Characterization, Modeling, Monitoring and Remediation of Fractured Rock

Characterization, Modeling, Monitoring and Remediation of Fractured RockThe Royal Society of Canada is pleased to announce the publication of a new Expert Panel report from its sister academy, the National Academies in the United States. The full report can be accessed online. Leslie Smith

Dr. Leslie Smith, FRSC, has kindly provided a Canadian perspective on this report and its relevance to Canada and Canadians. Dr. Smith is a professor of hydrogeology in the Department of Earth, Ocean and Atmospheric Sciences at the University of British Columbia, where he holds the Cominco Chair in Minerals and the Environment. His research program includes studies of hydrologic processes in mine waste materials, surface water - groundwater interactions, and prediction of solute transport in fractured rock masses. He also serves as a consultant for private agencies and government, providing peer review of project work at mine sites and hazardous waste management facilities.

Characterization, Modeling, Monitoring and Remediation of Fractured Rock – A Canadian Perspective

There are innumerable sites in Canada where the migration of contaminants through the subsurface is controlled by the occurrence of open fractures in bedrock. The characterization of groundwater flow through fractured rock, the analysis of solute transport along connected fracture pathways and contaminant transfer between fractures and the adjacent intact rock is inherently difficult, posing many challenges. Length scales of fractures span many orders of magnitude; from small discontinuous cracks to fault zones that may be tens of metres in width and extend laterally from many kilometers. Some fractures are open; many are closed or infilled with mineral precipitates. Some fault zones act as large-scale preferential pathways for groundwater flow; others are infilled with clay-like materials that cause the fault to act as a barrier to groundwater flow. Predictions of contaminant migration in fractured rock systems are subject to significant uncertainty.

The Academy study on fluid flow and solute transport in fractured rock systems was supported by the U.S. Nuclear Regulatory Commission, the U.S. Department of Energy, and NASA. The report provides an excellent overview of the current state of practice and new innovation in the characterization of fractures rock systems, and suggests information requirements for effective management and remediation of sites with contaminated groundwater. The nature of fractured rock environments in the United States and impacts on groundwater quality due to past industrial practice are no different than the conditions we face here in Canada; the learnings discussed in this report and the recommendations provided by the Committee are directly relevant in the Canadian context.

The mandate of the committee was to identify areas where research and development could improve the current state of the art in subsurface characterization, modeling, monitoring and remediation; where incorporation of scientific and technical advances could improve the current state of the art; and where enhanced science-based understanding could inform federal regulations, policies and guidance. The first two themes underpin the third theme, which reflects the goal that government regulation recognizes the reality of the challenges faced in the management and remediation of contaminated sites where fractured bedrock is present in the subsurface.

The report reinforces the long-held view that a broad-based and integrated approach is required at fractured rock sites; incorporating geologic, geophysical, geomechanical, hydrologic and biogeochemical concepts and tools to build a conceptual framework for identifying transport pathways, controlling processes, and fracture – rock matrix interactions. The key role of storage of solutes in the rock matrix bounding fractures is given great weight in the Committee’s assessment of requirements for reliable and cost-effective management of contaminated sites. The Committee identified better understanding of microbiological processes occurring in a fractured rock setting as an important research opportunity because the subsurface microbial community can be a rate-determining factor in the time required for remediation of a broad class of contaminants. It is noted that while considerable progress has been achieved in the development of site characterization methods and tools over the past twenty years, additional research is needed to bridge the gap between field measurements made at the borehole scale, cross-borehole measurements, and the larger scale required for site-wide assessments, which can extend over hundreds of metres.

Groundwater hydrologists use mathematical models of fluid flow and solute transport as a principal means of interpreting site data, updating the conceptual model that underpins the numerical model, and to predict the fate and transport of contaminants. In Canada, as elsewhere, there is sometimes a contentious debate on the type of model that should be used for site assessments and prediction of future impacts. Some models treat the rock mass as equivalent to a granular porous medium; some models include an interacting fracture domain and a second domain representing the rock matrix; while other models incorporate an explicit representation of the fractures in the model structure. As our modeling tools have become more powerful, it is sometimes (perhaps often) the case that more is expected in terms of reliable model prediction than can reasonably be provided, especially in the absence of field measurements that can be used to calibrate a solute transport model. The Committee addresses these issues in their report, providing valuable recommendations on best-practice approaches to model selection and model construction that recognize the challenges in modeling complex systems typical of fractured rock settings.

The Academy committee also calls for better recognition by regulators of the time scales involved in the remediation of contaminated sites underlain by fractured media, which can require decades, with projections at some sites extending to centuries. It is important that the general public also recognize the time requirements typically associated with site remediation to better inform their view of risks associated with groundwater contamination. Time scales that are driven by the presence of contaminants entering matrix blocks between the fracture pathways must be considered in setting realistic cleanup goals. This view reflects experience in the United States, but parallel conditions are encountered in Canada. The Committee concludes with the optimistic view that with interdisciplinary and adaptive approaches to site characterization and management, it is possible to analyze a fractured rock environment with acceptable levels of uncertainty and reliability.

The focus of the Academy report is principally on industrial sites, and sites requiring active remediation to restore groundwater quality. In Canada, in addition to these cases, the findings of this study are also relevant in other sectors of our economy, such as the mining industry. In many parts of Canada mines occur in remote areas where bedrock occurs at shallow depths. As part of an environmental impact assessment for a proposed mining operation, a proponent is required to assess seepage from their proposed waste disposal facilities (tailings, waste rock piles) to the surrounding environment, including nearby streams and lakes. Often, the scale of these facilities is larger than an industrial site and the footprint of the area that must be examined is considerably larger. Interest here focuses as much on estimating seepage volumes and flow directions as it does on predicting travel times and solute concentrations in assessing potential solute loads to receiving waters. Reliable prediction of seepage volumes requires sound estimates of the bulk hydraulic conductivity of the rock mass. Identification of preferential flow paths through fracture zones or connected fracture paths; or the determination that this type of pathway does not exist in the vicinity of the proposed disposal site; is often an issue of debate in the review of environmental impact assessments. The large scale of these operations, sometimes covering areas of several km2 to tens of km2, and the typical financial resources available to carry out site characterization and data interpretation, suggest that while the Academy report provides a good starting point, there is merit in promoting a review of current approaches and research needs for project developments with these larger environmental footprints. Similar questions will arise when Canada moves forward with the evaluation of potential sites for geologic disposal of its high-level nuclear wastes.