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. Note:Page 61 is missing.