Coprostanol levels and organic enrichment in sediments of the Bilbao estuary (north of Spain)

An assessment was made of the distribution of the faecal sterol coprostanol (COP) in sediments in the Bilbao estuary. Concentrations of COP ranged from 135.15 to 10.77 μg g^?1 dry weight, from the inner to the open estuary, with a mean value of 51.43 μg g^?1; the highest concentrations were found near the mouths of several tribularies flowing into the estuary. Comparisons with other published values indicate that sediments from the Bilbao estuary are affected by sewage pollution in a similar degree to that reported for other estuaries in the world. Organic enrichment and anaerobic sediment conditions prevail in three-fourths of its watercourse bed. Since the spatial distribution of several organic sedimentary parameters could be clearly correlated with COP levels, we can conclude the existence of major pollution by the addition of untreated sewage, coming from mainly domestic sources. The implementation of a sewerage treatment plan by the local water authority needs the monitoring of reliable sewage tracers, such as faccal sterols, to assess the effectiveness of the plan in reducing the effects of pollution.