Measurements and modeling of gas fluxes in unsaturated mine waste materials

Accurate measurements and predictions of surface CO₂ fluxes are needed to quantify biogeochemical reaction rates in unsaturated geologic media and soils. However, no standard appears to exist for establishing the accuracy of field measurements of soil respiration rates. As a result, a technique to measure CO₂ fluxes from the soil surface to the atmosphere was recently developed and verified in mesocosms over the range of CO₂ fluxes reported for field conditions. The method, termed the dynamic closed chamber (DCC), was shown to accurately measure CO₂ fluxes from ground surface to the atmosphere in mesocosms. The main advantage of this direct technique is the almost instantaneous estimation of the CO₂ flux. Although the DCC is a promising technique, its ability to accurately quantify surface CO₂ flux under field conditions remains to be verified. The field application of the DCC is investigated in this thesis with a particular focus on quantifying reaction rates in waste-rock piles at the Key Lake uranium mine in northern Saskatchewan, Canada. It should, however, be noted that the dominant geochemical reactions in the two waste-rock piles at the Key Lake mine were not typical of acid rock drainage (ARD) waste-rock piles. The CO₂ fluxes measured in this study occur in the organic material underlying the waste rocks, in contrast to ARD waste-rock piles where O₂ consumption and CO₂ production are the results of sulphide oxidation and carbonate buffering. This work provided a complete suite of measurements required to characterize spatial distribution of CO₂ fluxes on larger-scale studies of waste-rock piles. There has been no previous field-scale study to quantify CO₂ fluxes across a waste-rock pile. The ability of the DCC method to accurately quantify field soil respiration was demonstrated by comparing the DCC fluxes to those obtained using two other CO₂ flux measurement techniques: the static closed chamber (SCC) and eddy covariance (EC) methods. The DCC yielded comparable data but had distinct advantages over the two other methods in terms of speed and repeatability. The DCC was also used to investigate CO₂ fluxes under the climatic variables (e.g., rainfall and evaporation) that affect soil water content at the Deilmann north (DNWR) and Deilmann south (DSWR) waste-rock piles, at the Key Lake uranium mine. The effects of rainfall events on waste-rock surface-water conditions and CO₂ fluxes were of short duration. A simple model for predicting the effects of soil water content on CO₂ diffusion coefficient and concentration profiles was developed. The model was verified with measured CO₂ fluxes obtained from mesa-scale columns of unsaturated sand. Verification of the model showed good agreement between predicted and measured data. The model was subsequently used to predict CO₂ diffusion and concentration profiles in response to changes in soil water contents in the piles and also to predict surface CO₂ fluxes from the DNWR and DSWR for a 6-d test period [August 1 (day 3) to August 6 (day 8) 2002] following a 72.9 mm precipitation event over the initial 48-h [July 30 (day 1) to July 31 (day 2) 2002]. The model predicted surface CO₂ fluxes trends that were very similar to the measured surface CO₂ fluxes from the DNWR and DSWR piles during the test period. Based on the tests conducted in this thesis the DCC method has shown to be suitable for field applications to quantify CO₂ fluxes and to characterize the spatial and temporal dynamics of CO₂ fluxes from unsaturated C-horizon soils and waste-rock piles.

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