The Diavik waste rock project: Water flow through mine waste rock in a permafrost terrain

Abstract

A field experiment is being carried out at the Diavik diamond mine in northern Canada to investigate the influence of unsaturated flow behavior on the quality of drainage from mine waste rock piles in a region of continuous permafrost. This paper is part of a series describing processes affecting the weathering of waste rock and transport of reaction products at this site; here the focus is on unsaturated water flow and its role in mass loading. Two 15m-high instrumented test piles have been built on 60m by 50m collection systems, each consisting of lysimeters and a large impermeable high-density polyethylene (HDPE) liner. Collection lysimeters are installed nearby to investigate infiltration in the upper 2m of the waste rock. Porosity, water retention curves, and hydraulic conductivity functions are estimated from field measurements and for samples ranging in size from 200cm3 to 16m3. Net infiltration in 2007 is estimated to have been 37% of the rainfall for mean annual rainfall conditions. Early-season infiltration freezes and is remobilized as the waste rock thaws. Wetting fronts migrate at rates of 0.2–0.4md−1 in response to common rainfall events and up to 5md−1 in response to intense rainfall. Pore water and non-reactive solutes travel at rates of <10−2 to 3×10−2md−1 in response to common rainfall events and up to 0.7md−1 in response to intense rainfall. Time-varying SO4 mass loading from the base of the test piles is dictated primarily by the flow behavior, rather than by changes in solute concentrations.