Hydrocarbons in Soils of the Tamsagbulag Oilfield (Eastern Mongolia)

Oil production facilities exercise severe technogenic impact on dry-steppe chestnut soils (Kastanozems) of the Tamsagbulag oilfield. This paper estimates the degree of soil contamination with petroleum hydrocarbons and mechanical disturbance of the soil cover. It has been established that technogenic impact on chestnut soils due to oil production is manifested in a high variability of physicochemical parameters in the surface horizon: aqueous extract mineralization varies from 32.5 to 325.0 mg/L; specific electrical conductivity, from 65.4 to 647.0 μS/cm; while concentration of hydrocarbons (HCs), from 7 to 647 mg/kg. Only in a few cases, HC concentrations exceed the background level established for oil production zones (100 mg/kg) by 3.2 and 6.5 times. The HC proportion in the total organic carbon content (C_org) varies in a wide range: from 0.01 to 6.20%. Gas chromatographic analysis of the molecular-weight distribution of n‑alkanes in the HC composition showed that high molecular weight (C₂₇–C₃₉) homologues with high enough transformation degrees of individual odd-numbered n-alkanes prevail in the surface soil horizon. The proportion of medium molecular weight homologues (∑C₂₂–C₂₅) does not exceed 8% of the total content of n-alkanes. In the soil microbial community, heterotrophic bacteria (HB) predominate among ecological–trophic groups of microorganisms (unicellular bacteria, actinomycetes, and micromycetes); the proportion of oil-oxidizing bacteria in the total HB count reaches 9.1–39.3%. Degradation of petroleum HCs is accompanied by the formation of a wide range of volatile organic compounds. The quantitative and qualitative composition of oil components in the surface horizon is determined by two processes: HC vertical migration and transformation. Together, these processes offset the accumulation of oil in the surface soil horizon.