Distribution of sulfur and oxygen isotopes in rivers of eastern himalayan states: Implications for riverine sulfate source partitioning in coal mining areas of northeast India

Abstract

Stable isotopes of sulfur and oxygen in water can provide valuable information about sources, geochemical and biogeochemical processes, and conditions influencing source to sink movement of sulfur in hydrological systems. Although research on stable isotopes of sulfur and oxygen in water has gained momentum worldwide, there are limited study from water bodies in the eastern himalayan states of northeast India. The present study was conducted in the eastern himalayan states of northeast India, in rivers flowing nearby Namchik-Namphuk Coalfields in Arunachal Pradesh, Makum Coalfields in Assam, coal mining areas near Changki village in Mokokchung district of Nagaland, and Bapung coalfields in Jaintia Hills region of Meghalaya. Water samples were collected from twenty-one (21) sampling stations and analyzed for major ions, sulfur and oxygen stable isotopes (δ34SSO4 & δ18OSO4) in sulfate and hydrogen and oxygen stable isotopes (δDH2O  & δ18OH2O ). δ18OH2O and δ18OSO4, coupled with δ34SSO4 are used to partition riverine sulfate sources in water. pH of water samples in study area ranged between 2.90 to 7.95 and ionic abundance was in order SO42- > Ca2+ > Mg2+ > HCO3- > Na+ > Cl- > K+ > NO3- > NH4+ > F-. Sulfate ion ranged between 9.29 to 5051.57 mg/L. δ34SSO4 ranged between -12.87 to 9.75 ‰ (V-CDT) and δ18OSO4 ranged between -17.32 to 22.36 ‰ (V- SMOW). δ18OH2O ranged between -7.91 to -3.94 ‰ (V- SMOW) and δDH2O ranged between -49.57 to -28.69 ‰ (V- SMOW). The difference between oxygen isotope compositions in SO42- and H2O (Δ 18OSO4-H2O) ranged from -11.26 to 28.96 ‰. Oxidation of sulfides contributed sulfur largely in rivers impacted by acid mine drainage (AMD) followed by evaporites weathering and dissolution in the study area. Evaporation and mineral dissolution play dominant roles in AMD impacted water. The results from study will help in defining measures for remediation of polluted rivers in the study area.