The influence of connected heterogeneity on groundwater flow and salinity distributions in coastal volcanic aquifers

Geological heterogeneity is a key factor affecting rates and patterns of groundwater flow and the evolution of salinity distributions in coastal aquifers. The hydrogeologic systems of volcanic aquifers are characterized by lava flows that can form connected geologic structures in the subsurface. Surface-based geostatistical techniques were adopted to generate geologically-realistic, statistically equivalent model realizations of a hydrogeologic system based on that of the Big Island of Hawaii (conduit models). The density-dependent groundwater flow and solute transport code SEAWAT was used to perform 3D simulations to investigate subsurface flow and salt transport through these random realizations. Statistically equivalent geological systems generated with sequential indicator simulation (SIS) and equivalent homogeneous systems were also simulated for comparison. Simulation results show that conduit realizations tend to have more complex salinity distributions with larger mixing zones for which the center of mass is farther offshore compared to homogeneous and SIS realizations. The zone of groundwater discharge is also farther offshore than for homogeneous and SIS models, with higher point fluxes of both fresh and saline water and greater spatial variability. This highlights the importance of the geometry of geologic features in coastal groundwater flow and solute transport processes in highly heterogeneous aquifers. It also points to the importance of preferential flow, with implications for both coastal groundwater resource management and solute fluxes to the ocean.