Assessment of mercury and methylmercury pollution with zebra mussel (Dreissena polymorpha) in the Ebro River (NE Spain) impacted by industrial hazardous dumps

Large amounts of industrial waste containing high concentrations of mercury (up to 436 microg/g) are dumped in a reservoir adjacent to a chlor-alkali plant in Flix (Catalonia, Spain), on the lower Ebro River. In order to assess the spatial redistribution of mercury from the point source and its bioavailability to the aquatic food web, zebra mussels (Dreissena polymorpha) were collected at several sites. The highest total Hg (THg) and methylmercury (MeHg) concentrations ever reported for zebra mussels were found (THg: 0.02 to 0.81 microg/g ww; MeHg: 0.22 to 0.60 microg/g ww). At the most polluted site, close to the waste dump, the mean values were 20 times greater than the local background level. Concentrations decreased with increasing mussel size at all sites. The MeHg/THg ratio was ca. 60% (range: 50-80%). A comparison of similar size classes clearly indicated the hot spots of Hg bioavailability to the aquatic food web and downstream transport.