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  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.
Overview of Groundwater monitoring networks
The St. Johns River Water Management District operates and maintains groundwater monitoring networks used to evaluate the response of the hydrogeologic system to changes in climate and the demand to meet growing water supply needs. The goal of groundwater monitoring is to improve the understanding of the hydrogeologic system through a long-term, systematic data collection of groundwater levels and quality from monitoring wells and discharge and water quality from springs. Long-term data collection provides the data needed to evaluate current aquifer conditions, to detect changes and trends over time, and to develop groundwater models used in water supply planning.
Groundwater monitoring networks provide the data and information used to evaluate the hydrostratigraphic aquifer and confining units in the district. The geologic structure and lithostratigraphy of peninsular Florida control the nature and distribution of the hydrostratigraphic units and have a distinct influence on the storage, movement and quality of groundwater. The hydrostratigraphic units in the district include the surficial aquifer, the intermediate confining unit, and the units within the Floridan aquifer system. The Floridan aquifer system includes the Upper Floridan aquifer (UFA), the middle confining units I and II, and the Lower Floridan aquifer. The UFA includes the Ocala permeable zone (upper part of the UFA), the Ocala low permeable zone, and the Avon Park permeable zone (lower part of the UFA).
Resource assessment staff responsibilities include:
- Construct and maintain monitoring network wells for hydrogeologic evaluations and for long-term data collection of groundwater levels and water quality
- Maintain spring network for discharge, pool elevation and water quality data collection
- Evaluate and refine the networks to optimize spatial and temporal data collection to meet water management needs
- Evaluate and analyze the hydrologic and geologic data collected from the networks using statistical and mapping methods to produce useful information that increases our understanding of the hydrogeologic, climatic and human factors that affect groundwater resources
- Distribute and publish the network data, information and hydrogeologic evaluations as required by users for water supply planning, minimum flows and levels, consumptive use permitting and other district programs
Groundwater level monitoring
The groundwater level monitoring network was developed based on knowledge of the hydrogeologic system and the spatial, temporal and statistical evaluation of historical water level data. As of September 2012, there are 740 wells in the groundwater level monitoring network in the district.
Groundwater levels reflect the balance between recharge to, storage, and discharge from the hydrostratigraphic aquifer units in the groundwater system. The physical and hydraulic properties of the aquifers, climatic conditions and human factors control this balance. Groundwater levels in peninsular Florida generally show a natural cyclical pattern of seasonal fluctuation, typically rising in the summer and fall seasons and falling in the winter and spring due to the effects of climate and groundwater demand for various uses. The magnitude of water level fluctuations can vary from season to season and year to year in response to climatic events of varying magnitude and duration. The record of groundwater level measurements over time, displayed as a hydrograph, reveals general hydrologic trends and provides information about how aquifers with different hydrogeologic characteristics respond to climatic conditions and groundwater use.
Groundwater flow patterns in the hydrogeologic system are controlled primarily by the geologic characteristics and hydraulic heads, or hydraulic pressure differences, among the aquifer units. The distribution of hydraulic heads is controlled by the relative hydrostratigraphic unit elevations, the location and effectiveness of recharge and discharge areas, aquifer confinement, and aquifer permeability and transmissivity. Well clusters at many monitoring locations provide data on the characteristics of the aquifer units with depth. Well clusters at a particular site typically include two to four observation wells that monitor distinct permeable zones.
Water levels for most wells in the network are monitored continuously as part of a telemetry system or by digital recorders. Manual water level readings are taken monthly or at lesser intervals for some wells. Because the extent and nature of groundwater resources are not confined by political boundaries, the St. Johns District also obtains hydrologic data from neighboring water management districts and the state of Georgia for use in hydrogeologic investigations.
Groundwater quality monitoring
The major focus of the district’s groundwater quality monitoring is the Floridan aquifer system, which is the primary source of water for public supply and other uses in most areas. Most of the 340 monitoring wells in the water quality network (as of September 2012) monitor the Ocala permeable zone of the Upper Floridan aquifer, with additional wells monitoring the Avon Park permeable zone (lower permeable zone of the Upper Floridan), and the Lower Floridan aquifer. Water quality samples are also collected at 21 springs.
The chemical composition and the physical properties of water in the hydrostratigraphic aquifer units represent the net effect of the processes that have dissolved, altered or precipitated the chemical constituents. Rainfall chemistry, land surface features, soil types, and recharge, discharge, and leakance relationships among aquifers influence groundwater quality. The lithology, structure and porosity of aquifer materials and the residence time of water affects the precipitation and dissolution of minerals as water moves along flow paths. Land uses, groundwater withdrawals and irrigation may also have an impact on groundwater quality.
The groundwater quality monitoring network provides the data and information used to characterize the major ion chemistry of the aquifer units and to describe a water quality variable’s spatial variability, temporal trend and changes with depth at a specific location. The network is designed to minimize wells and sampling frequency in areas that have shown little water quality variability over time. This allows for increased well coverage and sampling frequency in areas that have significantly increasing trends in water quality variable concentrations, in areas near the potable and nonpotable groundwater interface, in areas of seawater intrusion along parts of the Atlantic coast, and in areas with projected Floridan aquifer water level drawdowns.
Wells are sampled at frequencies of once or twice per year, depending upon the historical water quality data and the location of the well relative to areas of groundwater quality concern. Wells and springs are sampled for the chemical constituents (major cations and anions) typically used as general indicators of groundwater quality. The major cations include calcium, magnesium, sodium and potassium; anions include chloride, sulfate and alkalinity. The concentration of total dissolved solids is also analyzed. Field measurements include temperature, pH, specific conductance and dissolved oxygen. Springs are sampled quarterly, with samples also analyzed for nutrient parameters. Spring water quality monitoring provides vital information about the chemistry and nutrient content of baseflow to stream and river systems.
For additional information, see the report describing the St. Johns River Water Management District groundwater quality monitoring network, an assessment of the major ion chemistry of water for each aquifer, and geochemical patterns of water in the Floridan aquifer system.