Design and application of a dynamic closed chamber system for measuring CO2 flux from unsaturated C-horizon soils and waste-rock piles to the atmosphere

Abstract

A method based on a dynamic closed chamber system (DCCS) for measuring CO₂ effluxes from unsaturated soils and waste-rock piles was developed and tested in well-constrained unsaturated minicosms (0.58 m dia. x 1.2 m thick), a mesocosm (2.4 m dia. x 3.2 m thick), and waste-rock piles. The DCCS method was compared and evaluated against traditional soil respiration measurement methods (e.g., static flux chamber method, and concentration-gradient calculations). Results of the studies showed that the DCCS yielded accurate measurements of CO₂ fluxes and that the static chamber underestimated actual effluxes especially when using long adsorption times (>24 h). The concentration-gradient method yielded reasonable estimates of CO₂ effluxes but was time consuming and required the determination of moisture content to calculate gas diffusion coefficients. Results of studies carried out on waste-rock piles, showed that the DCCS is a relatively quick technique (2 to 10 min) that can be used to quantify spatial and temporal distribution of CO₂ fluxes at the field scale. The DCCS yielded field measurements of CO₂ fluxes that were reproducible over time at individual chamber locations. The average CO₂ fluxes obtained over the summer study-period of 2000, for Deilmann north and Deilmann south waste-rock piles were 181 ± 41 mg CO₂ m⁻² h⁻¹ (n = 27) and 205 ± 60 mg CO₂ m⁻² h⁻¹ (n = 48), respectively. A statistical Student's test yielded no significant difference between the two sets of data. As a result, the two mean values were combined to yield an overall mean value of 193 ± 73 mg CO₂ m⁻² h⁻¹ (n = 68) for the two waste-rock piles. Most temporal changes were attributed to seasonal variations in temperature (7.9 to 16.5 °C). The DCCS has the advantage of being able to measure CO₂ fluxes in situ at the same locations using the same chambers without greatly disturbing the soil. The results of this study will be used in future modeling exercises to quantify the rates of sulfide oxidation and carbonate buffering reactions in these waste-rock piles.