FIELD EXPERIMENTS IN A FRACTURED CLAY TILL .1. HYDRAULIC CONDUCTIVITY AND FRACTURE APERTURE

Field values of horizontal hydraulic conductivity measured in the upper 1.5-5.5 m of a weathered and fractured clay-rich till were strongly influenced by smearing around piezometer intakes, which occurs during augering, and by the physical scale of the measuring device. Values measured in conventional augered piezometers were typically 1-2 orders of magnitude lower than those measured in piezometers designed to reduce smearing. Measurements of hydraulic conductivity in small-scale seepage collectors or piezometers, which typically intersect fewer than 10 fractures, vary over a much greater range, 10(-10) to 10(-6) m/s, than large-scale values based on infiltration into 5.5-m-deep trenches which intersect thousands of fractures (range 10(-7) to 3 x 10(-7) m/s). Values of hydraulic fracture aperture, 1-43 mum, and fracture porosity, 3 x 10(-5) to 2 x 10(-3), were calculated using the cubic law with fracture orientation/distribution measurements and the small-scale hydraulic conductivity measurements. This paper provides the first reliable determination of the magnitude and spatial distribution of hydraulically derived fracture parameters in a clay deposit. The absence of such data has, until now, severely limited the application of quantitative groundwater flow and contaminant transport models in this type of deposit.