Intrinsic bioremediation of a petroleum hydrocarbon-contaminated aquifer and assessment of mineralization based on stable carbon isotopes

This study presents a stepwise concept to assess the in situ microbial mineralization of petroleum hydrocarbons (PHC) in aquifers. A new graphical method based on stable carbon isotope ratios (δ¹³C) was developed to verify the origin of dissolved inorganic carbon (DIC). The concept and the isotope method were applied to an aquifer in Studen, Switzerland, in which more than 34,000 liters of heating oil were accidentally released. Chemical analyses of ground water revealed that in this aquifer locally, anaerobic conditions prevailed, and that PHC mineralization was linked to the consumption of oxidants such as O₂, NO_3- , and SO<SUB>4</SUB>^2- and the production of reduced species such as Fe<SUp>2+</SUp>, Mn²⁺, H₂S and CH₄. However, alkalinity and DIC balances showed a quantitative disagreement in the link between oxidant consumption and DIC production, indicating that chemical data alone may not be a reliable assessment tool. δ¹³C ratios in DIC have been used before for bioremediation assessment, but results were reported to be negatively influenced by methanogenesis. Using the new graphical method to display δ¹³C data, it was possible to identify anomalies found in methanogenic monitoring wells. It could be shown that 88% of the DIC produced in the contaminated aquifer originated from microbial PHC mineralization. Thus, the new graphical method to display δ¹³C ratios appears to be a useful tool for the assessment of microbial hydrocarbon mineralization in a complex environment.