Efficacy of two different reclamation strategies to improve chemical properties and to reduce Al toxicity in a lignite mine dump during a 20-year period

In this work, the efficacy of two different strategies, used for 20 years (1992-2012) to restore a lignite mine dump, was investigated. The reclamation strategies were selection of the sterile materials, avoiding surface placement of pyritic materials, or surface application of topsoil material. Representative plots corresponding to each of both strategies were selected, as well as a control plot where reclamation techniques were not applied. An increase in total C, soluble Al, and exchangeable Al, as well as a decrease in pH, electrical conductivity, sulfate, and exchangeable cations concentrations took place during the study period. Avoiding pyritic materials on surface was the strategy giving the best acid-base conditions in the whole period, showing higher pH values (6.1-6.7) than application of topsoil (4.7-5.3), as well as the highest exchangeable Ca values (0.71-6.83 vs. 0.55-3.00 cmol((+)) kg(-1)), and less exchange Al (0.22-2.80 vs. 0.99-3.72 cmol((+)) kg(-1)) and soluble Al (0.04-0.60 vs. 0.19-37.47 mg L-1). Regarding fractionation of Al in soil solution, labile forms predominated throughout the study period, although organic forms (nonlabile Al and acid-soluble Al) showed an increase over time, which was in accordance with an increase in C concentration. The most toxic Al species (Al3+ and Al-OH) were less abundant where pyritic materials were not placed on surface, compared with plots treated with topsoil (which did not differ from control plot for most parameters). The results of the study indicate that surface application of topsoil would not be justified as strategy to effectively restore lignite mine dump areas.