Total oil and grease analysis were performed in a specialised laboratory. The determinations were made in the whole body of mussels (pooled samples per site and per sample period). Briefly, mussel homogenates were prepared in an appropriate buffer and used as sample for a solid phase extraction procedure using a column for extraction based on hydrophobic interactions (ISOLUTE® SPE Column). The sample was acidified to pH 1.8–2.0 to neutralize fatty acids and no solvation was required. The extraction column was rinsed with 10 mL of acidified reagent water, pH ∼ 2 and the sample was loaded at rates not exceeding 10 mL min−1. The column was rinsed with 10 mL of acidified reagent water and analytes were eluted with 2 × 2 mL of trichlorotrifluoroethane. Petroleum hydrocarbons were determined collectively by the partition-infrared method, which is designed for samples that might contain volatile hydrocarbons that otherwise would be lost in the solvent-removal operations of gravimetric methods, and because provides the needed precision for low levels of oil and grease (