Faecal sterols as indicators of sewage contamination in estuarine sediments of the Tay Estuary, Scotland: an extended baseline survey

Abstract. Sterol ratios are used to identify sources, occurrence and partitioning of faecal matter in sediments of the Tay Estuary, Scotland. The 5β/(5α+5β) ratio is used to discriminate between sewage and biogenic sterol sources by comparing the concentrations of coprostanols to cholesterol plus coprostanols. This index shows unambiguous sewage pollution in the Invergowrie Bay area (values >0.7). The coprostanol/epicoprostanol index is used to differentiate between human and non-human faecal inputs. Ratios confirmed the primary source as human-derived faecal material. The coprostanol/cholesterol ratio was calculated in order to elucidate the contribution of different biogenic sources to the sedimentary sterol budget. Ratios of >1 clearly indicate faecal sterol sources. Invergowrie Bay displayed no sterol signature other than sewage. A biogenic source of cholesterol influenced total sterol concentrations upstream of the City of Dundee. Attention is directed to the potential role of density fronts in compartmentalization of faecal material in bottom sediments.