TRI-COUNTY/ELGIN LANDFILLS ARE FUNCTIONALLY ONE CONTIGUOUS DISPOSAL UNIT WITH SEPARATE OWNERSHIP AND OPERATING HISTO- RIES. THE SITE TOTALS APPROXIMATELY 66 ACRES, OF WHICH ALL BUT ABOUT 4 ACRES CONTAINS FILL MATERIAL. THE SITE ACCECT- ED SOLID AND LIQUID COMMERCIAL/INDUSTRIAL WASTE FROM 1961 THROUGH 1976. ALTHOUGH SOME RECORDS OF WASTE TYPE AND QUANTITIES EXIST FOR THE TRI-COUNTY LANDFILL, LOCATION OF HAZARDOUS WASTE DISPOSAL IS NOT KNOWN FOR EITHER LANDFILL. IN 1984, THE U.S. EPA DETECTED VOLATILE ORGANIC COMPOUNDS (VOCS) IN GROUNDWATER MONITORING WELLS DOWNGRADIENT OF THE SITE. APPROXIMATELY, 10,000 RESIDENTS WITHIN 3 MILES OF THE SITE USE GROUNDWATER AS THEIR SOURCE OF DRINKING WATER.