Formatted Title
A Novel Approach for Mapping Hydraulic Connectivity in Fractured Bedrock Aquifers
Background/Objectives
Groundwater flow and contaminant transport in bedrock predominantly occurs in hydraulically active and connected fractures. Successful remediation of contaminated fractured bedrock aquifers requires an understanding of the connectivity and hydraulic properties (e.g., hydraulic conductivity, storage, and hydraulic diffusivity) of bedrock fractures. Many groundwater remediation technologies require direct contact between the remedial additives (e.g., amendments and reactants) and the contamination in fractures. Most groundwater remediation designs for fractured bedrock aquifers fail because interconnectivity of hydraulically active fractures observed in vertical boreholes (i.e., intra-borehole connectivity) is not evaluated. Evaluation of the interconnectivity of hydraulically active fractures is crucial to achieve the necessary contact between remedial additives and the contamination in bedrock fractures. Traditional approaches for evaluating hydraulic interconnectivity in fractured bedrock aquifers (e.g., pumping tests, pulse interference tests, and packer tests) are not expeditious and expensive. We have developed an approach for mapping hydraulic connectivity in fractured bedrock during the borehole drilling phase of bedrock characterization as an alternative to the traditional approaches. This talk will discuss successful application of this method at four chlorinated solvent sites overlying crystalline, metamorphic, and sedimentary fractured bedrock.
Approach/Activities
Surface geophysics should be completed, where possible, to identify water-bearing fractures or fracture zones to define drilling locations for bedrock boreholes. While advancing the drill stem using air rotary or diamond coring drilling technologies, a pressure pulse is generated upon intersecting a hydraulically active fracture. Pressure transducers are placed in existing bedrock monitoring wells to record the arrival of the pressure pulses generated by the drilling activities. Completed bedrock boreholes are tested using geophysical, hydrophysical, and hydraulic logging tools to define the depths and orientations of hydraulically active fractures. The arrival time of the pressure pulse(s) (i.e., propagation of change(s) in hydraulic head) observed in the pressure transducer records is compared to depths of hydraulically active fractures interpreted with downhole geophysical logging tools. Monitoring wells are constructed in the recently drilled borehole(s) to target individual fractures or fracture zones for future pressure pulse mapping. The pressure pulse data collected with this approach are compared to data collected with traditional aquifer characterization methods to evaluate the validity of this approach.
Results/Lessons Learned
Our data show that the arrival time of the pressure pulse is indicative of the hydraulic diffusivity of the connected fracture network intersecting the drilled borehole and the monitored well network. We will show borehole geophysical logging and FLUTe™ Transmissivity Profiling data that confirm the presence of interconnected and hydraulically active fractures intersecting the drilled boreholes. We will show how the results from this method are used to map the three-dimensional hydraulic connectivity of the fracture network. We will show results from this method compared to results from traditional pumping tests conducted in the same fracture network mapped using this approach. Those results indicated similar hydraulic behavior, which verified the effectiveness of this method. We will show examples of how hydraulic connectivity can be quantified. We will show how the hydraulic connectivity data are used to optimize remedial strategy. Lastly, we will discuss important considerations for method implementation and limitations.