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Simulation of Transient Groundwater Flow in the Surficial and Floridan Aquifer Systems in East-Central Florida

Project Chief: Nick Sepulveda
Cooperator: St Johns River Water Management District
Period of Project: October 2008 - September 2012

Problem Statement

Figure 1. Areal extent of the east-central Florida study area, Central Florida Coordination Area, and model area, including lakes, streams, and springs.

As part of an ongoing water resources planning process, the St. Johns River Water Management District (SJRWMD) has undertaken efforts to evaluate the environmental impacts of proposed surface-water withdrawals from the St. Johns River. The planning process has progressed in phases. The primary goal of the Phase I groundwater modeling was to predict changes in discharges of groundwater into the St. Johns River resulting from increased surface-water withdrawals. In Phase II, the model was updated to simulate transient groundwater flow in the Surficial Aquifer System (SAS) and the Floridan Aquifer System (FAS) in east-central Florida (fig. 1). This update was necessary to assess the effects of projected increases in groundwater withdrawals from the FAS. The model will be used to evaluate future impacts on wetlands, streams, lakes, springs, and groundwater levels in the FAS caused by projected increases in groundwater use.

Objectives

The objective of the project was to develop a 3-dimensional transient groundwater flow model for east-central Florida.

Figure 2. Correlation chart showing the relation between lithostratigraphic and hydrostratigraphic units in the East-Central Florida Transient (ECFT) model area

Approach

The East-Central Florida Transient (ECFT) model used MODFLOW-2005 to simulate 7 layers that span the thickness of the surficial aquifer system (layer 1), the intermediate confining unit or intermediate aquifer system (layer 2), the Ocala permeable zone (layer 3), the Ocala low permeable zone (layer 4), the Avon Park permeable zone (layer 5), the middle confining units I and II (layer 6), and the Lower Floridan aquifer (layer 7) (fig. 2).

The ECFT model has the following model capabilities:

(1) simulates 3-dimensional flow

(2) uses general-head boundary conditions to simulate flow through the lateral boundaries of the model domain as well as springs contributions

(3) uses the Green-Ampt infiltration (GAI) algorithm to partition rainfall into infiltration and runoff

(4) calculates Hortonian runoff from GAI

(5) calculates Dunnian runoff by using the UZF1 package

(6) routes runoff to lakes and streams

(7) routes streamflow using the SFR2 package

(8) simulates water-surface elevation at the lakes using the LAK7 package

(9) calculates the temporal distribution of recharge rates to the SAS by routing infiltration through the unsaturated zone,

(10) simulates only the freshwater section of the ECFT model area shown in figure 1, and

(11) allows for the simulation of the impact of land-use changes by adjusting the hydraulic conductivity of the top soils, which results in the calculation of updated infiltration and runoff rates from the GAI algorithm.

Steady-state models for WY1999 and 2003 were developed to generate the horizontal and vertical hydraulic conductivity distributions using the parameter estimation software PEST. The specific yield and specific storage were calculated from the calibration of a two-year transient model that used the hydraulic conductivity fields derived from the steady-state calibration. The period of simulation for the calibration of the storage properties was WY2005-2006, the last two years of the 12-year simulation period (1995 to 2006).

Information Product

Sepulveda, Nicasio, “Hydrogeology and Transient Simulation of the Groundwater Flow System in East-Central Florida”, U.S. Geological Survey Scientific Investigations Report, (in review).