Copper, Zinc and Iron Contamination in Wellington Streams After Rainfall Events

Abstract

This work set out to determine the concentrations of dissolved copper (Cu), zinc (Zn) and iron (Fe) during base and wet weather flow at streams throughout the Wellington region. The secondary objective was to investigate possible sources of heavy metals during rainfall events. The concentrations of the three dissolved trace metals Cu, Zn and Fe were measured at 13 sites on five streams during base flow conditions and during runoff events (wet weather flow) in the Wellington region between January and July 2011. More than 240 base flow and 100 wet weather flow samples have been analysed for the three dissolved metals. Additionally, rainfall, roof runoff and paved surface runoff samples have been collected and analysed. The analysis was performed by Flame Atomic Absorption Spectroscopy (FAAS). A pre-concentration procedure using Chelex-100, a chelating polymeric resin bead, was developed and successfully used to enhance the concentrations of dissolved Cu and dissolved Zn. The recorded data were compared to the recommended long-term (chronic) toxicity triggers; the Australian and New Zealand Environment and Conservation Council freshwater toxicity trigger values (ANZECC (2000) TV) for dissolved Cu and Zn, and the Canadian trigger value (CTV) for dissolved Fe. Additionally, the concentrations of dissolved Cu and Zn in storm water samples were compared against the recommended short-term (acute) toxicity triggers, the United States Environmental Protection Agency (USEPA 2006) Criteria Maximum Concentrations (CMC). The medians of dissolved heavy metals concentrations; Cu, Zn, and Fe, all of which are potentially toxic to aquatic life, exceeded the long-term (chronic) toxicity guidelines at one of the studied sites for Fe, nine sites (69%) for Cu and 10 sites (77%) for Zn in base flow conditions. Comparison of base flow monitoring data to previous reports showed that the concentrations of the studied metals have increased over the last five years. Storm water (wet weather flow conditions) contained elevated levels of dissolved heavy metals in comparison to base flow concentrations which is consistent with what has been reported previously. Dissolved Cu and Zn exceeded the acute toxicity criteria at sites of suburban residential areas. The median of dissolved Fe concentration exceeded the sustained toxicity exposure trigger at eight of the studied sites (61%). Distinct catchment type contaminant concentrations (dissolved Cu and Zn) were observed during storm runoff events with a concentration pattern of suburban residential > commercial > light residential > rural catchment. Dissolved Fe exhibited a similar pattern but in this case the concentration in rural catchments was higher than in light residential catchments. These observations were attributed to the high traffic of vehicles passing nearby the area; accumulated particulate materials; and corrosion of materials containing heavy metals, such as galvanised and copper roofs, gutter systems and building construction materials. The strongest and most obvious first flush effect was observed with dissolved Cu followed by Zn where the phenomenon was observed in six storm runoff events for Cu and five events for Zn. The first flush effect of dissolved Fe was present in three out of eight storm runoff events. The concentrations of dissolved metals were of the same order of magnitude as those previously reported for the Wellington region, but this work recorded the highest expected concentrations, particularly, for dissolved Fe. The reported data are consistent with data sets from other New Zealand regions. The investigation of possible sources of dissolved heavy metals in storm runoff samples showed that rainfall water contained markedly elevated concentrations of dissolved Zn and smaller Cu concentrations, 0.04-0.075 and 0.0018-0.01 mg/L respectively, in comparison to the ANZECC (2000) TVs, 0.008 and 0.0014 mg/L respectively. The concentrations of dissolved Fe were below the CTV level of 0.3 mg/L. Most studies conducted in New Zealand did not take into account the atmospheric precipitation contribution to the elevated concentrations of dissolved metals during runoff events. Roof runoff samples had similar dissolved Cu and Fe concentration to those recorded in atmospheric rainfall water, but Zn was found to be higher in galvanised roof runoff. First flush samples from roof runoff had higher concentrations of all three metals than the delayed runoff samples, indicating the presence of accumulated particles containing metals. Paved surface runoff samples had concentrations of dissolved Cu and Zn higher than the chronic toxicity triggers, but the medians did not exceed the acute toxicity guidelines. The value of the median for the concentrations of dissolved Fe was below the CTV criteria. Similar results have been published for surface runoff in New Zealand and the international literature related to this field.