EXTENDING COASTAL HYDROGEOLOGY FROM ONSHORE TO OFFSHORE: GEOLOGIC AND HYDROLOGIC CONNECTIONS THROUGH THE CONTINENTAL SHELF (Invited Presentation)

Recent work has shown that fresh and brackish groundwater may exist offshore along many world coastlines. While this offshore freshened water may reflect climatic changes over glacial cycles, it may also reflect connections between onshore and offshore aquifers over tens of kilometers or more. These salinity distributions, unpredicted by theory, may change the way we think about our coastal hydrogeologic systems. On one hand, we may have more fresh groundwater available than previously thought, but on the other hand we have to consider connected pathways to the offshore. This could require a shift in the way we manage coastal groundwater resources. We conducted a series of numerical simulations of variable-density groundwater flow and salt transport in heterogeneous aquifers extending from onshore through the continental shelf. We show that geologic heterogeneity results in preferential flowpaths that create complex salinity distributions with fresh groundwater extending far offshore. These salinity distributions create density-driven circulation of saline groundwater through the shelf that greatly exceeds previous estimates but agrees with observations. We consider the influence of pumping, both onshore and offshore, and demonstrate both the potential benefit of the offshore resource and the vulnerabilities created by unpredictable onshore-offshore connections. This analysis demonstrates the importance of considering the large-scale land-sea system in understanding and managing fresh groundwater resources, nearshore marine ecosystems, and the chemistry of the ocean over geologic time. We suggest large-scale coastal hydrogeology as a frontier for the future, and look forward to new datasets to ground-truth this theoretical analysis.