Flow and transport in novel subsurface flow systems

Abstract

The ability to predict long-term water quality and drainage rates from mine waste facilities, and importantly, the effect of alternative closure strategies on the management/treatment of waste rock and heap leach facility seepage, is greatly complicated by the high degree of variability typically found in mine waste physical and hydraulic properties. Additionally, climatic conditions, waste rock geometry, unreclaimed and reclaimed surface conditions and surface water management are significant factors in solution and air flow. This presentation reviews the controlling processes for solution flow, air flow and solute transport in what can be best described as novel subsurface flow systems, with implications for focused site characterization and for supporting conceptual and mathematical models of observed and future behavior of mine waste hydrogeology.Solution flow in waste rock/heap leach materials occurs predominantly as matrix flow through small diameter pore space created by the fine-grained fraction and as preferential flow through larger macropores created by larger particles. Under low net percolation rates, solution flow primarily occurs via matrix flow, but as infiltration exceeds the hydraulic capacity of the matrix, flow can occur via macropores. The presence of macropores can result in the redistribution of water content as a pressure wave such that the hydraulic response in a wetted system (i.e. a heap leach facility) can be approximated by kinematic wave theory. Conversely, convective air flow occurs primarily in larger macro-pores with gas diffusion rates controlled by moisture retention relationships. Fine-grained waste typically has higher moisture retention and a lower proportion of macropores than coarse-grained waste rock. Waste material bulk densities are affected by the waste facility height as overburden pressure consolidates the waste material and reduces the porosity and the pore size distribution; typically, there is a decrease in the larger pore diameters, depending on clastsupported porosity.The amount of net percolation needed to "wet-up" waste rock and cause seepage is also significantly affected by the waste gradation and climate; large waste rock facilities in arid sites can theoretically take hundreds of years to wet up, whereas heap leach material is already wetted and in a drainage state. The solution residence time describes the porewater and solute travel time through the facility; the hydraulic response time describes the time between an infiltration period (i.e. wet season) and the observable response in facility seepage rates. The solution residence time dictates how long geochemical reactions can occur; the relative hydraulic response (increase/decrease) in waste facility seepage rates can be orders of magnitude faster than the solution residence time. Consequently, waste material hydraulic characterization efforts should focus on the use of field and laboratory methods that can measure matrix flow properties, the effect of large particles on macropore flow, and effects of ...