The report is a study of a regional aquifer in central Illinois and the public water supplies that depend on it for all, or most of their drinking water needs. The intent of the study is to bring together and consolidate into one report, several different aspects of wellhead protection, engineering, and hydrology. The report may be used by the communities included in the study area to implement local wellhead protection measures and to evaluate future needs in terms of water quality and quantity. Several different sources of information were utilized in preparing the report, including regional and site-specific geologic and hydrogeologic data. Computer modeling using Visual MODFLOW, MODPATH, SURFER, ARCINFO, and ArcView2 Geographic Information Systems (GIS) software was used to delineate capture zones for the public water supply wells, as well as evaluating geology, aquifer recharge and regional flow directions.