Authors: Romig, DE; DeCastro, CEB; Rabinowich, MS; Buchanan, NG; Roberts, R; Krueger, CB

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Romig, DE, DeCastro, CEB, Rabinowich, MS, Buchanan, NG, Roberts, R & Krueger, CB 2023, 'Successful revegetation of run-of-mine overburden soil covers without soil amendments: Copper mine closure studies in a semi-arid environment', in B Abbasi, J Parshley, A Fourie & M Tibbett (eds), Mine Closure 2023: Proceedings of the 16th International Conference on Mine Closure, Australian Centre for Geomechanics, Perth,

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Although most native plant species are well adapted to low fertility soil conditions, revegetation practitioners frequently recommend amending soil covers at considerable expense. But are soil amendments truly necessary for reclaiming semi-arid mine sites? Revegetation progress on unamended run-of-mine (ROM) overburden soil covers was monitored at three reclaimed sites for six to 12 years at two copper mines in New Mexico, USA. Monolithic, evapotranspiration (ET) soil covers (60 to 120 cm [2 to 4 ft] thick) were constructed using suitable overburden (leached cap) that was salvaged during pit development. Two sites were reclaimed using granite (GR) overburden and a third site was covered with a mixture of unmineralized rhyolite and leached granodiorite (RLG). Covers were placed over acid-generating waste and leach stockpiles. Cover construction primarily focused on material management of the ROM to reduce the rock content to approximately 50% by volume to improve seedbed conditions and water-holding capacity of the ET cover system. Both the GR and RLG soil covers are moderately coarse-textured sandy loams (USCS silty sands) with circumneutral to slightly alkaline pHs and are non-saline and non-acid generating (low sulfur). No soil amendments were used in the reclamation, though both ROM materials are considered infertile having low macronutrient (1-2 mg/kg nitrate) and organic matter content (<0.1% organic carbon). Sites were drill seeded with native seed mixes followed by mulching with long-stem grass hay. Periodic quantitative monitoring included assessment of the reclaimed site’s canopy cover, shrub density and plant diversity compared to a native reference area. Each site has progressed steadily towards meeting or exceeding the native reference area canopy cover performance standard that is used for financial assurance release. All sites experienced significant growing season drought for at least two years during establishment. Total canopy cover at one GR site was 58% in Year 6 and 64% in Year 12, exceeding the total canopy cover of the native reference area in both years. The second GR site experienced significant drought during establishment but after six growing seasons still achieved 80% reference area canopy cover standard. Vegetation of RLG covers exceeded the reference area standard within eight years following seeding. Each site also recruited numerous desirable plant species, including several native legumes and other perennial forbs. Six years after seeding, the GR sites averaged 63 additional plant species beyond the original seed mix while the RLG site had 86 volunteer species after seven years. This work demonstrates that soil amendments are not always required to revegetate infertile ROM cover materials. Unamended overburden covers can support diverse and productive reclaimed plant communities that are resilient and capable of sustaining themselves under the adverse conditions typical of semi-arid environments.

Keywords: overburden cover systems, run-of-mine, stockpile closure, revegetation, semi-arid reclamation

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