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Model Boundaries
The Florida Water Science Center (FLWSC) conducts hydrologic projects to improve understanding of the movement of groundwater and surface water using modeling simulations. Since the1980s, the FLWSC has completed over 50 hydrologic modeling projects. The model boundaries for about 30 projects completed by FLWSC hydrologists are highlighted below. Some of these projects also are described in the "Projects" web page under "Hydrologic Modeling".
Hydrologic Models
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Report Name: Exchanges of Water between the Upper Floridan Aquifer and the Lower Suwannee and Lower Santa Fe Rivers, Florida
Lead Author: J.W. Grubbs
End Date: 9/30/2007
Model Software: MODFLOW, MODBRANCH
Description: Exchanges of water between the Upper Floridan aquifer and the Lower Suwannee River were evaluated using historic and current hydrologic data from the Lower Suwannee River Basin and adjacent areas that contribute groundwater flow to the lowest 76 miles of the Suwannee River and the lowest 28 miles of the Santa Fe River. These and other data were used to develop a computer model that simulates the movement of water in the aquifer and river systems, as well as exchanges between these systems. Simulations were completed over a range of hydrologic conditions. The model used to predict the hydrologic response to a set of hypothetical water-use scenarios.
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Report Name: A Coupled Surface Water and Groundwater Model to Simulate Past, Present, and Future Hydrologic Conditions in DOI Managed Lands
Lead Author: Melinda A. Lohmann
End Date: 9/30/2009
Model Software: FTLOADDS - Flow and Transport in a Linked Overland/Aquifer Density-Dependent System (SWIFT2D)
Description: The USGS developed a coupled surface-water/groundwater numerical code named the Flow and Transport in a Linked Overland/Aquifer Density-Dependent System (FTLOADDS) to represent the surface water and groundwater hydrologic conditions in south Florida, specifically in the Everglades ecotone. The FTLOADDS code combines the two-dimensional hydrodynamic surface-water model SWIFT2D, the three-dimensional groundwater model SEAWAT, and accounts for water exchanges and salt flux between the surface water and groundwater. The code was applied within two major testing regions: 1) the Southern Inland and Coastal Systems (SICS) model domain and 2) the Tides and Inflows in the Mangroves of the Everglades (TIME) model domain. An initial application of the code within the SICS domain only utilized the SWIFT2D surface-water code. Later applications in the SICS domain used an updated version of FTLOADDS, in which SWIFT2D was coupled to the SEAWAT groundwater model code. The TIME model domain utilizes the further enhanced version of FTLOADDS, which includes enhancements to the wetting and drying routines, changes to the conceptualization of frictional resistance terms, and calculations of evapotranspiration. In 2006, FTLOADDS was modified again to represent Biscayne Bay and surrounding areas. The first objective of this project was to update and reconfigure the FTLOADDS modeling code to include all version modifications and enhancements.
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Report Name: Do seismically-imaged sag structures in Biscayne National Park influence submarine groundwater discharge?
Lead Author: J.N. King
End Date: 9/30/2011
Model Software: SEAWAT
Description: A numerical model of variable density flow and constituent transport was used to investigate how 11 seismically-imaged sag structures recently defined by the USGS in Biscayne National Park might influence both submarine groundwater discharge (SGD) and marine chemistry. The model quantifies the portion of SGD forced by (1) the terrestrial hydraulic gradient, (2) buoyancy associated with the mixing of ocean and terrestrial waters, and (3) Kohout convection. Portions of SGD forced by other mechanisms, such as bioturbation or surface-gravity waves, are not described. The model also was used to investigate whether these structures influence the transport of heat and dissolved solids by SGD, and to partition SGD as a function of a source aquifer-system. The model is based on a USGS
hydrostratigraphic framework of the Florida peninsula. Maximum inner sag diameters range between 167 and 4886 m in the horizontal. The inner sag diameter is defined by a transition from concave to convex. The shallowest identified structure is approximately 100 m below sea level, within the intermediate confining unit. The deepest structure is approximately 750 m below sea level, within the Eocene strata of the Floridan aquifer system. Some structures may extend deeper, but interpretable reflection patterns do not exist within lowest parts of the profiles. The framework is represented in the model with 12 layers. The domain extends from the Everglades to submarine outcrops in the Atlantic Ocean. Model parameters are estimated by minimizing an objective function that describes the match between simulated and observed head. Sag structures are represented within the model by 116 conductance elements, which connect 11 of 12 model layers. Structure conductance is not currently well understood and requires further investigation. Conductance is represented in the model with a range of values.
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Report Name: Potential for Saltwater Intrusion into the Lower Tamiami Aquifer near Bonita Springs, Southwestern Florida
Lead Author: Barclay W. Shoemaker
End Date: 9/30/2003
Model Software: MODFLOW, UCODE, SEAWAT
Description: A study was conducted to examine the potential for saltwater intrusion into the lower Tamiami aquifer beneath Bonita Springs in southwestern Florida. Field data were collected, and constant- and variable-density groundwater flow simulations were performed that: (1) spatially quantified modern and seasonal stresses, (2) identified potential mechanisms of saltwater intrusion, and (3) estimated the potential extent of saltwater intrusion for the area of concern.
MODFLOW and the inverse modeling routine UCODE were used to spatially quantify modern and seasonal stresses by calibrating a constant-density groundwater flow model to field data collected in 1996. The model was calibrated by assuming that hydraulic conductivity parameters were accurate and by estimating unmonitored groundwater pumpage and potential evapotranspiration with UCODE. Uncertainty in these estimated parameters was quantified with 95-percent confidence intervals. These confidence intervals indicate more uncertainty (or less reliability) in the estimates of unmonitored groundwater pumpage than estimates of pan-evaporation multipliers, because of the nature and distribution of observations used during calibration. Comparison of simulated water levels, stream flows, and net recharge with field data suggests the model is a good representation of field conditions.
Potential mechanisms of saltwater intrusion into the lower Tamiami aquifer include: (1) lateral inland movement of the freshwater-saltwater interface from the southwestern coast of Florida; (2) upward leakage from deeper saline water-bearing zones through natural upwelling and upconing, both of which could occur as diffuse upward flow through semiconfining layers, conduit flow through karst features, or pipe flow through leaky artesian wells; (3) downward leakage of saltwater from surface-water channels; and (4) movement of unflushed pockets of relict seawater. Of the many potential mechanisms of saltwater intrusion, field data and variable-density groundwater flow simulations suggest that the saline concentration at the spring is most sensitive to upconing, and that lateral encroachment is also important. This interpretation is uncertain, however, because the predominance of saltwater intrusion through leaky artesian wells with connection to deeper, more saline, and higher pressure aquifers was difficult to establish.
Effective management of groundwater resources in southwestern Florida requires an understanding of the potential extent of saltwater intrusion in the lower Tamiami aquifer near Bonita Springs. Variable-density, groundwater flow simulations suggest that when saltwater is at dynamic equilibrium with 1996 seasonal stresses, the extent of saltwater intrusion is about 100 square kilometers in area and 70,000 hectare-meters volumetrically. The volumetric extent of saltwater intrusion was most sensitive to changes in recharge, groundwater pumpage, sea level, salinity of the Gulf of Mexico, and the potentiometric surface of the sandstone aquifer, respectively.
