properties will be characterized, guided in part by the data needs of the models. The proposed work will have educational impacts, and results will provide insight to managers who regulate nitrogen loading to improve coastal ecosystem health. The project will be carried out in coordination with investigations of carbon-mineral interactions along a salinity gradient proposed as part of a Critical Zone Observatory in the Christina River Basin of Delaware and Pennsylvania.