T264 Coastal Surface Water–Groundwater Interactions

Wednesday, 7 November 2012
1:30 PM - 5:30 PM
207A

SALINITY DISTRIBUTION UNDER A BEACH OF A TRANSGRESSIVE BARRIER ISLAND

Paper number: T264-13
Program: Technical Sessions
Time: 4:25 PM – 4:40 PM

Authors

Abstract

The role of beaches in coastal biogeochemistry is becoming more apparent, but the groundwater flow that controls these processes is complex. In simple conceptual models, a barrier island beach contains a transition zone between a freshwater upland and the ocean, with less dense freshwater existing as a lens above saline groundwater. More recently, it has been shown that tidal pumping and wave forcing alter the hydrology in the subsurface, creating additional groundwater exchange in an upper saline plume that forms landward of the larger freshwater-saltwater transition zone. Studies involving the freshwater-saltwater interface are typically done in homogenous beaches, but not all beaches are homogenous. How does more complicated beach stratigraphy, for example of the type associated with transgressive barrier islands, affect beach hydrogeology?

Cabretta island, a Holocene transgressive barrier island located seaward of Sapelo Island, GA has been continuously monitored through a transect of piezometers across the island, instrumented with pressure transducers. Stratigraphy in the beach consists of a homogenous sand unit interrupted by a ~2-3m thick silt/clay layer at ~2m depth. This distribution of sediments in the subsurface is characteristic of a transgressive barrier island as it migrates landward with rising sea level and greatly impacts groundwater flow paths. Salinity distributions in the beach and upland were also imaged for both a summer spring and neap tide using electrical resistivity tomography, in order to more effectively understand the freshwater/saltwater interface. Freshwater is channeled below the beach confining unit. This confining unit also limits the depth of the upper saline plume. Salinities measured around the low tide line have varied from ~34 to ~18 ppt, suggesting significant variation in freshwater discharge.