Groundwater Simulation Modeling and Potentiometric Surface Mapping, McHenry County, Illinois

Abstract

Illinois State Water Survey researchers conducted two studies to support water resources planning in McHenry County, Illinois. The first was an investigation to map heads in the shallow aquifers of McHenry County, and the second was a project to develop and use a computer model to simulate groundwater flow in the aquifers supplying the county. This report summarizes the hydrogeology of McHenry County and the surrounding region, discusses historical and future groundwater pumping, describes the methods employed to measure and map shallow heads in McHenry County, presents and discusses the potentiometric surface maps developed from the measured heads, summarizes the methods and datasets used to develop the groundwater flow model, and presents and discusses groundwater flow model results. We mapped 329 water levels measured in 2011 in wells finished in 5 shallow aquifers in McHenry County, including sand and gravel aquifers and the underlying Shallow Bedrock Aquifer. The water levels are strongly influenced by connections between the aquifers, which equalize heads between aquifers, and between the aquifers and surface waters, which equalize surface water elevation and head in the connected aquifer. The shallowest of these aquifers are completely desaturated in areas of dissected topography and in elevated areas adjacent to steep slopes, where any water entering the unit from above can readily drain out. The measured water levels suggest that heads in the shallow aquifers were about 2 feet higher in 2011 than in 1994, suggesting that changes in pumping rates and distribution, climate, land use, land cover, and other factors have not resulted in a countywide decline in shallow aquifer heads during the period from 1994 to 2011. A groundwater flow model was developed to provide planners and researchers with anunderstanding of the consequences in McHenry County of groundwater development in the county and surrounding areas of Wisconsin and Illinois. The 2.9-million cell MODFLOW model simulates groundwater flow under transient conditions in all major aquifers underlying McHenry County and represents pumping from over 8700 wells in the McHenry County region. The hydrogeology of the region is represented with 26 layers. The model is used to quantify drawdown and reduction in natural groundwater discharge to surface waters resulting from historical pumping from 1864 to 2009 and estimated pumping, under three plausible scenarios of groundwater development, from 2010 to 2050. Simulations show that the impermeable upper bedrock materials underlying the Shallow Bedrock Aquifer and overlying the Ancell Unit aquifer strongly influence groundwater circulation in the aquifers underlying McHenry County. The impermeable upper bedrock greatly limits leakage into the deep aquifers underlying it, which include sandstones of the Ancell, Ironton-Galesville, Eau Claire, and Mt. Simon Units. The comparatively low transmissivity of these deep aquifers also limits eastward movement of water from north-central Illinois and south-central Wisconsin, where the impermeable upper bedrock is generally absent, toward cones of depression in heavily pumped areas of northeastern Illinois and southeastern Wisconsin. Model simulations reflect the influence of these factors, showing that drawdown in the deep aquifers increases from west to east across McHenry County, exceeding 400 feet in the southeastern part of the county in 2009. Drawdown under scenarios of future pumping increases to 2050, and model simulations show that, for the simulated annualized pumping rates, Ancell head decreases to within 50 feet of the top of the Ancell Unit within McHenry County by 2050 under the most extreme pumping scenario. 2 The shallow aquifers overlying the impermeable upper bedrock materials, which include the Shallow Bedrock Aquifer and unconsolidated sand and gravel aquifers contained within the overlying Quaternary materials, are affected by significantly less drawdown than the deep aquifers, although this drawdown could still cause well failures in affected areas. The largest cones of depression surround public water system wells and commercial/industrial wells in and near Woodstock, Algonquin, Carpentersville, Cary, and Crystal Lake. Less drawdown affects the shallow aquifers because they receive replacement water at significantly greater rates than do the deep aquifers. Since this replacement water originates as captured surface water, however, withdrawals from the shallow aquifers, although they result in less drawdown, cause reductions in natural groundwater discharge, and these reductions may affect base flows in streams and water levels in lakes and wetlands. Model simulations show that natural groundwater discharge in the McHenry County area has been reduced by about 11.5 percent by pumping of groundwater. Watersheds that have experienced the greatest reductions are those of the Crystal Lake Outlet and the City of Woodstock (Silver Creek). Although these streams and many others in McHenry County receive discharges of treated effluent from wastewater treatment plants at rates that compensate in quantity for these reductions in natural groundwater discharge, the effluent differs in quality from natural groundwater, and it is discharged at point locations rather than by diffuse seepage along stream channels.Recommendations for further work include efforts to refine the model as well as modeling studies to simulate alternative scenarios of groundwater development in McHenry County. Such simulations can provide planners with guidance to minimize and/or distribute unwelcome impacts from pumping.