By Dr Mautushi Das
The unscientific and unplanned practice of rat hole mining in Meghalaya is an age-old phenomenon which has led to the environmental degradation over a period of time. The imbalance on environment created during and after the coal mining operations could be understood from the long term ecological after-effects. Water bodies like rivers, streams and springs which support rich biodiversity and traditional agriculture and serve the purpose of drinking and irrigation are adversely affected by contamination of Acid Mine Drainage (AMD) originating from mine spoils and leaching of heavy metals. The colour of water in these mining areas, which varies from brownish to reddish orange, speaks for itself about the degree to which it has been polluted. Consequently, the rivers and streams of the area show low pH, high conductivity, high concentration of sulphates, iron, many toxic heavy metals, low dissolved oxygen and high BOD. All these parameters characterize the degradation of water quality and diminish the life-supporting function of water there. This has eventually resulted in a drastic loss in biodiversity in all the contaminated water bodies which is threatening the ecological balance of the area.
Mining activities across the world have led to the contamination of many rivers and streams with toxic heavy metals, which have often obscured the natural loading of metals to the water and sediment. In a detailed study conducted by the writer on different streams located in active and abandoned mining site, and coal storage areas of Wapung, Ladrymbai, Khliehriat, Dkhiah found very low pH which ranged from 2.0 to 4.2, while high conductivity ranged from 233 to 954.25 µS/cm. Turbidity ranged from 1.16 to 10.77 NTU, dissolved oxygen ranged from 2.85 to 7.26 mg/l throughout the study period of 3 years. Sulphate content was also very high in mine impacted streams where it varied from 42.01 to 188.12mg/l during the study. The streams are toxic in nature diminishing most aquatic lives from the system.
Algae, the primary producers of any aquatic body, are also prominent features of many AMD impacted systems worldwide. Interestingly, I found that this group of lower plant with tremendous tolerance to survive in stress conditions were present and luxuriantly growing in all the coal mine impacted streams studied. The structure and function of algal communities in any aquatic system usually depend on the physical and chemical characteristic of water (physical and nutrient concentration) along with its other climatic factors prevailing in that area. Diversity and abundance of algal assemblages in a particular system thus reflect the ecological condition of that aquatic environment. Species composition, diversity, abundance, dominance of different algal groups were studied in different selected streams to determine the extent of changes occurred in the algal assemblages due to change in water characteristics mainly in AMD streams which resulted from extensive coal mining .Only 18 algal species belonging to four different classes of Chlorophyceae, Bacillariophyceae, Euglenophyceae and cyanophyceae were recorded from the streams in different seasons. These species were encountered every month and with high cell density dominating the impacted systems. Few tolerant diatom species like Frustulia rhomboides, Kutzing., Navicula cryptocephala Kutzing., N. microspora Kant and Gupta., N. viridis Kutzing., Pinnularia viridis (Nitz.) Ehrenberg., Eunotia exigua (Brebisson ex kutzing), Euglena mutabilis Schmidtz, a member from Euglenophyceae and two filamentous green algae Microspora quadrata Hazen. and Klebsormidium acidophilum Novis. dominated the AMD streams with huge number. An exclusive and distinctive feature of these streams was the presence of long thick bight green colored algal mat covering the whole stream bed which composed of two filamentous green algae Microspora quadrata Hazen. and Klebsormidium acidophilum Novis. Fascinatingly this algal mat was present in all the AMD impacted streams of Jaintia hills district and composed of the same two species mentioned above. 98% of the mat was composed of Microspora quadrata Hazen.and 2% by Klebsormidium acidophilum Novis. Statistical analysis like linear regression showed decrease in pH level in gradient of stress increased the growth and dominance of these two tolerant species. Productivity which is measured by chlorophyll a content also confirmed the fact that decreasing pH, and increasing metal concentrations resulted in increased productivity of these species.
