On the long-term hydraulic gradient in the thick clayey aquitard in the Sarnia region, Ontario

Much of the southwestern pad of Ontario between Lake Huron and Lake Erie ha s a thin freshwater aquifer overlain by an aquitard of Late Pleistocene cla yey glaciolacustrine deposits and underlain by a thick Devonian shale aquit ard. In a large area east of the St. Clair River, where the Quaternary aqui tard is 30-50 m thick, groundwater of Pleistocene origin (identified by O-1 8 and H-2 signature) occurs in the aquifer and in the bottom part of the Qu aternary aquitard. Numerous piezometer nests in the aquitard show a downwar d hydraulic gradient with depth. In some areas, the aquitard, has downward gradients only in the upper part and upward gradient in the lower part, ind icating a transient condition, The piezometer nests in the clayey aquitard also show an increase in CT concentration with depth. Long-term. piezometer monitoring at two sites show a major shift in the aquitard hydraulic gradi ent since 1983 and a large rise in head in the underlying aquifer, Analyses of initial aquifer water levels, reported in well drilling records, indica te a large decline in the potentiometric surface of the aquifer between the 1940's and the 1970's followed by a recent rise in the surface in part of the, region. This pattern is consistent with well drilling and water use re cords indicating that 7000 wells were installed in the aquifer in the three decades since 1940 and that groundwater use has greatly diminished in the, past 10-15 years due to rural pipeline distribution of lake and river wate r. The hydraulic gradient in the aquitard is slowly adjusting to the rise i n the aquifer potentiometric surface. One-dimensional solute transport mode lling provides close matches to the vertical profiles of Cl- migrating upwa rd from the aquifer since deglaciation, 15 000 - 18 000 years before presen t, by diffusion with little or no advection. The lack of advection indicate s a near-neutral long-term hydraulic gradient. As the withdrawal rate of wa ter from the aquifer continues to decline, it is expected that the hydrauli c head in the aquitard in much of the area westward of the recharge area wi ll continue to adjust for many decades.