Delineating the drivers of groundwater flow at a back barrier island – marsh transect in coastal Georgia
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Groundwater plays an important role in coastal regions by delivering freshwater and nutrients to near shore and salt marsh environments. To quantify groundwater flow, salinity, temperature and pressure, sensors were installed in shallow piezometers along a transect behind Blackbeard Island, GA. Located approximately 90 m from the nearest tidal creek, the transect reaches from a back barrier island to a hammock and into the adjacent marsh. Coincident with each well, vibracores were taken and sediment permeability was estimated from grain size measurements. Combined with horizontal pressure gradients, this allowed quantification of groundwater flow. Subsequently, the impact of forces that govern groundwater movement such as tidal inundation, subsurface propagation of pressure signals, and variations in fluid density was assessed. First, the propagation of pressure in the subsurface was investigated using a one-dimensional model and was found to likely only have a minor effect at the location of the well transect. Then, measured pressure gradients were separated into contributions from piezometric head changes and density changes. Density changes were responsible for typically <10% of groundwater flow and the effect was most dominant at the hammock. To delineate the contribution of tidal flushing, a classical harmonic analysis of the pressure time series was performed. Results of this ongoing effort show that tidal flushing plays the primary role. Alternate drivers, including precipitation events, are most evident in periods with low tidal forcing. Understanding current drivers of groundwater movement will help predict hydrological response to changing forces and the potential for saltwater intrusion.