THE THOMPSON CHEMICALS SITE IS LOCATED IN AN INDUSTRIAL AREA ALONG THE MISSISSIPPI RIVER IN ST. LOUIS, MO. A NUMBER OF DIFFERENT INDUSTRIAL FACILITIES HAVE OPERATED AT THE SITE SINCE THE LATE 1800'S. THESE OPERATIONS HAVE INCLUDED A COAL TAR PROCESSING FACILITY, A STORAGE AND MANUFACTURING FACILITY FOR WOOD TREATING PRODUCTS, THE PRODUCTION OF 2,4,5,T AND 2,4-D ACID PESTICIDES AND THE MANUFACTURE OF AGENT ORANGE. THE SITE IS CURRENTLY IN USE AS A BULK TERMINAL FACILITY FOR SOLVENT PRODUCTS. IN THE MID-1980'S, THE SITE CAME TO THE ATTENTION OF REGION VII WHEN SEEPAGE WAS OBSERVED AT THE GROUND SURFACE FROM A BURIED TANK OF CREOSOTE MATERIAL, WHICH HAD BEEN LEAKING. DURING THE LATER 1980'S AND EARLY 1990'S, SIGNIFICANT SAMPLING OF THE SITE WAS UNDERTAKEN BY EPA AND ITS CONTRACTORS. THE SAMPLING DOCUMENTED THE PRESENCE OF POLYNUCLEAR AROMATIC HYDROCARBONS, VOLATILE ORGANIC COMPOUNDS AND DIOXIN IN THE SOILS AT THE SITE, ABOVE LEVELS OF CONCERN TO REGION VII. IN 1987, SUPERIOR OIL COMPANY, INC., THE CURRENT OWNER OF A PORTION OF THE SITE AND THE CURRENT OPERATOR OF THE SITE, CONDUCTED A REMOVAL PURSUANT TO AN ADMINISTRATIVE ORDER ISSUED BY EPA REGION VII TO REMOVE THE BURIED TANK REFERRED TO ABOVE. IN 1991-2, SUPERIOR AND THE MISSOURI PACIFIC RAILROAD CO., ALSO AN OWNER OF A PORTION OF THE SITE, CONDUCTED ANOTHER REMOVAL, PURSUANT