The design and installation of a field instrumentation program for the evaluation of soil-atmosphere water fluxes in a vegetated cover over saline/sodic shale overburden

Abstract

The mining of oil sands near Fort McMurray, Alberta, involves the stripping of salinesadie overburden to gain access to the oil-bearing formation. The overburden is placed in mined out pits and surface dumps and is re-contoured before being capped with a mandated 1 m soil cover. The potential for slope instability, subsidence, and salinization resulting from the character of the saline-sodic material and its interaction with fresh water makes it imperative that the amount of precipitation percolating below the root zone be minimised. Syncrude Canada Ltd is conducting a large scale cover trial at the Mildred Lake mine in order to assess the performance of different reclamation strategies.

Four 1 hectare prototype covers were placed on an area referred to as the SW30-Dump in order to study the basic mechanisms controlling moisture movement within the cover systems. Three covers were constructed in 1999 with configurations of 1.00 m, 0.50 m and 0.35 m thick and consisted of a thin layer of peat overlying varying thickness of secondary soil. A fourth study site was established on a recently reclaimed watershed capped with a 1.00 m cover of peat/secondary mix in 1996.

A field instrumentation program was carried out consisting of detailed monitoring of matric suction, volumetric water content and temperature within the different soil profiles, as well as measurements of runoff, interflow and site-specific meteorological conditions. Generally, all instrumentation performed well and was found to correctly measure the soil-atmosphere fluxes required to assess each cover alternative.

Evaluation of the covers revealed that the 1m layered cover was the only one to maintain sufficient soil moisture for all monitored growing seasons. A significant portion of the available soil water was held within the peat layer. The other three covers lacked the required storage during drier periods and on numerous occasions soil suction values exceeded the wilting point for vegetation. Interflow measured from the covers was negligible, while high intensity rainfall events and large snow melt events caused preferential flow events in all but the 1 m layered cover. Moisture contents in the shale overburden fluctuated only 1 to 2 percent during the period monitored, indicating nominal percolation into the shale beyond the cover/overburden interface.