[en] The University of Ottawa was contracted by the NWMO to optimize the high-temperature vacuum-distillation extraction (VDE) technique for porewater characterization of low permeability rock, and to further assess the reliability of the method. As a part of this work program, a modified micro sampling technique (μVDE) was developed to optimize the vacuum distillation method for analysis of smaller samples of low water content. Optimization was achieved by undertaking complete closed-system sample crushing in stainless steel containers fitted with high-temperature silicone septa - n order to facilitate transfer of water vapour directly to the extraction vial under dynamic vacuum while heating the sample. An extraction temperature of 150℃ was determined experimentally to be the optimum for extraction of porewater. A dual transfer line was also developed to accommodate ultra-low water content samples. The μVDE method has several key advantages over the previous VDE method, including less potential for evaporative loss during sample crushing and loading, reduced extraction times, smaller sample size - enabling precision sampling of heterogeneities - and potential for reliable analysis by isotope ratio infrared laser spectroscopy (IRIS), provided there is no interference from residual hydrocarbons. The μVDE methodology was bench-tested with a series of control tests (using lab water of known isotopic concentration within a known matrix), including testing of porewater extracted from preserved core samples. Initial μVDE testing was completed on archived core material (DGR-6), which showed some evidence of evaporation due to repeated exposure/sampling and/or larger headspace in the extraction vial. Test waters extracted by closed-system high-temperature vacuum distillation (μVDE) to a smaller extraction vial show good reproducibility (±2.0‰ for δD and ±0.5‰ for δ¹⁸O; within the analytical uncertainty of isotope analysis by IRIS). The new method also was further demonstrated using freshly-drilled core from DGR-8, which showed good agreement with previous DGR1-6 results obtained by VDE. The results of the μVDE testing demonstrates that it is an appropriate method for characterization of the ultra-low permeability, low water content and high salinity porewaters in the sedimentary rocks underlying southern Ontario. (author)