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Water supply

Groundwater modeling

Groundwater flow simulation models

The groundwater flow models developed by and for the district incorporate the McDonald and Harbaugh (1988) modular, three-dimensional, finite-difference, groundwater flow model (MODFLOW) developed for the USGS. A number of criteria were considered in selecting MODFLOW. The ability to account for multiple aquifers and semiconfining units was a primary consideration. This ability was necessary to account for the interaction between the aquifers that comprise the Floridan aquifer system as well as the interaction between the Floridan aquifer system and the overlying surficial aquifer system. Other essential requirements included the ability to account for heterogeneity in the physical properties of the aquifers and semiconfining units of the Floridan aquifer system and of the upper confining unit; and the ability to represent complex lateral boundary conditions. In addition to meeting all of these criteria, MODFLOW is well documented and has been applied successfully in numerous other groundwater modeling studies.

Groundwater solute transport (saltwater intrusion) simulation models

Currently, the district is using two solute transport model codes. They are (1) Density-dependent Solute Transport Analysis finite-element Model(DSTRAM), Hydrogeologic, Inc. and (2) Saltwater Intrusion Model for Layered Aquifer Systems (SIMLAS), Huyakorn, et al. 1993.

DSTRAM is a three-dimensional finite element code that simulates fluid flow and solute transport in saturated porous media. The code is capable of performing several types of analysis. These include groundwater flow analyses, trace concentration solute transport analyses, and density dependent coupled flow and transport analyses. Each analysis can be performed in areal plane, a vertical cross-section, an axisymmetric configuration, or a fully three-dimensional mode. Because of its special design features, DSTRAM is capable of handling a wide range of complex three-dimensional, steady-state or transient, field problems.

SIMLAS is a finite-element code that enables simulations of the interaction between fresh and saltwater in multi-aquifer, groundwater flow systems. The transition zone between fresh and saltwater in SIMLAS is approximated as a sharp interface. Groundwater flow in SIMLAS is represented by three governing equations: groundwater flow equation representing the freshwater portion of the flow system, a groundwater flow equation representing the saltwater portion of the flow system, and an equation derived by Hubbert (1940) that enables determination of the elevation of the interface as a function of fresh and saltwater hydraulic heads and assigned density values. These equations are solved simultaneously in SIMLAS for each time step of a model simulation.

Groundwater optimization modeling projects

Optimization modeling involves the development of a systematic method of determining optimum water supply strategies that satisfy various environmental and hydrologic requirements. The purpose of this type of water supply strategy is to optimize the pattern of water supply development and usage to meet projected needs. This resource management problem requires the use of optimization modeling to identify desirable scenarios of resource allocation; otherwise, resources may not be used in the most effective and efficient manner. When environmental impacts are also incorporated the allocation problem expands to include identifying feasible scenarios that must also satisfy environmental constraints (i.e., groundwater quality standards, minimum water levels, etc.). To balance projected needs against available sources, it is possible that the management problem may become one of balancing projected development against adverse environmental impacts.

Optimization modeling for groundwater resource allocation is typically achieved through a combined simulation/optimization method. Groundwater flow simulation models predict pressure heads and fluxes resulting from specified initial conditions, boundary conditions, and pumping rates. An optimization model consists of an objective function, or quantity that is minimized or maximized, and a set of constraints, or conditional statements that must be satisfied. An optimization model can incorporate information from a groundwater simulation model. Such a combined simulation/optimization approach contains simulation equations and optimization algorithms. The simulation equations assure that the management model correctly emulates the aquifer responses to internal and external fluxes. The combined approach identifies groundwater withdrawal schemes that optimize the formulated objective function. A combined simulation/optimization model computes the optimal pumping strategy directly under specified constraints. In a groundwater management model, withdrawal rates represent the stimuli and pressure heads or fluid fluxes represent the system response. Many simulation/optimization models represent the relationship between the aquifer system response and its stimuli by incorporating the unit matrix response approach or the embedded method. The unit matrix response method requires decision variables only at those points identified as control points. Since the response equations are developed only at points of interest it is not necessary for equations to be developed for each grid cell within the aquifer system which allows for the dimensionality of the management problem to be significantly reduced when compared to other simulation/optimization techniques such as the embedded method. Several earlier studies utilizing the combined optimization/simulation model approach are of note: Demas and Burger [1995] developed an optimization model for the east-central Florida region. These optimization models served as the basis for present model development. Since the optimization models yield approximations for aquifer response, the true simulation model response is obtained with from the optimization model withdrawal strategy.

Analytical solutions for groundwater flow problems


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St. Johns River Water Management District
4049 Reid Street, Palatka, FL 32177
(800) 725-5922