Benchmarking of the Isotope Diffusive Exchange Method Using Opalinus Clay and Queenston Shale Rocks Equilibrated with Synthetic Saline Solution

This study aims to test recent developments for advancing the isotope diffusive exchange technique in order to adapt it to high salinity porewaters and, in a more general way, to solutions with low water activities. This report documents work performed to: 1) investigate factors that might influence the results of the isotope diffusive exchange technique for saline solutions through solution-solution experiments; and 2) benchmark the adapted diffusive exchange method using rock samples previously equilibrated with synthetic solutions of known composition (chemical and isotopic). Two different rocks have been used to perform the benchmarking tests: 1) Queenston Shale from Ontario, Canada; and 2) Opalinus Clay from the Mont Terri Underground Research Laboratory (URL), Switzerland. Solution-solution diffusive-exchange experiments showed that perturbing factors include: 1) the water activity mismatch between the test and sample solutions; 2) the weight difference between the test and sample solutions; and 3) contrasting chemical compositions (NaCl versus CaCl2) between test and sample solutions. Benchmarking of the isotope diffusive exchange method using rock samples equilibrated with synthetic solutions has tested: 1) whether or not the isotopic composition obtained by the diffusive exchange method actually corresponds to the composition of the porewater; and 2) if there are additional perturbing factors, i.e., other than those identified in the solution-solution experiments. Equilibration of Opalinus Clay and Queenston Shale rocks (which are saturated with porewater at experiment initiation) with 0.3 and 5 molal NaCl, and 2.5 and 5 molal CaCl2 synthetic solutions has been performed successfully by immersing 2 to 4 cm diameter rock pieces into air-tight PVC containers for periods of 62 and 90 days, respectively. Equilibrium was apparently attained in about 1 day for Opalinus Clay rock samples, and 4 days for the Queenston Shale rock samples. The rock samples never disaggregated during these experiments, even at 0.3 molal NaCl solution concentrations. The density of the equilibrated rocks, in all cases, is lower than that of the intact porewater-saturated rock and increases with increasing salinity of the synthetic solution. This is probably because the confining pressure in the equilibration experiments is low compared to in-situ conditions. The rock density correlates with its water content, which is higher in low salinity solutions. Data obtained from Opalinus Clay and Queenston Shale rock experiments indicate that the diffusive exchange method gives reliable results of the isotope composition (δ18O and δ2H) of the porewater at all salinities when the chemistries of the porewater and test waters are similar. Results for δ18O values are shifted up by about 1‰ for contrasting chemistries (NaCl versus CaCl2), and insignificantly for δ2H. The diffusive exchange technique gives reliable results for rocks with water content as low as 0.5 wt%. For unknown reasons, the use of the diffusive exchange method for obtaining the water content of the rock gave good results at high salinities (≥2.5 molal), but overestimated values at low salinities (0.3 molal).