2.1. Study Area and Sites Description
The South Ural region is a part of the Urals, stretching from the Ufa River to the Ural River (
Figure S1). From east and west, it is bounded by the East European and West Siberian plains.
The climate in the South Ural region is continental. Winter (mid-November to March) is long, severe, and clear sunny days prevail. There are blizzards and snowstorms, especially in February (2–16 days a month). Average monthly air temperature in January is −22 °C (minimum −45 °C). Sometimes, Atlantic cyclones bring thaws with sleet, rain, and fog (1–6 foggy days per month). Snow cover appears in October. The height of the snow cover in March is 40–80 cm. Spring (April to May) is cool with frosts. It is characterized by an alternation of cloudy days with rain and sleet and clear sunny days. Snow cover melts in mid-April, in some years it lasts until early May, the spring thaw lasts more than one month. Frosts occur until the first half of July. More precipitation falls on the western (windward) slopes of the Southern Urals (550–650 mm, and in some places more) than on the eastern (leeward) slopes (400–450 mm). The wettest months are in summer, when about half of the annual rainfall falls; the winter period accounts for no more than 25% of the annual rainfall.
The Chelyabinsk coal basin is located along the eastern slope of the Urals. Coal seams were discovered in 1832. In 1934, the Korkinsky open-cut mine No. 1 was commissioned. It flourished in the 1970s and 1980s, when up to 3 million tons of coal were mined annually. Over the 70 years of its operation, the Korkinsky open-pit mine became one of the largest open-pit mines in Russia, the largest coal mine in Europe. Nowhere else is coal open-cut mined from a depth of 540 m. The bottom of the quarry is 250 m below sea level, and half a kilometer below the erosion base of this area. In November 2017, a decision was made to end coal mining at the Korkinsky open-pit mine. Its reclamation began in 2020. Over the past year, most spontaneous fires were stopped at the Korkinsky mine as a result of the implementation of measures for the recultivation of the area: deciduous trees were planted, the upper edge of the section was strengthened, and a fire extinguishing system was built. Currently, air quality is negatively affected by the quarry itself and the long ridge of spoil heaps resulting from many years of field development. In winter, houses in the nearby Korkino and Roza settlements are heated with coal and wood, only a small number of households are supplied with gas. This also influences the PM levels in the air.
The locations of sampling sites are shown in
Figure 1. The sampling sites were chosen with a radial distribution around the Korkinsky mine and with linear distributions to assess the effects of prevailing winds and the influence of Korkino and Roza. The coordinates and site descriptions are presented in
Table 1. A flowchart showing the methodological steps for environmental characterization and analysis of snow cover is presented in
Figure 2.
At the time of sampling in January 2020, the Korkinsky mine was surrounded by a security fence. All snow samples were taken in safe areas, outside the fence. A total of 56 snow cover samples were collected from 12 sites. We chose sites to the north, south, east, and west of the mine (
Figure 1). Snow samples were collected directly near the mine fence (Sites 1, 4, 7, 10), and at a distance of 100 and 500 m from the mine fence (Sites 2, 5, 8, 11 and 3, 6, 9, 12, respectively).
2.2. Snow Cover Studies
Samples were collected on 5 January 2020 after a snowfall. The snow cover settled at the end of November 2019. Snow depth on the date of collection of samples was 25–30 cm. In total, 56 snow samples were taken within the investigated territory (see
Figure 1) at 12 sites, each consisting of 3–5 individual samples taken by plastic pipe with the cross-section area of 20 cm
2 (one plastic pipe corresponded to one snow core) [
14].
Snow cover samples were melted in the laboratory at 20–22 °C. Previous studies indicated that the concentrations of metal(loid)s can vary significantly between filtered and unfiltered snow samples loaded with higher concentrations of mineral dust [
14,
15]. Immediately after melting, filtration through cellulose acetate membrane filters with a 47 mm diameter and 0.45 μm pore size (EMD Millipore, Burlington, MA, USA) was applied to isolate filtrate with dissolved (or soluble) forms of metal(loid)s and particles on the filter with suspended (or insoluble) forms. Filters with a pore diameter of 0.45 μm are widely used to separate dissolved and suspended metals when studying the composition of snow cover [
14,
15,
16,
17]. Filters were dissolved using nitric acid and hydrogen peroxide in closed Teflon beakers at 95 °C for 16 h [
18].
The samples were analyzed for metal(loid)s (Al, As, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Sb, Sr, and Zn) using ICP-MS (Perkin Elmer ELAN 9000). All the readings were taken three times. The detection limit for most elements in solution was 0.01 ppb. The chemical composition of the melt water and undissolved solids were equal to the concentrations of the dissolved and suspended forms of metal(loid)s in snow cover in μg/L.
We used certified reference materials GSO 10413-2014 CO of the structure (agrochemical indicators) of cespitose and podsolic srednesuglinisty soil (I BEND Rosselkhozakademiya’s VNIIA, Russia, Moscow) and GSO 7186-95 of loess soil (Bronnitsky geological and geochemical expedition “Institute of Mineralogy, Geochemistry and Crystal Chemistry of Rare Elements”, Russia, Bronnitsy). The recoveries for elements ranged from 87% to 119%. The total dissolved solids of filtrates were measured in situ by a Portable Meter (Multitest KSL-111, Semico Ltd., Novosibirsk, Russia). Filtrate pH was measured in situ by a Portable Meter (Multitest IPL-513, Semico Ltd., Novosibirsk, Russia).
The cellulose acetate membrane filters containing particulate fractions were characterized by scanning electron microscopy (SEM). SEM analyses were performed on a Jeol JSM-7001F Scanning Electron Microscopy Complex, 30 keV, EDS Oxford INCA X-max 80, WDS Oxford INCA WAVE, EBSD, and HKL. The samples were coated with carbon using a hard vacuum system for wet specimens.