Fate and Transport of Petroleum Hydrocarbons in Vadose Zone: Compound-specific Natural Attenuation

Groundwater contamination caused by petroleum hydrocarbon (PHC) spills mostly from oil industry is a major environmental concern worldwide. However, infiltration into groundwater is decreasing due to the natural attenuation processes of PHCs in vadose zone, which acts as a safeguard of invaluable groundwater resource against contamination. This study was conducted to determine the retardation capacity of vadose zone and its influence factors based on investigations of a petroleum-contaminated site in NE China. Column leaching experiments in homogeneous and heterogeneous soils were utilized to simulate the actual infiltration process, which aimed to understand the variation of PHC compounds in vadose zone and to examine the effects of soil and water properties on the diversification of the compounds by using gas chromatography-mass spectrometry (GC-MS). The results showed that adsorption and biodegradation are dominant processes and 84 %, 76 %, and 66 % of the organic contaminants were entrapped in fine, medium, and coarse sands, respectively. This was mainly caused by the adsorption coefficient (K-d), which was linked with the soil properties; more specifically, smaller soil aggregates mean a higher K-d value and such discrimination also exists among petroleum compounds. Real-time polymerase chain reaction (RT-PCR) and culture-based methods were applied to identify the degrading microorganisms. Results demonstrate that these microorganisms could degrade compounds such as chainalkanes (ChA), cyclo-alkanes (CyA), and aromatic (Ars) into asphaltenes (Asp). The microorganism population increased with biodegradation products and the consequence of biodegrading capacity was (from high to low): ChA, CyA, and Ars; chemical analyses in the heterogeneous soil experiment indicated that concentration of the biodegradation products in leachate was negatively correlated to dissolved oxygen (DO) as a consumption of oxidants but positively correlated to electrical conductivity (EC) and pH of water. Enzyme activities and microorganism population of soil were positively correlated to concentration of biodegradation products.