Abstract
The effects of simplifying hydraulic property layering within an unconfined aquifer and the underlying confining unit were assessed. The hydraulic properties of lithologic units within the unconfined aquifer and confining unit were computed by analyzing the aquifer-test data using radial, axisymmetric two-dimensional (2D) flow. Time-varying recharge to the unconfined aquifer and pumping from the confined Upper Floridan aquifer (USA) were simulated using 3D flow. Conceptual flow models were developed by gradually reducing the number of lithologic units in the unconfined aquifer and confining unit by calculating composite hydraulic properties for the simplified lithologic units. Composite hydraulic properties were calculated using either thickness-weighted averages or inverse modeling using regression-based parameter estimation. No significant residuals were simulated when all lithologic units comprising the unconfined aquifer were simulated as one layer. The largest residuals occurred when the unconfined aquifer and confining unit were aggregated into a single layer (quasi-3D), with residuals over 100% for the leakage rates to the confined aquifer and the heads in the confining unit. Residuals increased with contrasts in vertical hydraulic conductivity between the unconfined aquifer and confining unit. Residuals increased when the constant-head boundary at the bottom of the Upper Floridan aquifer was replaced with a no-flow boundary.
Highlights
Groundwater provides much of the fresh drinking water to more than 1.5 billion people in the world (Clarke et al, 1996) and in the United States more that 50 percent of citizens rely on groundwater for drinking water (Solley et al, 1998)
These 3D groundwater flow models, regional in nature, were developed to simulate temporal variations in recharge to the surficial aquifer system (SAS) and pumping from the Upper Floridan aquifer (UFA), and to evaluate how simulated flow changes in the SAS and UFA might be affected by model layering simplification through the computation of composite hydraulic properties for the SAS and intermediate confining unit (ICU)
Each model had 400 rows and 400 columns, with no-flow conditions imposed along the lateral boundaries, either a constant-head or no-flow boundary at the bottom of the UFA, and a specified recharge rate imposed at the top of the SAS
Summary
Groundwater provides much of the fresh drinking water to more than 1.5 billion people in the world (Clarke et al, 1996) and in the United States more that 50 percent of citizens rely on groundwater for drinking water (Solley et al, 1998). The model grid used for the analysis of the aquifer-test data consisted of rows of uniform thickness of 0.305 m in the SAS, ICU, and UFA (vertical discretization) with a uniform hydraulic property assigned to each model layer (row) representing the lithologic unit identified in figure 5. The simulated horizontal hydraulic conductivity for the UFA at the Carrot Barn (30.1 m/d) and Lyonia Preserve (82.5 m/d) well fields were obtained from model-simulated values reported by McGurk & Presley (2002) These hydraulic properties for the UFA are not used in the simulated layer simplification process because composite hydraulic properties are derived only for the lithologic units in the SAS and ICU. At the Carrot Barn well field, average hydraulic conductivity values from slug test data for wells with open intervals to lithologic units 2 to 5 in the SAS were 9.15, 0.62, 1.85, and 5.7 meters per day, respectively. The identification of this variability in hydraulic conductivity resulting from slug tests and aquifer tests is a result of the high degree of discretization used to identify the vertical extent of uniform soil grains of each lithologic unit as shown in figures 2 and 5
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