Improving crystalline bedrock aquifer conceptual models using novel discrete fracture network methods

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Fractured bedrock aquifers are widely used for domestic and municipal water supply in Canada and internationally. Understanding fracturing in bedrock formations is also critical to assessing deep geological repositories and petroleum exploitation. Where only thin (or no) glacial deposits cover rock formations, shallow crystalline bedrock aquifers can be vulnerable to contamination from human activities. Fractures are primary conduits for contaminant transport. Crystalline fractured bedrock aquifers underlie much of central and eastern Canada (e.g., the Canadian Shield) and eastern, north-central and northwestern United States. These aquifers have low rock matrix porosity and thus low capacity for contaminant diffusion. Water and contaminants travel predominantly through rock fractures at often high groundwater velocities and thus there is potential for rapid transport rates. In order to help protect these aquifers for water supply, robust methods are required to improve the hydraulic characterization and understanding of crystalline aquifers. The systematic Discrete Fracture Network (DFN) Approach (Parker, 2007) has been widely used by researchers in the G360 Centre for Applied Groundwater Research group to characterize sedimentary bedrock aquifers for flow and contaminant transport applications. The objective of the proposed research program is to apply several novel pillars of this DFN Approach in order to: 1) use the methods for the first time in the crystalline bedrock of the Canadian Shield as opposed to sedimentary bedrock (e.g., dolostone); and 2) compare new results to previous hydraulic characterization research conducted using different methods (i.e., straddle packer slug tests, pumping tests and down hole video-logging) and to contaminant transport studies conducted at the field sites. The results will be applied to improve conceptual models for flow and contaminant transport for shallow fractured crystalline bedrock aquifers. This research will include specifically: 1) depth-discrete measurements of permeability and flow using specialized flexible and impermeable borehole liners (FLUTe™); 2) high resolution temperature profiling for identifying hydraulically active fractures; 3) novel cross-hole tests using pressure, heat and dye as tracers coupled with the liners, interpreted via numerical modeling; and 4) post hydraulic characterization water quality (nitrate and turbidity) monitoring using in situ probes and liners. This research economizes by using existing extensive well infrastructure drilled into the Canadian Shield at the Tay River Field Site (Levison et al., 2012) and Kennedy Field Station (Elmhirst and Novakowski, 2012). About 20% of the land surface comprises crystalline shield rock (Gustafson and Krásný, 1994). A thorough understanding, using multiple lines of evidence, of fracture parameters including apertures, transmissivity, spacing, and connectivity are required for all flow, contaminant transport and remediation efforts, with applications in water resources engineering, hydrogeology and natural resource exploitation. Novel methods of the G360 DFN Approach will be applied in a research environment with the opportunity to compare results to previous characterization efforts and field experiments. Also, a unique in situ nitrate probe and fluorometer will be tested with lined boreholes. This research will contribute to important applications for water supply and contaminant migration, including source protection, and provide consultants and researchers effective, cost efficient tools for rapidly characterizing crystalline aquifers. The research will be carried out by doctoral, Master’s and undergraduate students who will gain valuable training applicable to work in industry or continued research.