Episodic high intensity mixing events in a subterranean estuary: Effects of tropical cyclones

Hydrostatic balances between fresh and saline groundwater and saline surface water control the physical and chemical framework of subterranean estuaries, but they are responsive to high frequency (waves and tides), low frequency (seasonal recharge patterns), and episodic (storm) events. In this study, we document a salinity and pressure perturbation to the subterranean estuary in east-central Florida during and after the passage of Tropical Storm Tammy on 04 Oct 2005–05 Oct 2005 and Hurricane Wilma on 24 Oct 2005. These storms reversed hydraulic gradients, forced lagoon water into the aquifer, and shifted the outflow face landward. Salinity at 1.5 m and 2.5 m below a common datum converged on similar values intermediate between fresh and lagoon water salinities. The outflow face reestablished pre-storm conditions after 80 days at 15 m offshore, but more than 160 days at 30 m offshore, confirming that both the flow field and fluid sources control the position of the subterranean estuary. Episodic, high intensity events could influence the biogeochemical setting of the subterranean estuary and the overlying water body by altering redox conditions in the subterranean estuary during the landward migration of the dispersive mixing zone, increasing short-term discharge of potentially contaminated groundwater, and/or changing pore fluid residence time within the seepage face and along the mixing zone–seepage face front. Fluid discharge from submarine aquifers and associated chemical loading to overlying surface-water bodies have been the focus of a great deal of research during the last decade. The discharging fluid, collectively referred to as submarine groundwater discharge (SGD), is composed of terrestrially recharged groundwater and recirculated seawater. Within the aquifer, these two chemically contrasting fluids mix to form what is often referred to as the ‘‘subterranean estuary’’ (Moore 1999). A number of recent studies have investigated the role of the subterranean estuary on the biogeochemical cycling of metals, contaminants, and macroand micro-nutrients (e.g., Simmons 1992; Krest et al. 2000; Cable et al. 2002), thus providing the physical and chemical framework for these dynamic systems and their effects on chemical transport. Most studies are limited in temporal monitoring and many use synoptic data sets, such measurements result in an incomplete understanding of how subterranean systems respond to perturbations (e.g., seasonal recharge, human consumption, storms, etc.). Only a few studies have documented how SGD varies with changes in forcing mechanisms. Michael et al. (2005) noted a distinct seasonal pattern in SGD reflecting recharge of the surficial aquifer and changes in the position of the subterranean estuary in Waquoit Bay, Massachusetts. Extensive harmful algal blooms (HABs) in 2005 along the west-central Florida coast were blamed on the active 2004 Atlantic hurricane season, which dropped substantial rainfall on the peninsula (Hu et al. 2006). These researchers hypothesized HABs were fed by large SGD-derived nitrogen fluxes driven to the coastal ocean by this precipitation and subsequent recharge. Along the North Carolina continental shelf, Moore and Wilson (2005) noted 1–2uC temperature oscillations 4 m below seafloor following the passage of Hurricanes Dennis and Floyd in 1999. These temperature anomalies were attributed to enhanced pore-water exchange between the permeable seafloor sediments and the ocean during the passage of large storm waves. Here we investigate perturbations to the subterranean estuary associated with the passage of tropical cyclones and quantify how the subterranean estuary responds to these episodic events both spatially and