Concentrations, mobility, and potential ecological risks of selected metals within compost amended, reclaimed coal mine soils, tropical South Sumatra, Indonesia

Abstract

Coal production in tropical areas of southeast Asia, the majority by opencast mining, has grown exponentially since the late 20th century, raising concerns about its impacts on terrestrial and aquatic ecosystems. In this study, potential impacts of six metals (Al, Fe, Mn, Ni, Pb, Zn) in reclaimed mine soils amended with compost were examined at the PT Bukit Asam-Tanjung Enim mine in South Sumatra, Indonesia. The analysis was carried out by measuring and comparing metal concentrations in the soil matrix and soil waters at three reclaimed sites of differing age and compost treatments. Calculated soil enrichment factors and risk quotients (for water and soil) based on multiple background and toxicity criteria suggest the risk posed by metals within the reclaimed soils and soil water was negligible. Differences between treated and untreated sites suggest the influence of organic matter (OM) on dissolved metal concentrations varied between metals. Insignificant statistical differences (P < 0.05) in dissolved Mn or Zn concentrations between control and compost treated sites suggest that dissolved organic carbon (DOC) did not influence Mn or Zn soil water concentrations. In contrast, control plots had the lowest dissolved soil water concentrations of Al, Fe, and Pb; maximum concentrations were measured at a 1-year old compost-amended site whose soils had the highest C contents. These data suggest that the dissolution of OM, and the formation of DOC, may have increased the solubility of Al, Fe, and Pb within the reclaimed soils. However, no significant differences in metal concentrations were observed between surface- and subsurface soil layers, suggesting that migration was limited at all sites, including those treated with compost. Metal mobility and risks were likely reduced by multiple factors, including (1) relatively low metal concentrations and a lack of acid-producing sulfide minerals in the reclaimed topsoil, and (2) the fine-grained nature of the reclaimed soils, which limited the rate of soil water flow and increased the sorption potential of the soil matrix.