Effects of local soil chemical and hydraulic nonequilibria on element budgets of disturbed sites

Open-cast lignite mining is causing large-scale and long-term disturbance, resulting in a destruction of existing ecosystems and creation of landscapes formed by overburden spoil piles. Overburden sediments can largely differ from parent geological material of previously existing and undisturbed sites. It is expected that new soils and ecosystems will develop from overburden sediments with element budgets differing with respect to rates and directions of development from the undisturbed situation. Amelioration measures further alter the conditions. Typical for such mine soils is a small-distance spatial heterogeneity as a result of mixing of chemically and physically different sediments. Initially, highly different components, such as pyrite/alkaline ash, sand/clay/coal fragments, lignite/soil organic matter, or saline/nonsaline zones, can exist directly next to each other. For such small-distance spatial variability, predictions of long-term development of soils and sites are challenging. Despite disturbed soils show hardly any diagnostic horizons, soil profiles are characterized by distinct depth gradients of soil chemical conditions. Precipitation of secondary minerals, water repellency features of the lignite components, and preferential flow paths may depend on the smalldistance spatial heterogeneity. Irregular flow patterns could be observed by dye-tracer studies. Element budgets of disturbed sites are characterized by extremely high soil solution concentrations of most elements. Although solute concentrations decrease with site age, they are still several orders of magnitudes higher compared to undisturbed sites even 50 years after disturbance. Chemical conditions change more rapidly along preferential flow paths than in other soil regions, by enhanced dissolution and transport, thereby affecting plant root growth and soil development. Better understanding of the effects of local non-equilibrium conditions in geochemical and physical properties will help to improve the long-term prediction of the development of disturbed sites.