## 2 Research Methods and Results
The density was calculated using the following formula:  
\[\rho=(m_{1}-m_{2})/V, \tag{1}\]  
where \(m_{1}\) is the mass of sandy soil with the loosest or densest structure, g; \(m_{2}\) is the mass of the glass flask, g; V is the volume of sand filling up the flask, cm\({}^{3}\). The results of performed calculations indicate that under contamination, soils have lower densities in the loosest and in densest conditions compared to conditionally pure samples (Table 1). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{
\begin{tabular}{c} **Laboratory** \\ **Sample** \\ \end{tabular} } & \multicolumn{3}{c}{**Loose Structure**} & \multicolumn{3}{c}{**Dense Structure**} \\ \cline{2-7}  & **Density, g/cm\({}^{3}\)** & **Average Value 1**, & **Density, g/cm\({}^{3}\)** & **Average Value 1**, \\  & **from** & **to** & **g/cm\({}^{3}\)** & **From** & **To** & **g/cm\({}^{3}\)** \\ \hline \multirow{3}{*}{1–6} & \multirow{3}{*}{1.41} & \multirow{3}{*}{1.43} & \multicolumn{3}{c}{Conditionally pure sand} \\  & & & 1.42 & 1.71 & 1.72 & 1.72 \\ \cline{1-1}  & & & \multicolumn{3}{c}{Contaminated sand} \\ \cline{1-1}  & & 1.31 & 1.34 & 1.32 & 1.51 & 1.52 & 1.51 \\ \hline \hline \end{tabular}  
* Calculated as a weighted average value.  
\end{table} 
Table 1: The density measurements for the sands under study. Figure 4: Triangular diagram of grain-size distribution for studied sands: green dots—conditionally pure sands; red dots—sands covered by hydrocarbon films. The density of contaminated loose and dense sands reduces by 0.10 g/cm\({}^{3}\) and 0.21 g/cm\({}^{3}\) respectively. In percentage terms, the density of contaminated sand in a loose structure compared to conditionally pure sand decreased by 7%, and in a dense composition by 12%. This could be explained by the aggregation of mineral particles. Aggregates, which constitute mineral grains cemented by hydrocarbon films, take more space per unit of volume compared to uncontaminated particles. This has a direct impact on the porosity of sand. Previous studies investigating properties of sands under hydrocarbon contamination demonstrated that, along with the density being reduced from 1.43 down to 1.01 g/cm\({}^{3}\) and from 1.68 to 1.25 g/cm\({}^{3}\) for loose and dense samples, respectively, porosity increased from 0.47 to 0.58 and from 0.37 to 0.49, respectively [19]. The lower density and higher porosity of contaminated sands lead to a change in permeability. To quantify the water permeability of sandy soils, laboratory tests with soil specimens were carried out. According to the normative technique, the permeability coefficient was measured while maintaining a permanent hydraulic gradient throughout the entire test (\"steady-state filtration condition\") [21]. For this purpose, a permeameter--a filtration tube designed by SPECGEO--was used. The results acquired allowed the analysis of the relationship between the filtration coefficient and and density. To perform such an experiment, specimens of sandy soils with the loosest and densest structures were prepared. Once the necessary sand density was reached and the apparatus was filled with distilled water, permeability tests were started. Over the course of the experiment, the time needed to filter a given volume of water was measured, while the head gradient was maintained at the level of 1.0. A total of 24 tests were completed--12 for each type of sand. The results acquired were used to develop a plot relating the filtration coefficient (at T = 10 \({}^{\circ}\)C) and sand density (Figure 5). Figure 5 shows that, despite differences in loose sand density, contaminated specimens had almost comparable permeability relative to conditionally pure sands. The filtration coefficient of contaminated sands was about 18.7 \(\div\) 20.0 m/day compared to 17.3 \(\div\) 19.8 m/day for pure sands. In the case of dense specimens, the permeability appeared to be significantly different. The filtration coefficient for contaminated sands ranged from 5.1 to 6.5 m/day, for pure sands, it varied from 11.2 to 13.0 m/day.