Estimating spatial distributions of hydraulic parameters for a two-scale structured heterogeneous lignitic mine soil

Abstract

In lignitic mine soils, monitoring of water and solute fluxes is required to assess and mitigate the impact of acid mine drainage on groundwater quality and the success of revegetation. These soils exhibit an appreciable heterogeneity of hydraulic properties, which originates both from the spatial distribution of lignitic components and inclined sedimentary structures. Both types of heterogeneity may lead to preferential flow. Water and element balances are mostly calculated by using one-dimensional (1D) numerical models, which do not account for preferential flow effects. Application of more complex model approaches is hampered by the lack of information about the spatial distribution of hydraulic parameters. The objective of this study is to assess the spatial distribution of hydraulic parameters for a sandy lignitic mine soil site based on visible structures in a 2D-vertical excavated soil cross-section. The hydraulic properties are calculated from the measured bulk densities and texture data, using the Kozeny–Carman-equation for the saturated hydraulic conductivity, K s, and the Arya–Paris approach for the water retention characteristics, ψ( θ). The effective porosity for K s is reduced and the water retention increased in proportion to the lignite contents. Small-scale variability of hydraulic parameters is evaluated by using photographs of lignitic fragment distributions. For the discernable inclined layer structures, with a width of approximately 40 cm and a dip of 35°, K s values in the range between 2.5 and 250 cm/d are estimated. The variability of the water retention characteristics is less pronounced. The predicted ψ( θ) functions are more comparable with loamy than sandy soils. The small-scale intra-layer variability of hydraulic parameters is higher than for lignite-poor mine spoil sediments and for most natural soils. The results may help to improve the analyses of flow and transport in lignitic mine soils by providing 2D two-scale parameter distributions for numerical simulations.