Georgia coastal sound science initiative 2005 — what have we learned?

Abstract

Groundwater pumpage has resulted in saltwater contamination of the Upper Floridan aquifer at the northern end of Hilton Head Island, South Carolina, at Brunswick, Georgia, and near Jacksonville, Florida. This saltwater contamination has constrained further develop-ment of the aquifer in the coastal area and created competing demands for the limited supply of water. The Georgia Coastal Sound Science Initiative was initiated during 1999 to provide scientific support for development of the Georgia Department of Natural Re-sources, Environmental Protection Division’s water-man-agement strategy for the Georgia coastal area. Implemen-tation of the water-management strategy is scheduled for January 2006. In support of the Coastal Sound Science Initiative, the U.S. Geological Survey is evaluating ground-water flow, saltwater contamination, and alternative water sources in the Georgia coastal area and adjacent parts of South Carolina and Florida (Fig. 1). Other participants in this multidisciplinary effort include the South Carolina Department of Health and Environmental Control; Skida-way Institute of Oceanography; Georgia Water Resources Research Institute; and several private consulting firms. The initiative has provided new insight into multilayer groundwater flow systems, provided a better delineation of saltwater contamination in the Floridan aquifer system, and improved our understanding of alternative water sources in the coastal area. From these studies we have learned that: ● Offshore drilling near Savannah, Georgia, and Hilton Head Island, South Carolina, indicates that saltwater is entering the Upper Floridan aquifer though paleochannels that have thinned breached the Upper Floridan confining unit and provide a hydraulic connection with seawater or overlying saltwater-bearing units. ● In addition to paleochannel pathways, porewater analysis of core samples indicate that saltwater probably is migrating downward through the Upper Floridan confining unit and toward the Upper Floridan aquifer along the hydraulic gradient created by pumping from the aquifer since about 1900. ● New test wells and water sampling by the South Carolina Department of Health and Environ-mental Control indicate that there are three major chloride plumes in the Upper Floridan aquifer at the northern end of Hilton Head Island and in the marshlands behind the island. Previous studies had suggested the presence of only one plume in the northern Hilton Head Island area. ● Geologic assessment of Oligocene and younger sediments has provided insight into the deposi-tional environments, lithology, and areal extent of the Brunswick and surficial aquifer systems, which have been identified as potential alterna-tive sources of water; and aquifer testing at several sites has provided insight into the water-bearing properties of these aquifer systems. ● Deep test wells completed at seven locations have improved characterization of the geology, ground-water quality, and hydraulic properties of the Floridan aquifer system in coastal Georgia. One of the test wells—at St. Simons Island, Georgia—revealed the unexpected absence of the Fernandina permeable zone of the Lower Floridan aquifer—the source of chloride contamination at nearby Brunswick, Georgia.