Geochemical transformations in liquid and solid phases of forest-steppe soils in the affected area of Moscow brown coal basin (Russia)

Long-term coal mining (more than 50 years) in the Moscow basin has a complex negative effect on soils. Because of underground mining at coal fields spoil heaps with a high content of iron sulfides, aluminosilicates and organic carbon of coal origin were formed. Oxidation of sulfides and acid hydrolysis of aluminosilicates in waste dumps results in the producing of toxic sulfuric acid, Al and Fe sulfates (Nordstrom and Alpers 1999). Acid mine drainage (AMD) entering from eroded spoil heaps, leads to physico-chemical and morphological changes in soil characteristics. On foreslopes around spoil heaps technogenically transformed soils are common. Our study aimed at evaluation of post-mining geochemical evolution of chemical composition and properties of solid and liquid soil phases.

We examined two key sites within abandoned coal mine fields in the central part of the Moscow basin. Predominant natural soils are Greyic Phaeozems and Haplic Chernozems (WRB 2014) (Grey forest and Leached Chernozems in Russian classification).

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, heavy metals (HM) and organic carbon) by standard methods. The composition of clay minerals in soils were determined by X-ray diffractometry. The saturation degree of soil solutions by gypsum, iron and aluminum hydroxides was estimated.

Properties of technogenic soils differ significantly from natural soils. We observed the transformation of the composition of soil solutions. Key geochemical processes at mine sites in contaminated soils were: (1) acidification and Fe-Al-SO₄ salinization of entire soil profile along with the increment in H⁺ and Al³⁺ ions content; (2) cation exchange, leading to displacement of Cа²⁺ and Mg²⁺ ions by Al³⁺, H⁺ and, probably, by Fe²⁺ and Fe³⁺ in soil cation-exchange complex (CEC); (3) alteration of radial differentiation of organic carbon and carbonates in soils; (4) clay mineral transformations.

Topsoil features a high content of technogenic organic carbon (reaches 12%) due to the inflow of coal material particles from the dump. Ca²⁺ and Mg²⁺ ions predominate (for 70 to 90%) in CEC of natural soils. Exchangeable Al³⁺ accounts for more than 75% of the acidity formation in transformed soils. The share of exchangeable Ca²⁺ and Mg²⁺ in CEC of contaminated soils depletes on 22-38%.

Extracted soil solutions from polluted soils are heavily oversaturated by Al hydroxides. Even though the activity of Ca²⁺ and SO₄^2- ions in some samples reaches the gypsum saturation level, gypsum neoformations are not distinguished morphologically.

The content of Co, Сu, Ni and Zn in displaced solutions of transformed soils in tens or even hundreds times exceeds the background values. The clay minerals of natural soils are represented by kaolinite, illite, vermiculite and mixed-layer minerals. The sharp increase in smectite fraction (up to 75-80%) and slightly in chlorite fraction was revealed in transformed soils.

Post-technogenic soils have no analogues in natural forest-steppe landscapes of the Russian Plain.