South Bison Hill Soil Capping Research Synthesis

In 1999, Syncrude Canada Ltd. (Syncrude) initiated a multi-disciplinary study to evaluate the performance of various reclamation capping thicknesses for saline/sodic (Clearwater Formation) overburden fill material at Syncrude’s Mildred Lake mine. An instrumented watershed was created at Syncrude’s South Bison Hill (SBH) that included three one-hectare reclamation plots to test soil capping thicknesses of 35, 50 and 100 cm. The prescribed thickness of reclamation material at the time of the capping study was 100 cm for Clearwater fills.
Current regulatory approvals mandate a cover thickness of 150 cm, an increase to the 100 cm standard in place prior to 2006. Syncrude has implemented a total reclamation capping thickness of 150 cm for Clearwater overburden material at Mildred Lake since imposition of this standard. While intended to mitigate risks (primarily salinization of the soil cover), the increase in soil cover thickness has resulted in the need to salvage and stockpile larger volumes of soil. This in turn has resulted in an increase in the disturbance footprint, reduced availability of water to downstream features, and increased costs of reclamation.
In 2012, Syncrude established a multi-disciplinary research team to synthesize existing data, collect additional data, and conduct modeling to clarify the current performance of different cover thicknesses and project future performance under different climatic conditions. The goal of the study was to determine the minimum soil cover depth required to deliver  an appropriate amount of water for targeted boreal forest vegetation, without compromising the quality of the cover by salt ingress. Fieldwork was completed in the summer and fall of 2012.
The SBH research team conducted a risk assessment to identify potential performance risks associated with plant growth on the various soil cover thicknesses using a Failure Mode and Effect Analysis approach.
The 2012 field program was conducted for a 49 ha study area on SBH that included a portion of the dump plateau, adjacent slopes and the original 1999 capping study area. A field survey and sampling campaign assessed the effects of reclamation capping thickness on soil salinity and both above-ground and below-ground vegetation response. The results of the 2012 program are included in Appendix A to F. A modeling study was conducted to evaluate the sensitivity of water storage and water dynamics to reclamation cover thickness, and a long-term assessment of evapotranspiration and net ecosystem exchange monitoring data was conducted for the site. The scientific research compilation indicates that a capping thickness of 50 cm has been sufficient to ensure an equivalent forest capability to date, with little to no increase in vegetation growth with capping thicknesses greater than 50 cm.
However, since the site has experienced near normal growing season precipitation, the vegetation may experience reduced performance during a prolonged drought on soil covers less than 50 cm. The study water modeling suggests a soil cover of 75 cm is sufficient  in providing enough soil-water to vegetation during drought conditions. Conversely the  research shows that a thick capping (>100 cm) is likely to reduce the amount of water released
downstream to adjacent wetlands, while allowing more extensive salt ingress into the capping soils.
It is the professional opinion of the research team that a thickness of 75 cm will provide adequate water supply to vegetation during droughts while also allowing for the release of water to the downstream reclaimed landscape.