Drivers of groundwater flow at a back barrier island – marsh transect

Groundwater plays an important role in coastal regions by delivering 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 located behind Blackbeard Island, GA, approximately 90 m from the nearest tidal creek, reaching from a back barrier island to a hammock and into the adjacent marsh.  Coinciding with each well, vibracores were also collected and sediment permeability was estimated from grain size measurements.  Combined with pressure gradients, this allowed an estimate of groundwater flow.  To better understand groundwater dynamics, pressure data was normalized to a fixed datum.  Subsequently, forces that govern groundwater movement such as tidal inundation, tidal propagation, and variations in fluid density were quantified.  First, the role of the propagation pressure waves in the subsurface was investigated using a one-dimensional model.  Results show that pressure propagation would only have a minor effect at a distance of 70m from the tidal creek.  To investigate the role of variations in salinity, measured pressure gradients between adjacent wells were then separated into freshwater head and density changes.  Density changes are only responsible for up to 12% of groundwater flow.  In order to investigate the drivers influencing pressure gradients due to head changes, and to delineate the contribution of tidal flushing, the pressure time series were studied using Fourier analysis.  Preliminary results of this ongoing effort indicate that tidal flushing plays the primary role.  Understanding these drivers of groundwater today will help predict the response to changing forces, e.g. due to storms, changes in surface runoff due to coastal development or sea level rise.