Analytical determination of copper speciation in Oregon highway runoff

Abstract

Low concentrations of dissolved copper have been shown to adversely affect the olfactory system of endangered salmonids, impairing their ability to avoid predators and likely increasing predation. It is believed that only the free ionic (Cu2+ free) and weakly complexed forms are bioavailable to organisms; these forms typically account for a small fraction of the total dissolved copper. To determine the threat of copper to salmonids, the speciation of copper in aqueous environments is essential. Copper speciation has been examined in freshwater, seawater, estuaries, and rainwater. However, no such studies have focused on stormwater, a major source of dissolved copper in natural water bodies. To bridge this gap in knowledge, competitive ligand exchange – adsorptive cathodic stripping voltammetry (CLEACSV) techniques developed for seawater were adapted to stormwater. CLE-ACSV is unique among speciation techniques in that it allows the characterization of the ligands in the system as well as free copper concentrations. Major alterations of seawater CLE-ACSV techniques include the shift to a lower pH (6.80) and the addition of an electrolyte to increase solution ionic strength to 0.05M. The results of this research show that free copper concentrations at the monitored sites ranges from 10−13.49 to 10−6.74 M. Additionally, free copper concentrations are best correlated with dissolved copper and hardness levels. Ligand concentrations correlate with dissolved organic carbon (DOC) measurements and (along with copper-ligand stability constants) vary widely within and between sites. Finally, modeled Cu2+ free concentrations overestimate analytically determined concentrations.