P-wave, shear wave, advancing boreholes, and downhole geophysical techniques, were used to confirm the presence of a hydraulic window that connects the Sangamon River (Makinaw Member of Holocene Cahokia Alluvium the and Wisconsinian Henry Formation) and the pre-Illinoian Mahomet Sand Member of the Banner Formation. The Mahomet Sand Member of the Banner Formation is the primary aquifer for East Central Illinois. The relatively young age and lower chloride levels of waters in the Mahomet Sand aquifer beneath the Sangomon River indicated that a hydraulic window existed between the Mahomet Sand Member and the Sangomon River. The geophysical investigation indicated that a northwest-southeast trending tunnel valley beneath the Sangamon River in the Allerton Park area, Piatt County, Illinois provides the hydraulic window. This study, of the Mahomet Aquifer in Piatt County, was begun in response to changes of groundwater use patterns when municipal wells in nearby Decatur are pumped. The study was a collaborative project between the Illinois State Geologic Survey and Illinois State University and was supported by the Illinois Board of Higher Education.