Evaluation of Preferential Flow Processes in Reclamation Soil Covers

Abstract

To predict the effectiveness of land reclamation, it is important to understand how water and solutes are transported within reconstructed landscapes. The objective of this study was to examine the influence of preferential flow on salt leaching in reclamation soil covers. The study site was a reconstructed landscape where saline-sodic minespoil from oil sands mining was capped with layers of glacial and peat mix soil. Preferential flow was investigated using laboratory column experiments and in situ adsorptive dye and conservative tracer experiments.

Results from column experiments and dye tracer experiments indicate that preferential flow is an important and prevalent mechanism of solute transport. Column experiments, which used time-domain reflectometry to monitor the transport of a chloride tracer through an undisturbed core of peat mix soil, determined immobile water fractions (èim/è) ranging from 80-99% and diffusive mass transfer rates (á) between 0.15 - 2.0 h-1. Breakthrough curves showed the early arrival of chloride and extended tailing. Dye tracer experiments, in which Brilliant Blue dye was applied in solution to the soil surface, were carried out at 6 hillslopes plots. Approximately 24 hours after dye application, a vertical soil face was excavated to reveal stained flow patterns. Preferential flow as macropore flow, fingering, and / or funneling was observed at each plot.

Results from the conservative tracer field study indicated soil solutes were flushed by a combination of vertical and lateral flow processes. A large pulse of bromide and chloride was applied across the lower slope of the 0.35-m cover. Soil sampling at approximately 1 and 2 years later determined vertical leaching, lateral translocation downslope, and upwards movement of soil solutes. Matrix flow during the spring melt, combined with matrix flow and / or preferential flow during summer and fall periods, was responsible for the vertical leaching of solutes. Subsurface flow generated in response to the spring melt or due to differences in soil hydraulic conductivity was responsible for the lateral transport of solutes. As a result of advective or diffusive processes, solutes were transported upwards into the overlying soil. These results suggested that despite the existence of preferential flow, there were other mechanisms of solute transport which served to leach and flush salts from the soil.