Acid mine drainage (AMD) resulting from pyrite oxidation is produced in two large
waste rock dumps at La Mine Doyon located in Northwestern Quebec. A monitoring
program for the South Dump is designed to characterize the physical and geochemical
conditions prevailing during AMD production within this coarse unsaturated material.
Pyrite oxidation generates heat so, among other instrumentation, the program includes
6 wells drilled through the wastes equipped with thermistor strings reaching the
underlying soil and bedrock. Measured temperatures reach up to 65ºC halfway through
the 30 m thick waste rock dump. We report here on the analysis of the thermal data
gathered weekly for two years at the site.
We consider heat transfer within the dump by conduction through the bulk of the
material (rocks and water) and by advection due to fluid (mainly air) movement in the
pores. Seasonal air temperature variations induce cyclic temperature variations within
the dump. Fourier analysis is used to characterize these variations and provide the
amplitude and phase of the cyclic variations at each measured point. These
parameters allow for the evaluation of the bulk thermal properties of waste rocks. The
relationship between attenuation and phase shift also provides an estimate of the
magnitude and direction of advection. Air convection is driven by the steep
temperature gradients within the wastes which affect air density. The analysis also
reveals that the average heat stored within the dump has decreased significantly
during the monitoring period.
Once the thermal properties of the wastes are known, the heat generation rate is
calculated from the mean temperature profiles. Assuming most of the heat production
results from pyrite oxidation, the oxidation rate may be evaluated from the thermal data
independently of any AMD production model. The oxygen required for pyrite oxidation
could be supplied by diffusion or air convection in the wastes. The estimated oxidation
rate is actually too high to be sustained by oxygen diffusion alone and air advection is
seen to be the dominant oxygen supply mechanism. The thermal data provides strong
evidence for the presence of convection cells within the dump. This is confirmed by
thermographic surveys and by the oxygen concentrations measured within the dump.
A conceptual model of air convection patterns within the dump is proposed based on
the available monitoring data. Since air convection seems to be the main mechanism
responsible for the high AMD production rate in these waste rocks, control measures
aiming at a reduction in AMD should focus on means of reducing air convection.
