Determination of detailed stable isotope profiles and hydrogeologic conditions in a complex clay-rich aquitard, Saskatchewan, Canada

Abstract

Four techniques for obtaining δ¹⁸O and δ²H values in clay-rich geologic media were compared and evaluated. The results of this evaluation were then applied to a hydrogeologic investigation of a complex clay-rich aquitard system. The four techniques included mechanical squeezing, centrifugation, azeotropic distillation, and a direct soil-water equilibration technique. Direct CO₂-core equilibration yielded sufficiently accurate and reproducible δ¹⁸O results of pore water in clay-rich tills. Mechanical squeezing and centrifugation produced results similar to direct equilibration. Azeotropic distillation produced a high level of discrepancy in δ¹⁸O and δ²H results compared to the other methods. The results of this study suggested that direct equilibration is the best method for determining detailed δ²H and δ¹⁸O values of pore water in clay-rich aquitard systems. The aquitard system investigated in this study consisted of a thick Quaternary till overlying an intertill aquifer. A detailed vertical profile of pore water δ¹⁸O through the aquitard suggested that the till unit was divisible into three distinct hydrogeochemical zones: an upper zone (0 - 10.5 m), an intermediate (10.5 - 30m) zone, and a lower zone (30 - 62.5 m). A comparison of laboratory and field hydraulic conductivity (K) values suggested that the K in the upper 30 m of till is higher than the underlying till due to a system of interconnected fractures and sand layers. In contrast, results suggested that the lower zone (35 - 62.5 m) was not fractured but contained sand lenses or streaks. Results of K measurements supported the presence of three distinct zones suggested by the δ¹⁸O data. Bulk K was estimated to range from 3.2x10⁻⁹ to 3.4x10⁻⁸ m/s and 1x10⁻¹¹ to 1x10⁻¹⁰ m/s in the upper/intermediate and lower zone, respectively. Measured values of hydraulic head in the aquitard supported the contention that the lower zone had a significantly lower K than the overlying till. Although K in the upper and intermediate zones is high, these data suggest that active groundwater flow is limited to shallow localised zones. This investigation also shows that a three-dimensional study of groundwater flow at the site is warranted.