Time Scales of Variability in Unsaturated Soil Cover System Water Content and Temperature.

Mining operations worldwide are responsible for the generation of various waste streams. Unsaturated soil cover systems are constructed to minimize the interaction of meteoric water with underlying waste. The efficacy of the cover systems depend on the highly variable nature of water and energy. Unfortunately, the temporal scales at which variations in water and temperature occur is not known. The objective of this work was to employ the novel Hilbert Huang Transform to isolate the sources of variation in cover systems at a northern Alberta oilsand operation. Daily average soil temperature and volumetric water content values were collected at eight depths in a cover comprised of 20 cm organic / mineral mix, overlying 100 cm of till material, overlying the waste shale. Empirical mode decomposition for each depth produced a set of intrinsic mode functions, indicative of the scale at which variations in the dataset occur. Variability in temperature data from 5 cm to 180 cm depth was almost solely attributable to annual time scales, with a minimum variance contribution of 50%, an average of 65%, and a maximum of 89%. Total variance in water content data was also dominated by annual cycles, yet was less than 50%. With increasing depth smaller time scales became the dominant contributor to water contend data variance, indicating that the majority of the water dynamics are occurring at the till / waste interface. Water content data at increasing depth also saw substantial variance contributions from time scales greater than a decade, suggesting that a long term trend of hydraulic development is still taking place within the cover. Finally, data were compared to a control site, and were also correlated to air temperature, precipitation, and net radiation.  Data from this study will increase our understanding of cover system dynamics, allowing more sustainable designs.