High resolution multi-level monitoring for bedrock aquifers

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Source: Solinst Canada Ltd.

This presentation describes a comprehensive approach for determining the nature, extent, transport and fate of contaminants at industrial sites on fractured rock, with emphasis on sedimentary rock, referred to as the Discrete Fracture Network (DFN) approach. Development of this DFN approach began at a sandstone site in California in 1997, where the initial version of a method for measuring volatile organics (e.g. TCE and degradation products) in rock core was applied. Rock core contaminant analyses at this site, and at many other sedimentary rock sites where more advanced versions of the method have been applied, shows that at aged sites, nearly all of the contaminant mass now resides as dissolved and sorbed phase in the low permeability matrix blocks between fractures due to diffusion. Noteworthy is the fact that groundwater samples reflect contaminant concentrations in the advective fractures that are in strong disequilibrium with the matrix porewater. Therefore, the rock core analysis method is the essential means for determining the nature and extent of contamination and evaluating the long term fate. However, to understand the contaminant distribution and model the transport and fate, it is necessary to quantify various processes including groundwater flow in the fracture network and diffusion, sorption, and the influence of degradation in the matrix. Therefore, many other types of measurements are made on rock core samples and in the core holes as indicated in the chart shown in Figure 1. The core hole measurements include conventional open hole geophysics but rely primarily on new measurement types developed through collaborations, including new approaches for high precision straddle‐packer testing, FLUTe™ hydraulic conductivity profiling, high resolution temperature profiling inside lined holes, and comprehensive use of commercial multi‐level monitoring systems. The DFN method has been applied at sandstone and shale sites in the United States and is also being applied at contaminated sites in the Silurian dolostone belt of Ontario (Cambridge and Guelph) and also in uncontaminated areas of Guelph, where understanding of the dolostone aquifer is needed to support source water protection. This presentation shows examples of how the DFN method is being adapted to suit the local borehole conditions. Major components of this DFN approach are relevant to any fractured rock hydrogeology problem, and allow one to capitalize on the strong capabilities of existing DFN numerical models.

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