Stochastic Analysis of Groundwater Flow and Contaminant Transport in a fractured Rock System
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STOCHASTIC ANALYSIS OF GROUNDWATER FLOW AND CONTAMINANT TRANSPORT IN A FRACTURED ROCK SYSTEM F.W. SCHWARTZ', L. SMITH 2 and A.S. CROWE' 'Dept. of Geology, Univ. of Alberta, Edmonton, Canada 2Dept. of Geological Sciences, Univ. of British Columbia, Vancouver, Canada ABSTRACT It has been possible to develop a stochastic model for groundwater flow and mass transport in a fractured rock system. A large number of statistically independent realizations of a fracture network are generated from a set of probability distributions for parameters defining the fracture geometry. By solving for hydraulic head at the fracture intersections and using data on apertures and porosities, seepage velocities may be calculated and the transport equation solved for each trial of a Monte Carlo simulation. Model output consists of distributions of moving particles and various mass exit times. Applications illustrate the types of model results and the skewed character of particle distributions. I1TRODUCTION Very little work has been carried out in modeling mass transport in networks of discrete fractures in rocks. The most important existing work is that of Castillo et al. [1, 2] and Krizek et al. [3], who discuss the results of simulation and experimental studies designed to identify how features of the fracture geometry control the spread of contaminants. However, because their models involved an oversimplified geometry, with individual fractures that were either continuous through the region or unrealistically long, the applicability of these results is somewhat limited [4]. In most natural settings, fractures are not continuous within their own planes, and the continuity of the flow system usually develops as a result of the interconnection of fracture sets. The main objective of this paper is to describe a modeling concept for simulating mass transport in a discrete fracture network. The fracture networks we consider are much more general than those constructed by Castillo et al. [1, 2]. In addition, individual fractures are not continuous within their own plane. Another very important feature of this model is its ability to function as a stochastic simulator, providing estimates of the probability distributions on model output, given the uncertainty in specifying the fracture geometry. The results of simulations presented in this paper are designed to show how this modeling approach can assist in describing the character of dispersion in fractured media and in understanding the role of fracture geometry in controlling transport. DESCRIPTION OF THE MODEL Groundwater flow and mass transport are simulated in a rectangular flow domain with the top and bottom boundaries assumed to have no flow with respect to both water and mass (see Fig. la). Constant head values are assigned to the vertical side boundaries to develop a steady-state, two-
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458 dimensional system of flow in the fractures. It is assumed that the rock matrix is impermeable and has zero porosity. This assumption implies that all water and mass f
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