MT3D Model Output Of Relative Water-Quality Concentrations
Click here to enlarge imageSeveral field tests have been performed to gain a better understanding of the local hydrogeologic system. Surface geophysics were conducted using ground-penetrating radar (GPR) to assist in identification of sub-surface conditions. Of particular interest were discontinuities in the clay confining layers and karst related features such as sinkholes. Information obtained from the GPR survey and other available hydrogeologic information was then used to assist in determining locations for coring activities. Over 30 soil borings were made with undisturbed soil cores collected to analyze vertical permeability.
Aquifer testing was performed to estimate average values for aquifer characteristics. A tracer test was conducted during the aquifer test using a sodium chloride tracer. The tracer test was used to estimate porosity and dispersivity of the aquifer. A tracer test was also performed within a wetland to estimate porosity and dispersivity of the sediments beneath the wetland. Slug tests were performed in monitor wells to estimate characteristics of the surficial aquifer.
Computer Modeling
A ground-water flow model using MODFLOW was developed to simulate the flow system. A five-layer model was developed based on geologic information derived from the boring program. The model was calibrated to water levels measured in 17 wells across the site. The calibrated model was then used to simulate the effect of restoration of wetlands and lakes on the site.
The particle-tracking model MODPATH was used to track the movement and travel times of the simulated particles from the receiving wetlands and lakes to the production wells. The solute-transport model MT3D was used to estimate the effect of dilution and dispersion on the concentration of a parameter in the restoration water during movement through the ground-water system.
Water samples were collected from the monitoring wells, lakes, and the potential sources of restoration water. The water-quality analysis results will be used together with the results of the ground-water flow and transport modeling to perform a risk analysis for the use of surface water and reclaimed water for restoration of wetlands and lakes.
Upon completion of the well rehabilitation, the ground-water model will be revised to assess the potential effects of the restoration project. The results of the well rehabilitation and restoration activities should provide an important component for the overall management of water resources at the Section 21 Well Field. Water levels in wetland features should improve as a result of these efforts.
About the Author:
David A. Wiley, PE, is a principal and head of the Tampa office of Leggette Brashears & Graham, Inc. (LBG), a national groundwater and environmental engineering services firm. He has over 20 years of professional experience in all phases of project management, hydrogeologic evaluations for water resource projects, water supply development, well rehabilitation, water use permitting, computer modeling, contamination assessment, environmental assessments, surface water studies, aquifer storage and recovery, and mine dewatering projects.