Stable isotopes (δ18O,δ2H) of pore waters in clay-rich aquitards:: A comparison and evaluation of measurement techniques

Pore water collected from piezometers installed in a thick clay-rich till were used to compare and evaluate four techniques for obtaining deltaD and delta O-18 values in these media. The techniques included mechanical squeezing, centrifugation, azeotropic distillation, and a direct soil-water equilibration technique. Direct CO2-core equilibration yielded sufficiently accurate and reproducible delta O-18 results of pore water in clay-rich tills. In addition, this method eliminated the need for labor-intensive complete extraction of water from the geologic media. Mechanical squeezing and centrifugation produced results similar to direct equilibration. However, both of these methods exhibited a greater degree of variability and were laborious and more time consuming. Small differences in delta O-18 values between piezometer water and equilibrated, squeezed, and centrifuged samples suggested that each method collected different fractions of the clay-water reservoir. Although these subtle differences were not conclusive, they did suggest the presence of weakly bound water and highlighted the difference between these three techniques for determining the stable isotopic composition of pore water in clay-rich aquitards. Azeotropic distillation produced a high level of discrepancy in deltaD and delta O-18 results compared to the other methods. Incomplete extraction was considered the most probable cause of this error. The results of this study suggested that direct equilibration is the best method for determining detailed deltaD and delta O-18 values of pore water in clay-rich aquitards.