The Depth of Fractures and Active Ground‐Water Flow in a Clayey Till Plain in Southwestern Ontario

Abstract

AbstractThe 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 l 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.