Hydraulic Conductivity of a Fine-Grained Till, Cattaraugus County, New York

ABSTRACT Hydraulic conductivity of a fine‐grained till was evaluated by field and laboratory techniques as part of a study of the movement of radionuclides at a nuclear‐waste landfill in western New York. In general, both methods produced similar results. Hydraulic conductivity values determined from (a) slug tests of piezometers, (b) several types of laboratory tests on cores, and (c) calculation from mercury porosimeter tests, agreed within a factor of 25. Slug tests were analyzed by two methods: one assumed horizontal flow to the piezometer screen; the other assumed spherical, isotropic flow. Average horizontal hydraulic conductivity from slug tests of 12 piezometers was 6 × 10−8 cm/s (2 × 10−9 ft/s) by the first method and 2 × 10−8cm/s (7 × 10−10ft/s) by the second. Laboratory tests of core samples were designed to determine the extent to which anisotropy and increased pressures with depth may affect hydraulic conductivity. Consolidation tests were done on four core samples to determine changes in hydraulic conductivity with pressure. Vertical hydraulic conductivity of the samples decreased by 40 percent as pressure increased from near atmospheric to the 7 kg/cm2 (100 psi), a pressure that would prevail at a depth of 30 m (98 ft). This indicates that hydraulic conductivity of the till decreases with depth in response to overburden pressure. Despite evidence of higher horizontal than vertical permeability in one set of laboratory tests of till samples, other laboratory and field tests as well as analyses of ground‐water flow by a computer model indicate that the till is nearly isotropic. Hydraulic conductivity of the till was estimated with reasonable accuracy from mercury‐porosimeter tests. However, estimating hydraulic conductivity of fine‐grained samples from some regions by this method may be inappropriate because the method uses dry samples, which could severely affect the results if swelling clays are present.