By D.F. Payne, A.M. Provost, C.I. Voss, J.S. Clarke, United States Geological Survey, U.S.A.
REFERENCE: First International Conference on Saltwater Intrusion and Coastal Aquifers- Monitoring, Modeling, and Management. Essaouira, Morocco, April 23-25, 2001

Abstract

Increased water demand in coastal Georgia, U.S.A., has resulted in increased potential for ground-water contamination as saltwater is drawn toward major pumping centers. To characterize ground-water flow and encroachment of saltwater into freshwater zones, the U.S. Geological Survey, in cooperation with the Georgia Department of Natural Resources, is developing ground-water flow and variable-density transport models for coastal Georgia. These models will be used to predict the effects of various pumping scenarios on ground-water supplies, which will aid in the management of coastal ground-water resources. Field data collection concurrent with the modeling effort will provide model input and calibration data.

The potential for ground-water contamination is being studied using the three-dimensional variable-density ground-water flow and solute-transport simulator SUTRA. The primary objective is to characterize the temporally variable geometry and concentration gradient of the saltwater-freshwater transition zone. Preliminary modeling has focused on simulating two specific mechanisms by which saltwater may enter the aquifer:

1. entry of seawater into the aquifer through areas where erosion has partially or completely removed the overlying confining unit and resulted in subsequent lateral migration of seawater through the aquifer due to reversal of the normally seaward hydraulic gradients (Figure 1); and
2. upward vertical movement of saline water into the aquifer through vertical conduits, such as fractures or karstic features (Figure 2).

Preliminary model results (Figures 3 and 4) will be used to assess the viability of different hydrogeologic conceptual models, characterize relative time and length scales of saltwater movement by the two mechanisms, determine sensitivities of flow dynamics and solute distribution to hydrogeologic characteristics, and indicate what additional data-collection efforts would improve the models. Questions raised by preliminary modeling results include:

1. are the conceptual models reasonable?;
2. are the predicted time-scales of saltwater movement of the various mechanisms reasonable for the conceptual models?; and
3. what is the relative importance of different transport paths where multiple transport mechanisms are operating?