The Fractional Composition of Compounds of Metals and Sulfur in the Upper Layer of Soils of the Impact Zones of a Coal Mine (Central Russian Forest-Steppe)

Abstract—The unreclaimed dumps of the Moscow coal basin are a long-term spontaneous experiment reflecting the transformation of soils and toxic technogenic material under the influence of acid mine drainage with compounds of heavy metals and metalloids. In the Tula region, the particle size distribution, pH value, the electrical conductivity of a water extract, the concentration of total organic carbon and three mobile fractions, as well as the total content of Al, Ca, Fe, Mn, and S were obtained to analyze the transformation of the fractional composition of compounds of six elements in the soils of sulfuric acid geocomplexes at 11 key sites (a total of 57 samples). Heavy loamy chernozems that were not exposed to the waste heap contain up to 9% total organic carbon, have a near-neutral environment, and low electrical conductivity. In these, the average mobility of elements (in %) decreases in the series: Ca (62) > Mn (35) > Al, Fe (14) > S (3). Among the mobile compounds of Ca, exchangeable ones predominate, while Al, Fe, and Mn are extractable in 1 N HNO₃ at close levels of the S concentration of the three studied mobile fractions. Medium-heavy loamy toxylithostrats of the bare surface of the waste heap with a predominantly very acidic environment often have sulfate salinity. In these, the average mobility of elements decreases in the series Ca (80) > Fe (36) > S (15) > Al, Mn (3). Among the mobile compounds of Ca, Mn, and S, exchangeable compounds predominate, that is, Al and Fe, extracted with 1 N HNO₃. Acid mine drainage and solid matter from the waste heap form toeslope talus that bury highly productive chernozems. In terms of the fractional composition of the Al, Ca, Fe, Mg, Mn, and S compounds, the bare soils of the talus are close to the toxic substrates of the waste heap and the grass-covered soils of the toeslope talus are close to the background chernozems. Relative to the waste heap material, the content of mobile compounds and total Ca and S is reduced in the displaced substrate from toeslope talus. Presumably, due to the influence of vegetation in the grass-covered substrate, the contents of mobile Mn compounds and the mobility of Al are significantly increased, while due to the lower supply of substances from the body of the waste heap, the mobility of Fe is reduced. The AUx horizon of chernozems has increases in the content of mobile compounds and total S, as well as water-soluble Ca²⁺ and Mg²⁺ with a decrease in the content of mobile compounds and total Mn.