SITE-SCALE SATURATED ZONE TRANSPORT MODEL PREDICTIONS USING ALTERNATE FLOW MODELS AT THE YUCCA MOUNTAIN SITE

Alternate flow models have been developed resulting from a revised hydrogeologic framework model, additional water-level data, revised recharge distribution, updated boundary fluxes, additional permeability data, and further evaluation of the flow models. The resulting specific discharge from the alternate flow models (AFMs) is in the range of 0.51 to 0.653 m/year, significantly lower than the value of 1.3 m/year for the base case flow model (BFM). The flow paths resulting from the AFMs are generally deeper and more southerly than those from the BFM. The fraction of each flow path in volcanics versus the alluvium for the AFMs is greater than that for the BFM. This is important because in this transport model the volcanics are represented as a dual-porosity medium with low effective porosities (and resultant high fluid flow velocities), significant matrix diffusion, and retardation in the matrix, while the alluvium is represented by a single porosity medium with much higher effective porosities (with resultant lower fluid flow velocities), and retardation but no matrix diffusion. Colloid-facilitated transport is important in both media. For conservative radionuclides, the median transport time (time required for 50% of the input mass to breakthrough at the boundary to the accessible environment at 18 km) for the AFMs ranges from 6500 to 9100 years, as compared to the value of 705 years for the BFM. Thus the AFMs lead to significantly slower transport of the radionuclides as compared to BFM. This shows that the form of the flow model is an important source of uncertainty in transport predictions. Comparison of AFMs with and without matrix diffusion in the volcanics shows that matrix diffusion leads to significant retardation of the radionuclides, and this mechanism is more pronounced at the lower velocities encountered in AFMs. The median transport time for a moderately sorbing radionuclide such as Np using an AFM is about 75,000 years, considerably greater than the corresponding value of about 26,000 years for BFM. In conclusion, the AFMs lead to median transport times significantly longer than that for the BFM.