Hydrological performance of soil and vegetation covers impact in mine rehabilitation: results of a preliminary modelling study

Rehabilitation in the mining context refers to restoring the natural characteristics such as land stability, vegetation, soil functions, biodiversity and hydrological cycle. The main aim of mine rehabilitation is to construct environmentally sustainable landforms and to restore their ecosystem services, either to a site specific stable equilibrium or ideally to its previous state. The objective of this work is to build a predictive and decision making tool using hypothetical modelling to simulate water fluxes for two different scenarios in terms of different soil cover depth including vegetation (grass). Hypothetical hydrological modelling was performed using the HYDRUS-1D with one-dimension water flow modelling based on the Richards equation and hydraulic functions of van Genuchten-Mualem model. The soil and coal hydraulic parameters were derived from laboratory tests using the extended evaporation method. Water flux modelling was performed for 2021 using the climatic data from Latrobe Valley (Victoria, Australia) meteorological station, where the coal and soil samples were collected. Two scenarios were selected which varied in the depth of soil cover and coal layer, both with grass vegetation on top of the soil columns. The first scenario (S1) had 50 cm of soil cover, while the second scenario (S2) had 100 cm of soil cover on top of coal material, respectively. Modelling results revealed that soil water content and fluxes were directly reflecting the precipitation pattern and the most limiting factor in downward water flow was the low permeability of the coal layer. The hydraulic parameters for coal show large water retention capacity at very low hydraulic conductivity. The shallower soil cover in the S1 scenario resulted in higher soil water content during the period of intense rainfall and resulted in larger and faster initiation of surface runoff. The thicker soil cover layer resulted in larger infiltration rate and root water uptake which was however limited when the soil was fully saturated in both scenarios. Interestingly, very similar bottom flux in both scenarios even with two different coal layer depth (i.e., 30 cm vs 80 cm) were recorded. Water balance results indicate increase in potential of storing water in the S1 scenario which has a thicker coal layer due to its high water retention capacity. However, at this point it is not clear to what extent stored water from coal can be available for plants. Beside valuable research insights in terms of soil cover design, hypothetical modelling will assist in preventing experimental design flaws and providing a more efficient, robust controlled experiment performed in a next study phase.