Ok Tedi Mining Ltd undertakes a tree planting programme to rehabilitate dredged areas
IN 1984, the State of Papua New Guinea (State) approved Ok Tedi Mining Limited (OTML) to dispose its waste rock and tailings from the mining operations into the river system following the failure of the tailings dam. To minimise the impacts on the environment and the livelihoods of the communities, OTML is implementing several mitigation programmes that have been approved by the State. These programmes which have been implemented since 1997 are:
- Dredging of the sediment to reduce riverbed level which in turn reduces over-bank flooding and forest dieback;
- Extracting pyrite from the tailings to minimise the risk of acid rock drainage and improve water quality;
- Adding limestone at the mine waste dumps and mill to reduce the risk of acid rock drainage and improve water quality; and
- Rehabilitating the stockpile to restore plants and animals’ life and the people who depends on those ecosystem services for their livelihood.
The dredging project, which is located at Bige in the Lower Ok Tedi, is about 80km from the mine. Dredging was commissioned in 1998 to reduce sediment load reaching the Lower Ok Tedi and Fly River system. The dredge removes at least 10 million cubic meters of sand annually from the river which constitutes 60 per cent of the sediment load passing through the dredge slot. The slot is 840m x 50m x12m and is designed to capture sediments as they pass through this section of the river. The dredged sand is hydraulically placed in engineered stockpiles on the east and west banks at Bige covering an area of 1000 ha of land that have been impacted by dieback in the mid-1990s. The remaining 40 per cent, comprising mainly silt and mud passes through to the Fly River. Most of this material will settle in the lower Middle Fly region.
After more than two decades of dredging, at a cost of US$55million (about K193million at current rates) per annum, the riverbed levels have reduced by two to four meters in the lower Ok Tedi, and have stabilised including the Middle Fly region. Over-bank flooding and the associated forest dieback have decreased compared to 1996/97 period and recovery of some forest species are observed in the lower Ok Tedi and upper middle Fly River as a result of bed level reduction. Forest and woodland dieback are expected to continue in some parts of the floodplain but at a much slower rate. The reduction and stabilisation of riverbed is beginning to restore food resources and habitat quality for fish and other species. The restored riparian vegetation provides a key source of food for species that feed on insects, plant leaves, roots and seeds. Fish monitoring data in the last few years from Kuambit and Erekta, located downstream, proximal to the dredge site, showed greater numbers and diversity of small fish indicating the positive impact of dredging on the downstream environment. Dredged sand contains contaminant metals such as copper, lead, zinc and cadmium and are devoid of essential nutrients and detritus to support plant growth. These present challenges that need to be managed to achieve the success criteria specified for the stockpile in the Mine Area Rehabilitation Plan (MARP). The MARP calls for a stable landform that comprise a self-sustaining vegetation that comprise native species. To achieve the MARP requirement OTML ensures that:
- Stockpile is designed and constructed to achieve a stable landform which include surface drainage controls
- Geochemistry of the dredged sediments, particularly the final cover materials, meets the geochemical criteria set to reduce the risk of acid rock drainage (ARD) by more than 80 per ceent.
- Studies are conducted to improve species selection, planting/seeding methods and surface treatments. The rehab method now used have culminated over many years of trial.
- The performance of the rehab programme is monitored to achieve the MARP requirement.
Native pioneer species
The model for Bige stockpile rehabilitation is to introduce native pioneer species capable of establishing, reproducing, stabilising and gradually ameliorating the sand substrate through leaf litter production and root respiration.
As shaded canopy and soil development advances, ongoing rehabilitation would introduce additional early and mid-successional forest species. Over time the species from the surrounding forest are expected to colonise naturally via seeding by birds, bats, rats and other wildlife vectors that are attracted to the developing forest. Since 2017, OTML adopted the Ecosystem Function Analysis (EFA) and Landform Function Analysis (LFA) methodology developed by CSIRO (Tongway and Lindley, 1995) to assess the performance of the revegetation programme. The EFA/LFA monitoring constitutes tracking three landform functions (stability, infiltration and nutrient cycling) and three ecosystem functions (forest succession, forest structural complexity and cultural plants) and many field attributes under each function. Monitoring data from the last three years has shown positive trajectory towards achieving a self-sustainable vegetation that comprises a diversity of native plants and trees that are of cultural importance. The EFA/LFA methodology also allows for management intervention if the score for a function or any of the attributes within each function is not performing to expectation. The stockpile rehabilitation programme epitomises the 2021 Word Environment Day theme of “Ecosystem Restoration”.
More than 30 tree species
Over US$600,000 (K2.1 million) per year is spent on the ecosystem restoration programme on the stockpiles and a dedicated rehabilitation team with 15 fulltime national employees manage the programmes from seed germination and seedling care at the nursery yard to tubestock planting in the field. More than 50,000 tree seedlings from over 30 species were produced in the nursery in 2020 of which 33, 860 tubestock trees were planted out to cover 45 ha of the stockpile. Over 200 ha of stockpile of a total of 1,000 ha have been revegetated with 176, 899 tree seedlings between 2010 and 2020. Species selection of the stockpile is based on local community values and importantly functional trait for secondary forest tress that can perform in severely degraded conditions as pioneers to create a conducive microclimate that will promote natural forest succession. OTML is committed to conduct business in a manner that balances the environmental impacts, economic needs of PNG and the expectation of the local community it operates in through continuous improvement in its environmental performance. OTML had spent K3 billion on environmental mitigation and restoration projects since 1998 which is testament to its commitment towards minimising the impacts on the environment and the livelihood of the communities.
- Story and pictures supplied by OTML Communication and Public Relations Department