Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Geophysics

Supervisor

Robert Schincariol

Abstract

Potential thermal impacts from below-water-table aggregate extraction on a cool-water stream were investigated by monitoring thermal plumes, moving through an unconfined glacial-outwash aquifer, and assessing their subsurface persistence. The growing demand for aggregate and increased pressure to pursue extraction in ecologically sensitive areas has driven the need for this work. During a 10-year period, ground and surface water temperatures were measured monthly, including two periods of intensive monitoring (22 months and 2.5 years). The aquifer hydraulic conductivity (K) is quantified at the laboratory and field scale. The mean K’s from the multi-scale tests depend on test-support volume and span two-orders of magnitude, 1.8×10–4 to 1.7×10–2 m s–1. The effective thermal conductivity λ is characterized at an unprecedented level of detail by: (i) measuring the thermal conductivity of the soil solids, ls using the steady-state divided-bar apparatus and estimating conductivity from mineral composition; (ii) measuring the volumetric water content and porosity using cross-hole ground-penetrating radar; (iii) evaluating four models used to predict the apparent thermal conductivity, l, of variably saturated soils (iv) calculating the l field on a 0.25-m square cell grid using measured data and the selected model, and (v) simulating thermal transport within the two-dimensional domain using a finite-element numerical model. The apparent thermal conductivity in the saturated aquifer ranges from 2.14 to 2.69 W m-1 K-1 with a mean of 2.42 W m-1 K-1. These measurement and model methods may be used at other sites to construct thermal conductivity distributions for similar glacial soils. The annual temperature amplitude in the pit is 10ºC greater than the up gradient groundwater, resulting in alternating warm and cool plumes that persist in the aquifer for 11-months and migrate up to 250 m down gradient. The observed plume velocity (1.2 m d–1) lags the groundwater velocity (2.8 m d–1) due to thermal retardation. Using field data a conceptual model is developed, and implemented in a three-dimensional finite-element numerical model. While this work focused on plume migration, these results demonstrate that assessing impacts on the aquatic community requires an integrated, multi-disciplinary study. This work can guide such assessments.


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