The US Army Corps of Engineers has recently studied potential effects on the Upper Floridan aquifer (UFA) due to a proposed harbor expansion of the Port of Savannah (Supplemental Studies to Determine Potential Groundwater Impacts to the Upper Floridan Aquifer, October 2007). The proposed Savannah Harbor Expansion Project (SHEP) consists of deepening approximately 35 miles of navigation channel.
The objective of the most recent study was to determine if deepening the Savannah Harbor channel has the potential to impact water quality in the UFA, the primary source of drinking water in the coastal area. The study focuses on the Miocene age upper confining unit of the Floridan aquifer (i.e., confining layer), which in some areas of the present harbor is exposed in the bottom of the navigation channel. Special emphasis was placed on the role of buried paleochannels that have cut into the confining layer.
The clay-rich, low permeability confining layer protects the underlying porous limestone strata. Prior to the 1880’s, wells drilled into the artesian aquifer would yield a head of water up to 35 feet above mean sea level (MSL) in the Savannah-Hilton Head, South Carolina area. However, since the 1880’s, due to increasing withdrawals of water from the aquifer, a resulting cone of depression in the Savannah area has lowered the water level in the aquifer to as much as 130 feet below MSL. The net effect of this lowering of water level has reversed the natural predevelopment flow of groundwater from the aquifer upward through the confining layer to a downward flow of water through the confining layer toward the center of the area of greatest pumping from the aquifer (Savannah). Since much of the area within the drawdown cone of depression is overlain by saltwater, chloride levels in the UFA in the Savannah area are expected to increase.
Removing additional confining layer material during the dredging process would effectively reduce the thickness of the layer; therefore, it was important to determine what effect this may have on the level of chlorides in the UFA due to any potential increase in the rate of downward leakage of saltwater.