07/05/2023
By James Heiss
Candidate Name: Olasunkanmi Olorunsaye
Defense Date: Wednesday, July 19, 2023
Time: 1-2 p.m. EST
Location: 212, Olney Hall, North Campus and Zoom
Thesis/Dissertation Title: The influence of geologic heterogeneity on the onset of salt fingering flow in beach aquifers
Advisor: James Heiss, Ph.D., Environmental, Earth and Atmospheric Sciences, UMass Lowell
Committee Members: Kate Swanger, Ph.D., Department of Environmental, Earth and Atmospheric Sciences, UMass Lowell; Kyra Adams, Ph.D., NASA Jet Propulsion Laboratory, Caltech
Abstract:
Saltwater-freshwater mixing zones in beach aquifers support biogeochemical reactions that moderate chemical loads in fresh groundwater discharging to marine ecosystems. Existing laboratory and numerical modeling studies have demonstrated that fluid density gradients in the mixing zone can lead to free convection and the formation of density instabilities, or salt fingers, under certain hydrologic, morphologic, and hydrogeologic conditions. However, salt fingers have not been observed in real-world beach aquifers despite a growing body of field studies investigating intertidal mixing zones. In this study we used geostatistical methods to generate randomly distributed assemblages of fine and medium sand and incorporated those geologic realizations into variable-density variably-saturated flow and salt transport simulations to explore the influence of geologic structure on mixing zone stability in tidally-influenced beaches. Results show that geologic heterogeneity inhibits salt finger formation and promotes a stable intertidal mixing zone due to enhanced dispersion. This effect is highest for intermediate proportions of fine and medium sand and for more laterally connected geologic architecture. Compared to equivalent homogeneous models, heterogeneous sediments produce mixing zones that are up to 23% smaller and 8 times more stable under the range of heterogeneous scenarios considered. The models indicate that geologic heterogeneity may explain the lack of field observations of salt fingers in real-word intertidal mixing zones. The findings have implications for predicting the onset of free convection in beaches and for understanding intertidal pore water biogeochemistry and chemical fluxes to the ocean.