THE ROLE OF PALEOCHANNELS IN SALTWATER INTRUSION IN THE SAVANNAH HARBOR AREA

The US Army Corps of Engineers is studying the potential effects on the Upper Floridan aquifer due to a proposed harbor expansion of the Port of Savannah. Deepening the existing navigation channel would reduce the thickness of the Floridan aquifer upper confining unit; therefore, it is imperative to gain a better understanding of the hydraulic characteristics of the confining layer underlying the Savannah River. The current study focuses primarily on the “area of concern” from Field's Cut to approximately two miles offshore of Tybee Island, where the confining layer naturally thins and the Floridan aquifer is closer to ground surface. Special emphasis is being placed on the role of several deep buried paleochannels that have cut down into the confining layer and how their presence affects saltwater intrusion into the Floridan aquifer.

A detailed subbottom seismic survey provided a comprehensive data set of the stratigraphy underlying the navigation channel within the area of concern. Initially, data was acquired along each edge of the navigation channel, and supplemental track lines were established in areas where prominent paleochannels passed under the navigation channel. The supplemental track lines were oriented such that the paleochannels were spatially well defined in seismic profiles.

The seismic profiles generated from the survey were used to better understand the three-dimensional relationship of the navigation channel, paleochannels, and the confining layer, and numerous core borings were used to verify the seismic profile interpretations. In addition, porewater samples of the confining layer and paleochannel fill material were extracted from core samples to create vertical profiles of chloride penetration. The porewater profiles were incorporated into a three-dimensional coupled ground-water flow and solute transport model to test various scenarios related to the hydrologic system in the immediate vicinity of the navigation channel. The combination of seismic, geochemical, and ground-water modeling data results indicates that paleochannels may locally enhance saltwater leakage, but any additional contribution of chloride from the paleochannels is negligible when compared to the total contribution from other saltwater sources outside the paleochannels along the river bottom.