Stable isotopes (delta O-18,delta H-2) 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 w ere used to compare and evaluate four techniques for obtaining deltaD and d elta O-18 values in these media. The techniques included mechanical squeezi ng, centrifugation, azeotropic distillation, and a direct soil-water equili bration technique. Direct CO2-core equilibration yielded sufficiently accur ate and reproducible delta O-18 results of pore water in clay-rich tills. I n addition, this method eliminated the need for labor-intensive complete ex traction of water from the geologic media. Mechanical squeezing and centrif ugation 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 pie zometer 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 t he presence of weakly bound water and highlighted the difference between th ese three techniques for determining the stable isotopic composition of por e water in clay-rich aquitards. Azeotropic distillation produced a high lev el of discrepancy in deltaD and delta O-18 results compared to the other me thods. Incomplete extraction was considered the most probable cause of this error. The results of this study suggested that direct equilibration is th e best method for determining detailed deltaD and delta O-18 values of pore water in clay-rich aquitards.