Discrete-depth measurements of natural gradient groundwater flow in fractures using borehole tracer methods

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Maldaner, Carlos Henrique
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University of Guelph

This thesis aims to quantify groundwater flow under ambient conditions in depth-discrete fractures using measurements in bedrock boreholes. The first method applied, known as the point dilution method, is well established and is performed in this study to serve as a benchmark for values of groundwater velocities in the study area. To improve on standard point dilution testing, a packer system was adapted to efficiently perform four types of hydraulic tests and tracer dilution tests in short intervals without moving the packers. This combined method is performed in multiple depth intervals of a fractured dolostone corehole (GDC-5) located in Guelph, Ontario, Canada, allowing estimation of groundwater velocities assuming possible numbers of hydraulically active fractures. The second method, is a new technique proposed in this thesis which uses heat as a tracer to quantify flow in fractures. Heat is added at a constant input along a composite fiber optic cable installed behind an inflated flexible and impermeable borehole liner, pushing the cable against the entire length of the borehole wall, and avoiding cross-connected flow inside the borehole. The temperature along the borehole wall is measured continuously along the cable using fiber-optic distributed temperature sensing. Heat transfer is enhanced where groundwater flow occurs in hydraulically active fractures and it is estimated from the measured effective thermal conductivity. Volumetric groundwater flow is estimated for the first time using the active DTS method in sealed boreholes assuming a single fracture per interval which can result in unreasonable high values. The method is advanced by considering uncertainties including multiple fractures and their spacing per interval, influences of the cable position in the borehole relative to the groundwater flow direction, and heat input (duration) during the test. This thesis shows for the first time that active DTS in sealed boreholes reliably estimates groundwater flow in bedrock fracture zones with substantial advantages in time and cost efficiencies over other methods, especially removal of borehole short-circuiting effects. This methodology may change the abilities of the groundwater profession to understand groundwater and contaminant travel times. This thesis is organized into three independent manuscripts prepared for journal publication, plus overall introductory and conclusion chapters.

fractured rock, average linear groundwater velocity, hydraulic tests, fracture aperture, point dilution method, straddle packers, tracer dilution test, volumetric groundwater flow rate, dolostone, groundwater, active distributed temperature sensing, rock thermal conductivity, natural gradient, thermal response tests, sealed boreholes, DTS, bedrock aquifer