Quantifying Long-Term Seawater Circulation in Coastal Aquifers: A Novel Geochemical Approach Validated at Indian River Bay, Delaware

Submarine groundwater discharge (SGD) significantly influences ocean chemistry, yet quantifying solute fluxes remains challenging due to the complex interplay of freshwater and saltwater within coastal aquifers operating at different temporal and spatial scales. The key lies in differentiating distinct saltwater flux components and characterizing their end-member compositions.

We developed a novel geochemical approach to isolate and quantify long-term density-driven seawater circulation in coastal aquifers. By compiling an extensive global dataset of coastal groundwater chemistry from onshore wells, we identified systematic deviations from conservative mixing models: enrichment in Ca and Sr and depletion in Na and K. These signatures reflect water-rock interactions occurring over multi-year timescales during mostly density-driven circulation, distinct from rapid tidal/wave-driven exchanges that show conservative mixing. Our novel approach quantifies the long-term SGD component by comparing major element enrichment and depletion in subterranean estuary samples (collected from seepage meters and piezometers) against an end-member composition derived from our global compilation of onshore well data.

To validate our methodology, we applied our mass balance approach to Indian River Bay, Delaware. Based on Ca and Sr enrichment (12 and 0.24 meq/L, respectively) and K depletion (5 meq/L), we calculated long-term circulation at 9±4% of total saline SGD. After correcting for wave-driven circulation, both fresh SGD and long-term circulation represent ~1% of total SGD, consistent with global estimates and extrapolating to 1.2-3.6×10³ km³/y globally. Sr-based flow field mapping further constrains circulation patterns within the coastal aquifer.

This study demonstrates that geochemical tracers can effectively partition SGD components across spatial scales, providing a framework for quantifying long-term seawater circulation impacts on coastal and ocean biogeochemistry.