Quantifying Density-Driven Circulation in Coastal Aquifers Based on Water Geochemistry

The importance of submarine groundwater discharge (SGD) to coastal aquifers' biogeochemistry is well established. It is also acknowledged that seawater that circulates in the coastal aquifer comprises most of the SGD. However, the circulation mechanisms vary by their spatial and temporal scales, from short-term/small-scale circulation driven by tides and waves, through seasonal exchange driven by sea- or groundwater-level changes, up to long-term/large-scale circulation driven by density differences. Although short-term circulation has been shown to affect groundwater chemistry and potentially modify the composition of seawater for some elements, the long-term processes have the potential to affect elements that are controlled by long-term geochemical processes. Previous studies show that the amount of seawater circulating through the long-term processes may be relatively large, especially in a heterogeneous medium. However, field-based estimation of the long-term circulation is challenging due to the difficulty in isolating the long-term process. Therefore, we need new methods to understand the actual effect of SGD and seawater circulation in aquifers quantitatively. Preliminary results from Indian River Bay, Delaware, and the Eastern Mediterranean (EM), show potential for identifying long-term circulation in the aquifer, based on the geochemistry of the groundwater. Our results from seepage meters in Indian River Bay show that some of the groundwater compositions lie on a conservative mixing line, while others show a typical behavior of enrichment in Ca and Sr and depletion in K. Based on the geochemistry, the long-term circulation discharge is ~10% of the total saline water discharge. In addition, ²³⁴U/²³⁸U and ⁸⁷Sr/⁸⁶Sr isotopic ratios in circulated seawater within the EM coastal aquifer show gradual change with distance from the shore. ⁸⁷Sr/⁸⁶Sr decreases and ²³⁴U/²³⁸U increases compared to seawater ratios. The ²³⁴U/²³⁸U change occurs faster than the ⁸⁷Sr/⁸⁶Sr change, and therefore these isotopes can be used for identifying the relative timescale of water-rock interaction. Based on existing data of Ca, K and ⁸⁷Sr/⁸⁶Sr and their oceanic budgets, the long-term seawater circulation may be estimated to be between 4% and 20% of river discharge and thus has a significant role in ocean chemistry.