Use of CTRW for Prediction of Radionuclide Migration in Fractured Tuff

Abstract:

Non-local contaminant transport methods have been extensively studied as an alternative for the classical Advection Dispersion Equation (ADE) to model particle migration in heterogeneous media and in regions with geologic patterns that shape secondary porosity. The challenges encountered in fractured media are usually more complex than un-fractured porous media because of the irregular connectivity patterns between individual fractures, large number of parameters, and wide distribution of parameter space. The Continuous Random Time Walk (CTRW) methodology provides a framework for modeling non-Fickian transport through fracture networks by employing probabilistic distributions to generate particle jump lengths and residence time spanning over orders of magnitude. We apply CTRW framework to model transport of radionuclides in the fractured volcanic tuff of Western Pahute Mesa located at the Nevada National Security Site (NNSS). By analyzing borehole data recorded at the NNSS, statistical attributes of fracture parameters are derived that are used to generate discrete fracture network (DFN) realizations. Through convolution of both particle travel time and fracture length distribution, transport is modeled on a continuum of spatial scales via the CTRW technique and the predictions are compared against DFN results to ascertain the efficacy of upscaling.