Data on physico-chemical characteristics and elemental composition of gray forest soils (Greyzemic Phaeozems) in natural-technogenic landscapes of Moscow brown coal basin

Waste rocks material and acid mine drainage (AMD) in sulfur coal mining areas of Moscow brown coal basin lead to significant transformation of landscape components (soils, surface, and groundwaters). Most of the abandoned sulfide-bearing spoil heaps have not been reclaimed and toxic products of their weathering cause the risk of long-term soil contamination. In this article, we report original data on some physico-chemical properties and elemental composition of liquid and solid soil phases, waste dumps and AMD from twо abandoned spoil heaps of the Moscow basin and adjacent territories (the Tula region, Central Russia). Soil samples were collected from each genetic horizon of soil depth profile at sites affected by waste dumps and mine subsidence, as well as at natural sites. Waste material was sampled from the different parts of the spoil heaps. Sampling of AMD was performed in technogenic reservoirs near waste dumps. In displaced liquid phases (by ethanol) from soils and waste dump material, natural superficial waters and AMD pH-value, electrical conductivity (EC), the content and composition of readily soluble salts (by high-performance liquid chromatography (HPLC)), as well as titratable acidity (H+and Al3+) and, water-soluble Fe (using UV/Vis spectrophotometry) were measured. In bulk soil samples organic carbon (Corg), exchangeable cations (Cа2+, Mg2+, H+, Al3+ in KCl-extracts) and hydrolytic acidity (in CH3COONa-extracts) were determined. The obtained data can be used to understand the behavior of сhemical elements in soil profiles polluted by coal mining; the negative impact of mine wastes on soil salinity; when identifying pollution levels of potentially hazardous elements in soils affected by coal mining and for complex remediation of spoil heaps in Moscow brown coal basin.


Data source location
Data source location The sampling sites were located in the southern part of Moscow brown coal basin (the Tula Region, Russia). Natural landscapes are watersheds and gentle slopes with deciduous forest, mixed-grass meadows, and fallow lands of the northern part of the Central Russian upland. Prevailing soils are Greyzemic Phaeozems Albic (IUSS Working Group WRB, 2015

Value of the Data
• First open access complex database on physico-chemical properties of solid (exchangeable cations) and liquid (soil solutions) phases, levels of macro-and microelements and particle size of the transformed soils, waste dumps and acid mine drainage in sulfur coal mining areas of the Moscow brown coal basin in Central European Russia. Reported complex data are the key to understanding the geochemical processes occurring in soils polluted by coal mining. • The data can be used by researchers to understand migration and accumulation of с hemical elements in soil profiles affected by coal mining as well as to forecast the negative impact of mine wastes on soil salinity status. • The data obtained might be helpful in the identification of pollution levels and in the geochemical assessment of technogenic anomalies of potentially hazardous elements in the soils affected by coal mining. • The data may be useful for policy makers to develop programs for complex remediation of spoil heaps and forest-steppe landscapes in the Moscow brown coal basin. Fig. 1 shows sampling locations of soils, waste dumps, natural superficial waters, and AMD at two key sites. The number of each sampling point is supplemented by a capital letter «S», «D», «F» or «W» («S» for soil, «D» for waste dumps, «F» for AMD (filtrated waters) and «W» for natural superficial waters). Photos of the soil profiles are presented in Fig. 2 . Description of the sampling points location and morphological properties of natural soils, and soils with technogenic transformations are given in Table 1 . Table 2 contains data on the selected chemical properties (pH value, electrical conductivity (EC), content and composition of readily soluble salts, titratable acidity, water-soluble Fe 2 + and Fe 3 + ) of natural superficial waters and AMD released from the spoil heaps. Data on chemical properties (pH value, readily soluble salts, titratable acidity, water-soluble Fe 2 + and Fe 3 + ) of displaced liquid phases from soils and waste dump material are given in Table 3 . Content of organic carbon (C org ), pH value of KCl-extracts, concentrations of exchangeable cations and hydrolytic acidity of natural soils, and soils with technogenic transformations are shown in Table 4 . Data on distribution of five grain-size soil fractions (10 0 0-250, 250-50, 50-10, 10-1 and < 1 μm) are presented in Table 5 . Concentrations of macroelements (Fe, Si, Al, Ca, Mg, Ti, S, P, K) and microelements (Mn, V, Cr, Ni, Zn, Pb, Sr) in reference and transformed soils of the key site «Kireevsk» are shown in Table 6 .  Table 1 . Abbreviated names of places of sampling are the same as given in Fig. 1 .

