THE DEPTH OF FRACTURES AND ACTIVE GROUNDWATER-FLOW IN A CLAYEY TILL PLAIN IN SOUTHWESTERN ONTARIO

The St. Clair Plain in southwestern Ontario is underlain by extensive clayey till deposits which are generally 30 m to 40 m thick. The tills have vertical fractures near the ground surface, and this study investigates the depth of those fractures. Observations in test pits show that most weathering features along fractures reach a depth of only 2.5 m to 4 m, though isolated major fractures extend past the pit bottom depth of 5.6 m at two sites. This study also investigates the depth of active ground-water flow, which is defined as the ground-water flow in the clayey deposit which occurs in fractures. Water-level response tests show bulk hydraulic conductivities of the fractured till ranging from > 10(-7) to 10(-8) cm/s. Seasonal variations in hydraulic head profiles suggest that fractures influence ground-water flow to maximum depths ranging from 5 m to more than 10 m at the 10 sites studied. The water table is usually within 2 m of the surface, but piezometers went dry to depths of 2 m to 4.9 m during dry periods in 1987 and 1988. Tritium levels greater than 1 T.U., indicating the presence of post-1952 water, are found to depths of 7.5 m at all sites and to depths exceeding 12 m in two cases. Tritium has moved to the base of deep, open fractures by active ground-water flow and has travelled 1 or 2 m beyond that depth by molecular diffusion. Observations of seasonal water-level variations and tritium sampling show the most promise for determining the maximum depth of hydraulically active, vertical fractures. The maximum depth of active ground-water flow in fractures varies widely, ranging from 5 m to more than 10 m at the 12 sites studied on the St. Clair Plain.