Soil reconstruction and heavy metal pollution risk in reclaimed cultivated land with coal gangue filling in mining areas

Abstract

Coal gangue filling reclamation is an important means to restore coal mining subsided cultivated land. However, coal gangue can release heavy metals, threatening the overlying soil environment. During the reconstruction of reclaimed mine soils (RMSs), soil properties evolve in a complex manner and influence the distribution of heavy metals, forming complex network relationships that differ from those of non-subsided cultivated soils (NCSs). It is difficult to determine the mechanism of RMS reconstruction and heavy metal accumulation and distribution by studying the trends of only a few variables. Complex network theory (CNT), however, can reveal the original internal characteristics and mechanisms of the system in a comprehensive and effective way. This study aimed at revealing the characteristics and mechanisms of soil reconstruction and heavy metal distribution in RMSs from a holistic perspective. Reclaimed cultivated land with coal gangue filling in high-groundwater-level mining areas in Jining City, Shandong Province, China, was selected for the study. First, soil samples were collected from depths of 0–20, 20–40, and 40–60 cm, and a total of 21 soil indicators were determined. Then, CNT was applied to explore the special soil property relationships and key factors in the RMS reconstruction process. Finally, the intrinsic mechanism of RMS reconstruction and heavy metal distribution was revealed by comparison with NCSs. The results showed that coal gangue filling reclamation and reconstruction activities led to increases in bulk density (BD), decreases in nutrients, and the accumulation of heavy metals. These activities also changed the relationship strength and cooperation efficiency among soil properties. BD reduction due to soil aggregate recovery increased microbial activity and accumulated organic carbon and nutrients, which became essential for RMS recovery. The filled coal gangue became a new source of heavy metals for the RMSs, with water-dissolved colloids acting as carriers of the heavy metals.