Fracture effects in the shallow groundwater zone in weathered Sarnia-area clay

The Sarnia area of southwestern Ontario is underlain by thick deposits of clay-rich glacial till. From ground surface to between 4 and 6 m depth the clay till is fractured, oxidized, and penetrated by root holes. The water table fluctuates seasonally between the ground surface and the bottom of the weathered zone. Water-level response tests in conventional piezometers installed in augered holes in the weathered zone typically provide very low values of hydraulic conductivity (10 −7 -10 −9 cm/s), which are similar to values obtained from the deeper unweathered and unfractured till. The augering process creates a smeared clay zone along the borehole walls that significantly reduces measured hydraulic conductivity values. In this study, the weathered-zone hydraulic conductivity was measured using unconventional piezometers, a large-diameter well, and a tracer experiment between two test pits. The smeared zone was removed or reduced around these installations, and the measured weathered-zone hydraulic conductivity was 10 −5 -10 −7 cm/s, much higher than in the deep unweathered clay. These results, together with depth profiles of groundwater tritium and monitored water-level fluctuations, indicate that the weathered zone has hydraulic conductivity governed by fractures and has significant hydrologic activity in the fractures. Key words: clay, fractures, hydraulic conductivity, permeability, piezometer, tritium, tracer experiment, weathered zone.