COASTAL WATERSHEDS AND BEYOND: HEAT TRACER EVIDENCE FOR GROUNDWATER-SEAWATER EXCHANGE FAR FROM SHORE

Watersheds are the fundamental unit of hydrology, and the interplay between watershed processes and marine processes in coastal systems is critical to understanding the exchange of solutes at the land-ocean interface. The concept of a watershed begins to weaken sharply at the shoreline, however. Our ability to monitor flow also weakens with distance offshore, so that our knowledge of porewater-seawater exchange more than a few km beyond the shoreline is very sparse. In this zone poreflow has the potential to be strongly influenced by ocean currents and waves. This talk focuses on analyses of heat tracer data collected from below the seafloor at distances of 5 to 50 km offshore in the South Atlantic Bight, including adventures in submarine field methods and the development of new tools to invert thermal data to estimate flushing depths and groundwater flow velocities. Flushing was approximated in 1-D thermal models using an apparent thermal diffusivity. Very shallow (3 to 12 cm depth) observations reveal periodic flushing caused by a combination of tidal currents and thermal instability during cold intrusions onto the continental shelf at a site 50 km offshore of St. Catherine's Island, GA. Deeper (0.25 to 3 m) observations from a new wellfield installed 5-20 km offshore of Charleston, SC, reveal strong seasonal changes that suggest large-scale flushing to depths of 50 cm. Scour and sediment migration may contribute to porewater-seawater exchange.