The origins of hydrocarbons trapped in the lake of berre sediments

For decades, the Lake of Berre (France) - a heavily contaminated ecosystem - has received hudge amounts of hydrocarbons from refineries, petrochemical plants, transportation systems, in addition to those originating from marine and terrestrial biogenic inputs. The purpose of this work was to identify the sources of hydrocarbons trapped in the sediment, using characterization indices, based on n-alkane and isoprenoid abundance ratios obtained from GC/MS analysis. The upper sediment was sampled from 5 widely-separated stations. In each station, the subsurface (0–10 cm) and the underlaying (10–20 cm) sediment were separately analyzed by gravimetry, GC/FID and GC/MS. Total hydrocarbon contents ranged from 63 to more than 6100 mg.kg−1 1 of dry sediment. Qualitative analysis of the saturated hydrocarbon fraction demonstrated the presence of weathered petroleum residues of mixed geochemical origins, as evidenced by the presence of different cyclic alkanes known as biomarkers. Total polyaromatic hydrocarbon contents ranged from 29 to more than 2300 mg.kg−1 1 of dry sediment, associated with molecular indexes suggesting the occurence of combustion processes prior to sedimentation. Petrogenic, pyrolitic, and biogenic sources were discriminated, with reference to previous surveys of the same area or of adjacent areas. With regard to biogenic hydrocarbons, terrestrial sources seem generally predominate, except in the Northwestern area where n-alkane and isoprenoid abundance ratios are more characteristical of organic matter from a marine origin.