Detail observation of the mat in various streams gives an idea as to how these tolerant algal species (which are growing producing huge biomass in the field prevailing condition) could be used to recover the contaminated systems. So, a detailed study was carried out by firstly analysing the mat from different AMD impacted streams to discover the accumulation capacity of different metals by the algal mat using Atomic absorption spectrophotometer. Field condition study revealed that metal like iron and lead were manifold higher in algal mat compared to that in water. Linear regression analysis showed significant positive correlation between Fe, Pb, Mn, Ni content in water with that in algal biomass. In case of Zn, no such significant correlation could be observed. In general, Fe content was maximum followed by Zn, Mn, Pb, Ni and Cr in water whereas accumulation was maximum for Fe, followed by Mn, Pb, Zn , Ni, and Cr in the algal mat collected from different AMD streams. The trend was as follows:
Water= Fe> Zn> Mn> Pb> Ni > Cr. Algae= Fe> Pb>Mn> Ni>Zn> Cr
So, it was successfully demonstrated that mat composed of Microspora quadrata and Klebosormedium acidophilum has immense capacity to accumulate toxic metals which is the primary cause of water bodies becoming poisonous and unfit for drinking and irrigation. Therefore, the most dominant algal taxa sample was brought to the laboratory and grown in controlled cultured condition. Four metals like iron, zinc, manganese and lead were tested in laboratory condition by Microspora quadrata at different concentrations and at different exposure time. There was significant increase of metals in algal tissue when the exposure time and metal concentrations were increased. Of all the metals (except Zn) tested on Microspora quadrata at different concentrations and exposure time increased when metal concentrations in feed solution decreased with each exposure time. 95-98% of Fe and Pb removal was observed by the alga. 93-95% removal of Zn was observed till 4th day exposure thereafter reducing the removal potential. 60-75% Mn removing potential of the alga was also recorded during different days exposure. Moreover, productivity of Microspora quadrata grown at different pH level (2.0 to 8.0) was maximum at pH level of 2.0 to 4.0.The alga grows the best when it is grown in low pH level. These experimental results supported the results obtained from field conditions and confirmed the acidophilic nature of the alga and tolerance of the alga to extreme acidic condition. Ultra structural studies showed that the metal adsorption was restricted to outer wall of Microspora quadrata filament and did not affect the other cellular organelles.
Despite the complex pattern of polluted environments generated by the combination of these various agents of stress, algal communities were a reliable indicator of AMD. Reduction of species number to as low as 18 in AMD streams clearly indicated the level of degradation of water bodies due to ongoing coal mining. Dominance of two filamentous algae Microspora quadrata Hazen. and Klebsormidium acidophilum Novis covering the streams with huge biomass showed the tolerance of these two green filamentous algae to extreme acidic condition.
Recently the interim order of National Green Tribunal was passed following an application filled by all Dimasa Students’ Union, Dima Hasao District Committee against the state of Meghalaya. The petitioners contended that the water bodies which are sources of water for human consumption and other related activities were severely contaminated in the downstream areas of Assam due to Acid mine drainage from the mining areas of Jaintia hills located in the upstream. Most of the streams and rivers located in Jaintia hills and other parts of the state where rat hole method of mining is being practised is also facing tremendous water pollution problem as the water bodies are loaded with heavy metals and extreme acidity. Preliminary survey of many streams and rivers of Jaintia hills and Garo hills showed the presence of the above mentioned alga in the contaminated systems itself. As the state is facing lack of potable water and severe health problems in case of consumption. Therefore, recovery of the systems will be a matter of concern for the state if ongoing primitive method of mining is stopped. In case government has any management plan to recover the acidic water bodies to reutilize the systems, this alga which grows luxuriantly in coal mining impacted sites, could be suggested as a useful plant material to remove toxic metals from AMD impacted systems of the region at lower costs and reduced impact on the ecosystem. Once the metals are removed from the contaminated water, acidity can be controlled by using different chemicals like limestone, hydrated lime, pebble quicklime, soda ash, caustic soda, ammonia etc.