Sulphur and oxygen stable isotopes in acid mine drainage impacted rivers of Meghalaya (India): deciphering the sulphide oxidation pathways.

Sulphur and oxygen stable isotopes of sulphate have been used to trace the sources of sulphur into aquatic systems. These isotopes have also been used to understand the transformation and fate of sulphur in the water bodies contaminated by AMD discharge from active and/or abandoned mines. Stable isotopes of oxygen in dissolved sulphate (δ¹⁸O_SO4) and water (δ¹⁸O_H2O) have helped to decipher the sulphide oxidation pathways and estimate their contributions. The present study is focused on analysing the composition of sulphur and oxygen stable isotopes of sulphate and oxygen stable isotope in AMD and Lunar-Lukha River water flowing through the coal mining area of the East Jaintia Hills District, Meghalaya, in order to decipher the sulphide oxidation pathways. The results showed that the sulphur stable isotope of sulphate (δ³⁴S_SO4) ranged between -12.5 and -8.0 ‰ (VCDT). The oxygen isotope of sulphate (δ¹⁸O_SO4) ranged between 1.4 and 2.0 ‰ (VSMOW). The oxygen isotope of water (δ¹⁸O_H2O) was distributed between -6.2 and -4.2 ‰ (VSMOW). Pyrite oxidation was found to be the dominant source of sulphate in the Lunar-Lukha Rivers. The results of the stoichiometric isotope balance model showed that 68-83 % of sulphate derived Fe³⁺ oxidation pathway, with a high portion of sulphate oxygen derived from water. The sulphite-water oxygen exchange model revealed the release of intermediate sulphoxyanions, suggesting the presence of an oxidation pathway of sulphide minerals to sulphate via sulphoxyanions. The results from this study will be helpful in defining effective remediation strategies to mitigate AMD impacts.