## 2 Research Methods and Results  
In order to carry out experimental studies and comparative analysis of transformation in sandy soil composition and properties, which had been affected by hydrocarbon pollution, 12 samples of both contaminated and non-contaminated sands were taken (6 samples of each type). Sampling sites were confined to the area of an abandoned military shooting range, which was previously supported by the continuous use of oil hydrocarbons as fuel and lubricants. Prolonged use of oil products resulted in the contamination of soils which was specific to this territory. Contaminated areas in the form of concentric zones were easily identified by the dark-gray and black colors of sandy deposits and by the distinctive odor of oil hydrocarbons. During trial pit excavation, some sand intercalations containing oil hydrocarbons were found. By visually analyzing the collected sand samples, it was established that uncontaminated sands were characterized by yellowish-beige color, medium-grained structure (prevailing grain size 0.25-0.5 mm), and quartz-feldspar composition. In contrast to conditionally pure sands, contaminated samples had a dark-brown to black color and were similar to poorly cemented sandstone with a pronounced smell of oil hydrocarbons (Figure 1). To determine the amount of oil hydrocarbons in contaminated sandy soils a fluorometric analysis was performed using the analyzing device Fluorat 02-3M. The substance of such analysis performed according to the normative document (PND F 16.1:2.21-98) was to extract oil hydrocarbons from contaminated samples with a hexane solution and to determine their content using a chromatography column [20]. The results thus obtained showed that in test samples the oil hydrocarbon content was 17.5 g/kg. Figure 1: Visual distinctions between conditionally pure (**a**) and contaminated sands (**b**). The visual description of sand samples was complemented by a microscopic study that led to the discovery of hydrocarbon films on the surface of mineral particles in contaminated sands (Figure 2). The presence of oxidized hydrocarbon films on the surface of mineral particles in contaminated sands led to the formation of microaggregates ranging in size from 0.5 to 5.0 mm. Detected films played the role of cementing substances (Figure 3). Studying sand grain-size distribution by means of the sieve method (GOST 12536-2014) revealed that conditionally pure samples were within the range of medium-grained sands, whereas contaminated sands were classified as coarse and very coarse-grained (Figure 4). Analyzing the granular composition of samples under study shows that the formation of hydrocarbon films on the surface of sand grains and the building up of microaggregates results in increased coarseness. As seen in Figure 4, the content of particles with a size between 0.5 and 2.0 mm in conditionally pure sands varies from 12 to 22%, whereas in contaminated sands it exceeds 51%. Moreover, in contaminated samples, we observe the emergence of individual aggregates with sizes from 2.0 to 5.0 mm. Figure 3: Microaggregates of sand particles contaminated by oil hydrocarbons (the image was taken with a LEICA DM750 microscope). Figure 2: Image of sandy soils obtained using a LEICA DM750 microscope: (**a**) conditionally pure sands; (**b**) sands contaminated by oil hydrocarbons. The transformation of granular composition towards the increased content of large fractions affects the density of contaminated sands with both dense and loose structures. Measurements of sand density were based on a method, which used previously weighed glass flasks of the same volume. The loosest structure of sand samples was achieved by freely filling flasks through the funnel. The most compact structure was attained by filling the flasks with sand in small portions, which was followed by compaction using the impact of the tamper on the flask walls.