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

Abstract

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-C22 to 0.10 ± 0.08 month −1 for > 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.