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Mechanisms and spatial variability of rainfall infiltration on the Claude waste rock pile Bellehumeur, Tracy M.

Abstract

The transport of acid rock drainage from a highly heterogeneous and unsaturated waste rock pile is based on an understanding of fluid flow within a pile. This study provides a quantitative evaluation of spatial variability in rainfall infiltration and identifies relations between hydrologic pathways, contact areas and structural features in the Claude waste rock pile. Thirty-five fixed ring infiltrometer tests were conducted to measure infiltration capacity over the surface of the pile. Hydrologic pathways into the Claude pile were investigated by releasing dye staining tracers at the surface, followed by excavation and mapping of the stained waste rock. The west side of the pile is characterized by a coarse, 'untrafficked' surface, in which infiltration at the surface is immediate. Recharge on the east side of the pile is controlled by a fine traffic compacted material and surface topography. Intense rainstorms cause ponding and overland flow to concentrate in coarse drains at the bottom of small catchment areas. In an intense rainstorm, the majority of recharge is through the drains on Claude east. In a typical low intensity rainstorm, the majority of recharge is through the fine matrix material on Claude east. Following the infiltrometer test, a volume of dye tracer, rhodamine WT, was ponded in the ring infiltrometers. Excavation provided a visual record of Darcy flow and flow through macropores. Dye distribution decreased rapidly and converged in macropores near the surface. Three large-scale dye tracer releases (800L to 2400L) were applied at the catchment drains on Claude east and distributed over a small area on Claude west. Excavation of the tracer revealed that infiltrating water migrates over the surface of coarse particles, avoiding the fine matrix material (boulder hopping). Significant lateral spreading was caused by ponding on old traffic surfaces encountered below the pile surface. Two mechanisms of flow were observed during excavation of the large-scale tracer tests, preferential flow in macropores, and boulder hopping. Darcy flow was observed in the fine, traffic compacted layers. This thesis documents the strong influence of macropore flow and preferential flow on the distribution of rainfall recharge into the Claude pile.

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