Validation and testing of the VAM2D computer code

Abstract

This document describes two modeling studies conducted by HydroGeoLogic, Inc. for the U.S. NRC under contract no. NRC-04-89-O90, entitled, "Validation and Testing of the VAM2D Computer Code." VAM2D is a two-dimensional, variably saturated flow and transport code, with applications for performance assessment of nuclear waste disposal. The computer code itself is documented in a separate NUREG document (NUREG/CR-5352, 1989). The studies prsented in this report involve application of the VAM2D code to two diverse subsurface modeling problems. The first one involves modeling of infiltration and redistribution of water and solutes in an initially dry, heterogeneous field soil. This application involves detailed modeling over a relatively short, 9-month time period. The second problem pertains to the application of VAM2D to the modeling of a waste disposal facility in a fractured clay, over much larger space and time scales and with particular emphasis on the applicability and reliability of using the equivalent porous medium approach for simulating flow and transport in fractured geologic media. Reflecting the separate and distinct nature of the two problems studied, this report is organized in two separate parts. iii TABLE OF COINTENTS Section

On the effective continuum method for modeling multiphase flow, multicomponent transport, and heat transfer in fractured rock

“…Once fluid saturations, S,,and S , , , in fracture and matrix system are determined, separately, the effective relative permeabilities for the ECM calculations are evaluated using Equation (8) For a 2-D transport in a regular, two-orthogonal, parallel fractures system, Kool and Wu (1991) provide an alternative, approximate correlation for effective dispersivities as, for effective longitudinal dispersivity along the x-direction and for effective transverse dispersivity to the x-direction. These two dispersivities were derived using a two-region type model of transport in an aggregated soil (van Genuchten and Dalton, …”

Comparing Discrete Fracture and Continuum Models to Predict Contaminant Transport in Fractured Porous Media

“…In contrast, when the EPM model is applied, an equivalent dispersivity incorporates the effects of both the porous matrix and the discrete fractures. The expression used to evaluate this equivalent longitudinal dispersivity is (Kool and Wu ) …”

“…The determination of the effective dispersivities is one of the most critical steps in the application of the EPM approach. The velocity contrast between porous rock matrix and fractures is treated as an apparent dispersion process, which is reflected by large dispersivity values in computer simulations (Kool and Wu ). Thus, the EPM model may not appropriately capture the fracture/matrix interactions and the mechanical dispersion transport mechanism in the simulations presented here because it only considers diffusion in the immobile region.…”

Performance assessment for the disposal of low-level waste in the 200 West Area Burial Grounds

“…The results from independent v e r i f i c a t i o n and benchmark t e s t i n g of an e a r l i e r version of the VAM3D-CG code were reported by Huyakorn and Panday (1993). 2D version (VAM2D) of VAM3D-CG was validated by Kool and Wu (1991) (Table 6 2 The PRP comnends the PA preparers on the extent of the draft PA.…”