Changes of solute transport characteristics in soil profile after mining at an opencast coal mine site on the Loess Plateau, China

Abstract

Dramatic changes in soil during opencast coal mine activities inevitably change soil water and solute movement. The objectives of this study were to assess the effects of mining on solute transport in different soil profiles and to find the relationships between solute transport parameters and soil physicochemical characteristics. After taking undisturbed soil columns from the different soil profiles of natural land (NLs) and reclaimed mine land (RLs), ammonium nitrate displacement studies were conducted, and the breakthrough curves (BTCs) of NO3− and NH4+ were obtained. Due to mining and reconstruction, soil texture became coarser, bulk density increased 2.77–15.15%, cation exchange capacity (CEC), soil organic matter (SOM), and total nitrogen (TN) content decreased 19.54–37.25%, 53.66–72.05%, and 42.58–58.55%, respectively. Additionally, the distributions of the physicochemical properties changed. The BTCs of NO3− for all soil columns can be well explained by both the convection-dispersion equation (CDE) and the two-region model (T-R). The transport parameters average pore water velocity (v), dispersion coefficient (D), and dispersivity (λ) were lower in RLs, which indicates that solutes became difficult to penetrate after mining. A significantly correlated relationship of bulk density and soil texture with the v, D, and λ parameters only occurred in the RLs samples. This result indicates that transport processes in NLs columns became more complicated and that soil texture and bulk density played an important role in the transport process of RLs columns. The two-site model could well describe NH4+ transport for both NLs and RLs and provided a slightly better fit in RLs. This finding also gave evidence that RLs became homogeneous after mining and reconstruction. The parameter retardation factor (R) was significantly correlated to CEC, bulk density and soil texture in RLs. These results suggest that soil becomes homogeneous after mining and reconstruction and that high bulk density and fine soil texture could decrease the risk of leaching of solutes.