Transport and geochemical processes controlling the chemical and isotopic composition of pore waters in the upper 45 m of a thick, surficial, clay-rich till (Battleford Formation) were studied. The upper 3 m of the till is oxidized and fractured. The remaining 77 m of the till (the aquitard) is nonfractured and unoxidized. Concentrations of total dissolved solids, SO42-, Na+, Mg2+, and K+ decrease with depth through the unoxidized zone to a depth of about 15 m below ground, below which the concentrations remain constant. A similar trend was observed for alkalinity; however, the decrease in concentration occurs over 20 m. In contrast to these ions, Ca2+ concentrations increase with depth through the upper 20-m unoxidized zone, below which the concentrations remain relatively constant. The distribution of dissolved ions shows the presence of two end-members: elevated solute concentrations in the oxidized zone attributed to geochemical weathering that occurred since the start of the Holocene and connate Pleistocene-age water at depths between 20 and 45 m. Abiotic and biotic geochemical reaction rates in the aquitard were shown to be extremely slow or not occurring. ¿ 2000 American Geophysical Union