Reclamation of mining dumps: Is mining waste suitable for forest restoration?

~ Prachi Thatte

With 664 million tonnes (Mt) of coal, India was the third largest producer in 2014-15, next to China (3474 Mt) and the USA (924 Mt). The Indian government has announced an ambitious plan to produce 1500 Mt of coal by 2020, at an annual growth rate of almost 20%. In order to meet this target, massive expansion of open cast mines is envisaged. About 80% of India’s coal reserves lie in the central Indian landscape and much of it is under forests. Destruction of forests is inevitable for open-cast mining. Along with deforestation, direct and indirect mining activities change the landscape surrounding the mine. Direct activities include removal of the top soil, followed by excavation of overburden and then coal extraction. Indirect activities include tree felling for constructing roads, houses and other infrastructure, thus increasing the anthropogenic impact on the surrounding landscape.

A coal mine. Photo by Nitin Kirloskar.

Under the Forest Conservation Act, 1980, whenever forest land is diverted for non-forest use, compensatory afforestation (CA) needs to be carried out on an equal amount of non-forest land, or double the amount of degraded forest land. It is usually recommended that CA should be done at the point closest to where diversion is taking place. Mining companies often reclaim the overburden dumps for afforestation. But can these overburden dumps, after reclamation, support similar species of trees which were found in the forest that was cleared? How different are the physical and chemical properties of the dump soil compared to the soil found in the surrounding areas? In order to answer these questions, Jitendra Ahirwal and Subodh Kumar Maiti from the Indian School of Mines, Dhanbad evaluated the changes in soil properties due to direct and indirect mining activities around Ananta open cast mine in Odisha. They collected soil samples from 5 different sites in 2008:

1. Soil from Sal (Shorea robusta) forest patch- original habitat before the area was mined

2. Reclaimed mine soil (RMS)- While excavating coal from the mine, the overburden (rock and soil which lies above the coal) was dumped in a nearby area. In 2003 these dumps were reclaimed by planting fast growing species of trees such as Acacia and Cassia

3. Mine face topsoil, the uppermost layer of soil which is close to the excavated area

4. Soil from wasteland- Sal forest that was degraded due to human activities

5. Soil from agricultural area (mostly rain-fed paddy cultivation).

Within each of these sites, five 10m x 10m plots were marked and soil samples were collected from different depths (0-20cm, 20-40cm and 40-60cm from the surface). Physical and chemical properties of the samples were analysed in the laboratory, except the infiltration rate or the speed at which water enters the soil which was measured at each site. The study finds that the physical and chemical properties of the reclaimed soil is drastically different from the soil in the Sal forest. Soil from wasteland and Sal forest was found to have similar physical composition (approx. 84% sand, 12% silt and 4% clay). The soil in all other land uses had a very different composition with significantly lower amount of sand. Sal forest soil had low pH and salinity and high nutrient content. Whereas the soil associated with mining activities, both the reclaimed as well as topsoil were found to have higher pH and salinity and low nutrient content.

Sal forest. Photo by Ruchi Singh Rao.

Although wasteland and intact Sal forests were similar in many respects, nutrient content and moisture of the soil was much lower in the wasteland. The rate at which water seeps into the soil was found to be fastest in sal forest followed by wasteland and was the lowest in reclaimed area. A high rate is favorable for downward movement of water from surface which recharges the ground water stores and reduces run-off to a great extent. Sal trees are known to flourish in such well drained, slightly acidic soil. The study concludes that removal of natural Sal prior to mining and reforestation with fast growing tree species during reclamation, will not recover original forest composition due to complete alteration of soil properties. The authors recommend adding a layer of topsoil (excavated from another mine site) on the surface of the reclaimed dump. They suggest using a mixture of grass and legumes as it helps in restoring a degraded site. Legumes are plants which have special type of bacteria associated with their roots that can use Nitrogen directly from the air and provide it to the plants. When they die, they decompose fast and increase the nutrient content of the soil. However these techniques may still not be enough for restoration of soil quality and hence the original habitat. Several studies all over the world have shown that it is usually not feasible to replicate the exact conditions which existed before mining. Massive alteration of physical and chemical properties and soil drainage preclude this. Habitat reclamation should hence focus on planting species which fulfill the same role in the ecosystem as the original ones. The aim should be to create a stable ecosystem similar in composition and function to the one that existed before mining but not necessarily identical to it. Such habitat reclamation approaches have not been extensively used in India. In fact, according to a 2014 Comptroller and Auditor General (CAG) report, of the 1,03,390 hectares over which compensatory afforestation had to be done since 2002, the environment ministry had a record of afforestation on only 7% of the land. Compensatory afforestation initiatives need to catch-up soon if we are to cope with the massive expansion of the coal mines that is envisaged in the next decade. Whether the envisaged expansion is necessary to meet the energy demands of the country and if compensatory afforestation can actually compensate for the lost forests are discussions for another time. Original Paper: Ahirwal, J and Subodh Kumar Maiti (2016). Assessment of soil properties of different land uses generated due to surface coal mining activities in tropical Sal (Shorea robusta) forest, India. CATENA, 140:155–163. DOI: 10.1016/j.catena.2016.01.028. Available at: http://www.sciencedirect.com/science/article/pii/S0341816216300455