The Society for Ecological Restoration (SER 2004) defines ecological restoration as ‘the process of assisting the recovery of an ecosystem that has been degraded, damaged or destroyed’. The process of natural succession, which happens concurrently with ecosystem development takes time. Several ecosystem functions of mined land require a long time to develop through natural processes. Therefore, it is indispensable to incorporate some ecosystem management tools that can accelerate the recovery of mine degraded land. Mine degraded sites are either reclaimed—returning a derelict site to some use or rehabilitated, creating conditions for a new and substantially different use of the mining site or restored—recreating conditions suitable for previous use of area based on the end land-use of the sites. Generally, ecological restoration with forestry as end land-use is carried out in two stages:
■ Physical, technical or engineering restoration: It is a high cost, low risk restoration activity, but constitutes over 60 to 90 per cent of the total cost of restoration; and
■ Biological restoration: It is a low cost, high risk restoration work and requires multidisciplinary inputs.
Coal production and pollution
A fossil fuel contributing to about 60 per cent of the energy mix in India, coal is a high demand commodity. India consumes 8 per cent of the total world’s coal, making it the third largest consumer as well as the third-biggest greenhouse gas emitter. In terms of production, India ranked third globally with 639.23 million tonnes in 2015-16 and 724.71 million tonnes projected for 2016-17, next to China and USA (Ministry of Coal, 2017).
Coal mining in India started over 240 years ago around 1774 in the Raniganj coalfield. However, coal production was boosted only after nationalisation of coal mines in 1972. India has 308.8 billion tonnes of coal reserves in seven states—Jharkhand (26 per cent), Odisha (25 per cent), Chhattisgarh (18 per cent), West Bengal (10 per cent), Madhya Pradesh (8.7 per cent), and Maharashtra (3.7 per cent) (Ministry of Coal, 2014).
Indian coal is of Gondwana origin with low sulphur and high ash content ranging from 30 to 50 per cent (Xia et al., 2016), compared to an ash content of 15-20 per cent found in developed countries. This high ash content is responsible for generation of huge amounts of fly ash—a by product of combustion in thermal power plants.
As high as 92 per cent of coal is mined by open cast method. As most of the coal deposits in India are locked under forest cover, a substantial amount of coal production through surface mining involves the complete removal of vegetation, soil cover and overlying waste rocks. This causes loss of forests and biodiversity, habitat fragmentation and deterioration ecosystem services, which inherently damage and pollute the environment. Massive degradation of land, complete change in land use, deterioration of aesthetics, severe air pollution with marked increase in particulate matter and dust, water pollution and siltation, are very common in coal mining areas. Increase in respirable particulate matter—RPM or PM10, suspended particulate matter (SPM) and settable dust (dust fall) are thus of great concern.
Surface coal mining process and generation of mine waste dump
To reach the coal seam, the overlying rocks above the uppermost coal seam and the inner-burden between the lower coal seams are removed in parallel strips (pits) across the coalfield until the area is mined. After being drilled and blasted, rocks covering the shallowest coal seam is removed using shovel-dumper combinations, draglines and auxiliary excavating equipment. Till mining reaches a desired depth, all the topsoil, subsoil and overburden materials are dumped outside—‘external dump’. Once these dumps are inactive, they are stabilised by topsoil blanketing. Sometimes, they reach a height of 90-100 m from the ground surface.
As mining progresses and several rows of spoil piles are created, grading of the piles begins, using bulldozers to create a post-mining topography commonly known as ‘back-filling’. The reclamation activities include the regrading of mine spoil—a term used for overburden material removed during opencast mining. It creates a post-mining topography including restoring drainage networks, replacing salvaged topsoil, re-establishing vegetation cover and implementing erosion-control measures. Development of forestry as post mining land use is relatively easier, compared to external dumps as regrading, redistribution of topsoil and run-off management are simpler.
Characteristics of mine waste
The properties of mine spoils are directly controlled by the physical and geochemical properties of the rock strata from which they are derived. Once the mine spoils are physically reclaimed and utilised to support plant growth (with or without topsoil), they are considered as minesoils.
Minesoils are pedogenically young soils developed from the mixtures of fragmented and pulverized rock material—anthrosols. Minesoils developing on the surface mined lands are characterised by high rock fragments, ranging from 40-60 per cent, low water holding capacity, compacted surface (high bulk density) and low infiltration rate compared to native soils. Use of heavy earth moving machinery (HEEM) during technical reclamation resulted in compact minesoils, which exhibit low porosity and poor aeration.
