Modeling of Groundwater Flow in a Karst Aquifer in Northeast Iowa

Groundwater contamination has been a major environmental concern in northeast Iowa for many years. The Paleozoic Plateau region of northeast Iowa differs from most other parts of the state because the bedrocks are at or near the surface. Carbonates dominate the bedrock landscape of the region and are subject to karst development by groundwater movement. Only thin, isolated remnants of loess and glacial-drift deposits remain in the region. The open nature of the karst system makes the groundwater resource highly susceptible to chemical or biological contamination. The Williams Creek watershed is located within the Paleozoic Plateau region and was selected for this study. The creek originates near Postville, Iowa, flows northeast through the center of the watershed and discharges into the Yellow River. The Postville Wastewater Treatment Plant is located at the head of Williams Creek and consequently drains its wastewater effluent directly into this stretch of the creek. The rural population is concerned that water quality throughout the watershed has diminished in recent years. The purpose of this study was to collect water quality data and construct a groundwater flow model of the area in order to gain a better understanding of the contaminants and contaminant transport within the watershed. Groundwater Modeling Systems (GMS) version 3.0 software was used to construct a conceptual model based on the geologic and hydrogeologic data of the site. Fourteen private wells were selected for monthly water table measurements for ten months. A steady-state flow model was built and calibrated against the long-term average water table conditions of the watershed and spatially-varied recharge rates were obtained. The transient simulation was then carried out using monthly hydraulic head data and time-dependent recharge rates were obtained. Karst conditions coupled with contaminant sources such as pesticide applications, hog confinement lots, and wastewater treatment facilities are widespread in this watershed. Results from the simulations will be discussed to show how karst conditions affect groundwater and contaminant flow in a limestone aquifer.