USGS Georgia Water Science Center

USGS Scientific Investigations Report 2005-5089

Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of South Carolina and Florida—Predevelopment, 1980, and 2000

This report is available online in pdf format (10 MB): USGS SIR 2005-5089 (Opens the PDF file in a new window. )

Dorothy F. Payne, Malek Abu Rumman, and John S. Clarke

U.S. Geological Survey Scientific Investigations Report 2005-5089, 91 pages (Published 2005)

ABSTRACT

Cover photograph: Great Egret, Clam Creek, Jekyll Island, Georgia  Photograph by Alan M. Cressler, U.S. Geological Survey, 2000A digital model was developed to simulate steady-state ground-water flow in a 42,155-square-mile area of coastal Georgia and adjacent parts of South Carolina and Florida. The model was developed to (1) understand and refine the conceptual model of regional ground-water flow, (2) serve as a framework for the development of digital subregional ground-water flow and solute-transport models, and (3) serve as a tool for future evaluations of hypothetical pumping scenarios used to facilitate water management in the coastal area.

Single-density ground-water flow was simulated using the U.S. Geological Survey finite-difference code MODFLOW-2000 for mean-annual conditions during predevelopment (pre–1900) and the years 1980 and 2000. 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. A combination of boundary conditions was applied, including a general-head boundary condition on the top active cells of the model and a time-variable fixed-head boundary condition along part of the southern lateral boundary.

Simulated heads for 1980 and 2000 conditions indicate a good match to observed values, based on a plus-or-minus 10-foot (ft) calibration target and calibration statistics. The root-mean square of residual water levels for the Upper Floridan aquifer was 13.0 ft for the 1980 calibration and 9.94 ft for the 2000 calibration. Some spatial patterns of residuals were indicated for the 1980 and 2000 simulations, and are likely a result of model-grid cell size and insufficiently detailed hydraulic-property and pumpage data in some areas. Simulated potentiometric surfaces for predevelopment, 1980, and 2000 conditions all show major flow system features that are indicated by estimated peotentiometric maps.

During 1980—2000, simulated water levels at the centers of pumping at Savannah and Brunswick rose more than 20 ft and 8 ft, respectively, in response to decreased pumping. Simulated drawdown exceeded 10 ft in the Upper Floridan aquifer across much of the western half of the model area, with drawdown exceeding 20 ft along parts of the western, northern, and southern boundaries where irrigation pumping increased during this period.

From predevelopment to 2000 conditions, the simulated water budget showed an increase in inflow from, and decrease in outflow to, the general-head boundaries, and a reversal from net seaward flow to net landward flow across the coastline. Simulated changes in recharge and discharge distribution from predevelopment to 2000 conditions showed an increase in extent and magnitude of net recharge cells in the northern part of the model area, and a decrease in discharge or change to recharge in cells containing major streams and beneath major pumping centers.

The model is relatively sensitive to pumping and the controlling head at the fixed-head boundary and less sensitive to the distribution of aquifer properties in general. Model limitations include: (1) its spatial scale and discretization, (2) the extent to which data are available to physically define the flow system, (3) the type of boundary conditions and controlling parameters used, (4) uncertainty in the distribution of pumping, and (5) uncertainty in field-scale hydraulic properties. The model could be improved with more accurate estimates of ground-water pumpage and better characterization of recharge and discharge.


CONTENTS

Abstract

Introduction

Purpose and scope

Description of Study Area

Previous Investigations

Hydrogeology

Geologic Setting

Hydrogeologic Units

Surficial and Brunswick Aquifer Systems

Floridan Aquifer System

Ground-Water Flow System

Predevelopment

Modern Day: 1980 and 2000

Recharge and Discharge

Ground-Water Pumpage

Ground-Water-Level Trends

Saltwater Contamination

Simulation of Ground-Water Flow

Spatial Discretization

Model Layering

Hydraulic Properties

Boundary Conditions

Vertical Boundaries

Lateral Boundaries

Offshore Boundary

Pumpage

Model Calibration

Calibration Targets

Recharge

Steady-State Simulation of Predevelopment Flow System

Ground-Water Flow

Water Budget

Steady-State Simulation of 1980 and 2000 Flow System

Calibration of Simulated Head

Ground-Water Flow

Vertical Distribution of Head

Water-Level Change

Predevelopment to 2000

1980 2000

Water Budget

Seaward and Landward Flow

Model Sensitivity

Composite-Scaled Sensitivities

Pumping Sensitivity

Fixed-Head Boundary Condition Sensitivity

Model Limitations

Summary

Selected References

Appendix A. Observed and Simulated Ground-Water Levels, 1980 and 2000

Appendix B. Transient Response Testing

 


REPORT AVAILABILITY

This report is available online in pdf format (10 MB): USGS SIR 2005-5089 (Opens the PDF file in a new window. )
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