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Hydrologic processes in unsaturated waste rock piles in the Canadian subarctic

University of British Columbia

Hydrologic processes occurring within waste rock situated in a semi-arid continuous permafrost environment at the Diavik Diamond Mine waste rock research project, located in Northwest Territories, Canada, were studied. Characterization of water flow through, and hydrologic properties of, waste rock is complicated by the heterogeneous clast composition, the spatial arrangement of these clasts, internal structures, and the unsaturated nature. Results are presented from experiments on six active zone lysimeters (AZLs), each with an approximate 5 cubic metre volume, along with three instrumented test piles, each with an approximate 3,000 square metre footprint and approximate 15 metre height. A basal drain collection system and individual basal collection lysimeters underlie each test pile to monitor water discharge. The test piles and AZLs were instrumented with monitoring devices designed to collect geochemical, thermal and hydrological data including moisture content, pore-water samples, pore-gas samples, temperature, thermal conductivity, air permeability, matric potential, and microbiology samples. Results indicate that infiltration into the waste rock was dependent on the magnitude and timing of rainfall. Rainfall received after net radiation peaked in June permitted greater infiltration. Once infiltrated, water flow through the matrix was observed to be the dominate transport mechanism under average rainfall conditions despite a grain size distribution with less than 18% of the waste rock composed of the finer than 5-mm fraction. Only in response to large storm events was water flow through preferential flow paths observed. Continuous monitoring of water discharge at the base of the test piles indicated the rate of initial wetting front advancement under average precipitation was about 7.5 m.yr-¹ and under drier conditions was about 6 m.yr-¹. Based on solute transport determined from a tracer test at the AZLs, the average advective velocity during the active flow period was between 1.0 and 1.4 m.yr-¹. Through the first three years of study, the active layer of the uncovered test piles extended the entire depth of the pile, although frozen areas within the core may remain. Each year the discharge period was shorter and less pile volume thaws, consistent with an overall cooling of the lower central core of the test piles.

Text

2014-08-08

Attribution-NonCommercial-NoDerivs 2.5 Canada

http://hdl.handle.net/2429/49951

Geological Sciences

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/2.5/ca/