Metal speciation in urban runoff
A literature review has identified free and weakly complexed dissolved as well as solid surface associated, metal species as those forms most likely to be bio available. To analyse bio available Zn, Cd, Pb and Cu fractions in urban runoff a speciation scheme was devised which provided a relatively rapid technique for separating the dissolved and suspended solid phases of storm water into appropriate fractions. A new method, termed Dialysis with Receiving Resins, which responds to bio available metal species by incorporating a specially developed cysteine resin, within a dialysis membrane, has been devised and tested, A small scale gullypot study allowed the identification of six major processes which affect metal mobilisation and transport. The relevance of these processes is discussed with reference to gullypot outflow loadings of the different metal fractions. Acid rain mobilisation was found to greatly increase the levels of bio available free and weakly complexed metal fractions. Mass balance studies showed that Zn, Cd and Cu have a residence time in the road surface/gullypot system of little more than one storm event whereas Pb tends to be more persistent due to its predominant association with the solid phase. The speciation scheme was also applied to the analysis of metal fractions in storm water outfall samples collected from urban catchments in Sweden and the U.K. Significant concentrations and loadings of metals were found for both catchments with, on average, bio available metal forms representing 68% of the Zn, 76% of the Cd, 66% of the Pb and 32% of the Cu. Individual metal species were found to load at different rates onto metal chemographs and can be explained in terms of controlling parameters, as well as the influence of mobilisation and transport processes. Cadmium and Cu concentrations in storm water were regularly found to exceed Water Quality Standards and also to accumulate to significant levels in the Dialysis with Receiving Resins method. These metals may therefore represent a direct threat to receiving water quality.