Controls on the distribution of major ions in pore waters of a thick surficial aquitard

Transport and geochemical processes controlling the chemical and isotopic c omposition of pore waters in the upper 45 in of a thick, surficial, clay-ri ch till (Battleford Formation) were studied. The upper 3 m of the till is o xidized and fractured. The remaining 77 m of the till (the aquitard) is non fractured and unoxidized. Concentrations of total dissolved solids, SO42-, Na+, Mg2+, and K+ decrease with depth through the unoxidized zone to a dept h of about 15 m below ground, below which the concentrations remain constan t. A similar trend was observed for alkalinity; however, the decrease in co ncentration occurs over 20 m. In contrast to these ions, Ca2+ concentration s increase with depth through the upper 20-m unoxidized zone, below which t he 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 si nce the start of the Holocene and connate Pleistocene-age water at depths b etween 20 and 45 m. Abiotic and biotic geochemical reaction rates in the aq uitard were shown to be extremely slow or not occurring.