The Sheffield low-level radioactive-waste disposal site is located on 20 acres of rolling terrain about 3 miles southwest of Sheffield, Illinois. Twenty-one trenches were constructed and filled with radioactive waste from August 1967 through April 1978. Forty-three test wells were installed by the U.S. Geological Survey on and adjacent to the site. Continuous cores were collected from 36 wells to help in defining the subsurface geology. The wells have been used for water sample collection and to monitor water-level changes. A tunnel, 6.5 feet in diameter by 290 feet in length, was constructed beneath four burial trenches to provide access for collection of hydrologic and geologic data.
Pennsylvanian shale and mudstone deposits are overlain by Pleistocene glacial deposits consisting of the Teneriffe Silt, Glasford Formation, Roxana Silt, Peoria Loess, Parkland Sand, Cahokia Alluvium, and Henry Formation. Three till units of the Glasford Formation, the Hulick Till Member, the Radnor Till Member, and Till A have been identified on the site. Stratigraphic position indicates that the Hulick Till Member and Till A are probably variations of the same till. A continuous pebbly sand deposit, classified as part of the Toulon Member, extends across the middle of the site and continues off site on the northeast and southwest corners. Because of its relatively high hydraulic conductivity, this deposit will be a controlling factor in shallow groundwater movement and in any radionuclide migration.
Ground water at the site is derived through infiltration of precipitation and as underflow from adjacent highlands. Precipitation averages 35 inches per year, 1 or 2 inches of which probably recharge the ground water. Runoff is estimated to be 12 to 15 inches per year and evapotranspiration about 20 inches.
The fluctuation of water levels has been about 2.5 feet in hilltop wells, 3.6 feet in sidehill wells, and 5.9 feet in valley wells.
Hydraulic conductivity of the materials comprising the hydrogeologic system vary widely from about 2.8 to 2.8 x 10^-6 feet/day. Tritium in ground water near the southeast corner of the site has moved about 25 feet per year since June 1975.