Multi-year hydrologic response of experimental waste-rock piles in a cold climate : active-zone development, net infiltration, and fluid flow

• Main Author: Fretz, Nathan Mackenzie
• Language: English
• Published: University of British Columbia 2013
• Online Access: http://hdl.handle.net/2429/43923

A large-scale field study was constructed at the Diavik Diamond Mines, NT, Canada, to examine the hydrologic, geochemical, microbiologic, gas-transport, and heat-transport mechanisms that influence drainage water quality from waste-rock. This thesis focuses on the hydrology component of the project, with the aim of better understanding fluid flow through mine waste-rock at multiple scales in cold climates. The hydrologic response of two large-scale, experimental waste-rock piles (test piles) and a series of smaller-scale lysimeters (active-zone lysimeters) were monitored. Both the test piles and the active-zone lysimeters were constructed on top of water collection systems in order to monitor outflows, and were instrumented with tensiometers, moisture content sensors, and thermistors. Active-zone development, net infiltration, matrix wet-up, wetting front movement, and outflow at the test piles and active-zone lysimeters are examined. Above-freezing air temperatures result in an active-zone developing inwards into the test piles during the summer months, allowing net infiltration at the thawed surface and fluid flow through thawed regions to occur. Over a five year period the average percent net infiltration into the active-zone lysimeters and crowns of the test piles was estimated to be 43%, 57%, 15%, 41%, and 58% of natural rainfall in 2007, 2008, 2009, 2010, and 2011, respectively. An indirect evapotranspiration computation suggests that net infiltration from rainfall events <5 mm/d is restricted to periods early (May) and late (September to mid-October) in the rain-season, while net infiltration in June, July, and August is restricted to rainfall events >5 mm/d. TDR sensors are used to monitor initial wet-up of the matrix fraction and wetting front velocities. Approximate spatial uniformity of wetting front velocities for all but the most intense rainfall events, and positive correlation between outflow rate and solute/dissolved metal concentration from effluent samples, suggest that the dominant mechanism of fluid flow at the test piles and active-zone lysimeters is porewater displacement in the matrix from propagating pressure waves, as opposed to preferential mechanisms. Over a 5 year period outflow below the batters of the test piles was significantly greater than outflow below the crowns, even after the matrix material below the crowns completely wet-up. === Science, Faculty of === Earth, Ocean and Atmospheric Sciences, Department of === Graduate