Deciphering the origin and controlling factors of mercury in reclaimed soils: a case study in Pingshuo opencast coalmine of China

Considering the significant influence of mercury (Hg) contamination on the land reclamation inopencast coalmine, the spatial distribution patterns and ecological risks of Hg were investigated and the regulating factors of Hg mobility were determined in the South Dump of the Pingshuo opencast coalmine. The results show that the total Hg (HgT) contents of most soil samples (83.7%) vary from 6 to 50 mu g kg(-1), while the potential ecological risk index (EIHg) values of most samples (79.8%) are lower than 80, indicating that most reclaimed soils are in relatively good conditions and the soil samples at high to very high ecological risk are mainly collected near the backfilled coal gangue. Moreover, the kriging maps of the geo-accumulation index (I-geo) indicate that the uncontaminated areas (I-geo < 0) and Hg-contaminated areas (I-geo > 0) in topsoil (0-10 cm) are roughly divided by an east-west arc while the Hg-contaminated areas in other soil horizons are characterized by a point distribution pattern. The slight Hg contamination in topsoil is mainly triggered by the atmospheric Hg deposition from the nearby coal-fired power plant, while the Hg contamination in other soil horizons should be attributed to the weathering and spontaneous combustion of coal gangue. On the other hand, Pearson's correlation analyses show that HgT contents were positively correlated with clay (r = 0.31, P < 0.01) and SOC (r = 0.53, P < 0.01) contents. This study can provide some insight for the land reclamation measures in the opencast coalmine.

Automated summary

The study on mercury contamination in the South Dump of the Pingshuo opencast coalmine shows that most soil samples have moderate levels of total Hg, with low ecological risk index values. Contaminated areas are mainly near coal gangue-filled areas, with different distribution patterns in topsoil and other soil horizons. The slight contamination in topsoil is due to nearby coal-fired power plant emissions, while contamination in other soil horizons is linked to coal gangue weathering and spontaneous combustion.