Incorporation of Geostatistics into Flow and Transport Simulations

The zeolitic threshold method used in previous chapters is replaced with more rigorous geostatistical modeling. With this more sophisticated modeling approach, we examine whether using a threshold cutoff for zeolitic abundance is an appropriate upscaling method for characterizing small scale heterogeneity. Geostatistical techniques are used to develop variograms, kriging maps, and conditional simulations of zeolitic abundance. The kriging maps are used to estimate the uncertainty in the mineralogical model. The conditional simulations result in equally probable maps of zeolitic abundance. Using the algorithms discussed in Chapters 2 and 3, a conditional simulation of zeolitic abundance was mapped on top of the geologic model. We investigate the relationship between percent alteration, permeability changes due to alteration, sorption due to alteration, and their overall effect on radionuclide transport. A key conclusion to the study is that the retardation due to sorption predicted by a conditional simulation is much larger than the retardation predicted by the zeolite threshold method. The reason for larger retardation of 237Np for the conditional simulation is a small but significant Kd at locations with zeolite abundance less than 10%. For the purposes of abstracting these results, setting nonzero Kd’s in the vitric tuffs may be an effective way to mimic the more rigorous conditional simulations. Rocks with high zeolitic abundance may not be as effective in retarding radionuclides such as 237Np and Uranium since these rocks also in general have low permeability and contaminants can only enter these regions through molecular diffusion. In addition, any fractures in these highly zeolitized rocks will provide a mechanism for fluid and radionuclides to bypass these regions. In future studies, multiple conditional simulations will be performed using linear coregionalization models to strengthen the conclusions drawn in this study.