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Report Name: Central Broward Saltwater Intrusion Model
Lead Author: Jeremy White
Model Software: MODFLOW, SWR1, SEAWAT
Description: The model uses MODFLOW and the SWR1 process for groundwater/surface water flow and SEAWAT from salt transport. The models have/will be subject to formal parameter estimation in high dimensional parameter space. Formal predictive uncertainty analysis also will be completed for the Central and Southern models. Ultimately, the models will be used to support allocation decisions in Broward.
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Report Name: Estimating Nitrate Concentrations in Wells and Springs (2002-2005) in the Surficial and Upper Floridan Aquifer, Dougherty Plain and Marianna Lowlands, Florida, Georgia, and Alabama
Lead Author: Christy A. Crandall
End Date: 9/30/2012
Model Software: MODFLOW 2000, MODPATH
Description: A steady-state regional groundwater flow model was used to provide flow boundaries for two local-scale models that were in turn, used to delineate areas contributing recharge to three wells and three springs in the Upper Floridan aquifer (UFA) of the lower Apalachicola-Chattahoochee-Flint basin. One local-scale model was constructed for the UFA in the upper Chipola basin and provides nitrate concentration estimates from 2002 through 2050 for Jackson Blue Spring, Baltzell Spring Group, and Sandbag spring. The other local-scale flow and particle-tracking model, located in the Ichawaynochaway basin, provides nitrate concentration estimates for 2002 through 2050 for three wells that have long-term nitrate concentration data available.
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Report Name: N/A
Lead Author: Christy A. Crandall
End Date: 2012
Model Software: MODFLOW 2000, MODPATH
Description: A steady-state regional-scale groundwater flow model was used to provide boundary flows to the Chipola local-scale groundwater flow model. This model, built in the MODLFOW 2000 framework, covers the Surficial and Upper Floridan aquifers in the upper Chipola River basin of the lower Apalachicola-Chattahoochee-Flint River Basin, an area where the Upper Floridan aquifer is unconfined. The flow model results were used with particle tracking to identify contributing areas and to estimate groundwater travel times to Jackson Blue Spring, Baltzell Spring Group, and Sandbag Spring. Particle tracking also was used as a tool to predict future nitrate concentrations at the 3 springs using 3 management scenarios: 1) Nitrate input to groundwater will remain at 2001 levels; 2) Nitrate input to groundwater will decrease at a rate of 4 percent per year from 2002 through 2050, and 3) Nitrate input to groundwater ceased in 2002. All concentrations estimates were calculated for the period 2002 through 2050.
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Report Name: N/A
Lead Author: Christy A. Crandall
End Date: 2012
Model Software: MODFLOW 2000, MODPATH
Description: A steady-state regional-scale groundwater flow model was used to provide boundary flows to the Middle-Flint local-scale groundwater flow model. This model, built in the MODLFOW 2000 framework, covers the Surficial and Upper Floridan aquifers in the Ichawaynochaway and Spring Creek basins of the lower Apalachicola-Chattahoochee-Flint River Basin, an area where the Upper Floridan aquifer is unconfined. The flow model results were used with particle tracking to identify areas contributing recharge and groundwater travel times to 3 wells. Particle tracking also was used as a tool to predict future nitrate concentrations at the 3 wells using 3 management scenarios: 1) Nitrate input to groundwater will remain at 2001 levels; 2) Nitrate input to groundwater will decrease at a rate of 4 percent per year from 2002 through 2050, and 3) Nitrate input to groundwater ceased in 2002. All concentrations estimates were calculated for the period 2002 through 2050.
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Report Name: Simulations of groundwater flow and particle tracking analysis in the area contributing recharge to a public-supply well near Tampa, Florida, 2002-05
Lead Author: Christy A. Crandall
End Date: 9/30/2009
Model Software: MODFLOW 2000 and MODPATH
Description: The TANC local-scale groundwater flow and particle-tracking model uses MODFLOW 2000 and MODPATH. The model was used to simulate groundwater flow and estimate nitrate transport to a public supply well in the city of Temple Terrace, Florida.
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Report Name: Hydrogeologic investigation and simulation of groundwater flow in the Upper Floridan aquifer of north-central Florida and southwestern Georgia and delineation of contributing areas for selected City of Tallahassee, Florida, water-supply wells
Lead Author: Hal Davis
End Date: 9/30/1996
Model Software: MODFLOW
Description: A 4-year investigation of the groundwater flow system in Leon County, Florida, and surrounding counties of north-central Florida and southwestern Georgia began in 1990. The purpose of the investigation was to describe the groundwater flow system and to delineate the contributing areas to selected City of Tallahassee, Florida, water-supply wells. The investigation was prompted by the detection of low levels of tetrachloroethylene in groundwater samples collected from several of the city’s water-supply wells.
Hydrologic data and previous studies indicate that groundwater flow within the upper Floridan aquifer can be considered steady state; the Upper Floridan aquifer functions as a single water-bearing unit; recharge is from precipitation; and discharge occurs as spring flow, leakage to rivers, leakage to the Gulf of Mexico, and pumpage. Measured aquifer transmissivities ranged from 1,300 ft2/d (feet squared per day) to 1,3000,000 ft2/d.
Steady-state groundwater flow in the Upper Floridan aquifer was simulated using a three-dimensional groundwater flow model. Transmissivities ranging from less than 5,000 ft2/d to greater than 11,000,000 ft2/d were required to calibrate to observed conditions. Recharge rates used in the model ranged from 18.0 inches per year in areas where the aquifer was unconfined to less than 2 inches per year in broad areas where the aquifer was confined.
Contributing areas to five Tallahassee water-supply wells were simulated by particle-tracing techniques. Particles were seeded in model cells containing pumping wells then tracked backwards in time to define recharge areas. The contributing area for each well was simulated twice, once assuming a porosity of 25 percent and once assuming a porosity of 5 percent. A porosity of 25 percent is considered a reasonable average value for the Upper Floridan aquifer; the 5 percent porosity simulated the movement of groundwater through only solution-enhanced bedding plains and fractures. The contributing areas were generally elliptical in shape, reflecting the influence of the sloping potentiometric surface. The contributing areas delineated for a 5 percent porosity were always much larger than those determined using a 25 percent porosity. The lowest average groundwater velocity computed within a contributing area, using a 25 percent porosity, was 1.0 ft/d (foot per day) and the highest velocity as 1.6 ft/d. The lowest average groundwater velocity, determined using a 5 percent porosity, was 2.4 ft/d and the highest was 7.4 ft/d.
The contributing areas for each of the five wells also was determined analytically and compared to the areas derived by the numerical model. The upgradient width of the simulated contributing areas calculated with the numerical model was larger than the upgradient width of the analytically-determined contributing areas for four of the five wells. The numerical model is thought be a more realistic representation of the contributing areas because of the ability to simulate wells as partially penetrating and by incorporating complex, three-dimensional aquifer characteristics, which analytical method could not.