Dataset area and objects
The dataset area is situated in the northern part of Central Russian upland and belongs to the southern part of Moscow brown coal basin (the Tula Region, Russia). Watersheds and gentle slopes are occupied by deciduous forests with lime, maple and oak and mixed-grass meadows.
Natural soils of the dataset area are Greyzemic Phaeozems Albic [10] (gray forest soils in Russian classification [3] ), silty, heavy loamy on mantle loams. Because of the high percentage Table 6 Concentrations of the selected macro-and microelements in soils. of ploughed land (up to 70%) arable and post-arable soils on fallow lands are widespread. In karst sinkholes and local depressions, Gleyic Phaeozems prevail. Due to the technology of underground mining, conical spoil heaps of waste rocks 40-60 m high were formed on the land surface. Spoils of the Moscow brown coal basin comprise of iron sulfide-bearing carbonaceous black greasy clays with kaolinitic clays, brown coal layers, loams, sandy loams, and quartz sands, as well as pyrite crystals with СаСО 3 (calcite) and FeCO 3 (siderite) impurities in clays [8] .

Macroelements
The weathering of the spoil heaps led to the formation of deluvial-proluvial tailings of sandyclay gangue with a high content of sulfides (mainly pyrite and marcasite), as well as organic carbon of coal origin. Technogenic deposits that overlap soils could be up to several dozen centimeters in thickness. The waste material and AMD released from the spoil heaps were strongly acidic ( рН< 4.5) due to continuous oxidation of sulfides and subsequent releasing of toxic sulfuric acid, as well as the formation of ferric and ferrous iron sulfates [6] . Oxidation of aluminosilicates in clay minerals (predominantly kaolinite and illite) by acidic waters resulted in the formation of toxic aluminum sulfate in soils [2 , 7] . Leaching of the gangue also led to migration of the potentially hazardous elements to the soil. Besides, the profile of the overlapped soils had specific morphological properties: carbonaceous-humus films on faces of structural units and secondary gypsum neoformations along with admixture of pyritized fragments and carbonaceous particles in soil pore space.
Because of dewatering of abandoned coal mines, in coal mining areas dips and subsidence up to 6 m deep were formed [9] . Changes in moisture conditions led to the development of semihydromorphic soils with different grades of gleying. In the soil profiles of mine subsidence areas, muck accumulation and formation of the organogenic horizon were observed.
The key geochemical processes in Greyzemic Phaeozems at mine sites were as follows: (1) acidification and Fe-Al-SO 4 salinization of soil profile along with the increasing of H + and Al 3 + ions content; (2) cation exchange, leading to the displacement of C а 2 + and Mg 2 + by Al 3 + , H + and by Fe 2 + cations in soil ion-exchange complex; (3) accumulation of potentially hazardous elements. That resulted in the formation of technogenic variations of Greyzemic Phaeozemsacidified, compacted, carbonized, base-unsaturated, and salinized soils.

Sampling procedure
Sampling of soils, waste dump material, natural superficial waters, and AMD was performed at tw о abandoned spoil heaps of Moscow brown coal basin: Smirnovskaya-6 (the key site «Кireevsk) and Skuratovskaya-6 (the key site «Tula») and adjacent territories.
Soil samples were taken from the central part of each genetic horizon and in 8 soil pits (4 for each key site) down to 110-150 cm. For each horizon, three subsamples were collected from different walls of the pit to form a composite sample. Reference soils were sampled within 500 m from the spoil heaps. Soils with technogenic transformations were sampled in areas of the deluvial-proluvial deposits and the release of AMD. Two soil pits at the key site «Tula» were located in the mine subsidence area.
Waste material was sampled by drilling down to a depth of 100 cm at the foothill of the Smirnovskaya-6 waste dump and the slope of the Skuratovskaya-6 waste dump.
Natural surface waters samples were taken at 4 locations: from the ponds in karst holes (2 samples) and rivers (2 samples) at a distance of about 500 m from the spoil heaps. The sampling of AMD was performed at 3 points of its release (from waterlogged reservoirs), at the foothills of the waste dumps.

