An investigation into zinc and suspended solids in urban stormwater and their effect on water quality in the Heathcote River

Abstract

A study of sediment and zinc concentrations in stormwater from the Garlands Road outfall was carried out to gain an understanding of the effects these two contaminants have on water quality in the Heathcote River and to investigate if any relationship existed between the two contaminants.

Sampling was undertaken at three sites, the Garlands Road stormwater outfall and two sites on the Heathcote River, one upstream of the outfall beside the Garlands Road Bridge and another approximately 103 metres downstream of the outfall. Sampling was carried out during four separate storm events at low tide. Two sampling runs were undertaken within the first hour of a storm event to represent contaminant loadings in the first flush while the other sampling runs were conducted at two hours and five hours into a storm event respectively. Sampling was undertaken using the grab sample technique and the samples were analysed for suspended solids and zinc by the Environment Canterbury Water Laboratory.

The results of the study found suspended solid concentrations from the stormwater outfall were up to twenty times higher than the suspended solid concentrations in the Heathcote River. Suspended solid concentrations were higher during the 'first flush' period when compared to samples taken outside this period. There appeared to be little impact on the downstream suspended solid concentrations as a result of the discharges from the Garlands Road stormwater outfall. Overall the suspended solid concentrations in the Lower Heathcote River were low, being between 13 and 67mg/L at all sites, despite the dirty appearance of the water in the study area.

Zinc levels followed the same trend as suspended solids being substantially higher in the stormwater samples than in the river samples. There was little noticeable difference in upstream results compared with downstream, despite the influence of high loadings from the Garlands Road stormwater outfall. Overall zinc levels in the stormwater were higher than typical urban stormwater and in the Heathcote River itself zinc levels on two of the four sample days breached the Australia New Zealand Environment and Conservation Council guidelines for zinc concentrations in a highly modified ecosystem.

The correlation between suspended solids and zinc showed a strong positive relationship based on the small number of samples taken. This supports other research that has found direct relationships between suspended solid loadings in urban runoff and heavy metals.

The stormwater network was investigated using drainage maps from the Christchurch City Council. This study found only one site in the catchment with any form of stormwater treatment. All other stormwater discharged untreated from hardstand areas and roofs into the reticulated network and then into the Heathcote River.