Metal speciation in separate stormwater systems

Biogeochemical processes, affecting metal speciation in a gullypot system and at stormwater outfalls, are investigated. Ionic Pb and Cu species released from road sediments by acid rain are scavenged by dissolved organic material and suspended solids as a result of a rise in pH through the road/gullypot system. Cadmium and Zn tend to remain in the dissolved phase. Bacterial activity and acid dissolution produce increases in dissolved metal in the gullypot liquor and it is these metals which contribute to the early storm profile. Metals in basal gullypot sediments are readily mobilised during high volume/intensity storms. The resulting stormwater contains dissolved ionic forms of Cd and Zn, while Pb is mostly adsorbed to suspended solid surfaces. Copper also binds to solids, although approximately 50% is transported by dissolved organic material (molecular weight ~1000-5000). For the separation of directly toxic metal species, anodic stripping voltammetry at polymer coated electrodes is preferred. Lead and Cu are present as iron/humic colloids and organic complexes respectively, which are not directly toxic to algae. Cadmium is predominantly ionic and inorganically complexed and therefore directly toxic. It is recommended that the highly toxic lipid soluble metal species should be analysed in stormwater.