THE SEASONAL HYDROLOGIC CYCLE: A DRIVER OF SALINE WATER EXCHANGE BETWEEN AQUIFERS AND THE COASTAL OCEAN

Fresh and saline groundwater flowing from coastal aquifers into the ocean comprise submarine groundwater discharge (SGD). This outflow is an important pathway for transporting nutrients and contaminants across the sediment-water interface, which can adversely affect coastal ecosystems. Field measurements during five summers in Waquoit Bay, Massachusetts reveal the pattern and composition of submarine groundwater discharge. Outflow follows a bimodal distribution with maximum discharge occurring in two alongshore bands: brackish outflow nearshore and saline discharge offshore. Most of the total outflow in the summer is saline, yet net seawater inflow over a tidal cycle is negligible. Circulation mechanisms such as tides, waves, and hydrodynamic dispersion cause significant saline groundwater discharge, and are potentially important for chemical loading to estuaries. However, these mechanisms can explain only a fraction of the observed saline outflow in Waquoit Bay and produce zero net flow over a tidal cycle. We propose that large-scale saline discharge is caused by seasonal movement of the freshwater-saltwater interface within coastal aquifers that is driven by, but lags, seasonal recharge. A series of idealized simulated systems demonstrates this process for a range of realistic aquifer parameters, and a time lag between maximum recharge and simulated peak discharge may explain the observed net discharge during times of low recharge. Winter hydraulic gradient measurements in Waquoit Bay indicate inflow in the zone of peak summer saline discharge, confirming seasonal variation in SGD at this field site.