Laboratory methods
AMD and superficial waters were sampled into 500 mL chemically inert plastic containers. The containers were filled up to the lid to avoid the degassing of the water and were placed into a portable refrigerator at 4 °C. The samples of water and AMD were filtered through 0.45-μm PVDF filters (MillesHV, Millipore) and analyzed in the laboratory using highperformance liquid chromatography (HPLC) within 24 h after sampling.
The collected samples ( n = 100) of waste dumps, technogenic deposits, and soils, were airdried at temperature < 40 °C and were crushed to a particle size of 1 μm for physico-chemical analyses. Prepared bulk samples were stored in special sealed plastic containers. Fresh soil and technogenic deposits samples were used for soil solutions displacement. Soil solutions were displaced by ethanol (Ischtscherikow-Komarova method) [4] . The samples of soil and technogenic deposits at natural moisture were sieved through a 3 mm mesh sieve, then placed in plastic tubes with an inside diameter of 4 cm and 100 cm in height to a bulk density of 1-1.2 g cm −3 for soil solution extraction by displacement with ethanol according to Ischtscherikow-Komarova method [4] . Soil solutions (20-40 mL) were collected in chemically inert plastic containers. The testing of the solution for alcohol was made organoleptically. The measurements of each sample were performed in one replicate.
Electrical conductivity (EC) of soil solutions was measured using the conductometer Seve-nEasy S30 (Mettler Toledo, Switzerland). Anions (Cl − , SO 4 2 − ) and cations (Ca 2 + , Mg 2 + , K + , Na + ) in soil solutions were measured by HPLC using a Styer chromatograph (Aquilon, Russia). The results of measurements of soil solutions are given in mmol c dm −3 . The calculation of the ionic ratios, as well as their proportion in the sum of anions and cations, was performed for ion concentrations, expressed in mmol c dm −3 . The content of H + and Al 3 + ions in soil solutions (the sum of H + and Al 3 + is equal to titratable acidity) was determined by titration to pH 8.2 using a 0.01 М NaOH solution. The total alkalinity of soil solutions was determined by acid-base titration using a 0.01 М H 2 SO 4 solution to pH 4.4. [5] . Total mineralization (in mg L −1 ) was evaluated by summarizing concentrations of all determined elements.
After the displacement of the soil solution, soil samples were removed from the columns. These samples were used to determine the exchange cations. Exchangeable cations (C а 2 + , Mg 2 + , H + , Al 3 + ) were determined in bulk soil samples by extraction with 1 M KCl solution at a ratio soil:solution as 1:2.5. The content of exchangeable Ca 2 + and Mg 2 + in KCl-extracts was determined by titration using a 0.05 М EDTA solution [5] . Exchangeable acidity (the sum of H + and Al 3 + ) was released upon exchange by a buffered 1 M KCl solution at the soil to solution ratio of 1:2.5.
The suspension was shaken and filtered. The content of acidic components was determined in the filtrate by titration to pH 8.2 using a 0.01 М NaOH solution [5] . Hydrolytic acidity was measured upon exchange with 1 М CH 3 COONa solution at the soil to solution ratio of 1:2.5.
The content of water-soluble and exchangeable (KCl-extracts) Fe 2 + and Fe 3 + in soil and soil solutions was measured by UV/Vis spectrophotometry with α-α-dipyridile using Odyssey DR 20 0 0 spectrophotometer (Hach, USA). The pH-values of soil solutions and KCl-extracts were measured by the potentiometric method using Expert 001 ionometer (Econics Expert, Russia). The total content of C org in soil was determined by K 2 Cr 2 O 7 oxidation method. The grain size distribution in soil and technogenic deposit samples was quantified using Analysette 22 MicroTec plus (Fritsch, Germany) laser particle sizer. Samples were pre-treated for analysis by dispersing with 4% Na 4 P 2 O 7 without H 2 O 2 oxidation of organic matter. The particlesize classes were defined in accordance with the Russian conventional fraction groups [1] .
The total content of sulfur in technogenic deposits and soils was measured using portative analyzer WDXRF «Olympus Innov-X Delta» (Delta-X, USA). For XRF analysis, air-dried soil samples were crushed manually in an agate mortar to a particle size of ≤71 μm. The samples were pressed into cups 3 mm deep and specially made from boric acid.
Calculation of the results of soil, technogenic deposits and waste material analyses was done on the basis of oven-dried (at 105 °C) soil mass.