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Factors affecting the position of the freshwater-saltwater interface in Central and Southern Broward County, Florida

Project Chief: Joseph Hughes and Jeremy White
Cooperator: Broward County, City of Fort Lauderdale, City of Hollywood, City of Dania Beach, City of Hallandale
Period of Project: October 2010 – September 2014

Problem Statement

Figure 1 – Mapped Hydrography within the model domains.

In coastal aquifers, a dynamic balance exists between the seaward flow of fresh groundwater and the inland flow of saline groundwater. Changes in hydrologic conditions from either the landward side, such as a net decrease in the seaward flux of freshwater, or the seaward side, such as an increase in sea level, can cause movement of the transition zone between fresh water and saline water. Inland movement of the transition zone is commonly referred to as saltwater intrusion and poses a serious threat to fresh drinking-water supplies in coastal regions. Groundwater models can be used to simulate the position of the transition zone, as well as provide guidance to resource managers tasked with making water-resource allocation decisions which may affect the position of the transition zone. However, groundwater models used to simulate saltwater intrusion are more complicated than models routinely used to simulate the effects of a well field because the density contrast between fresh water and seawater affects flow patterns. Thus, models designed to simulate saltwater intrusion must include the effect of density variations on groundwater flow and must also represent the transport and mixing of fresh and saline groundwater.

Objectives

The purpose of this study was to evaluate the ability of a variable-density groundwater flow and solute transport model to simulate the historical pattern and rate of saltwater intrusion in Central and Southern Broward County, Florida. Specific project objectives were to:

1. Develop a three-dimensional dispersive saltwater intrusion model for the central and southern part of Broward County to simulate hydrologic conditions for the period from the early 1900’s to the present

2. Use the historical data record (groundwater heads and salinities) to history match aquifer flow and transport parameters and determine if a parsimonious numerical model is capable of representing the observed saltwater intrusion patterns

3. Conduct a formalized sensitivity analysis to determine the types of field data that are most useful for calibrating the numerical model, as well as the model parameters that have the most influence on the simulated saltwater intrusion patterns (fig. 1).

4. Quantify the relative importance of various hydrologic mechanisms for causing movement of the transition zone between freshwater and saltwater

Figure 2 – Predictive simulation results for the northern portion of Broward County.   Shown is the impact of various sea level rise (SLR) rates on the predicted viability of a public supply well field

Approach

To achieve the project objectives, a three-dimensional variable-density groundwater flow and solute-transport model was developed for Central and Southern Broward County. The SEAWAT computer program, which is a combined version of MODFLOW and MT3DMS, was used to simulate advective and dispersive variable-density groundwater flow and transport. The SWR1 process was used to represent the extensive surface water system in the central and southern portions of the county (fig.2).

Concurrent to model development was a major data collection effort which included a wide range of subsurface information and historical water level, salinity, and injection/abstraction records. These data not only were used to form a conceptual model of the system, but also provide conditioning for the history matching process.

Results

Not yet available.

Information Product

A USGS Scientific Investigations Report (SIR) will describe the factors affecting the position of the freshwater saltwater interface in Broward County Florida.

A journal article will present the predictive uncertainty analysis of an integrated variable density surface-water/groundwater model in high-dimensional parameter space.