Diagnostics of the Hydrocarbon Status of Soils when Selecting Background Sites around Oil Fields

Abstract In regions where hydrocarbons are extracted, there remains a challenge in identifying uncontaminated (notionally background) land plots, necessary for developing soil quality standards with respect to petroleum hydrocarbon (PH) content and assessing the level of soil contamination by petroleum. This paper examines approaches to diagnosing the hydrocarbon status of soils at oil fields. The composition and profile distribution of normal alkanes (n-alkanes) as biomarkers of transformation of hydrocarbons, including petroleum products, in soils of the tundra zone, northern taiga, and dry and dry steppe zones are studied. Changes in the composition and profile distribution of n-alkanes in oil-contaminated soils compared to background soils are identified, and the possibility for identifying self-purification processes based on the results of PH composition assessment is demonstrated. Homologues C25, C27, C29, and C31 predominate in the composition of n-alkanes in soils of the tundra and taiga zones; their genesis is associated with the decarboxylation of fatty acids entering the soil with plant residues. In the soils of the dry steppe zone, the predominant ones are n-alkanes of the ligroin-kerosene fraction with a characteristic low proportion of high-molecular homologues. Estimated coefficients based on the composition of n-alkanes (∑C15C17C19, ∑(C20–C25), ∑(C26–C40)) allow us to judge about the rate of self-purification of oil-contaminated soils. The intensity of n-alkane transformation has been shown to be statistically significantly associated with soil properties, natural and climatic factors, as well as the composition of crude oil and the level of oil contamination. When selecting background analogues of oil-contaminated soils in tundra and taiga, it is advisable to take into account the nature of the distribution of even and odd homologues of n-alkanes, the value of the sum of odd n-alkanes (ΣC27C29C31) along with the total content of PH, when selecting background analogues of oil-contaminated tundra and taiga soils. In soils of the steppe and dry steppe zones, under conditions of rapid mineralization of soil organic matter and agricultural use, better markers of background soils are the carbon preference index (CPI) of n-alkanes, the ratio between n-alkanes with long and short carbon chains (C ≥ 25 and C < 25 – LSR), and the sum of n-alkanes Σ(C20–C25), markers of microbial degradation of hydrocarbons.