Oil spill in the Río de la Plata estuary, Argentina: 2-hydrocarbon disappearance rates in sediments and soils

The 6-month assessment of the oil spill impact in the Río de la Plata described in the preceding paper [Colombo, J.C., Barreda, A., Bilos, C., Cappelletti, N., Demichelis, S., Lombardi, P., Migoya, M.C., Skorupka, C., Suárez, G., 2004. Oil spill in the Río de la Plata estuary, Argentina: 1 - biogeochemical assessment of waters, sediments, soils and biota. Environmental Pollution] was followed by a 13- and 42-month campaigns to evaluate the progress of hydrocarbon decay. Average sediment hydrocarbon concentrations in each sampling include high variability (85-260%) due to contrasting site conditions, but reflect a significant overall decrease after 3 years of the spill: 17 ± 27, 18 ± 39 to 0.54 ± 1.4 μg g−1 for aliphatics; 0.44 ± 0.49, 0.99 ± 1.6 to 0.04 ± 0.03 μg g−1 for aromatics at 6, 13 and 42 months, respectively. Average soil hydrocarbon levels are 100-1000 times higher and less variable (61-169%) than sediment values, but display a clear attenuation: 3678 ± 2369, 1880 ± 1141 to 6.0 ± 10 μg g−1 for aliphatics and 38 ± 26, 49 ± 32 to 0.06 ± 0.04 μg g−1 for aromatics. Hydrocarbon concentrations modeled to first-order rate equations yield average rate constants of total loss (biotic + abiotic) twice as higher in soils (k = 0.18-0.19 month−1) relative to sediments (0.08-0.10 month−1). Individual aliphatic rate constants decrease with increasing molecular weight from 0.21 ± 0.07 month−1 for isoprenoids and n-C27, similar to hopanes (0.10 ± 0.05 month−1). Aromatics disappearance rates were more homogeneous with higher values for methylated relative to unsubstituted species (0.17 ± 0.05 vs. 0.12 ± 0.05 months−1). Continued hydrocarbon inputs, either from biogenic (algal n-C15,17; vascular plant n-C27,29) or combustion related sources (fluoranthene and pyrene), appear to contribute to reduced disappearance rate. According to the different loss rates, hydrocarbons showed clear compositional changes from 6-13 to 42 months. Aliphatics disappearance rates and compositional changes support an essentially microbiologically-mediated recovery of coastal sediments to pre-spill conditions in a 3-4 year period. The lower rates and more subtle compositional changes deduced for aromatic components, suggest a stronger incidence of physical removal processes.