EVAPORATION FROM WASTE-ROCK AND RESTORATIONSURFACES: A COMPARATIVE ANALYSIS

摘要

Mining operations in northern Saskatchewan and Alberta result in the creation of waste-rockoverburden piles that are left in-situ on the landscape. In the restoration/reclamation process, an accurateevaluation of the water balance and net percolation through waste-rock is critical to the development ofdecommissioning plans for these wastes. One of the most difficult components of the water balance toquantify is evaporation, yet its accurate determination is essential in evaluating the water available for soilmoisture storage, plant growth and deep drainage. The quantity of deep drainage is of particular concernas effluent may carry dissolved contaminants released form the waste rock as a result of mining andsubsequent geochemical weathering. Additionally, evaporation will influence the upward movement ofsalts, which may inhibit plant growth. Evaporation and the surface energy balance were measured usingthe eddy covariance method from two waste-rock sites: 1) Deilmann South Waste-Rock pile (DSWR), afine to medium grained sand waste-rock without a vegetation cover near Key Lake, Saskatchewan (6 June– 25 August, 2002), and 2) South Bison Hill (SBH), a vegetated organic soil covered waste-rock pile with ashale substrate near Fort McMurray, Alberta (18 May – 24 August, 2003). Both sites had distinctevaporation and surface energy budgets based upon their characteristics. The lack of vegetation at theDSWR site and high surface reflectivity reduced available energy for turbulent fluxes compared with SBH,whereas the vegetation at SBH moderated turbulent fluxes. The greatest fraction of energy balancepartitioning at DSWR was into latent heat, followed by soil heat flux and sensible heat flux respectively. Incontrast, SBH had similar mid-day partitioning between latent and sensible heat flux and a reduced soilheat flux. At DSWR the ratio of sensible to latent heat (the Bowen ratio) remained stable, averaging 0.45± 0.10 over the course of the summer season, whereas at SBH, vegetation growth cycles and precipitationmoderated the Bowen ratio. The Bowen Ratio was typically <1 at the onset of the study and reachedvalues near 1 during the main growing season. As senescence began and rainfall declined in August, theBowen increased to >1. Actual evaporation rates for DSWR averaged 1.79 mm/day compared with 2.44mm/day for SBH. At both sites, actual evaporation was significantly less than potential evaporation. ThePenmann-Monteith combination equation and the decoupling coefficient (?) were used to further evaluatethe radiative, aerodynamic and vegetative controls on ET. At both sites net radiation is the dominant factorin controlling evaporation, and where present, vegetation plays a critical moderating role.