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Report Name: Nitrate-N Movement in Groundwater from the Land Application of Treated Municipal Wastewater and Other Sources in the Wakulla Springs Springshed, Leon and Wakulla Counties, Florida, 1966-2018
Lead Author: Hal Davis
End Date: 9/30/2010
Model Software: MODFLOW
Description: The City of Tallahassee began a pilot study in 1966 at the Southwest Farm spray field to determine whether disposal of treated municipal wastewater using center pivot irrigation techniques to uptake nitrate-nitrogen (nitrate-N) is feasible. Based on the early success of this project, a new, larger Southeast Farm spray field was opened in November 1980. However, a recent 2002 study indicated that nitrate-N from these operations may be moving through the Upper Floridan aquifer to Wakulla Springs, thus causing nitrate-N concentrations to increase in the spring water. The increase in nitrate-N combined with the generally clear spring water and abundant sunshine may be encouraging invasive plant species growth. Determining the link between the nitrate-N application at the spray fields and increased nitrate-N levels is complicated because there are other sources of nitrate-N in the Wakulla Springs springshed, including atmospheric deposition, onsite sewage disposal systems, disposal of biosolids by land spreading, creeks discharging into sinks, domestic fertilizer application, and livestock wastes.
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Report Name: Simulation of Groundwater Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida
Lead Author: Nick Sepulveda
Model Software: MODFOW 2005
Description: A groundwater flow model, based on MODFLOW 2005, named the East-central Florida Transient model, simulates flow in the Upper Floridan Aquifer system for the time period from 1995 to 2006. The model uses the LAK7, UZF1, and SFR2 packages. The model not only simulates groundwater flow, but also simulates the surface-water system (SFR2 package, LAK7 package) and the unsaturated zone (UZF1 package). The SFR2 package routes flow and calculates groundwater exchanges in the 320 stream segments. The LAK7 package simulates water-surface elevation and groundwater exchanges at 345 lakes. The UZF1 package simulates unsaturated zone processes.
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Report Name: Documentation of a Conduit Flow Process (CFP) for MODFLOW-2005
Lead Author: Barclay W. Shoemaker
Lead Author: Barclay W. Shoemaker
End Date: 9/30/2008
Model Software: MODFLOW, CFP (Conduit Flow Process)
Description: The Lake Belt CFP model was built for testing non-laminar flow calculations within MODFLOW-CFP.
This report documents the Conduit Flow Process (CFP) for the modular finite-difference groundwater flow model, MODFLOW-2005. The CFP has the ability to simulate turbulent groundwater flow conditions by: (1) coupling the traditional groundwater flow equation with formulations for a discrete network of cylindrical pipes (Mode 1), (2) inserting a high-conductivity flow layer that can switch between laminar and turbulent flow (Mode 2), or (3) simultaneously coupling a discrete pipe network while inserting a high-conductivity flow layer that can switch between laminar and turbulent flow (Mode 3). Conduit flow pipes (Mode 1) may represent dissolution or biological burrowing features in carbonate aquifers, voids in fractured rock, and (or) lava tubes in basaltic aquifers and can be fully or partially saturated under laminar or turbulent flow conditions. Preferential flow layers (Mode 2) may represent: (1) a porous media where turbulent flow is suspected to occur under the observed hydraulic gradients; (2) a single secondary porosity subsurface feature, such as a well-defined laterally extensive underground cave; or (3) a horizontal preferential flow layer consisting of many interconnected voids. In this second case, the input data are effective parameters, such as a very high hydraulic conductivity, representing multiple features.
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Report Name: Groundwater Contributing Area model for the Biscayne aquifer in Miami-Dade County
Lead Author: Linzy Brakefield
Model Software: MODFLOW/MODPATH
Description: A Groundwater Contributing Area model was constructed for the Biscayne aquifer in Miami-Dade County. The model extent is the same as the USGS BISCAYNE model. The model is a MODFLOW/MODPATH model for determining contributing areas to major well fields in Miami-Dade County. Contributing areas were defined with an unconstrained Monte Carlo ensemble which uses hydraulic conductivity distribution generated by sequential Gaussian simulation.
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Report Name: Monitoring, modeling, management, and mitigation: potential for densification, subsurface disposal, and transport of buoyant wastewater treatment plant effluent in Miami-Dade County, Florida, USA
Lead Author: J.N. King
End Date: 9/30/2010
Model Software: SEAWAT
Description: Miami-Dade County is transitioning the primary means of treated wastewater (effluent) disposal for its 2.4 million residents from ocean outfall to subsurface injection, to protect the coastal environment, quality of life, and local economies. Miami-Dade County injects effluent (injectate) into the Boulder Zone, a karst stratum located one kilometer below land surface with a 3-centimeter-per-second hydraulic conductivity, which contains saline water with a density similar to seawater. Potential exists for transport of buoyant injectate through the following pathways: (1) Boulder Zone outcrops to the ocean, (2) breaches in confinement caused by natural, structural-collapse systems, and (3) construction related breaches in confinement, which may connect the Boulder Zone to Underground Sources of Drinking Water (USDWs). Miami-Dade County continuously monitors physical and water-quality characteristics within 21 injection wells and 21 single or multi-zone monitoring wells at two wastewater treatment plants. Injectate contains elevated levels of both nutrients and organic wastewater compounds (OWCs), such as human hormones, pharmaceuticals, and antibiotics. Some OCWs are endocrine disruptors. The 13-cubic-meter-per-second, 2006 effluent outflow is expected to increase 25 percent by 2025. The objective of the present work is to describe modes of injectate transport in Miami-Dade County, and characterize injectate densification with seawater (IDS), a management technology with the potential to mitigate transport of injectate from the Boulder Zone to both the ocean and to USDWs. Subsurface effluent disposal is (1) modeled analytically with the variable density form of Darcy's Law, (2) modeled numerically with the governing equations for variable density flow and transport in porous media, and (3) characterized with a bench-scale analog model. The numerical model was calibrated to monitoring data to estimate hydrogeologic parameters with a formal inverse method, such that an objective function is minimized. The objective function describes the degree to which model results fit observations. IDS is a mitigation technology that decreases effluent buoyancy, and dilutes concentrations of OWCs and other effluent constituents. Existing ocean outfalls can be employed to deliver sufficient quantities of seawater to densify injectate. Transport of densified injectate is a function of hydrogeologic parameters, injection scenarios, and USDW fluid density.
