Along-shore movement of groundwater and its effects on seawater-groundwater interactions in heterogeneous coastal aquifers

Studies of coastal groundwater dynamics often assume two-dimensional (2D) flow and transport along a shore-perpendicular cross-section. We show that along-shore movement of groundwater may also be significant in heterogeneous coastal aquifers. Simulations of groundwater flow and salt transport incorporating different geologic structure show highly three-dimensional (3D) preferential flow paths. The along-shore movement of groundwater on average accounts for 40%-50% of the total flowpath length in both conduit-type (e.g., volcanic) heterogeneous aquifers and statistically equivalent (e.g., deltaic) systems generated with sequential indicator simulation (SIS). Our results identify a critical role of three-dimensionality in systems with connected high-permeability geological features. The 3D conduit features connecting land and sea cause terrestrial fresh groundwater to migrate seaward and increases the rate of SGD compared to equivalent homogeneous, SIS and corresponding 2D models. In contrast, in SIS-type systems, less-connected high-permeability features produce mixing zones and SGD nearer to shore, with comparable rates in 3D and 2D models. Onshore, 3D Heterogeneous cases have longer flowpaths and travel times from recharge to discharge compared to 2D cases, but offshore travel times are much lower, particularly for conduit-type models in which flow is highly preferential. Flowpath lengths and travel times are also highly variable in 3D relative to 2D for all heterogeneous simulations. This study highlights the importance of three-dimensionality and the geometry of geologic features in coastal groundwater flow and solute transport processes in highly heterogeneous aquifers. The results have implications for water resources management, biogeochemical reactions within coastal aquifers, and subsequent chemical fluxes to the ocean.