Source apportionment of polycyclic aromatic hydrocarbons and n-alkanes in the soil-sediment profile of Jianghan Oil Field, China

Surface soil in oil exploration area always contains high contents of polycyclic aromatic hydrocarbons (PAHs) and n-alkanes. To investigate the migration possibility of PAHs and n-alkanes from surface through aquitard and aquiclude to aquifer, the distribution, together with the source apportionment using several indicators, such as composition pattern, fluoranthene/(fluoranthene+pyrene) (Flt/(Flt+Pyr)), anthracene/(anthracene+phenanthrene) (Ant/(Ant+PA)), and the carbon preference index (CPI) of n-alkanes, in a 30-m-deep soil-sediment profile were studied. Results showed that there were considerable PAHs and n-alkanes not only in surface soil but also in aquitard, aquiclude, and aquifer sediments. The PAHs and n-alkanes in surface soil strongly suggested petroleum pollution. The high molecular weight PAHs and the n-alkanes with both long and short chains could not migrate into deep sediments as their sources in surface soil and deep sediment were different. Whereas the aquitard and aquiclude had significant input of low molecular weight PAHs (LMWPAHs) from petroleum sources, the LMWPAHs in confined aquifer suggested pyrogenic sources. Therefore, LMWPAHs migrated from surface to aquitard and aquiclude, but did not cause aquifer pollution in Jianghan Oil Field. However, the high mobility of LMWPAHs from surface to aquitard and aquiclude suggested that the long-term risk of groundwater pollution from oil exploration should be concerned.