An alternative view on the origin of chemical and isotopic patterns in groundwater from the Milk River Aquifer, Canada

Abstract

The Milk River artesian aquifer underlies 15,000 km2 of southern Alberta, Canada. It consists of thin (30–75 m thick) sandstone and is confined above by the Pakowki shale (typically 120 m thick) and below by the Colorado shale. The aquifer subcrops in southern Alberta and northern Montana. Groundwater movement is to the north, west, and east from the outcrop area (dominant recharge area). Cl− and I− concentrations increase in the direction of flow from less than 0.05 and 0.001 mmol/L, respectively, near the recharge area to more than 140 and 0.15 mmol/L at the northern edge of the aquifer. Similarly, waters become more enriched in oxygen 18 and deuterium from less than −21.0 and −167‰ near the recharge area to values approaching −8.0 and −70.0‰ in the north. Isotope values in the recharge area plot on the global meteoric water line and indicate that the recharge waters are isotopically unaltered meteoric waters. Downgradient from the recharge area the data deviate from the meteoric water line (slope of 6.3 instead of 8.0). Three mechanisms have been advanced to explain the origin of the chemical and isotopic patterns: the introduction of connate formation water through the Colorado shale and subsequent mixing with infiltrating meteoric water; a finite source of meteoric recharge mixing with more saline water in the aquifer; and chemical and isotopic enrichment due to ion filtration. Hydraulic conductivity and hydraulic head data, Cl− concentrations of drill cuttings and isotopic values from groundwater from the Colorado shale provide arguments for reevaluating these ideas. An alternative mechanism, based on diffusion between saline, isotopically enriched water in the Colorado shale and fresher, isotopically depleted water in the aquifer, is tested using an analytical mass transport model. Although uncertainty exists in the model parameters, results showed that the patterns in Cl−, I−, δ18O, and δ2H in the aquifer can be explained by aquitard diffusion. Further, aquitard diffusion must be considered when using chlorine 36 dating methods in aquifer‐aquitard systems such as the Milk River aquifer.