The influence of connected heterogeneity in hydraulic conductivity on groundwater flow and salinity distributions in a coastal volcanic aquifer

Michael, Holly A.The mixing zone of coastal aquifers, the region where fresh terrestrial groundwater interacts with saltwater, plays a critical role in solute flux to and from the ocean. Knowledge of the geologic structure and pathways of groundwater input to this zone is crucial for predicting effects on coastal ecosystems and ocean health. These same pathways may also allow for seawater intrusion, making knowledge of their influence a priority for both inland and offshore water chemistry. Groundwater and associated solutes can be preferentially channeled through highly connected geologic structures, such as lava tube systems in the volcanic aquifers of Hawaii. To better understand how the complex geology of volcanic systems affects water fluxes between land and sea in coastal regions, we created a geologic model composed of high-permeability lava tubes surrounded by lower-permeability lava and ash. The model was constructed with a surface-based geostatistical method using geometrical parameters extracted from process-based simulations. We investigated the effects of this hydraulic heterogeneity on the rate and spatial distribution of submarine groundwater discharge and subsurface salinity distributions with simulations of variable-density groundwater flow and salt transport. We also compared the connected hydraulically heterogeneous simulations to homogeneous simulations with upscaled equivalent permeability as well as simulations with a different heterogeneous geometry. We found that simulations with connected heterogeneity have larger mixing zones, higher volumes of recirculated seawater, and contained farther offshore freshwater and less onshore seawater. The results have implications for coastal water resource management and provide a better understanding of geologic controls on solute flux between land and sea in volcanic systems.University of Delaware, Department of Geological SciencesM.S