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Simulation of Saltwater Movement in the Upper Floridan Aquifer in the Savannah, Georgia–Hilton Head Island, South Carolina, Area, Predevelopment–2004, and Projected Movement for 2000 Pumping Conditions

This report is available online in pdf format (15 MB): USGS SIR 2006-5058 ()

Alden M. Provost, Dorothy F. Payne, and Clifford I. Voss

U.S. Geological Survey Scientific Investigations Report 2006-5058, 124 pages (Published 2006)

ABSTRACT

A digital model was developed to simulate ground-water flow and solute transport for the Upper Floridan aquifer in the Savannah, Georgia–Hilton Head Island, South Carolina, area. The model was used to (1) simulate trends of saltwater intrusion from predevelopment to the present day (1885–2004), (2) project these trends from the present day into the future, and (3) evaluate the relative influence of different assumptions regarding initial and boundary conditions and physical properties. The model is based on a regional, single-density ground-water flow model of coastal Georgia and adjacent parts of South Carolina and Florida.

Variable-density ground-water flow and solute transport were simulated using the U.S. Geological Survey finite-element, variable-density solute-transport simulator SUTRA, 1885–2004. The model comprises seven layers: the surficial aquifer system, the Brunswick aquifer system, the Upper Floridan aquifer, the Lower Floridan aquifer, and the intervening confining units.

The model was calibrated to September 1998 water levels, for single-density freshwater conditions, then refined using variable density and chloride concentration to give a reasonable match to the trend in the chloride distribution in the Upper Floridan aquifer inferred from field measurements of specific conductance made during 2000, 2002, 2003, and 2004. The model was modified to simulate solute transport by allowing saltwater to enter the system through localized areas near the northern end of Hilton Head Island, at Pinckney Island, and near the Colleton River, and was calibrated to match chloride concentrations inferred from field measurements of specific conductance. This simulation is called the "Base Case." Water-level residuals ranged from –5.3 to 23.4 feet for September 1998 conditions and single-density freshwater conditions. When chloride transport was simulated, water-level residuals ranged from –12.5 to 23.3 feet. The simulated chloride distribution captures the general trends in the field data. Chloride transport is sensitive to the permeabilities assigned to the confining units in the source areas and the porosity assigned to the Upper Floridan aquifer.

1. if present-day (year 2000) pumping conditions are maintained, plumes of saltwater in the Upper Floridan aquifer will continue to expand and move toward Savannah and across Hilton Head Island;
2. the rate of movement of the 250-mg/L (milligram per  liter) isochlor toward Savannah is between 144  feet per year and 190 feet per year and that the 250-mg/L isochlor could reach the pumping center at Savannah in 800 years;
3. if effective porosities are lower than those used in the model, as is likely, higher rates of solute transport would result; and
4. plumes may have occurred along the northern shore of Hilton Head Island before substantial development began in the mid-1960s, and lesser amounts of intrusion may have already occurred prior to the onset of pumping during 1885.

Model limitations include uncertainty in (1) field data, (2) the conceptual model, (3) the physical properties and representation of the hydrogeologic framework, and (4) uncertainty in the boundary and initial conditions. Results of simulations projected far into the future must be interpreted with caution because they are based on an assumed future pumping distribution and fixed boundary conditions, and because these conditions may differ substantially from those for which the model is calibrated.

CONTENTS

Abstract

Introduction

Purpose and scope

Approach

Description of Study Area

Previous Investigations

Acknowledgments

Hydrogeology

Geologic Setting

Hydrogeologic Units

Surficial and Brunswick Aquifer Systems

Floridan Aquifer System

Ground-Water Flow System

Predevelopment

Present Day: 1980 – 2000

Recharge and Discharge

Ground-Water Pumpage

Ground-Water Level Trends

Salinity Distribution

Possible Mechanisms of Saltwater Intrusion

Simulation of Variable-Density Ground-Water Flow and Solute Transport

Model Construction

Model Layering

Spatial Discretization

Hydraulic and Transport Properties

Boundary Conditions

Top and Bottom Boundaries

Lateral Boundaries

Pumpage

Model Calibration to Observed Head ("Head Calibration") and Flow-Model Sensitivity

Head Calibration Method

Head Observations

Head Calibration Results

Simulated Heads and Residuals

Calibrated Permeability Values, Confidence Intervals, and Correlations

Composite Scaled Sensitivities

Predevelopment Steady State

Evaluation of Steady-State Assumption

Flow Budget

Model Calibration to Observed Chloride ("Chloride Calibration") and Solute-Transport Model Sensitivity

Chloride Calibration Method

Base Case

Solute-Transport Model Sensitivity

Permeability of the Upper Confining Units in the Source Areas

Permeability of the Upper Florida Aquifer

Porosity

Dispersion

Surface-Water Chloride Concentration

Spatial Discretization

Temporal Discretization and Nonlinearity Iterations

Matrix Solver Convergence Tolerances

Summary of Solute-Transport ModelSensitivity

Variations on the Base Case

Variation 1: No Salt during 1965

Variation 2: Predevelopment Plumes

Variation 3: Model Truncated Below Upper Floridan Aquifer

Simulation of Future Saltwater Intrusion

Base Case

Variation 1: No Salt during 1965

Variation 2: Predevelopment Plumes

Variation 3: Model Truncated Below Upper Floridan Aquifer

Effect of Uncertainty in Porosity

Model Limitations

Field Data

Conceptual Model

Geometry and Physical Properties

Boundary and Initial Conditions

Numerical Approximation and Solution

Future Predictions of Chloride Distribution

Discussion

Summary and Conclusions

Selected References

Appendix A. Modifications to SUTRA Code

Irregular Three-Dimensional Meshes

Generalized Observation Points

Time-Dependent Pumping

Time-Step Cycling

Other Changes

Appendix B. Observed Specific Conductance Measurements and Estimated Chloride Concentration

Appendix C. Altitude of Top and Bottom Surface of Hydrogeologic Units

Appendix D. Solid Matrix Compressibility

Appendix E. Effect of Porosity on Rate of Transport

Appendix F. Distribution of Specified Pressure Applied at Model Boundaries

Appendix G. Observed and Simulated Water Levels

REPORT AVAILABILITY

This report is available online in pdf format (15 MB): USGS SIR 2006-5058 ()
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