How does aquifer connectivity impact groundwater flow to the sea and subsurface patterns of salinity?

Michael, Holly A.In large scale coastal aquifers, the quantification of subsurface salinity patterns is important to water resources management. Meanwhile, the quantification of amounts and patterns of ocean-aquifer exchange is important to protecting coastal waters and understanding long-term ocean chemistry. This study used numerical simulations to investigate the impacts of (1) geologic and hydrologic connectivity, (2) large scale geologic heterogeneity, and (3) model dimension on steady-state variable-density groundwater flow and salt transport on the continental shelf (200 km) scale. Heterogeneous hydraulic conductivity (K) fields with different geologic connectivities were generated geostatistically and equivalent homogeneous K values were computed. Groundwater flow and solute transport in each heterogeneous and equivalent homogeneous aquifer was simulated. Finally, the size and position of the mixing zone, the distribution and position of fresh and saline submarine groundwater discharge, the quantity of fresh recharge, and the quantity of saltwater circulation were quantified. The results show, first, that as aquifer connectivity increases in the horizontal direction, the fresh-saline mixing zone and fresh and saline submarine groundwater discharge occur farther seaward, are spread over a larger area, and become more variable. Next, aquifers with large-scale heterogeneity usually have a larger interface and freshwater that is present further seaward than homogeneous equivalent aquifers do. Third, saltwater circulation is significantly greater in heterogeneous aquifers compared to that of equivalent homogeneous aquifers. Finally, modeling aquifers in 2D rather than 3D exaggerates these results. This study has implications for understanding the variability of field data, improving numerical modeling of coastal groundwater systems, estimating the offshore extent of fresh water, and properly quantifying saltwater circulation on continental shelves.University of Delaware, Department of Civil and Environmental EngineeringM.C.E