Pedological properties and ecological implications of substrates derived 3 and 11 years after the revegetation of lignite fly ash disposal sites in Serbia

Abstract

The revegetation of disposal sites of lignite fly ash (FA) offers the best way to overcome the extremely serious problems which their deposition via sluicing into settling ponds can pose for the environment and human health. Even so, plant survival and development is limited by toxicity and the highly unfavourable physical, chemical and nutritive characteristics of FA. Hence, initiating soil formation processes at the ash deposit sites is of inestimable significance for the success of this reclamation process.

This case study investigates the influence of weathering and vegetation development on changes in selected physical and chemical properties of FA, as well as assessing the environmental risks of FA at three deposit lagoons (L0, L1 and L2) at the ‘Nikola Tesla-A’ thermal power plant in Obrenovac, Serbia, weathered for 0 (raw ash), 3, and 11 years respectively. The raw FA is characterised by a sandy texture, alkaline pH, high electrical conductivity (EC), a low cation exchange capacity (CEC), and low total nitrogen (N) and available P and K content, while the content of elements such as As, B, Cr and Cu is in the critical range for plants. Weathering and revegetation processes have brought about an increase in the clay and silt fraction, a reduction in alkalinity and salinity, and an increase in the cation exchangeable capacity (CEC) and N, P and K content, particularly in the surface layer (0–10 cm) at L2. Even though these changes point only to the initiation of soil formation processes, they lead to colonisation and in later stages an increase in the diversity of spontaneously colonising plants (55 species at L1 and 80 species and a greater value on the Shannon index of diversity at L2). At the same time, a reduction in the total content of As, B, Cr, Cu, Mn, Ni and Zn and their mobility was noted, as well as in individual and total contamination of FA over time. In this regard, our study, conducted at a large coal ash disposal site, can contribute to knowledge on the effects of weathering and the development of vegetation directly on ash itself and on changes in the physical and chemical properties of ash as important indicators of soil initiation and development on this substrate, with these processes being of exceptional importance for the successful ecological reclamation of ash deposit sites.