A groundwater flow model and time-related capture zone for the Harristown, Illinois Public Supply Well
Peterson, Steven M.
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As part of the Illinois Environmental Protection Agency’s recent wellhead protection efforts, a time-related capture zone was delineated for the Harristown, Illinois public supply well. Time-related capture zones, which define the area contributing groundwater to a well over a specified period of time, are useful tools for Wellhead Protection Area (WHPA) delineation. A time-related capture zone can be used to delineate a WHPA to protect the well against direct and indirect contamination, either by existing practices or future uses. The geology of the Harristown well site consists of Wisconsinan and Illinoian glacial deposits overlying poorly permeable bedrock of the Mattoon, Bond, and Modesto Formations. The Wisconsinan glacial deposits consist of till and outwash, the Illinoian glacial deposits are till, and the bedrock is mainly poorly permeable shale. The Harristown well discharges water from the unconfined Wisconsinan outwash aquifer. To delineate a five-year capture zone, a three-dimensional finite-difference groundwater flow model of the area was created using the GMS modeling software, which uses MODFLOW as the core code. The groundwater flow model consisted of over 1890 cells in each of five layers, with the top layer representing various soil and weathered till units and the bottom four layers representing the outwash aquifer and bounding till units. Twelve water levels were measured in private water supply wells during July of 1998 to serve as model calibration points. Simulated water levels were calibrated to within 1.25 meters for ten of the twelve points, and to within 1.9 meters to the remaining two points. Calibrated hydraulic conductivities, drain conductances, and recharge were all within typical ranges of published data for such sediments. The simulated heads output by the calibrated flow model were used as inputs to a MODPATH particle tracking simulation to produce the five-year capture zone for the Harristown well. The capture zone was found to have complex lateral and vertical distribution, and was influenced by the complex local geology of the shallow subsurface. A complex capture zone such as the one produced for the Harristown well could not have been accurately predicted using simpler techniques such as analytical flow equation solutions.