Groundwater in Fractured Rocks

IAH Selected Paper Series, volume 9

Jirí Krásný, John M. Sharp

• Publisher: CRC Press
• Year: 2007
• Print ISBN:
  9780415414425
• eBook ISBN:
  9781134122189
• Topic: Biological Sciences
• Subtopic: Environmental Science
• Index: Biological Sciences

The hydrogeologic environment of fractured rocks represents vital natural systems, examples of which occur on every continent. This book discusses key issues, methodologies and techniques in the hydrogeology of fractured rocks, summarizing recent progress and anticipating the outcome of future investigations. Forty-four revised and updated papers w

Numerical modelling of fractured environment

CHAPTER 37
A double continuum approach for determining contaminant transport in fractured porous media

Matthias Beyer and Ulf Mohrlok

Institute for Hydromechanics, University of Karlsruhe (TH), Kaiserstr. Karlsruhe, Germany

ABSTRACT: To predict the plume development in fracture-matrix systems without the detailed knowledge of the fracture network geometries, a double continuum approach is chosen. The complex transport behaviour can be adequately represented by two overlapping and interacting continua. An existing double continuum approach has been further developed for steady-state flow conditions by focusing on mass exchange terms between the fractures and the matrix system. This approach is implemented in the “Double Porosity MT3D” program and has been applied to a transport problem in a fractured sandstone formation. The parameter determination for the fracture continuum is the most important step to calibrate the double continuum model and to predict the plume development. Of particular interest was the determination of parameters by fracture network characteristics. The transverse dispersion coefficient and the hydraulic conductivity are directly dependent on the angles between the fracture directions and the hydraulic gradient. The larger the angle, the higher is the transverse dispersion coefficient and the lower will be the resulting effective hydraulic conductivity.

1 INTRODUCTION

The determination of plume development in fractured porous media is very complicated due to their extremely complex structure. Large fractures provide preferential pathways for regional fluid movement leading to a fast contaminant transport. The small fractures as well as the rock matrix are relevant in terms of storage and retardation, as their permeability values are some orders of magnitude smaller. The interactions between these parts of the system are characterized by specific exchange processes. The main exchange processes are given by matrix diffusion as a result of concentration differences between fractures and matrix, local advection due to pressure gradients between fractures and matrix, and regional advection due to a regional pressure gradient.

To describe flow and transport processes in fractured porous media, several model approaches are available depending on the scale of application and the geological information available (Figure 1). The simplest approach, single continuum representation, can be used for macroscale studies if a rough approximation is sufficient without representing the complex structure of the system.

A much better representation of the complex fracture system can be achieved by applying a discrete or coupled discrete model. Unfortunately, the investigation effort is very high due to the fact that the exact geometry and parameters of all individual fractures have to be determined. For this reason the discrete approaches are limited to local scale studies of well known investigation areas. Large scale applications require stochastic fracture network generations. Due to many uncertainties, the achieved results have to be averaged by Monte Carlo simulations resulting in a huge computational effort. A combination of discrete and continuum approaches can be used for investigations, where a limited number of known fractures dominate the system.

Figure 1. Different model representations for fractured aquifers (after Teutsch et al., 1991).

If a fractured porous medium is to be described without considering the exact fracture network geometries, a double continuum approach, based on averaged fracture-matrix parameters, should be used. The fractured porous medium is described by two overlapping and interacting continua. The first continuum represents the large fracture system, and the second continuum represents the small fracture system or the matrix. These two continua possess different flow, transport and storage parameters and are connected by the respective exchange terms. Detailed knowledge of the fracture network geometries are not required, but the complex transport behaviour can be adequately represented. In general two different types of double continuum approaches are known. Within the dual porosity approach only one continuum contributes to the regional flow, whereas the second continuum is a fluid and solute storage. A dual permeability approach consists of two continua which both contribute to the regional flow.

2 DP-MODFLOW – FLOW MODEL

Within a double continuum model the fracture system (f ) as well as the matrix system (m) are represented as separated continua. The groundwater flow of each continuum is described by Darcy’s law, which is given by

where Q is the flow rate through the cross sectional area A, K is the hydraulic conductivity of the medium and dh/dl is the hydrauli...

Table of contents

• Cover Page
• Title Page
• Copyright Page
• Preface
• About the editors
• Hydrogeology of fractured rocks from particular fractures to regional approaches: State-of-the-art and future challenges
• Hydrogeological environment of fractured rocks
• Conceptual models, groundwater flow and resources in fractured rocks
• Groundwater quality in fractured rocks
• Investigation and interpretation methods in fractured environment
• Anthropogenic impacts on fractured environment
• Numerical modelling of fractured environment