Natural degradation of low-level petroleum hydrocarbon contamination under crop management

PURPOSE: Natural degradation of petroleum hydrocarbons (PHC) is a crucial process to consider when managing contaminated soils. However, the degradation rate is dictated by site-specific environmental factors and contaminant characteristics, so it is highly variable. Thus, region-specific research is required to determine the capacity of natural degradation processes to reach cleanup goals. This study describes the natural degradation of low levels of Bakken crude oil contamination in the Northern Great Plains, USA. MATERIALS AND METHODS: Soils contaminated by PHC from an accidental release of Bakken crude oil in western North Dakota, USA, were used to construct research plots adjacent to an active remediation site. The plots had four different initial PHC concentrations, 1400, 700, 220, and 100 mg kg⁻¹, and they were monitored over two growing seasons under crop production. The wheat (Triticum aestivum L.) and field pea (Pisum sativum L.) grown in the plots were assessed for uptake of polycyclic aromatic hydrocarbons (PAH). RESULTS AND DISCUSSION: After two growing seasons under crop management, PHC concentrations were reduced between 46 and 53%. Neither the incorporation of composted manure nor the depth of sampling had any effect of PHC reduction, indicating that initial concentration is the driving factor in determining how long it will take to meet a given threshold. Half-life values ranged from 455 to 573 days, which is critical information for land managers determining the best remediation approach. Notably, crops grown in these contaminated soils did not show increased risk of plant uptake of PAHs. CONCLUSIONS: Overall, these findings indicate that soil PHC concentrations decrease while producing common agricultural crops in this region, and the crops grown in these contaminated soils show no increased risk to crop safety. Thus, depending on specific project goals, continuing a normal crop rotation may be a viable management strategy for low-level soil contamination in this region.