Bioavailability of trace metals in runoff from mining and mobilization of river transported metals in estuaries due to mixing of salt water

Summary

The speciation, bioavailability and toxicity of trace metals in acidic mine discharge (AMD) have been studied in the receiving River Stjørdal. Atlantic salmon (Salmo salar L.) parr and smolt were captured by electrofishing and placed in exposure cambers both at sites influenced by AMD and at reference sites. Mortality occurred rapidly for fish exposed in tributary water influenced by AMD. Sampling of gills from fish exposed to AMD demonstrated high levels of metal accumulation compared to reference fish, i.e. 20 µgCu/g gill d.w., 560 µg Fe /g gill d.w. and 30µg Al /g gill d.w. Based on in situ fractionation of the water influenced by AMD, the results demonstrated that the concentration of total Cu and cationic Cu species was 26±10 ug/L and 13 ±7 µg/L, respectively. Despite that a large fraction of Al and Fe, was associated with organic material in the freshwater (pH 5,7-6,5), the water was toxic. However, when saltwater was added to freshwater, metals associated with organic material (not bioavailable) in the freshwater could be mobilized. Experimental mixing of saltwater with river water to 6 ? was performed and Atlantic salmon were exposed to the mixture. Based on in situ fractionation of the mixture waters and sampling of gills from exposed smolt, the results demonstrated that Al and Fe-species were mobilised from colloidal river transported material, forming species that accumulated on gills in the freshwater ? saltwater mixture. Thus, AMD could cause critical water quality for fish in influenced freshwaters, and probably also in estuaries. This implies that processes related to bioavailability of metals and toxicity not only the freshwater influence by AMD, but also estuaries have to be included in risk assessments for AMD