DISTRIBUTION, ORIGIN, AND HYDRAULIC INFLUENCE OF FRACTURES IN A CLAY-RICH GLACIAL DEPOSIT

In the unconsolidated clay-rich glacial deposits underlying a site in southwestern Ontario, fractures and root casts greatly influence hydra ulic conductivity and groundwater flow. The fractures are predominantl y vertical and have visible oxidation staining from surface to a depth of 6 m. Root casts commonly occur along fracture surfaces in the uppe r 3 m, but can also occur as holes in apparently unfractured blocks. T he fractures are believed caused mainly by dessication during past per iods of low water table. This hypothesis is supported by a decrease in fracture density with depth and the presence of a stiff crust, presum ably caused by desiccation-induced consolidation. The random pebble fa bric and faint layering indicate deposition in a calm lacustrine envir onment, which precludes the possibility of the fractures having been c aused by overriding ice. Fractures were found below the depth of oxida tion staining (6 m) but most of these appear to have been caused by st ress-relief due to the excavation and subsequent drying. In the upper 3 m the fractures and root casts are responsible for field-measured hy draulic conductivity values that are up to 3 orders of magnitude great er than measured in the laboratory for samples of the unfractured matr ix. High values of field-measured hydraulic conductivity, seasonal hea d variations greater than 0.5 m, and high tritium content all persist below the depth of root casts, indicating that hydraulically conductiv e fractures do exist to depths of at least 6 m and possibly as gn:at a s 12-15 m, which is well below the depth of oxidation staining. Howeve r, there is some uncertainty in this assessment of the extent of hydra ulically conductive fractures because of the sensitivity to small leak s in the piezometer installations.