Abstract: Petroleum hydrocarbons (C₁₀-C₄₀) are composed of hydrocarbon compounds with different carbon numbers, which would have different degrees of ecological environmental risks. In this study, we analyzed the residual characteristics of petroleum hydrocarbons in each carbon segment in soils with different pollution histories in the oil exploitation area of Liaohe Oilfield. We evaluated and predicted the ecological environment risk of soil petroleum hydrocarbon pollution based on soil quality index and Monte Carlo simulation. The results showed that the content of residual petroleum hydrocarbons in the soil of oil field mining area varied greatly (17.8-2004.7 mg·kg^-1), and that the proportion of each carbon segment was different as follows: C₂₂-C₄₀ (53.00%) > C₁₃-C₁₆ (31.81%) >C₁₇-C₂₁ (15.17%) > C₁₀-C₁₂ (0.02%). The natural decay rate constants (λ) of petroleum hydrocarbons in different carbon segments ranged from 0.0286 to 0.0395 a^-1. The residue model and 95% confidence interval constructed based on λ reflected the accumulation and residue processes of petroleum hydrocarbons in each carbon segment. Risk assessment showed that short-term pollution (≤35 a) residual petroleum hydrocarbons led to low levels of ecological environment risks, while long-term pollution (>35 a) led to medium and low risks. Our results suggest that the overall ecological environment of oilfield area will be at a very high risk level for a long time under the condition of continuous pollution and no reasonable control.

Key words: petroleum hydrocarbon carbon segment, residual model, natural decay, pollution history, risk assessment.