Natural Attenuation Processes for Petroleum Releases in Karst Aquifers in Kentucky

Natural attenuation processes in karst aquifers are highly understudied. This study evaluates the occurrence of natural attenuation in a shallow karst aquifer affected by a continuous crude oil seep in Bowling Green, Kentucky, United States. To assess the relative contributions of recharge-driven advection/dilution and in situ biodegradation, high-resolution groundwater level and precipitation data were monitored together with groundwater hydrochemistry. The contaminant concentrations were strongly impacted by recharge events. Thus, the intense precipitation events rapidly flushed the contaminated well, reducing total petroleum hydrocarbon (TPH) concentrations below the analytical detection limit. Elevated TPH concentrations coincided with lower dissolved oxygen, nitrate, pH, and oxidation-reduction potential (ORP), and increased concentrations of iron and manganese in groundwater. Comparison with a nearby hydraulically connected uncontaminated well showed that these geochemical changes were restricted to the contaminated portion of the aquifer. The study provides field-based evidence that advection/dilution and localized biodegradation are relevant natural attenuation pathways in the karst aquifer investigated. Although biodegradation fingerprints were less pronounced than in intergranular aquifers, they consistently developed during periods of hydrocarbon accumulation. These results provide field-based evidence of natural attenuation in an active karst aquifer and provide a better understanding of petroleum hydrocarbon behavior.