Conclusions Conclusions

The research presented in this dissertation makes original contributions to understanding of the movement of thermal plumes from aggregate pits and to assessing their potential impacts on nearby cool- and cold-water streams. The contributions are:

1) Chapter 1 presents the results of a detailed field investigation. These field results provide evidence of the movement of thermal plumes and is the first

comprehensive field investigation of thermal plume movement from aggregate pits. The data collected shows that the thermal plumes do persist for distances exceeding 100 m and may persist beyond 200 m. The methods employed in this field investigation provide the basis upon which field studies at other locations may be designed. The linkage between the groundwater and stream habitat are discussed and provide a basis for considering what impacts thermal plumes may have on nearby cool- and cold-water streams. Cross-correlation of the pond temperature signal with the measured groundwater temperature signals down gradient of the pond is used to estimate the plume velocity. Furthermore, the thermal plume velocity is shown to lag the average linear groundwater velocity through the thermal retardation factor estimated to be 2.3 in this aquifer.

2) The aquifer hydraulic conductivity is measured at several scales and found the laboratory-measured hydraulic conductivities are up to two-orders of magnitude smaller than field-measured hydraulic conductivities. While field measured values provide better estimates for predicting thermal plume velocities care must be taken in selecting which field estimates are appropriate as estimates obtained from a pumping tests are over a factor of 2 larger than estimates obtained from

observing the plume velocity and values used in the calibrated groundwater flow and transport model.

3) Chapter 2 presents a method for constructing the two-dimensional thermal conductivity field for a glaciofluvial outwash deposit. The method couples field and laboratory methods to determine the bulk thermal conductivity of the aquifer solids, the volumetric water content, and the porosity of the aquifer with an approximating model for predicting the apparent thermal conductivity of variably- saturated soils. The Campbell model is shown to be the best-approximating model using the information-theoretic approach. The measured thermal conductivity values for the aquifer solids provide a dataset upon which to estimate the apparent thermal conductivity of similar porous media. Porosity was shown to strongly influence the thermal conductivity, indicating that in conduction dominated systems this parameter must be defined carefully.

4) The Campbell model is implemented into a finite-element density-dependent groundwater flow and thermal transport numerical model. Using the measured two-dimensional thermal conductivity field and the numerical model, we demonstrated that heterogeneous λ fields increase the thermal dispersion analogous to solute transport.

5) A three-dimensional conceptual site model is developed in Chapter 3 and implemented in a modified version of Heatflow. Model calibration and

verification demonstrate that thermal plumes from aggregate pits can be modelled successfully. Heatflow was compared to a number of benchmark tests to verify various physical processes were being considered correctly. A standards benchmark test for density-dependent thermal transport models is the Elder problem. New analytical solution to this problem are presented and Heatflow is shown to yield good agreement. This new analytical solution may be used as a benchmark test for other density-dependent thermal transport numerical models.

6) Simulation results indicated that in this outwash aquifer where the groundwater velocity is relatively high, the temperatures in the aquifer within the first 100 m of

the pond are dominated by the convective transport of the temperature

perturbation in the pond. Within this zone the annual temperature variation at the ground surface (largely transported by conduction) is masked by that of the pond. Beyond 100 m the temperature perturbation from the pond is attenuated by thermal retardation and thermal dispersion to the point where the influence of the temperature variation at the ground surface becomes evident in the measured subsurface temperatures. The simulations are in good agreement with the observed subsurface temperatures and support the conclusion that convective groundwater transport from a pond may influence subsurface temperatures well beyond 100 m down gradient.

7) The results from the numerical simulations are in good agreement with the groundwater temperatures measured at this site down gradient of the pit and presented in Chapter 1. The observed temperatures and simulated temperatures indicate that the thermal plume migrates up to 150 m down gradient of the pond. At this site, the groundwater discharges to streams that are well beyond this distance, and thus are not affected by these thermal plumes. Where pits are within 250 m of the stream, these results suggest there is the potential for the thermal plumes to alter the temperature of the groundwater discharging to the stream.

8) Simulation results demonstrate that where large groundwater velocities exist (typical of settings with aggregate extraction) heat transport is dominated by thermal convection and significant effort should be focused on obtaining representative estimates of the groundwater velocity (i.e., large scale hydraulic conductivity, porosity and hydraulic gradient). Groundwater velocity plays a dominant role in the distance of thermal plume transport. While hydraulic conductivity (and hence groundwater velocity) may be obtained by several methods, the groundwater velocities in the calibrated flow and transport model (~2.5 m d−1) best match our estimates of groundwater velocity obtained from the observed thermal plume movement (i.e., ~2.8 m d−1) which is essentially an

aquifer tracer test where heat is the tracer. Hydraulic conductivity obtained from a pumping test was a factor of two too large, suggesting care must be taken in selecting the appropriate methods of estimating hydraulic conductivity. Thermal conductivity appears to be less important in these settings where convection dominants plume transport and estimates of thermal conductivity based on the measured values presented here and in other studies may be acceptable provided the porous media are similar.