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Report Name: Axisymmetric simulation of aquifer storage and recovery with SEAWAT and the Sea Water Intrusion (SWI) Package for MODFLOW
Lead Author: Christian D. Langevin
End Date: 5/30/2006
Model Software: MODFLOW, SEAWAT
Description: SEAWAT and the Sea Water Intrusion (SWI) Package for MODFLOW were used to simulate hypothetical
aquifer storage and recovery (ASR) scenarios under constant- and variable-density conditions. To test
the codes, both models were used to represent two-dimensional axisymmetric groundwater flow near an ASR well. The models were “tricked” into representing axisymmetric flow by multiplying the input values for horizontal and vertical hydraulic conductivity, specific storage, and porosity by 2pr, where r is the radial distance from the well to the cell center. Additionally, the logarithmic transmissivity weighting option was used, instead of the harmonic weighting option, to ensure that the calculated horizontal conductances were consistent with axisymmetric flow. Results from constant-density simulations were in good agreement with the solution and the equation for an expanding cylinder. For variable-density conditions, SEAWAT and SWI results also were similar, provided that a large vertical hydraulic
conductivity value was used for the SEAWAT simulation. Results from a sensitivity analysis with
SEAWAT indicated that recovery efficiency is highly dependent on vertical grid resolution, vertical
hydraulic conductivity, native aquifer salinity, storage time, and hydrodynamic dispersion.
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Report Name: Simulation of Regional Groundwater Flow in the Suwannee River Basin, Northern Florida and Southern Georgia
Lead Author: Michael Planert
End Date: 9/30/2007
Model Software: MODFLOW 2000
Description: The Suwannee River Basin covers a total of nearly 9,950 square miles in north-central Florida and southern Georgia. In Florida, the Suwannee River Basin accounts for 4,250 square miles of north-central Florida. Evaluating the impacts of increased development in the Suwannee River Basin requires a quantitative understanding of the boundary conditions, hydrogeologic framework and hydraulic properties of the Floridan aquifer system, and the dynamics of water exchanges between the Suwannee River and its tributaries and the Floridan aquifer system.
Major rivers within the Suwannee River Basin are the Suwannee, Santa Fe, Alapaha, and Withlacoochee. Four rivers west of the Suwannee River are the Aucilla, the Econfina, the Fenholloway, and the Steinhatchee; all drain to the Gulf of Mexico. Perhaps the most notable aspect of the surface-water hydrology of the study area is that large areas east of the Suwannee River are devoid of channelized, surface drainage; consequently, most of the drainage occurs through the subsurface.
The groundwater flow system underlying the study area plays a critical role in the overall hydrology of this region of Florida because of the dominance of subsurface drainage, and because groundwater flow sustains the flow of the rivers and springs.
Three principal hydrogeologic units are present in the study area: the surficial aquifer system, the intermediate aquifer system, and the Floridan aquifer system. The surficial aquifer system principally consists of unconsolidated to poorly indurated siliciclastic deposits. The intermediate aquifer system, which contains the intermediate confining unit, lies below the surficial aquifer system (where present), and generally consists of fine-grained, unconsolidated deposits of quartz sand, silt, and clay with interbedded limestone of Miocene age. Regionally, the intermediate aquifer system and intermediate confining unit act as a confining unit that restricts the exchange of water between the overlying surficial and underlying Upper Floridan aquifers. The Upper Floridan aquifer is present throughout the study area and is extremely permeable and typically capable of transmitting large volumes of water. This high permeability is largely due to the widening of fractures and formation of conduits within the aquifer through dissolution of the limestone by infiltrating water. This process has also produced numerous karst features such as springs, sinking streams, and sinkholes.
A model of the Upper Floridan aquifer was created to better understand the groundwater system and to provide resource managers a tool to evaluate groundwater and surface-water interactions in the Suwannee River Basin. The model was developed to simulate a single Upper Floridan aquifer layer. Recharge datasets were developed to represent a net flux of water to the top of the aquifer or the water table during a period when the system was assumed to be under steady-state conditions (September 1990). A potentiometric-surface map representing water levels during September 1990 was prepared for the Suwannee River Water Management District (SRWMD), and the heads from those wells were used for calibration of the model. Additionally, flows at gaging sites for the Suwannee, Alapaha, Withlacoochee, Santa Fe, Fenholloway, Aucilla, Ecofina, and Steinhatchee Rivers were used during the calibration process to compare to model computed flows. Flows at seven first-magnitude springs selected by the SRWMD also were used to calibrate the model.
Calibration criterion for matching potentiometric heads was to attain an absolute residual mean error of 5 percent or less of the head gradient of the system which would be about 5 feet. An absolute residual mean error of 4.79 feet was attained for final calibration. Calibration criterion for matching streamflow was based on the quality of measurements made in the field. All measurements used were rated “good,” so the desire was for simulated values to be within 10 percent of the field measurements. All river reaches and springs were calibrated to within 5 percent, less than the 10-percent criterion of the measured discharge. Simulated transmissivity values range from 1,000 to 2 million feet squared per day. All relatively high values of transmissivity are associated with springs where the probability of fractures and dissolution has enhanced the primary permeablity of the limestone. The lowest transmissivity values are generally associated with areas of poor drainage where swamps or wetlands are present.
Model-simulated recharge values range from 0.5 inch per year (in/yr) in the confined area of the Upper Floridan aquifer in the northeastern and eastern part of the study area to 20 in/yr near Wacissa Springs. The initial estimate of 7 in/yr proved to be appropriate for most of the unconfined part of the Suwannee River Basin.
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Report Name: Summary of the Hydrology of the Floridan Aquifer System in Florida and In Parts of Georgia, South Carolina, and Alabama
Lead Author: Richar H. Johnson and Peter W. Bush
End Date: 9/30/1988
Model Software: N/A
Description: The Regional Aquifer-System Analysis (RASA) Program was started in 1978 following a congressional mandate to develop quantitative appraisals of the major groundwater systems of the United States. The RASA Program represents a systematic effort to study a number of the Nation's most important aquifer systems, which in aggregate underlie much of the country and which represent an important component of the Nation's total water supply. In general, the boundaries of these studies are identified by the hydrologic extent of each system and accordingly transcend the political subdivisions to which investigations have often arbitrarily been limited in the past. The broad objective for each study is to assemble geologic, hydrologic, and geochemical information, to analyze and develop an understanding of the system, and to develop predictive capabilities that will contribute to the effective management of the system. The use of computer simulation is an important element of the RASA studies, both to develop an understanding of the natural, undisturbed hydrologic system and the changes brought about in it by human activities, and to provide a means of predicting the regional effects of future pumping or other stresses.
The final interpretive results of the RASA Program are presented in a series of U.S. Geological Survey Professional Papers that describe the geology, hydrology, and geochemistry of each regional aquifer system. Each study within the RASA Program is assigned a single Professional Paper number, and where the volume of interpretive material warrants, separate topical chapters that consider the principal elements of the investigation may be published. The series of RASA interpretive reports begins with Professional Paper 1400 and thereafter will continue in numerical sequence as the interpretive products of subsequent studies become available.
