Geochemical alterations of forest soils in post-mining area (the Moscow brown coal basin, Russia)

Long-term coal mining in the Moscow basin has a complex negative impact on soils of natural ecosystems. Due to underground mining operations at coal deposits in the Moscow basin spoil heaps with a high content of iron sulfides and aluminosilicates were formed. Active oxidation of sulfides in waste dumps results in the producing of toxic sulfuric acid and iron sulfates (Nordstrom and Alpers 1999). Acid mine drainage (AMD) of sulfuric acid, Al and Fe sulfates as well as pyritized material, entering from eroded spoil heaps, results in physico-chemical and morphological changes in soil properties. On foreslopes around spoil heaps technogenically transformed soils are common. Our study aimed at evaluation of post-mining geochemical transformation of soil properties, which is adjacent to spoil heaps.

We observed two key sites within abandoned coal mines in the western part of Moscow basin (the Tula region, Russia). Prevailing natural soils are Umbric Albeluvisols and Umbric Podzols (sod-podzolic soils and sod-podzols in Russian classification). Soils with transformed water regime are formed in mine subsidence.

Soil samples and displaced soil solutions (by ethanol) were analysed for acid-base properties, content and composition of readily soluble salts, content of Fe²⁺ and Fe³⁺, H⁺ and Al³⁺, composition of exchangeable cations, total content of S, Al, Fe, heavy metals (HM) and organic carbon).

Properties of newly formed soils differ significantly from natural soils. We identified the transformation of the composition of soil solutions. The basic geochemical processes in contaminated soils are as follows: the acidification and changes in the composition of ions in soil solutions from bicarbonate-sulfate-calcium to sulfate-iron-aluminum-calcium; cation exchange, leading to the replacement of Cа²⁺ and Mg²⁺ ions by Al³⁺ and H⁺ ions, and, probably, by Fe²⁺ and Fe³⁺ in soil ion-exchange complex. Transformed soils were characterized by a very low degree of base saturation (less than 20%). Estimation of the saturation degree of liquid phases of transformed soils with poorly soluble compounds revealed a high oversaturation of soil solutions with Fe³⁺ and Al³⁺ hydroxides. The total content of HM in transformed soil profiles were lower than background values due to the removal of soil reserves of elements in highly acidic conditions (pH<4.5). Among the morphological features of transformed forest soils intensification of podzolization process (acid hydrolysis of fine clay fractions of aluminosilicates) as well as; ferrugination (segregation of ferric iron, mainly in amorphous or poorly crystallized forms) and carbon enrichment of coal origin can be noted. Due to AMD impact that had destroyed fine clay minerals, numerous clarified areas were formed, composed of quartz and feldspar. Semi-hydromorphic soils with signs of gleying and peat accumulation were formed in subsidence areas.