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The Discrete Fracture Network (DFN) approach for contaminated bedrock site characterization

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A comprehensive methodology known as the Discrete Fracture Network (DFN) Approach has been developed since the mid-1990’s as a framework for characterization of contaminated sites on fractured rock. This article provides an overview of the Approach and is followed by eight additional articles that describe the various methods that constitute this Approach. The ninth article also describes the general nature of contaminated sites on fractured sedimentary bedrock where the Approach is being comprehensively applied. The goal of this Approach, developed by a research team led by Dr. Beth Parker at the University of Guelph, Canada, is appropriate data acquisition followed by evaluation of the data for the improved understanding needed for science-based decision making for site characterization, risk assessment, and remediation. This field approach was developed to take advantage of the capability of powerful numerical models developed earlier in the 1980’s-1990’s for simulation of groundwater flow and contaminant transport and fate in rock with fracture networks and porous, permeable rock matrix blocks between fractures. These models had not been used to represent real-site conditions due to the lack of suitable field data (i.e., lack of measured input parameter values). The field information and the DFN numerical models are used to advance site conceptual models (SCMs) that are the essential basis for contaminated site decision-making.

This Approach is based on the premise that the characterization of contaminated sites and SCM development should be separate from and a prerequisite to long-term monitoring. In the DFN Approach, either an initial SCM or multiple SCMs (e.g., multiple hypotheses) guide data collection for the site. As application of the Approach progresses, the number of hypotheses is reduced and the SCM is advanced in an iterative manner, as key questions are addressed to an appropriate degree of refinement and reliability.

The DFN Approach for contaminated bedrock is based on the recognition that contaminant mass occurs in both the fractures and the porous blocks of rock existing between the fractures. Most contaminant mass commonly resides in the rock matrix, due to molecular diffusion and sorption, but most groundwater flow, and hence contaminant transport, occurs in the fractures. Therefore, emphasis in this Approach is directed at acquisition of data from both the fractures and the rock matrix. To date, the DFN Approach has only been comprehensively applied to sites on sedimentary bedrock where the rock matrix has porosity in the range of 4-20%, such that diffusion in the matrix is a strong process influencing contaminant behaviour.

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