Three-dimensional spatial variability of chemical properties around a monitored waste emplacement tunnel

Regulatory requirements and scientific needs require that the response of the geological system to emplacement of high level radioactive waste be monitored for long time periods. This monitoring activity is intended to establish the extent to which predicted behavior matches the actual response of the geological system to waste emplacement. To accomplish this goal, field measurements must be made at a spatial resolution that will determine whether the changes in parameters that are monitored conform to predicted evolutionary patterns. From the perspective of thermohydrological and geochemical parameters, key measurements will consider pore water compositional evolution and changes in matrix and fracture saturation in the near vicinity of waste emplacement tunnels. A massively parallel high performance computational platform (a 1200 processor IBM SP-2) was used to conduct three-dimensional, high resolution simulations to ascertain the spatial variability to be expected during a monitoring period. The results show that spatial variability in certain chemical parameters below waste emplacement tunnels provides robust targets for monitoring, but will require sampling on the scale of 10 s of centimeters in some locations, in order to rigorously test models. Chemical variability induced by relatively small changes in waste package heat output suggests that designing a monitoring program that will rigorously test model predictions will likely require high resolution, three-dimensional simulations of the "as-built" monitoring tunnels.