EFFECTS OF SEA-LEVEL RISE ON COASTAL AQUIFER SYSTEMS – POTENTIAL ECONOMIC AND ECOLOGICAL IMPACTS, EASTERN U.S

The majority of concerns related to sea-level rise along the Atlantic coast of the United States have been focused primarily on surface inundation from saline flooding and increased coastal erosion. However, the potential impacts of sea-level rise on groundwater flow systems may pose an even greater long-term risk for some of the nearly 60 million people who live in coastal counties along the eastern seaboard of the United States.

Previous studies have shown that an increase in sea-level position can affect groundwater flow in shallow, coastal aquifer systems, causing changes in water levels, surface-water discharge, and in the position of the freshwater/saltwater interface. Here we compare and contrast the effects of sea-level rise on three shallow, unconfined aquifers systems in different hydrogeologic settings along the Atlantic seaboard: (1) the glacially-deposited Cape Cod peninsula of southeastern Massachusetts, (2) the Assateague barrier island complex, eastern Maryland and Virginia, and (3) the carbonate system of southeastern Florida.

Regardless of hydrogeologic setting, vadose zone thickness is the key determinant for sea-level rise associated impacts in coastal groundwater systems because the vadose thickness determines the unsaturated volume available for future rises in the water table. The thick vadose zone in areas such as Cape Cod mitigates potential impacts to subterranean infrastructure such as a septic systems and basements, which are typically located well above the water table. The potential exists for the water table to intersect land surface when the vadose zone is thin, such as on Assateague Island and in southeastern Florida, thus no longer keeping up with the rate of sea-level rise. The depth to the interface between fresh and salty groundwater decreases accordingly, particularly near surface-water features, resulting in a thinning of the freshwater lens. Substantial changes in vadose zone thickness and the thickness of the freshwater lens will alter habitat suitability in barrier islands ecosystems and increase the potential for saltwater intrusion in public-supply wells in the shallow coastal carbonate aquifer system of southeastern Florida.