How do mine rehabilitation strategies affects soil heterogeneity and structure in the long term?

Open-cast or surface mining present a site-specific and a temporary use of land that unavoidably involves the displacement of the fertile topsoil layer and overall environmental change. Following mine closure, rehabilitation of mining areas is essential to build sustainable and stable landforms. Reclamation of soils is particularly challenging as their construction needs to take into account broader land reclamation analysis including soils, water, vegetation, stability and biodiversity effects. The objective of this study was to assess development in soil properties of rehabilitated soils since construction (1980, 1998, 2009, 2016, 2017) focusing on hydraulic and mechanical soil properties. The study was performed at an open-cast lignite coal mine site in south-east Australia (Victoria) in 2021. Soil hydraulic properties (SHP) were determined using the extended evaporation method while the mechanical properties were assessed using in-situ vane shear tests. The study indicated the relevance of a consistent soil cover design and construction. The soil hydraulic properties results showed a shift in soil water retention curve characteristic (1980 vs 2017 site), together with a decrease in saturated hydraulic conductivity (Ks), which can certainly influence soil water dynamics and increase surface runoff. The vane shear test showed large heterogeneity among the sites with rehabilitated sites indicating large internal variation compared to the reference site and generally higher shear resistance. Differences in soil hydraulic conductivity and higher SOC storage over time revealed developments in soil recovery, but the improvement in soil mechanical strength did not show any relation to soil properties affecting soil structural stability. Rather, the change was driven by the large variability in texturally affected pore size distribution among the sites. The observed small-scale heterogeneity of the rehabilitated soils is most likely explained by disturbance due to excavation activities and used rehabilitation methods as well as availability of the soil material. Closer monitoring of rehabilitated areas temporally and spatially as well as in improvement in topsoil cover design is recommended, as such heterogeneity leads to uncertainty in long-term sustainable landscape formation.