Seasonal Variation in Stable Isotopes and Physico-chemical Characteristics of AMD Affected Water Bodies in Northeast India

The northeastern states of India have extensive deposits of coal among other minerals. Coal mining activities contribute significantly to the economy of these states, providing livelihood to a substantial segment of the populace. Unscientific mining practices have contributed adversely to environmental health of the region, particularly to aquatic systems. Acid mine drainage (AMD) discharge from the mines pollute the streams and rivers, causing serious deterioration of the aquatic environment. Stable isotopes combined with physico-chemical parameters are a promising tool for understanding pollution sources, hydrological processes, factors influencing such processes, and fate of pollutants in hydrological systems. The present study was conducted in AMD affected streams & rivers flowing through the states of Arunachal Pradesh, Nagaland, Assam and Meghalaya in northeast India in two different seasons (pre-monsoon (PRM) & post monsoon (POM)). Water samples were analyzed for 24 physicochemical parameters including the major ions. Stable isotopes of oxygen & hydrogen in water samples were estimated. The results of both sampling seasons are significantly different and reveal that water samples of all sampling sites are dominated by presence of high SO₄^2- in both the sampling seasons; abundance of major ions during PRM are in order SO₄^2->Mg²⁺>Ca²⁺>HCO₃^->Na⁺>Cl^->NO₃^->K⁺>NH₄⁺>F^- and during POM are in order SO₄^2-> Ca²⁺>Mg²⁺>HCO₃^->Na⁺>Cl^->NO₃^->K⁺>NH₄⁺>F^-. The stable isotopes of water (δ¹⁸O & δD) analysis results indicated enrichment during the POM sampling season. δ¹⁸O (‰ V-SMOW)  ranged between -8.60 to -4.95 during PRM and -6.60 to -3.94 during POM, δD (‰ V-SMOW) ranged between -58.29 to -29.63 during PRM and -41.40 to -28.69 during POM. The deuterium excess (d-excess ‰) ranged between 5.70 ~ 17.94 during PRM and 1.57 ~ 14.49 during POM. The hydrochemical characteristics of water during both sampling seasons were deciphered through comprehensive analysis of hydrochemistry (piper diagram, durov plot, gibbs diagram, ion ratios, chadha’s plot) and stable isotopes. The results are discussed.