Modeling Submarine Groundwater Discharge (SGD) Driven by Tidal Pumping in a Heterogeneous Coastal Aquifer

This study quantitatively tests whether submarine groundwater discharge (SGD) is driven by tidal-induced seawater recirculation or terrestrial hydraulic gradients. SGD as an unseen phenomenon is now recognized as an important water pathway between land and sea. It is difficult to quantitatively predict the SGD owing to its significant spatial and temporal variability. We developed a two-dimensional hydrogeological model to simulate SGD from a coastal unconfined aquifer in the northeastern Gulf of Mexico where previous SGD tracer studies are available. A density-variable numerical code, SEAWAT2000, was applied to simulate the SGD at the study site. To accurately predict SGD, various influences factors such as conductivity heterogeneity, uncertain boundary conditions and tidal- pumping were systematically assessed. The tidal-influenced seawater recirculation and the freshwater-saltwater mixing zone under the various tidal patterns, tidal amplitudes, and water table heights were investigated. The modeling study was calibrated and validated from long-term, intensive measurements in the study area. The results indicate that the percentage of fresh SGD related to the total SGD ranges from 4%-50% under normal conditions. Based on simulations to two field measurements in summer and spring, respectively, the freshwater ratios are 9% and 15%, respectively. These results support the hypothesis that tidal pumping is the most important SGD driver at this coastal plain site. The estimates of total and fresh SGD are at the low and high ends, respectively, of the estimation ranges obtained from geochemical tracers (e.g, 222Rn).