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Report Name: Hypothesis testing of buoyant plume migration using a highly parameterized variable-density groundwater model at a site in Florida, USA
Lead Author: Alyssa M. Dausman
End Date: 9/30/2006
Model Software: SEAWAT
Description: The South District Wastewater Treatment Plant (SDWWTP), located in southeastern Miami-Dade County about 1 mi west of the Biscayne Bay coastline, is the largest capacity deep-well injection plant in the United States. Currently, about 100 Mgal/d of partially treated, essentially fresh (less than 1000 mg/L total dissolved solids) effluent is injected through 17 wells (each approximately 2500 ft below land surface) into the highly transmissive, lower-temperature, saline Boulder Zone composed of highly fractured dolomite. A thin confining unit called the Delray Dolomite, which is 8-16 ft thick, overlies the intended injection zone at the site. Although the Delray Dolomite has a vertical hydraulic conductivity estimated between 0.001 and 0.00001 ft/d, well casings for 10 of the 17 wells do not extend beneath the unit. A 700-ft-thick middle confining unit, with estimated vertical hydraulic conductivities between 0.1 and 28 ft/d, overlies the Delray Dolomite and separates it from the Upper Floridan aquifer.
Protected by the Safe Drinking Water Act (SDWA), the Upper Floridan aquifer contains water that is less than 10,000 mg/L total dissolved solids. In southern Florida, this aquifer is used for reverse osmosis, blending with other waters, and as a reservoir for aquifer storage and recovery. At the SDWWTP, ammonia concentrations that exceed background conditions have been observed in monitoring wells open in and above the middle confining unit, indicating upward vertical migration of effluent, possibly toward the Upper Floridan aquifer. The U.S. Geological Survey currently is developing a variable-density groundwater flow and solute transport model for the Floridan aquifer system in Miami-Dade County. This model includes the injection of treated wastewater at the SDWWTP. The developed numerical model uses SEAWAT, a code that calculates variable- density flow as a function of salinity, to capture the buoyancy effects at the site and along the coast.
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Report Name: Simulation of Groundwater Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida
Lead Author: Nick Sepulveda
End Date: 9/30/2002
Model Software: MODFOW 1996
Description: A numerical model of the intermediate and Floridan aquifer systems in peninsular Florida was used to (1) test and refine the conceptual understanding of the regional groundwater flow system; (2) develop a data base to support subregional groundwater flow modeling; and (3) evaluate effects of projected 2020 groundwater withdrawals on groundwater levels. The four-layer model was based on the computer code MODFLOW 96. The top layer consists of specified-head cells simulating the surficial aquifer system as a source-sink layer. The second layer simulates the intermediate aquifer system in southwest Florida and the intermediate confining unit where it is present. The third and fourth layers simulate the Upper and Lower Floridan aquifers, respectively. Steady-state groundwater flow conditions were approximated for time-averaged hydrologic conditions from August 1993 through July 1994 (1993-94). This period was selected based on data from Upper Floridan aquifer wells equipped with continuous water-level recorders. The finite difference grid was uniform and composed of square 5,000-foot cells, with 210 columns and 300 rows.
The active model area, which encompasses about 40,800 square miles in peninsular Florida, includes areas of various physiographic regions classified according to natural features. Hydrogeologic conditions vary among physiographic regions, requiring different approaches to estimating hydraulic properties for different areas. The altitudes of water levels for the surficial aquifer system and heads in the Upper Floridan aquifer, for time-averaged 1993-94 conditions, were computed by using a multiple linear regression of measured water levels in each of the physiographic regions.
Groundwater flow simulation was limited vertically to depths containing water with chloride concentrations less than 5,000 milligrams per liter. Water-level altitudes in the Floridan aquifer system beneath which chloride concentrations exceed 5,000 milligrams per liter were estimated from previously developed maps and analytical results of groundwater samples. Flow across the interface represented by this chloride concentration was assumed to be negligible.
The groundwater flow model was calibrated using time-averaged data for 1993-94 at 1,624 control points, flow measurements or estimates at 156 springs in the study area, and base-flow estimates of rivers in the unconfined areas of the Upper Floridan aquifer obtained by using a generalized hydrograph separation of recorded discharge data. Transmissivity of the intermediate aquifer system, Upper Floridan aquifer, and Lower Floridan aquifer; leakance of the upper and lower confining units of the intermediate aquifer system, the intermediate confining unit, the middle confining unit, and the middle semiconfining unit; spring and riverbed conductances; and net recharge rates to unconfined areas of the Upper Floridan aquifer were adjusted until a reasonable fit was obtained. Root-mean-square residuals between computed and simulated heads in the intermediate aquifer system, Upper Floridan aquifer, and Lower Floridan aquifer were 3.47, 3.41, and 2.89 feet, respectively. The overall root-mean-square residual was 3.40 feet. Simulated spring flow was 96 percent of the total measured (or estimated) spring flow in the study area.
Simulations were made to project water-level declines from 1993-94 to 2020 conditions. The calibrated flow model was used to simulate the potentiometric surfaces of the intermediate aquifer system, Upper Floridan aquifer, and Lower Floridan aquifer for 2020 using water-use projections provided by the Water Supply Assessment plans of the State Water Management Districts. Water-use projections for 2020 were based on estimated population growth and 1995 withdrawals. Heads in the Upper Floridan aquifer under projected 2020 water-use stresses were simulated for two scenarios: (1) assigning interpolated 1993-94 heads along the lateral boundaries of the Upper Floridan aquifer; and (2) assigning 1993-94 simulated flux rates across the same boundaries.
Projected 2020 groundwater withdrawals for municipal, industrial, commercial, agricultural, and self-supplied domestic uses was approximately 3,400 million gallons per day, an increase of about 36 percent from 1993-94. The largest projected drawdown in the potentiometric surface of the Upper Floridan aquifer, for both scenarios, was simulated in Orange County, with a drawdown of 10 feet in the central part of the County. Projected drawdowns of 6 feet were simulated in parts of Duval and Polk Counties.
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Report Name: Southern Inland and Coastal Systems (SICS) Model Development
Lead Author: Eric Swain
End Date: 9/30/2000
Model Software: SWIFT2D, MODFLOW, SEAWAT, FTLOADDS
Description: In order to determine the effects of freshwater inflows and dynamic forcing mechanisms on flow patterns and salinity conditions in the subtidal embayments of northeast Florida Bay, a mathematical/numerical hydrodynamic/transport model of the Southern Inland and Coastal System (SICS) area was to be developed, implemented, calibrated, and verified with field-collected data. The model can also be used to study the significance of terrain relief, such as the Buttonwood embankment, and dynamic effects, such as wind and weather fronts, on flow patterns and salinity conditions and to provide boundary-condition information in the form of fluxes and gradients for Florida Bay model development.
The approach for this study included selection, enhancement, and application of a numerical model capable of simulating flow and solute transport within the SICS area and into Florida Bay. Data were obtained from several sources to apply, calibrate, and test the model. Additionally, results from ongoing or recently completed process studies were used to develop the model.
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Report Name: South Broward Saltwater Intrusion Model
Lead Author: Jeremy White
Model Software: MODFLOW, SWR1, SEAWAT
Description: The model uses MODFLOW and the SWR1 process for groundwater/surface water flow and SEAWAT for salt transport. The models have/will be subject to formal parameter estimation in high dimensional parameter space. Formal predictive uncertainty analysis will also be completed for the Central and Southern models. Ultimately, the models will be used to support allocation decisions in Broward.
