Hydraulic Conductivity of a Glacial Till in Alberta a

Soils underlain at shallow depths (less than 1 m, 3 ft) by glacial till are generally considered undesirable for irrigation because of their unfavourable internal drainage characteristics. In some areas of southern Alberta, Canada, soils developed upon shallow tills have been irrigated successfully for over 60 years with no adverse effects on the soil. An investigation was conducted to describe the hydro‐geologic properties of till under one of these areas and to assess the properties with regard to drainage. Study techniques consisted of detailed test drilling and sampling, excavation of test pits, installation and monitoring of ground‐water instrumentation, field and laboratory hydraulic conductivity testing and tritium analyses of ground‐water samples. Two fracture sets were found in this till. Both sets of fractures produce secondary permeabilities which mask the low hydraulic conductivity of the till matrix (10 −10 m·s −1 ). Small‐scale fractures which have a fracture spacing of approximately 10 mm (0.4 in.) have an apparent mean hydraulic conductivity of 5 × 10 −9 m·s −1 , whereas large‐scale fractures which have fracture spacings from 20 mm (0.8 in.) to over 630 mm(2 ft) have an apparent mean hydraulic conductivity of approximately 2 × 10 −7 m·s −1 . The high hydraulic conductivity of the large‐scale fractures was corroborated by tritium analyses of ground‐water samples. Tritium analyses also indicate the presence of recent water at depth in the till. The large‐scale fractures, which control the bulk hydraulic conductivity of this till, provide conduits through which infiltrating water can be transmitted to the ground‐water regime. These fractures are believed to be the reason why this land has remained irrigable for over 60 years.