EVIDENCE OF DEEP-SEATED, VERTICAL GROUNDWATER PATHWAYS WITHIN THE GEORGIA COASTAL PLAIN

In the 1970s and 1980s, studies by the U.S. Geological Survey (USGS) delineated Mesozoic rift structures, collectively referred to as the South Georgia Rift, beneath the southeastern coastal plain of the United States. The present authors have found evidence that these basement faults have been reactivated over geologic time and propagate up through the coastal plain sedimentary sequence, creating permeable pathways for groundwater flow. In addition, other researchers have mapped Cenozoic joint sets in the coastal plain sequence associated with younger stress fields.

At Brunswick, Georgia, the USGS has documented the movement of deep hypersaline groundwater into the Upper Floridan aquifer along a 2,000 feet (610m) fault or solution conduit. Similarly, the present authors have collected geochemical data indicating the Upper Floridan aquifer at St. Catherines Island is experiencing vertical saltwater intrusion from a deeper aquifer. Sag structures have also been documented in the surficial aquifer on the island and are likely related to fracture zones and collapse of solution cavities in the Upper Floridan. At Little Blue Springs near Sylvania, Georgia, missing stratigraphic units and chemical analysis of an artesian spring indicate the presence of a fault with a vertical displacement of at least 200 feet (61m). Finally, rectangular drainage, compressed meanders, and scour pools at the confluence of the Ogeechee and Canoochee rivers strongly suggest deep-seated structural control.

Permeable pathways within the Georgia coastal plain are significant as they allow groundwater to move vertically between aquifer systems and the surface environment. Historically, these pathways created artesian springs and ecological niches in the surface environment. Modern pumping withdrawals, however, have altered the horizontal and vertical hydraulic gradients within the coastal plain. This has led to reduced artesian spring flow to natural ecosystems, and in some places, a complete loss of flow. Moreover, in areas where the vertical gradient has been reversed, these permeable pathways represent avenues for saltwater intrusion of the Upper Floridan aquifer, which serves as the regional water supply. As sea level continues to rise, the potential for saltwater intrusion of this critical resource will increase.