Modeling the groundwater flowsystem and five-year recharge area of the Fawn Hills subdivision, Peoria County, Illinois
Carney, Clint P.
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Development of wellhead protection programs by the Federal and Illinois Environmental Protection Agencies in the 1980’s led to several measures for protecting groundwater resources. This study is a result of a cooperative effort between the Illinois EPA and Northern Illinois University. Application of computer models to represent the groundwater flow system and five-year recharge area for the wells supplying the Fawn Hills Subdivision in Peoria County, Illinois is the primary focus of this work. The community wells utilize the highly productive Sankoty Aquifer. Located west of the Illinois River and the community of Chillicothe, the study area contains a complex arrangement of Quaternary deposits including sand, till, outwash, and alluvium. The aquifer is characteristic of alluvial valley aquifers found in major river valleys. Hydrologic and geologic information was obtained from state agencies and field data gathered in 1998. The system was represented as a three-layered, finitedifference model in the USGS MODFLOW code. Model development and execution was conducted through the BOSS Groundwater Modeling System (GMS) software. Model parameters were manipulated to calibrate heads to within ± 1 meter of field values. A sensitivity analysis revealed the flow model to be most sensitive to changes in hydraulic conductivity and recharge. Reverse particle tracking from the Fawn Hills wells using the USGS MODPATH code revealed a capture zone approximately 1.4 kilometers in length and 0.2 kilometers in width. The pathlines terminate in the subsurface beneath till units. A sensitivity analysis revealed that the size of the capture zone was influenced by porosity values, and that the orientation of the capture zone was most sensitive to hydraulic conductivity and recharge. Tests showed that the nearby Chillicothe community wells have no influence on dimensions of the capture zone, and that high-capacity irrigation wells to the south influenced the capture zone only when simulated recharge was reduced 50%. Threats to the water supply are primarily from agricultural activities or roadway spills. Higher than expected simulated recharge values on the northwestern boundary show that additional flow may be entering the system through sands buried in the Wyoming bedrock valley to the northwest.