Special features of soil development within overgrowing fly ash deposit sites of the solid fuel power plant

Abstract

Waste deposit sites of solid fuel power plants exist within the majority of large cities of the Urals, Siberia and the Far East. Fly ash deposit sites are often located in close proximity to residential areas and may be observed as a potential source of environmental hazard, which leads to the alienation of significant land lots. The problem of reclamation of ash dumps is of particular concern, and its solution is impossible without understanding the processes of vegetation and soil cover development within such territories. In this paper, we present the results of studying the processes of soil development at the sites of the abandoned ash dumps of the power station in the city of Tyumen that was formed during the work of the enterprise on local peat as a main fuel type. The fly ash deposit sites of Tyumen CHP-1 plant consist of two sites with a total area of 100 ha (Site 1 - 54.0 ha, Site 2 - 46.0 ha) (See Fig. 1). The disposal of ash and slag wastes was discontinued more than 30 years ago; currently, within the territory of the dumps there is a gradual overgrowing. The surface of the ash dumps is aligned, subsidence relief forms occur, and the elevation difference is 1-3 m. A more diverse and complex relief is typical of Site 1. The level of groundwater varies significantly, depending on the position in the relief. Modern vegetation is mosaic; two main types of plant communities are represented: woody-shrubby and grassy. We conducted field studies of the soil cover of the fly ash deposit sites of Tyumen CHP-1 plant in summer 2017. At each fly ash deposit site, a pair of soil profiles was created (See Fig. 1) under woody and grassy vegetation (Profiles 1 and 4, 2 and 3, correspondingly) (See Fig. 2). Diagnostics of soils was conducted according to the Russian Soil Classification considering additions proposed for technogenic soils (Shishov LL et al., 2004). The names of soils are also given according to the World Reference Base for Soil Resources 2015. Profile 1 (57°8'47,30''N, 65°38'8,70''E) is found at a local elevation in the peripheral part of Site 1 under depleted willow forest with willow and aspen undergrowth and a developed grassy layer. Profile 2 (57°8'23,50''N, 65°38'22,80''E) is confined to the surface of Site 2, complicated by subsidence forms under meadow-grassy association with the presence of cereals. Soil is Spolic Technosol (Epiarenic, Endoprotocalcic, Fluvic, Hyperartefactic, Laxic, Amphivitric). Profile 3 (57°8'23,90''N, 65°38'17,80''E), characterizes local decline at Site 2. Vegetation is represented by an aspen-willow forest with developed litter. Soil is Spolic Technosol (Endoprotocalcic, Fluvic Hyperartefactic, Laxic, Stagnic, Amphivitric). Profile 4 (57°8'38,10''N, 65°37'30,50''E) is found at Site 1 with a pronounced technogenic filler relief. Meadow vegetation with willow, aspen and sea-buckthorn undergrowth is characteristic. Soil is Spolic Technosol (Amphiprotocalcic, Fluvic, Hyperartefactic, Laxic, Amphivitric). We studied the diversity and intensity of soil-forming processes at meso-, micro- and submicroscopic levels: in transparent sections using polarization and inversion microscopes and a scanning electron microscope equipped with an attachment for elemental analysis. We determined the granulometric composition of the technogenic deposits on a laser particle analyzer with preliminary dispersion of samples with pyrophosphate (See Fig. 3). The color of soil horizons and anthropogenic substrate was estimated by the Munsell system. The results of the conducted studies indicate that the intensity of soil-forming processes in soils developed from ash differs depending on the position in the relief, the type of vegetation, the level of groundwater, and the heterogeneity of the manmade substrate. The studied soils correspond to Spolic Technosol in accordance with the World Reference Base for Soil Resources. Technosols developed within ash deposit sites inherit the properties and material composition of the technogenic substrate, which is an alternation of fly ash and ash residue layers. The main components of the soil-forming material are silicates, aluminosilicates and ferrosilicates, particles of underburn and microspheres (See Fig. 4 and 7). The processes of soil development occur synchronously with the processes of transformation of the material composition of the initial substrate. The main soil-forming processes are: metamorphism of mineral matter (disintegration, carbonatization) (See Fig. 5), reorganization of the soil mass (coagulation and herbaceous-root structuring), metamorphism of the organic matter (the intake and transformation of plant residues and, to a lesser extent, humification) (See Fig. 6), and migration of matter and gley formation. Several soil-forming processes are diagnosed only at micro- and submicromorphological levels. The quantity and diversity of pedogenic carbonates is an important indicator of the intensity of soil formation. Concretions and microcrystalline forms of calcite are most numerous and well expressed in soils with the most formed on well-drained areas in the peripheral parts of the ash dump under the sparse tree vegetation with a developed grassy layer.