## 3 Discussion  
Analyzing the results of laboratory studies of conditionally pure and contaminated sands revealed some patterns in changes in the sandy soil grain-size distribution and their physical-mechanical and hydrophysical properties that occurred under long-term hydrocarbon contamination. Detected patterns can be shown in the form the following scheme (Figure 9).  
Figure 8: Shear stress–displacement curves for dense sands: \(\sigma_{1}\), \(\sigma_{2}\), and \(\sigma_{3}\)—normal stress values, 100, 150, and 200 kPa, respectively.  
Figure 9: Scheme of sandy soil transformation under long-lasting hydrocarbon contamination.  
It is seen from Figure 9 that input of petroleum hydrocarbons in the upper part of the profile, namely the unsaturated zone built of air-dried sands, is accompanied by a sorption process [27; 28; 29]. According to published data, the sorption capacity of soils is measured by their granulometric size composition [30; 31; 32; 33]. The highest capacity, in relation to hydrocarbons, is typical of clay soils, which are able to sorb and retain more than 40 L/m\({}^{3}\). The lowest capacity is characteristic of coarse-grained sands, which is less than 8 L/m\({}^{3}\)[19; 34]. However, the nature of this phenomenon is quite complex. It is defined by two main factors: the formation of polar groups within oil hydrocarbons and the presence of unlike charges on the surfaces of sand particles [35].  
It is known that when oil hydrocarbons interact with air oxygen their active transformation generated by chemical oxidation occurs [36; 37]. Such a process results in the evaporation of light hydrocarbon fractions and in a growing number of polar groups (-OH,-COOH, etc.) within heavier hydrocarbons, promoting an increase in their activity rate [38; 39]. Functional groups composing oil hydrocarbons can be regarded as polyelectrolytes, which can be sorbed at the surfaces with unlike charges. Such sorption is considered to be irreversible [40; 41]. The number of retained hydrocarbon molecules depends on their composition, molar mass, sorption energy, and the properties of mineral grain surfaces in contact (Figure 10) [13].  
The present understanding of unlike charges forming on the surface of sand mineral particles, based on spectrophotometric studies, is that it relates to the charge heterogeneity, which is expressed by active center formation (Lewis acids and bases). Silicon atoms with unlike charges play the role of such centers, which may chemically react with adsorbed hydrocarbon molecules (Figure 11) [13].  
As implied above, it follows that the interaction of sandy soils with oil hydrocarbons is a complex process, the development of which depends on the sands' mineral and grain-size composition, as well as on the hydrocarbons' structure. It is worth noting that, in order to reveal mechanisms of oil hydrocarbon sorption on the surface of mineral particles, individual thorough studies are required. However, despite the insufficient knowledge of such interactions nature, results of the investigations carried out revealed some relationships between changes in contaminated soil composition and transformation of their physical-mechanical properties. Summing up the results, the following conclusions can be drawn.  
Figure 10: Some sorption mechanisms of oil hydrocarbons with polar groups, displaying alignment of molecules at the surface of mineral particles [13]: I–III—successive stages of sorption; A–C correspond to weak, intermediate, and strong interactions.