Paleoclimatic controls on natural tracer profiles in biogenic sedimentary formations of the Horonobe area, Japan

The experience gained in modeling the evolution, from past to present, of natural tracer profiles in geologic media can help support safety assessments of disposal concepts for radioactive wastes in deep geologic repositories because the assessments will be based on predictions using similar models of conditions that could evolve, from the present to the future, in the repository's near-field and far-field environments. Solute-transport models were developed in the present study using a forward modeling approach constrained by boundary conditions inferred from the paleo-hydrogeological evolution of the Horonobe area in Hokkaido, Japan and ranges in model parameter values determined in laboratory and field-based investigations at two deep borehole locations in the area. The models were calibrated by adjusting individual parameter values within limits imposed by the site-characterization data to optimize agreement between model predictions and observations. Results from the calibrated models were consistent with results from other studies suggesting that observed variations with sampling depth in Cl concentrations and 818O and 8D values resulted primarily from an advective transport regime that existed for a relatively brief period during the most recent glacial-interglacial cycle. Advective transport was possible during this time because thawing of a discontinuous permafrost zone starting about 18 ka allowed meteoric water to recharge the flow system and hydraulic gradients were larger than at present because sea levels were significantly lower than the current levels reached at the beginning of the Holocene (i.e., since about 12 ka). Similar climatic controls on the generation and duration of advective/diffusive transport regimes likely existed during nine previous glacial-interglacial cycles that occurred in the Horonobe area since regional uplift began about 1 Ma. If so, alternative versions of the transport models suggest the tracer profiles observed in this area may have evolved over a much longer cumulative period of advective transport and at lower Darcy velocities than assumed in the calibrated models but in a manner that is still compatible with ranges in model parameter values determined in laboratory and field investigations. Apparent differences in transport behavior at the two borehole locations considered in this study, which were situated only about 1 km apart, appear to have resulted from relatively small differences in accessible porosity and hydraulic conductivity, which in turn may have been controlled by local differences in fracture density and fracture connectivity.