Potential for intrinsic and enhanced crude oil biodegradation in Louisiana’s freshwater marshes

Abstract

This study determined the intrinsic rates of biodegradation of Louisiana “sweet” crude oil (LSCO) in aPanicum hemitomon freshwater marsh using kinetic microcosm studies and verified the results in a large intact core study. In addition, the potential to enhance biodegradation using inorganic nutrient additions was determined. These freshwater marsh soils have high intrinsic rates of degradation (2.0%/day) for the measured alkane fraction (C11–C66) and even higher rates (6.8%/day) for the measured polycyclic aromatic hydrocarbon (PAH) fraction (naphthalene, methylated naphthalenes, phenanthrene, and methylated phenanthrenes). However, there were compound-specific effects with intrinsic rates of degradation highest for the smaller alkanes (C<15) (8.5–2.1%/day), while rates for longer chain alkanes (C>15) were much lower (0.7–1.2%/day). Results from the intact core study indicate that these rates are similar to those experiencedin situ, with the exception of the PAH fraction, whose rate constants will be substantially lower than those determined in the kinetic study. Nitrogen (ammonium) was primarily the limiting nutrient and increased degradation rate constants (2–3 fold). Few differences were seen between different classes of alkanes after fertilization. Critical nitrogen loading rates (amount needed to produce significant degradation increases) were similar for both the microcosm and core study (2.2–8.8 mg NH₄ ⁺-N/g oil), while maximum rates of degradation were observed at higher loading rates (22–44 mg NH₄ ⁺-N/g oil). While crude oil degradation can be enhanced by fertilization, the benefits need to be weighed against the presence of high intrinsic biodegradation rates in these systems.