Geochemical characterization of mine overburden strata for strategic overburden-spoil management in an opencast coal mine

Opencast coal mining generates a large amount of mine overburden spoils and creates huge overburden dumps in and around the mining area. It is a big issue as most of the overburden materials (OBMs) are unsuitable for vegetation due to lack of true soil characteristics, and are generally contaminated with toxic heavy metals (TMs). High sulfur content in overburden spoils is another major concern because it contributes to acid mine drainage (AMD) and metal leaching to the surrounding environments. Here, we studied 15 overburden strata (OBS) and four coal seams lying vertically up to a depth of 35 m below ground in an active opencast coal mine located at Margherita (Assam). The pH of the OBS samples ranged from 2.68 to 7.74, and the sulfur (S) content from 194.31 mg kg −1 to 25,301.5 mg kg −1. The content of TMs also varied from stratum to stratum in overburden horizons, and the pollution index of Ni and Cr were found higher (Pi >2) than the other TMs (As, Cd, Cu, Zn, and Mn). Pb and Co were not detected in the OBS. Sulfur showed moderate to high negative correlation with pH, Cd, Fe, and Mn, whereas a significant positive correlation was observed among pH, Cd, Fe, and Mn content. The analyses of toxic metals and sulfur loads occurring in different horizons of coal overburden led us to develop an effective dumping model that could be useful for sustainable overburden management in any opencast mining