Pit lakes formed as discards of open pit mining operations in West Bengal and Jharkhand buzz with activity as a large number of migratory winged visitors visit the lakes to rest and refuel, providing a ray of hope for the large number of abandoned pit lakes that are found in mining areas of India. While mining remains a contentious issue leading to untold destruction of the forests, topography and the environment, making best use of the left over pit lakes in a sustainable manner could be a useful way forward.
What are pit lakes?
Pit lakes are formed when dug outs created during open mining operations get filled with water, either through groundwater recharge, surface water diversion, or functioning pumping following the cessation of mining operations. Pit lakes are thus different from natural lakes and have markedly higher relative depths.
The left over pit lakes harbour a large amount of water and can provide an alternative source of water in locations where water is scarce. Residents living around the pit lakes are known to use the water to meet their household water requirements. Thus knowing about water quality in the pit lakes can be critical in such circumstances.
Raniganj coal field (RCF) has a high proportion of pit lake clusters in West Bengal. Coal mining started in the Raniganj Coalfield (RCF) area in 1774 during the British East India period and covers an area of 1530 sq km, containing about 1306 sq. km of coal- bearing land. The surface mining processes have created drastic changes in the landscape and led to the formation of large overburden dumps, huge voids and pit lake ecosystems in the mining sites. Pit lakes can form in open cut mining pits, which extend below the groundwater table.
Once dewatering ceases, the groundwater, surface water and direct rainfall contribute to the formation of a pit lake. Coal mining in RCF has produced pit lakes that range in area from <1 to 70 ha surface area, <10 to 70 m depth, 5-80 years in age, with the oldest being nearly 100 years old.
There are about 78 old opencast coal pits surrounding 260 hectares of RCF containing a total volume of 4,41,17,700 m3, which can be a dependable aquatic resource. However, many of the pit lakes are seeing water quality degradation and water loss and there is no organised strategy for their monitoring and management.
Even though it is strictly illegal, liming, and fertilising lakes to stock them with fish (mostly Indian major carps) is rather common in these pit lakes leading to negative and irreversible changes in these ecosystems. Ecological assessment of the pit lakes to conserve their biodiversity is thus crucial to make use of these huge water storages. Because of their steep slope and the fact that most pit lakes are inaccessible to people, ecological evaluation of pit lakes is a difficult task.
Pit lakes are utilised by people living around them
While experts suggest that mine water of the coalfield is not satisfactory for first-hand use in drinking & domestic purposes and requires proper treatment before its utilisation, the pit lakes are utilised by a number of people living around it.
About 10 percent of rich, 60 percent of the middle class and 90 percent of the poor households use the mine pit water for domestic purposes. About 10 percent of the rich households and 60 and 80 percent middle-class and poor households respectively consume fish collected from the mine pits while 74 percent of poor households collect fuelwood and wild fruits of the forest from near the mine pits.
What do recent studies on the state of pit lakes in Raniganj show?
A study of pit lake habitats in Harabhanga, Dhandardihi, Searsole and Dalurbandh under RCF located in West Bengal, India showed that Harabhanga pit lake showed a lower dissolved oxygen and hardness of water mainly due to presence of calcium and magnesium ions in the water. The usage of soap, detergents and other cleaning products for washing practices in the Searsole Pit Lake contributed to a greater hardness level than other locations. In Harabhanga, Searsole and Dalurbandh, elevated nitrate and phosphate concentrations were found as they received domestic sewage through drainage systems from the nearby households.
The study also evaluated the density of rotifers to understand the health of the pit lakes. Rotifers are small invertebrates found in lakes (50-2,000 ΐm), have a short life span, but are qualitatively (species richness) and quantitatively (species abundance) important components of zooplankton in aquatic ecosystems. They typically represent more than 60 percent of the zooplankton population in lakes. They are extremely important in maintaining ecological balances in freshwater ecosystems as they form natural food links between primary producers (algae) and zooplanktivorous fish.
Rotifers are considered to be very effective as biological indicators because they are capable of rapidly exploiting suitable environments and are relatively tolerant of minor environmental changes. They are also indicators of the nutrient status and serve as food for higher-order species in water bodies such as lakes and ponds. The mean density of rotifers was 98.40 ind/l in the study, which showed less species diversity as younger pit lakes and the harsh environmental conditions resulted in less species diversity compared to normal lakes.
The study found that the rotifer density of the pitlake was low showing poor quality of the pit lakes, but the very presence of rotifers showed that the present pit lake conditions were capable of supporting ecologically diverse conditions.
Another study in the Eastern Coalfield regions found that 20 pit lakes out of the 62 documented ones needed urgent prioritisation and immediate conservation. A total of 10 areas were surveyed, which fell under the Eastern Coalfields region, namely Jhanjra, Pandabeswar, Kenda, Kunustoria, Kajora, Kenda, Sripur, Satgram, Salanpur, Sodepur, and Mugma.
The pit lakes in the area showed capability of natural restoration and water quality and soil quality were good which enabled rich growth of plant species both on aquatic and terrestrial habitats supporting avian and arthropodal richness. These areas were identified to be brought under immediate conservation for sustaining diversity for the future.
In another study, 16 pit lakes of Paschim Bardhaman district of West Bengal, India, were included for water quality analysis. The areas of the pit lakes varied from 3.31 to 92.21 acres and the mean depths varied from 13.72 to 126 m. All the pit lakes were sandy with mud and had permanently waterlogged hydro-periods. Coal mining was the prior mining activity at all sample sites. In terms of shape, most of the pit lakes could be considered irregular.
All the pit lakes had aquatic plants which were submerged and attached to the sediments in the littoral zone. Dalurbandh, Searsole, and Bonbedi pit lakes were polluted because of domestic sewage and cattle washing and bathing. The surface water quality of the lakes was alkaline and domestic use would require treatment and disinfection.
In this study, it was found that the average quality of the pit lake water as poor, but it had good zooplankton diversity, which indicated the presence of both phytoplankton and fish in the system. Thus, the reservoir was sustainable, and could be revived with proper management strategies.
Conserving pit lakes through pit lake management crucial
Sustainable plans to improve water quality and biodiversity in ‘pit lakes’ do not exist due to confusion over the usability of the lakes, lack of prior planning and involvement of ecologists only after the lake is filled and lack of adequate information on pit lake ecology. An integrated approach to pit lake management should involve ecologists in pit lake design, prioritising ecological progress and passive treatment in mine closure planning, ultimately empowering communities with post-mining options.
The studies show that while water in the pit lakes can be of poor quality, the lakes can be revived through sustainabe interventions and have the capacity to sustain biodiversity while supporting the basic needs of human populations residing in the area and can be useful in times of water insecurity.
It is important to bring the studied pit lakes under central and state government's schemes such as National Plan for Conservation of Aquatic Eco-systems (NPCA), (Government of India, 2013), West Bengal Wetlands and Water Bodies Conservation Policy, 2012 (Government of West Bengal, 2012). National as well as state policymakers can also adopt this framework for setting up conservation regulations for existing self-sustaining mining pit lakes.
