Driving Mechanisms of Submarine Groundwater Discharge: Review of Recent Advancements and Observations in a Cape Cod Estuary

Understanding of the physical forcing mechanisms that drive submarine groundwater discharge (SGD) is important for quantification of fluid fluxes and chemical loading that can affect coastal ecosystems. Different mechanisms drive flows of different origin, chemical composition, and subsurface residence time, complicating flux estimation. Recent studies have used field measurements and modeling to improve understanding of physical processes, controls on fluid fluxes, and temporal and spatial variability. A review of recent advancements in understanding of four drivers of submarine groundwater discharge is presented. Mechanisms discussed are: 1- fresh groundwater discharge due to an upland hydraulic gradient, 2- circulation of saltwater due to tides and waves, 3- saltwater circulation driven by density gradients along the freshwater-saltwater interface, and 4- offshore seasonal saltwater exchange.

The SGD due to each of these mechanisms has been quantified in an estuary on Cape Cod, MA using direct measurements and modeling. The spatial location of discharge due to each mechanism was estimated, and temporal variability investigated. Because the origin and flowpaths of waters driven by each mechanism is different, it is expected that each would contribute solutes in different concentrations. This is illustrated in Waquoit Bay with measurements of radium isotopes in porewater and direct groundwater discharge. Combination of fluid flux and measured radium activities indicates that 80% of the total 226Ra flux occurs in the zone of discharge driven by interface density gradients, which supplies an estimated 40% of fluid discharge. Similar disproportion between fluid and solute fluxes occurs in other discharge zones, indicating that physical forcing mechanisms and resulting flowpaths may have a strong control on chemical loading. This has implications for use of geochemical tracers in estimating SGD, and indicates that an understanding of both the driving mechanisms and associated solute concentrations is essential for accurate estimation of groundwater-derived chemical fluxes to coastal waters.