HIGHLAND VALLEY COPPER - Terrestrial Areas
Revegetation started in 1983 with the level areas at the Bethlehem and Lornex waste rock dumps. Tailings reclamation followed subsequently with the Bethlehem main tailings in 1983, Highmont tailings in 1989 and Trojan tailings in 1990.
In 1988 at the Bethlehem tailings site the coarse sand crest of the tailings dam was seeded and fertilized in the spring and planted with native shrub species in the fall (2000 stems/ha). Standard agricultural equipment was used for planting, which was the most economic option, and irrigation was used initially. Waste dump sites were seeded in the spring by tractor or helicopter with an agronomic mix of grasses and legumes, as well as high nitrogen fertilizer (400 kg/ha in the first year and 200 kg/ha for 2 to 3 years after) to help establish legume growth. In the consecutive fall, native shrubs were planted in groups or islands (1000 stems/ha). Native plant species included: willow, sagebrush, wild rose, Saskatoon, buffaloberry, and clematis.
Site preparation included regrading slopes to approximately 24 degrees or shallower. Level areas were ripped to reduce soil compaction.
Overburden Influence on Vegetation Establishment
Vegetation was established directly on some waste rock sites, while other vegetation was established on overburden-capped sites. There was limited success establishing vegetation directly on the Bethlehem waste rock dumps, so the dumps were capped with material from the Valley site.
The depth of overburden was found to be an important factor in root system development. Maximum rooting depth was approximately 75 cm on the capped sites. Sites with deeper overburden (>37 cm) had thicker vegetation cover than sites with shallower overburden (<20 cm). Legumes were found to be more prolific on sites with deeper overburden, while grass cover was more dominant on areas with shallower overburden. To achieve dense vegetation cover the minimum overburden requirement was found to be approximately 40 cm depth.
Available Water Storage Capacity (AWSC)
Sites with dense vegetation cover were found to have less than 50% coarse fragments and AWSC of 30 or greater
No relationship was found between material size, type, grading or texture, and vegetation cover. Field studies determined that AWSC of the soil was the main factor influencing vegetation cover on the reclaimed sites. For both the overburden capped and non-capped sites there was a strong correlation between the density of vegetation cover and the AWSC. Thicker vegetation cover was observed on sites with higher AWSC, while thinner vegetation cover was observed on sites with lower AWSC. Coarse fragments in waste rock reduce the AWSC and it was found that sites with lower coarse fragment content had higher AWSC and denser vegetation cover.
Sites with sufficient AWSC were also found to have self-sustaining vegetation after 3-4 years of maintenance fertilizing. In contrast, vegetation was not self-sustaining on sites with poor AWSC and nutrient holding capacity, such as on coarse tailings, waste rock areas resistant to weathering, and areas capped with insufficient depth of overburden.
Biosolids as Fertilizer
The addition of biosolids to capping overburden was investigated as a method to improve vegetation establishment. Biosolids amendments offer potential benefits to water storage capacity and nutrient storage capacity. Trials were initiated in 1996 and 1997 with biosolids from GVRD (now Metro Vancouver). Two treatments were applied: ‘topdressed’ and ‘incorporated’. Topdressed areas had biosolids and mulch applied to the surface of tailings and overburden capped waste rock areas that were previously not self-sustaining (although forages and native shrubs were present on these sites). Incorporated areas had biosolids that were tilled into the tailings and overburden, and then either seeded or planted with native shrubs.
By 1991 it was known that planting agronomic grass species may be required to stabilize the soil surface and control erosion in order to enable tree planting 10 to 15 years later. Of significance, it was identified that 'green' did not necessarily signify successful reclamation.
Slope Influence on Vegetation Establishment
Through reclamation monitoring it was observed that vegetative cover on reclaimed sites can be variable, irrespective of apparent similar conditions. On the waste rock sites level areas had higher total vegetation than sloped areas, with shallow slopes (<30 degrees) having more vegetation than steeper slopes (>30 degrees). The steeper slopes generally had higher content of coarse fragments, which was a contributing factor.
In 1990, a resloping study was conducted on the Lornex wasterock dumps. It was found that total vegetation cover was greater on the dumps that had been resloped and rounded, as opposed to left to the angle of repose.
Sites that were more densely vegetated also had higher organic matter, owing to the decomposition of root matter in the upper soil profile. It was also found that roots frequently extended to depths of 80 cm or more on areas with dense vegetation, while the maximum observed depth was 120 cm.
In 1995 trials using nitrogen-fixing bacteria (Azospirillum brasilense) to inoculate plants were started in an effort to find a method to reduce the need for nitrogen fertilizers on the reclaimed areas. This bacteria has been helpful in increasing water uptake and mineral uptake by grasses in other studies, and monitoring will continue to determine results.
Vegetation production was improved on topdressed and incorporated areas in the first few years following the start of trials. However, ASWC was still a limiting factor. Interestingly, molybdenum concentrations in alfalfa, wheatgrass, and willow were found to be lower in areas where biosolids were applied on both topdressed and incorporated treatments. This is an effect of the biosolids immobilizing the molybdenum present in the overburden. One future consideration in applying biosolids to reclamation areas is that multiple applications of biosolids (which can have elevated levels of metals) may increase other metals present in the growing medium, such that the grazing land use would be impacted. As well, soil water runoff tested from biosolids treated areas have indicated elevated copper, nitrate, molybdenum and zinc, which could pose an issue for aquatic and wildlife species.