Abstract

Traditional discrete fracture models implementing matrix diffusion can be computationally expensive and only applicable to simplified transport problems. Upscaling to a continuum model can reduce computational burden, but models based on only a primary continuum neglect fracture-matrix interaction. PFLOTRAN, a subsurface flow and reactive transport code, simulates a secondary continuum (matrix) coupled to the primary continuum (fracture) modeled as a disconnected one-dimensional domain using a method known as the Dual Continuum Disconnected Matrix (DCDM) model. This work presents several benchmarks to compare PFLOTRAN’s DCDM model to analytical solutions and a large-scale test problem in a one cubic km fractured domain modeling a conservative tracer with diffusion of the tracer into the rock matrix. The tracer was modeled using two different methods: first, with a Discrete Fracture Network (DFN) representation, and second, using the DCDM in PFLOTRAN. We find that the DCDM representation of the upscaled fracture network produces results comparable to the DFN and analytical solutions where available, verifying this method. We then apply the DCDM model to a fractured domain considering radionuclide isotope sorption, partitioning, decay, and ingrowth and find that radionuclide retardation is enhanced when considering these additional mechanisms.

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