THREE-DIMENSIONAL GEOLOGIC MAPPING AND HYDROGEOLOGIC INVESTIGATIONS TO SUPPORT GROUNDWATER MANAGEMENT IN MCHENRY COUNTY, ILLINOIS
Throughout the county, groundwater stresses are largest along the eastern margin, where withdrawals are mostly from basal sand and gravel aquifers. Groundwater-surface water interaction is a management concern in this area as many downstream communities and ecosystems rely on current river hydrology. Shallow groundwater demands in the north and west portions of the county are small with most water coming from deep bedrock aquifers. In the West, sparse data obscures our insight on geometry and character of geologic deposits. Groundwater demand in the southwest is under little stress, with water levels near land surface and very small gradients.
To address the need for 3-D geologic information, the ISGS is developing a geologic framework model (GFM) of the unconsolidated deposits in McHenry County. The geologic setting of the County includes a surficial fractured carbonate bedrock unit and 3 bedrock valleys filled with thick sequences of pre-Wisconsin and Wisconsin Episode sands and gravels. Modern rivers occupy 2 of these valleys. Multiple glacial advances left a series of glacial diamictons and intervening proglacial fluvial packages characterizing upland successions. The diamictons are generally absent in the valleys, leaving thick sand and gravel successions merged together. This forms an inter-connected system of layered aquifers and aquitards in the uplands merging into thick aquifers in the valleys.
A generalized version of the GFM has been delivered to hydrologists for groundwater flow modeling. The final GFM will be used to educate county officials and citizens, develop custom maps supporting the county’s Water Resources Action Plan, augment interpretation of groundwater flow modeling results, and guide county officials in water management decision making.