Isotope and ion geochemistry of groundwaters in the Milk River Aquifer, Alberta

Abstract

The Milk River artesian aquifer underlines an area of approximately 6300 km2 in southern Alberta. This aquifer fits almost exactly Chamberlain's (1885) concept of a classic artesian system. It crops out or is covered by a thin veneer of glacial drift in southern Alberta and northern Montana. It is believed that major recharge occurs in these areas (Meyboom, 1960). Structurally, the aquifer appears fan‐like in form, dipping gently to the east, north and west from its areas of outcrop. It is overlain by the Pakowki Formation which is a grey shale that acts as a confining bed. Analysis of stable isotope and major ion data for water samples collected from the aquifer reveals a variety of striking patterns. Oxygen‐18 and deuterium concentrations for groundwater from the recharging portion of the aquifer fall very close to the meteoric water line (Craig, 1961a) indicating that they are isotopically unaltered meteoric waters. Proceeding down‐dip in the aquifer groundwaters become enriched isotopically to a maximum of 70 and 12‰ with respect to deuterium and oxygen‐18 in the recharging waters. In addition, a systematic deviation (slope of 6.1 instead of 8) from the meteoric water line is also apparent. The most plausible explanation of these isotopic patterns is based on a process of dispersion or mixing on a regional scale. Waters probably enriched with respect to the isotopic composition of the aquifer recharge may be present and simply are being displaced by meteoric water, or leakage may have entered the aquifer from below progressively comprising an increased proportion of the aquifer water. This same type of mixing process appears to exert a major control on the major ion chemistry of the aquifer. In addition, chemical processes such as mineral dissolution and cation exchange play a role.