CHARACTERIZING HETEROGENEITY OF SURFICIAL AQUIFERS IN COASTAL GEORGIA USING COMPLEMENTARY OBSERVATIONS

Current scientific and management issues facing coastal stakeholders include saline water intrusion, groundwater eutrophication, water resource delineation, and erosion and sedimentation of barrier beaches. In coastal sedimentary sequences, groundwater flow is primarily controlled by heterogeneity in hydraulic properties across multiple spatial scales. However, standard hydrologic observations from discrete boreholes lack the spatial coverage necessary to completely characterize aquifer units. In this study we integrate geophysical, geological, and hydrologic observations to constrain both the local and regional scale hydrogeology of barrier islands and the fringes of adjacent estuaries in the Georgia Bight. Integration of such disparate data sets leads to a description of critical groundwater flow pathways, scales of aquifer heterogeneity, and the degree of interaction between groundwater and surface water near the edges of barrier islands. At the regional scale (cross-island and upland to salt marsh transects), analyses of continuous sediment vibracores, and coincident ground penetrating radar (GPR) images reveal distinct lithologic units, primary bedding structures, and the distribution of hydrofacies. Fine-grained sands (d₁₀=120 mm) dominate the sedimentary section in upland areas and also occur below a surface layer of muds in some marsh and tidal creek bank settings. Both short (0.1 km) GPR transects acquired near the edges of the barrier island and long (2 km) transects across the island show a distinct layer of dipping reflectors formed during accretion of the island. These reflectors, which are pervasive in many locations, imply heterogeneity in the gross hydraulic conductivity field and the presence of preferential flow paths within the surficial aquifer. At the scale of laboratory to field measurements, hydraulic conductivities determined using both direct (falling-head permeameter tests, pumping tests) and indirect (tidal pumping analyses) hydrologic methods are consistent with values estimated from traditional techniques based on grain size distributions, sorting, and clay and silt content.