Ecorestoration planning and implementation
Before re-vegetation planning, types of vegetation cover have to be planned. This depends on characteristics of the plant, nature of minesoils, quality of available topsoil, proximity to the nearby seed sources or natural seed banks, climatic conditions (rainfall, temperature) and proneness to anthropogenic disturbance. It is necessary that a clear objective of restoration and final land use at the end of mining operation is clearly defined by mine authority along with the consultation of regulators and local bodies. The restoration planning should be considered at an early stage of inception of project. Ecorestoration planning should have the following basic steps:
- Involvement of local community: mining is a temporary activity and as temporary occupiers of the land, the coal companies should aim for redevelopment of better post-mining land use by considering needs of the local communities.
- Removal and preservation of flora by developing a flora bank: this essentially consists of transplantation of native/valuable species and their documentation (local and scientific name, density, tree diameter at breast height, aerial height, crown cover, category of species, economic and medicinal values). The information must be displayed on a documentation board indicating importance of preservation. Before transplantation, it is to be ensured that soil is properly moistened to ease transplantations and the places for transplantation in newly restored sites are carefully chosen. Protection of remaining patches of the original vegetation should be encouraged so as to increase seed propagation and regeneration of native species.
- Top soil management: addition of topsoil is vital component to establish a self-sustaining cover during restoration process. It must be preserved wherever possible and its quality should be protected during moving and storage. Important aspects of topsoil management include—inventory and removal of topsoil, concurrent reuse and storage of topsoil (if required) including preservation of topsoil fertility by vegetating grass-legume mixture.
- u Technical aspects: the overall slope of the dump surface should not exceed 28o. To reduce the slope length, it is recommended that the height of the dump should be kept within 30 m with 10 m benching. A drainage system needs to be designed to control erosion and provide garland drains surrounding the dump. Scraped topsoil needs to be applied over the regraded surface (30-50 cm). Use of mulches is recommended to reduce surface compaction and overall amelioration of site conditions. Where little or no topsoil exists prior to mining, it may be necessary to amend or import soils depending on the final land use and site conditions.
- Revegetation Planning: a key element of successful restoration projects is the establishment of a succession based vegetation cover through plantation and application of grasses and legume seed mixtures. Tree species recommended for afforestation of coal mine dumps are khair (Acacia catechu), siris (Albizia lebbeck, A. procera), margosa (Azadirachta indica), and shisham (Dalbergia sissoo), to name a few.
Nowadays hydroseeding technique is used for fast and efficient application of seed mixture on slopes. Tillers of khus grass (Chrysopogon zizanioides) and citronella grass (Cymbopogon citrates) are also commonly used in slope and barrier constructed at the periphery of dumps, known as berm.
The revegetation planning should mimic surrounding non disturbed forest area or encourage specific ecosystem establishment, incorporating strengths of both native and non-native species. But unfortunately, in many instances dumps mine degraded lands are reclaimed by planting fast growing exotic tree species, without proper technical reclamation or even without soil in plantation pits.
Currently, grass-legume mixture (Stylosanthes hamata) are seeded as pioneer species on the barren dumps to enhance soil fertility before plantation of tree species. In many instances, dry mulches are also used to conserve moisture and provide substrate for ecosystem redevelopment.
Integration of ecological succession: this approach promotes development of native plant species—initially planted species will ameliorate dump surface and promote colonisation of native species. It is considered to be the bridge between initial colonisers and later developing vegetation (Fig. 1).
Ideally, one would like to have some statistics as to how many mines have been restored so far. However, there is no reliable information available in this regard, apart from some accounts of trees that are being planted.
Mining companies are now realising that reclamation by simple plantation of fast-growing trees are not going to restore a degraded ecosystem. There should be a paradigm shift to restore a degraded ecosystem through a ecological restoration approach. There is a close relationship between ecological understanding and successful ecorestoration. Technical aspects of restoration are the keys of ecological restoration of minesoils, which should be carried out in an orderly manner. This may be aimed at producing better land use and productivity prior to mining, re-grading and blanketing backfilled areas with soil. Salvaging and reusing topsoil carefully, designing appropriate control erosion measures to enhance stability of dumps are other ways. Minimising disturbance to the surface and groundwater, conserving floral patches and using seeds for regeneration of biodiversity, constructing/searching out water sources during extreme summer, monitoring the progress of ecosystem recovery, and post-caring of sites can also be experimented with. All these processes must be integrated with ecological engineering aimed at vegetation establishment and ecosystem creation in order to optimise land productivity and soil fertility. Consultation with local people must be a part of all effective decision-making processes. Finally, the development of the science of restoration should go hand in hand with achievements of successful restoration itself.