USGS

 

SIR 2004-5256
Prepared in cooperation with the
South Florida Water Management District

 

2005


Movement of the Saltwater Interface in the Surficial Aquifer System in Response to Hydrologic Stresses and Water-Management Practices, Broward County, Florida

Dausman and Langevin


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Abstract

Introduction

Purpose and Scope

Previous Studies

Acknowledgments

Hydrology of Southeastern Florida

Hydrogeologic Units and Aquifer Properties

Hydrologic Stresses

Rainfall and Evapotranspiration

Well-Field Withdrawals

Canal Stage

Saltwater Intrusion in the Biscayne Aquifer

Collection and Interpretation of Field Data

Well Selection and Continuous Monitoring

Influences on the Saltwater Interface

Tides

Rainfall and Control Structure Openings

Response to Water-Level Changes Near Structure S-36

Response to Water-Level Changes Near Structure S-13

Data Exploration with Artificial Neural Networks and Regression

Geophysical Logging

Numerical Simulation of Saltwater Interface Movement in Response to Water-Level Fluctuations

SEAWAT Simulation Code

Model Design

Initial Water Levels and Salinity Concentrations

Spatial Discretization

Boundary Conditions

Estimation of Representative Aquifer Parameters 

Ten-Year Simulation

Ground-Water Flow Patterns and Water Budget

Comparison of Model Results with Field Data

Evaluation of Saltwater Intrusion

Upconing

One-Foot Rule

SEAWAT Model Evaluation Scenarios

Canal Stage

Drought with Increased Ground-Water Withdrawals

Sea-Level Rise

Sensitivity Analysis

Model Limitations

Summary and Conclusions

References Cited

Appendix: Broward County Model

Abstract

A study was conducted to evaluate the relation between water-level fluctuations and saltwater intrusion in Broward County, Florida. The objective was achieved through data collection at selected wells in Broward County and through the development of a variable-density ground-water flow model. The numerical model is representative of many locations in Broward County that contain a well field, control structure, canal, the Intracoastal Waterway, and the Atlantic Ocean. The model was used to simulate short-term movement (from tidal fluctuations to monthly changes) and long-term movement (greater than 10 years) of the saltwater interface resulting from changes in rainfall, well-field withdrawals, sea-level rise, and upstream canal stage. The SEAWAT code, which is a combined version of the computer codes, MODFLOW and MT3D, was used to simulate the complex variable-density flow patterns.

Model results indicated that the canal, control structure, and sea level have major effects on ground-water flow. For periods greater than 10 years, the upstream canal stage controls the movement and location of the saltwater interface. If upstream canal stage is decreased by 1 foot (0.3048 meter), the saltwater interface takes 50 years to move inland and stabilize. If the upstream canal stage is then increased by 1 foot (0.3048 meter), the saltwater interface takes 90 years to move seaward and stabilize. If sea level rises about 48 centimeters over the next 100 year as predicted, then inland movement of the saltwater interface may cause well-field contamination.

For periods less than 10 years, simulation results indicated that a 3-year drought with increased well-field withdrawals probably will not have long-term effects on the position of the saltwater interface in the Biscayne aquifer. The saltwater interface returns to its original position in less than 10 years. Model results, however, indicated that the interface location in the lower part of the surficial aquifer system takes longer than 10 years to recover from a drought. Additionally, rainfall seems to have the greatest effect on saltwater interface movement in areas some distance from canals, but the upstream canal stage has the greatest effect on the movement of the saltwater interface near canals.

Field data indicated that saltwater interface movement includes short-term fluctuations caused by tidal fluctuations and long-term seasonal fluctuations. Statistical analyses of daily-averaged data indicated that the saltwater interface moves in response to pumpage, rainfall, and upstream canal stage. In areas near the canal, the saltwater interface is most affected by canal stage because water-management structures control the stage in the upstream part of the canal and allow movement of the saltwater interface. In areas away from the canal, the saltwater interface is most affected by pumpage and rainfall, depending on the location of well fields. Data analyses also revealed that rainfall changes the vertical flow direction in the Biscayne aquifer.

Results from the study indicated that upstream canal stage substantially affects the long-term position of the saltwater interface in the surficial aquifer system. The saltwater interface moves faster inland than seaward because of changes in upstream canal stage. For short-term problems, such as drought, the threat of saltwater intrusion in the Biscayne aquifer does not appear to be severe if the well-field withdrawal is increased; however, this conclusion is based on the assumption that well-field withdrawals will decrease once the drought is over. Sea-level rise may be a potential threat to the water supply in Broward County as the saltwater interface moves inland toward well fields.


Suggested Citation:

Dausman, Alyssa, and Langevin, C.D., 2005, Movement of the Saltwater Interface in the Surficial Aquifer System in Response to Hydrologic Stresses and Water-Management Practices, Broward County, Florida: U.S. Geological Survey: U.S. Geological Survey Scientific Investigations Report 2004-5256, 73 p.

U.S. Department of the Interior,
U.S. Geological Survey
Suite 106 & 107
9100 NW 36th St.
Miami, FL 33178

adausman@usgs.gov@usgs.gov


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