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Scientific Investigations Report 2009–5251

Effects of Sea-Level Rise and Pumpage Elimination on Saltwater Intrusion in the Hilton Head Island Area, South Carolina, 2004—2104

Dorothy F. Payne

ABSTRACT

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Saltwater intrusion of the Upper Floridan aquifer has been observed in the Hilton Head area, South Carolina since the late 1970s and currently affects freshwater supply. Rising sea level in the Hilton Head Island area may contribute to the occurrence of and affect the rate of saltwater intrusion into the Upper Floridan aquifer by increasing the hydraulic gradient and by inundating an increasing area with saltwater, which may then migrate downward into geologic units that presently contain freshwater. Rising sea level may offset any beneficial results from reductions in groundwater pumpage, and thus needs to be considered in groundwater-management decisions. A variable-density groundwater flow and transport model was modified from a previously existing model to simulate the effects of sea-level rise in the Hilton Head Island area. Specifically, the model was used to (1) simulate trends of saltwater intrusion from predevelopment to the present day (1885–2004) and evaluate the conceptual model, (2) project these trends from the present day into the future based on different potential rates of sea-level change, and (3) evaluate the relative influences of pumpage and sea-level rise on saltwater intrusion.

Four scenarios were simulated for 2004–2104: (1) continuation of the estimated sea-level rise rate over the last century, (2) a doubling of the sea-level rise, (3) a cessation of sea-level rise, and (4) continuation of the rate over the last century coupled with an elimination of all pumpage. Results show that, if present-day (year 2004) pumping conditions are maintained, the extent of saltwater in the Upper Floridan aquifer will increase, whether or not sea level continues to rise. Furthermore, if all pumpage is eliminated and sea level continues to rise, the simulated saltwater extent in the Upper Floridan aquifer is reduced. These results indicate that pumpage is a strong driving force for simulated saltwater intrusion, more so than sea-level rise at current rates. However, results must be considered in light of limitations in the model, including, but not limited to uncertainty in field data, the conceptual model, the physical properties and representation of the hydrogeologic framework, and boundary and initial conditions, as well as uncertainty in future conditions, such as the rate of sea-level rise.

First posted February 19, 2010

For additional information contact:
Director, Georgia Water Science Center
U.S. Geological Survey
3039 Amwiler Rd., Suite 130
Atlanta, GA 30360-2824
http://ga.water.usgs.gov/

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Suggested citation:

Payne, D.F., 2010, Effects of sea-level rise and pumpage elimination on saltwater intrusion in the Hilton Head Island area, South Carolina, 2004–2104: U.S. Geological Survey Scientific Investigations Report 2009–5251, 83 p.



Contents

Abstract

Introduction

Purpose and Scope

Methods

Previous Investigations

Acknowledgments

Description of Study Area

Hydrogeologic Setting

Groundwater Flow

Groundwater Pumpage

Saltwater Contamination

Conceptual Model

Sea-Level Rise

Simulation of Variable-Density Groundwater Flow and Solute Transport, Predevelopment–2004

Model Construction

Layering (Framework)

Spatial Discretization

Hydraulic and Transport Properties

Boundary Conditions

Top and Bottom Boundary Condition

Lateral Boundaries

Pumpage

Initial Conditions

Model Calibration

Simulated Predevelopment Conditions

Simulated Heads and Residuals, 2004

Simulated Chloride Distribution, 2004

Changes in Groundwater Levels and Chloride Concentrations at the Northern End of Hilton Head Island, 1960–2004

Comparison of Results to Original Model Results

Model Sensitivity

Top Boundary Condition–Water-Table Configuration

Overlying Confining-Unit Permeability

Saltwater-Source Areas

Freshwater-Recharge Areas

Upper Floridan Aquifer Permeability

Effective Porosity

Dispersion

Simulation of Sea-level Rise and Pumpage Elimination, 2004–2104

Scenario 1: Sea Level Continues to Rise at Current Estimated Rate

Scenario 2: Sea-Level-Rise Rate Doubles

Scenario 3: Sea-Level Rise Ceases at the 2004 Sea Level

Scenario 4: Pumpage is Eliminated while Sea Level Continues to Rise at Current Estimated Rate

Relative Effects of Sea-Level Rise and Pumpage Elimination

Model Limitations

Conceptual Model

Regional Flow System

Field Data and Physical Properties

Boundary and Initial Conditions

Calibration Approach

Future Predictions

Discussion

Summary and Conclusions

References Cited


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