Aquifer structure as a primary control on land-sea water exchange across the continental shelf

Land-sea water exchange and associated solute fluxes affect nearshore marine ecology on short timescales and ocean chemistry over geologic time. Evidence from radiochemical tracers suggests that groundwater discharge occurs across the continental shelf, offshore of predictions by large-scale numerical models and at rates greater than can likely be explained by small-scale physical mechanisms. We constructed geostatistical models of aquifer heterogeneity with three scales of connectivity in facies structure. Steady-state, variable-density groundwater flow and salt transport was simulated through the heterogeneous and equivalent homogeneous fields. Results show that increasing the shore-perpendicular connectivity of high- and low-permeability structures shifts both fresh and saline discharge offshore, enhances variability, and increases rates of saltwater circulation relative to fresh discharge. The existence of heterogeneity, independent of connectivity, similarly alters groundwater discharge rates and patterns relative to equivalent homogeneous models. These results suggest that aquifer structure has a strong control on water exchange rates and patterns across the continental shelf. This indicates that heterogeneity should be considered in both field and modeling studies of offshore aquifer-ocean exchange. Further, the influence of connectivity indicates that the nature of these exchanges may be predicted on a large scale by understanding of regional geologic settings along continental shelves.