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Report Name: Jacksonville Harbor Deepening
Lead Author: Jason Bellino and Rick M. Spechler
Model Software: SEAWAT
Description: Four cross-sectional models have been constructed for the U.S. Army Corps of Engineers St. Johns River saltwater intrusion modeling project. The SEAWAT model was used to determine areas that may be vulnerable to saltwater encroachment caused by further dredging of the Jacksonville Harbor proposed by the USACE to accommodate larger shipping vessels.
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Report Name: Tides and Inflows in the Mangrove Ecotone (TIME) Model Development
Lead Author: Raymond W. Schaffranek
Lead Author: Raymond W. Schaffranek
End Date: 9/30/2005
Model Software: SWIFT2D, WINBRANCH, SEAWAT, ADVFilter Programs, NetCDF
Description: A critical objective of the south Florida ecosystem restoration effort is to preserve ecological conditions that are consistent with habitat requirements. The duration, timing, and extent of wetland inundation in the southern Everglades have been greatly distorted as evidenced by shifts in biologic and vegetative species. Both natural and regulatory factors contribute to influence of hydroperiods making their precise evaluation and management difficult. This complexity is particularly problematic in the transition zone between the Everglades wetlands and coastal embayments encompassing the mangrove ecotone where freshwater inflow effects on salinities must also be considered. In order to correctly and sufficiently investigate flow effects on both hydroperiods and embayment salinities neither hydrologic processes affecting flows in the wetlands nor the dynamic effects of external forces such as tides and winds can be ignored. This project entails translation of findings from the Southern Inland and Coastal Systems (SICS) project and extension of the model westward to resolve boundary limitations and to enable concurrent analysis of wetland and tidal response throughout the entire saltwater-freshwater interface zone along the Gulf coast and Florida Bay. Extension of the SICS model westward will require the addition of continuous monitoring stations to supplement data from coastal creek stations and control structures needed to provide boundary conditions as well as the synoptic measurement of flows and water levels in the wetlands for use in model calibration and verification.
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Report Name: Application of Nonlinear Least-Squares Regression to Groundwater Flow Modeling, West-Central Florida
Lead Author: D.K. Yobbi
End Date: 9/30/2002
Model Software: MODFLOW 1996
Description: A nonlinear least-squares regression technique for estimation of groundwater flow model parameters was applied to an existing model of the regional aquifer system underlying west-central Florida. The regression technique minimizes the differences between measured and simulated water levels. Regression statistics, including parameter sensitivities and correlations, were calculated for reported parameter values in the existing model. Optimal parameter values for selected hydrologic variables of interest are estimated by nonlinear regression. Optimal estimates of parameter values are about 140 times greater than and about 0.01 times less than reported values. Independently estimating all parameters by nonlinear regression was not practical, given the existing zonation structure and number of observations, because of parameter insensitivity and correlation. Although the model yields parameter values similar to those estimated by other methods and reproduces the measured water levels reasonably accurately, a simpler parameter structure should be considered. Some possible ways of improving model calibration are to: (1) modify the defined parameter-zonation structure by omitting and/or combining parameters to be estimated; (2) carefully eliminate observation data based on evidence that they are likely to be biased; (3) collect additional water-level data; (4) assign values to insensitive parameters, and (5) estimate the most sensitive parameters first, then, using the optimized values for these parameters, estimate the entire data set.
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Report Name: Comparison of Estimated Areas Contributing Recharge to Selected Springs in North-Central Florida by Using Multiple Groundwater Flow Models
Lead Author: Barclay W. Shoemaker
End Date: 9/30/2004
Model Software: MODFLOW, PF Model
Description: Areas contributing recharge to springs are defined in this report as the land-surface area wherein water entering the groundwater system at the water table eventually discharges to a spring. These areas were delineated for Blue Spring, Silver Springs, Alexander Springs, and Silver Glen Springs in north-central Florida using four regional groundwater flow models and particle tracking. As expected, different models predicted different areas contributing recharge. In general, the differences were due to different hydrologic stresses, subsurface permeability properties, and boundary conditions that were used to calibrate each model, all of which are considered to be equally feasible because each model matched its respective calibration data reasonably well. To evaluate the agreement of the models and to summarize results, areas contributing recharge to springs from each model were combined into composite areas. During 1993-98, the composite areas contributing recharge to Blue Spring, Silver Springs, Alexander Springs, and Silver Glen Springs were about 130, 730, 110, and 120 square miles, respectively. The composite areas for all springs remained about the same when using projected 2020 groundwater withdrawals.
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Report Name: Evaluation of groundwater flow and hydrologic budget for Lake Five-O, a seepage lake in northwestern Florida
Lead Author: J.W. Grubbs
End Date: 9/30/1994
Model Software: MODFLOW88
Description: Temporal and spatial distributions of groundwater inflow to, and leakage from Lake Five-O, a soft water, seepage lake in northwestern Florida, were evaluated using hydrologic data and simulation models of the shallow groundwater system adjacent to the lake. The simulation models indicate that ground water inflow to the lake and leakage from the lake to the groundwater system are the dominant components in the total inflow (precipitation plus groundwater inflow) and total outflow (evaporation plus leakage) budgets of Lake Five-O. Simulated groundwater inflow and leakage were approximately 4 and 5 times larger than precipitation inputs and evaporative losses, respectively, during calendar years 1989-90. Exchanges of water between Lake Five-O and the groundwater system were consistently larger than atmospheric-lake exchanges. A consistent pattern of shallow groundwater inflow and deep leakage was also evident throughout the study period. The mean time of travel for groundwater that discharges at Lake Five-O (time from recharge at the water table to discharge at the lake) was estimated to be within a range of 3 to 6 years. Flow-path evaluations indicated that the intermediate confining unit probably has a negligible influence on the geochemistry of groundwater inflow to Lake Five-O. The hydrologic budgets and flow-path evaluations provide critical information for developing geochemical budgets for Lake Five-O, and for improving the understanding of the relative importance of various processes that regulate the acid-neutralizing capacity of soft-water seepage lakes in Florida.
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Report Name: Do seismically-imaged sag structures in Biscayne National Park influence submarine groundwater discharge?
Lead Author: Eve Kuniansky
Description: The Floridan aquifer system covers approximately 100,000 square miles in the southeastern United States in Florida and portions of Georgia, Alabama, Mississippi, and South Carolina. It is a sequence of carbonate rocks over 3000 feet thick in south Florida and thins towards the north. Typically split into the Upper Floridan aquifer, the middle confining unit, and the Lower Floridan aquifer, the majority of freshwater is contained in the Upper Floridan aquifer and is used for water supply. The Lower Floridan aquifer contains fresh to brackish water in northeastern Florida and Georgia, while in south Florida it is saline and used to dispose of effluent from wastewater treatment processes.
Precipitation in the southeastern United States is around 53 in/year. The majority of recharge to the Floridan aquifer system occurs in the areas where it is unconfined or semi-confined, approximately 10-25 in/year, whereas in the areas of confinement the recharge is less than 1 in/year. Natural discharge to streams and lakes occurs through springs or to the ocean through submarine groundwater discharge. Florida alone has 33 springs that discharge more than 64.6 million gallons per day per spring.
Groundwater wells for water supply from the Floridan aquifer system were first drilled in the late 1800's. Currently, the Floridan aquifer system supports almost 10 million people as their primary source of water and is one of the most productive aquifers in the world. Water from the Floridan aquifer system is used for public, domestic, and industrial water supply, with almost 50% of water being used for irrigation.
Project started in FY10 but suspended in FY11 when funds to the Groundwater Resources Program were reduced. In FY12, acting project chief and staff are completing documentation of datasets and hydrogeologic framework. Project will be restarted as funding to the Groundwater Resources Program become available.
For more information on the Floridan aquifer groundwater availability study go to http://fl.water.usgs.gov/FASWAM/.
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Report Name: Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast Lake Counties, Florida
Lead Author: Andrew M. O'Reilly
End Date: 9/30/1998
Model Software: MODFLOW88
Description: Wastewater reclamation and reuse has become increasingly popular as water agencies search for alternative water-supply and wastewater-disposal options. Several governmental agencies in central Florida currently use the land-based application of reclaimed water (wastewater that has been treated beyond secondary treatment) as a management alternative to surface-water disposal of wastewater. Water Conserv II, a water reuse project developed jointly by Orange County and the City of Orlando, began operation in December 1986. In 1995, the Water Conserv II facility distributed approximately 28 Mgal/d of reclaimed water for discharge to rapid-infiltration basins (RIBs) and for use as agricultural irrigation. The Reedy Creek Improvement District (RCID) began operation of RIBs in September 1990, and in 1995 these RIBs received approximately 6.7 Mgal/d of reclaimed water. Analyses of existing data and data collected during the course of this study were combined with groundwater flow modeling and particle-tracking analyses to develop a process-oriented evaluation of the regional effects of reclaimed water applied by Water Conserv II and the RCID RIBs on the hydrology of west Orange and southeast Lake Counties.
The groundwater flow system beneath the study area is a multi-aquifer system that consists of a thick sequence of highly permeable carbonate rocks overlain by unconsolidated sediments. The hydrogeologic units are the unconfined surficial aquifer system, the intermediate confining unit, and the confined Floridan aquifer system, which consists of two major permeable zones, the Upper and Lower Floridan aquifers, separated by the less permeable middle semiconfining unit. Flow in the surficial aquifer system is dominated regionally by diffuse downward leakage to the Floridan aquifer system and is affected locally by lateral flow systems produced by streams, lakes, and spatial variations in recharge. Groundwater generally flows laterally through the Upper Floridan aquifer to the north and east. Many of the lakes in the study area are landlocked because the mantled karst environment precludes a well-developed network of surface-water drainage.
The USGS three-dimensional groundwater flow model MODFLOW was used to simulate groundwater flow in the surficial and Floridan aquifer systems. A steady-state calibration to average 1995 conditions was performed by using a parameter estimation program to vary values of surficial aquifer system hydraulic conductivity, intermediate confining unit leakance, and Upper Floridan aquifer transmissivity. The calibrated model generally produced simulated water levels in close agreement with measured water levels and was used to simulate the hydrologic effects of reclaimed-water application under current (1995) and proposed future conditions.
In 1995, increases of up to about 40 ft in the water table and less than 5 ft in the Upper Floridan aquifer potentiometric surface had occurred as a result of reclaimed-water application. The largest increases were under RIB sites. An average travel time of 10 years at Water Conserv II and 7 years at the RCID RIBs was required for reclaimed water to move from the water table to the top of the Upper Floridan aquifer. Approximately 67 percent of the reclaimed water applied at the RCID RIB site recharged the Floridan aquifer system, whereas 33 percent discharged from the surficial aquifer system to surface-water features; 99 percent of the reclaimed water applied at Water Conserv II recharged the Floridan aquifer system, whereas only 1 percent discharged from the surficial aquifer system to surface-water features. The majority of reclaimed water applied at both facilities probably will ultimately discharge from the Floridan aquifer system outside the model boundaries.
Proposed future conditions were assumed to consist of an additional 11.7 Mgal/d of reclaimed water distributed by the Water Conserv II and RCID facilities. Increases of up to about 20 ft in the water table and 2 ft in the potentiometric surface of the Upper Floridan aquifer were simulated. The directions of reclaimed water movement through the groundwater system generally were similar to those under 1995 conditions. However, the greater reclaimed-water application rate at the RCID RIBs caused approximately half of the RCID reclaimed water to discharge to surface-water features and half to recharge the Floridan aquifer system.
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Report Name: Simulation of the Interaction of Karstic Lakes Magnolia and Brooklyn with the Upper Floridan Aquifer, Southwestern Clay County, Florida
Lead Author: M.L. Merritt
End Date: 9/30/2001
Model Software: MODFLOW-96, Three-Dimensional Method-of-Characteristics Solute-Transport Model (MOC3D)
Description: The stage of Lake Brooklyn, in southwestern Clay County, Florida, has varied over a range of 27 feet since measurements by the U.S. Geological Survey began in July 1957. The large stage changes have been attributed to the relation between highly transient surface-water inflow to the lake and subsurface conduits of karstic origin that permit a high rate of leakage from the lake to the Upper Floridan aquifer. After the most recent and severe stage decline (1990-1994), the U.S. Geological Survey began a study that entailed the use of numerical groundwater flow models to simulate the interaction of the lake with the Upper Floridan aquifer and the large fluctuations of stage that were a part of that process. A package (set of computer programs) designed to represent lake/aquifer interaction in the U.S. Geological Survey Modular Finite-Difference Ground-Water Flow Model (MODFLOW-96) and the Three-Dimensional Method-of-Characteristics Solute-Transport Model (MOC3D) simulators was prepared as part of this study, and a demonstration of its capability was a primary objective of the study. (Although the official names are Brooklyn Lake and Magnolia Lake (Florida Geographic Names), in this report the local names, Lake Brooklyn and Lake Magnolia, are used.)
In the simulator of lake/aquifer interaction used in this investigation, the stage of each lake in a simulation is updated in successive time steps by a budget process that takes into account groundwater seepage, precipitation upon and evaporation from the lake surface, stream inflows and outflows, overland runoff inflows, and augmentation or depletion by artificial means. The simulator was given the capability to simulate both the division of a lake into separate pools as lake stage falls and the coalescence of several pools into a single lake as the stage rises. This representational capability was required to simulate Lake Brooklyn, which can divide into as many as 10 separate pools at sufficiently low stage.
In the first of two calibrated models, recharge to the water table, specified as a monthly rate, was set equal to 40 percent of the monthly rainfall rate. The specified rate of inflow to the uppermost stream segment was set equal to outflows from Lake Lowry estimated from lake stage and the 1994-97 rating table. Leakage to the intermediate and Upper Floridan aquifers was assumed to occur from the surficial aquifer system through the confining layers directly beneath deeper parts of the lake bottom. A leakance coefficient value of 0.001 feet per day per foot of thickness was used beneath Lake Magnolia, and a value of 0.005 feet per day per foot of thickness was used beneath most of Lake Brooklyn. With these values, the conductance through the confining layers beneath Lake Brooklyn was about 19 times that beneath Lake Magnolia.
The simulated stages of Lake Brooklyn matched the measured stages reasonably well in the early (1957-72) and later (1990-98) parts of the simulation time period, but the match was unsatisfactory in an intermediate time period (1973-89). To resolve this discrepancy, the hypothesis was proposed that undocumented losses of water from Alligator Creek upstream from Lake Brooklyn or from the lake itself occurred between 1973 and 1989 when there was sufficient streamflow. The resulting simulation of lake stages matched the measured lake stages accurately during the entire simulation time period. The model was then revised to incorporate the assumption that only 20 percent of precipitation recharged the water table (the second calibrated model). Recalibration of the model required that leakance values for the confining units under deeper parts of the lakes also be reduced by nearly 50 percent. The stages simulated with the new parameter assumptions, but retaining the assumption of surface-water losses, were an excellent match of the measured values. The stage of Lake Magnolia was also simulated accurately. The results of sensitivity analyses show that simulated streamflow between Lakes Magnolia and Brooklyn tends to be water-budget controlled, and is not appreciably affected by the specified outflow altitude or channel characteristics of the receiving stream.
To match heads measured in observation wells of the surficial aquifer network, the assigned hydraulic conductivity values were zoned, and ranged from a minimum of 4 feet per day to a maximum of 400 feet per day in the first calibrated model. These values were reduced by about 50 percent in the second calibrated model. Differences between observation wells were noted in the abruptness of changes of measured head values, and in the relation of the timing of peak measured heads and simulated peak heads. These differences seemed to be correlated with the depth of the water table below land surface. Spatially uniform values of transmissivity were specified for the intermediate (10,000 feet squared per day) and Upper Floridan (100,000 feet squared per day) aquifers. Simulated heads in the Upper Floridan aquifer layer follow the trend of the heads measured in a long-term observation well with data beginning in 1960. This result suggests that the observed head decline could be explained entirely in terms of the stage decline in Lake Brooklyn and may not indicate a regional trend.
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Report Name: Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in the Ocala National Forest and Vicinity, North-Central Florida
Lead Author: Leel Knowles Jr.
End Date: 9/30/2002
Model Software: MODFLOW-2000
Description: The hydrogeology of Lake County and the Ocala National Forest in north-central Florida was evaluated (1995-2000), and a groundwater flow model was developed and calibrated to simulate the effects of both present day and future groundwater withdrawals in these areas and the surrounding vicinity. A predictive model simulation was performed to determine the effects of projected 2020 groundwater withdrawals on the water levels and flows in the surficial and Floridan aquifer systems.
The principal water-bearing units in Lake County and the Ocala National Forest are the surficial and Floridan aquifer systems. The two aquifer systems generally are separated by the intermediate confining unit, which contains beds of lower permeability sediments that confine the water in the Florida aquifer system. The Floridan aquifer system has two major water-bearing zones (the Upper Floridan aquifer and the Lower Floridan aquifer), which generally are separated by one or two less-permeable confining units.
The Floridan aquifer system is the major source of groundwater in the study area. In 1998, groundwater withdrawals totaled about 115 million gallons per day in Lake County and 5.7 million gallons per day in the Ocala National Forest. Of the total groundwater pumped in Lake County in 1998, nearly 50 percent was used for agricultural purposes, more than 40 percent for municipal, domestic, and recreation supplies, and less than 10 percent for commercial and industrial purposes.
Fluctuations of lake stages, surficial and Floridan aquifer system water levels, and Upper Floridan aquifer spring flows in the study area are highly related to cycles and distribution of rainfall. Long-term hydrographs for 9 lakes, 8 surficial aquifer system and Upper Floridan aquifer wells, and 23 Upper Floridan aquifer springs show the most significant increases in water levels and spring flows following consecutive years with above-average rainfall, and significant decreases following consecutive years with below-average rainfall. Long-term (1940-2000) hydrographs of lake and groundwater levels and spring flow show a slight downward trend; however, after the early 1960's, this downward trend generally is more pronounced, which corresponds with accumulating rainfall deficits and increased development.
The U.S. Geological Survey three-dimensional groundwater flow model MODFLOW-2000 was used to simulate groundwater flow in the surficial and Floridan aquifer systems in Lake County, the Ocala National Forest, and adjacent areas. A steady-state calibration to average 1998 conditions was facilitated by using the inverse modeling capabilities of MODFLOW-2000. Values of hydrologic properties from the calibrated model were in reasonably close agreement with independently estimated values and results from previous modeling studies. The calibrated model generally produced simulated water levels and flows in reasonably close agreement with measured values and was used to simulate the hydrologic effects of projected 2020 conditions.
Groundwater withdrawals in the model area have been projected to increase from 470 million gallons per day in 1998 to 704 million gallons per day in 2020. Significant drawdowns were simulated in Lake County from average 1998 to projected 2020 conditions: the average and maximum drawdowns, respectively, were 0.5 and 5.7 feet in the surficial aquifer system, 1.1 and 7.6 feet in the Upper Floridan aquifer, and 1.4 and 4.3 feet in the Lower Floridan aquifer. The largest drawdowns in Lake County were simulated in the southeastern corner of the County and in the vicinities of Clermont and Mount Dora. Closed-basin lakes and wetlands are more likely to be affected by future pumping in these large drawdown areas, as opposed to other areas of Lake County. However, within the Ocala National Forest, drawdowns were relatively small: the average and maximum drawdowns, respectively, were 0.1 and 1.0 feet in the surficial aquifer system, 0.2 and 0.8 feet in the Upper Floridan aquifer, and 0.3 and 0.8 feet in the Lower Floridan aquifer.
Projected 2020 withdrawals from the Floridan aquifer system caused decreases from average 1998 conditions in the following simulated flows: combined rates of excess evapotranspiration and excess overland runoff (which represent evapotranspiration and overland runoff that occur in excess of their assumed minimum rates); groundwater discharge to streams, lakes, and wetlands; and spring flow. The largest simulated flow decreases for first- or second-magnitude springs in Lake County were at Apopka (28 percent), Seminole (12 percent), and Bugg Springs (9 percent). The largest simulated flow decrease for first- or second-magnitude springs in the Ocala National Forest was at Juniper Springs (4 percent).
Particle-tracking analyses were used to delineate areas that contribute recharge to selected springs. Based on average 1998 conditions, the contributing area for Apopka Spring covers approximately 30 square miles and has an average contributing recharge flux of 15 inches per year, and the contributing area for Alexander Springs covers approximately 76 square miles and has an average contributing recharge flux of 18 inches per year. The contributing area for Alexander Springs changed little as a result of projected 2020 conditions because relatively little pumping exists in the vicinity of the spring's contributing area. However, the size of the contributing area for Apopka Spring decreased to 26 square miles and the average contributing recharge flux decreased to 13 inches per year as a result of projected 2020 conditions.
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U.S. Department of the Interior |
U.S. Geological Survey
