ASSESSING THE PERFORMANCE OF STORMWATER TREATMENT FACILITIES IN SUBURBAN CHRISTCHURCH

E. Harris, I. Cooper, & L. Allan (Pattle Delamore Partners Ltd)

ABSTRACT

This paper discusses the findings from a monitoring program at two stormwater treatment facilities constructed for Christchurch City Council (CCC) in the last 10 years in suburban Christchurch. Both locations contain first flush basins and wetlands, and service new residential subdivisions. The purpose of the monitoring program was to quantify the effectiveness of treatment provided by these facilities and to inform future decision making by CCC on capital/operational investment for these devices.

The sites investigated were the Prestons stormwater treatment facility, consisting of a wet first flush basin and a wetland, and the Knights Stream stormwater facility, consisting of a dry first flush basin and dry wetland. Autosamplers were used to collect samples through the treatment train of both facilities across nine rainfall events between 2018 and 2023. Composite samples at each autosampler site were prepared and analysed for a suite of common stormwater parameters including heavy metals, nutrients, and total suspended solids (TSS).

Results from Preston’s indicated that the majority of contaminant removal occurs through the wet first flush basin, not the wetland. Concentrations of dissolved reactive phosphorus, total and dissolved copper, and total and dissolved zinc, all exceeded the receiving environment standards prior to treatment in the wet first flush basin. However, all these contaminants met the receiving environment standards after treatment through the wet first flush basin but prior to treatment through the wetland. The results indicate that the wet first flush ponds provided the highest treatment efficiency while the wetland served to provide minimal polishing treatment.

The Knights Stream stormwater system appears to provide variable treatment. The dry wetland had effective removal of TSS, total lead, and total and dissolved zinc, however it had poor removal of dissolved copper and dissolved lead. In general, the results showed that the dry first flush basin provided higher treatment efficiency than the dry wetland but provided less reliable treatment than the wet first flush basin at Preston’s.

A key observation derived from this study pertains to the influence of loading rates on the efficiency of wetland treatment. The study indicated that both stormwater treatment wetlands exhibit sub-optimal treatment performance in comparison to international research findings. However, it is suspected that relatively low influent concentrations may be a significant factor in this comparatively poor level of treatment performance. The contributing catchments to both Prestons and Knights Stream facilities have relatively low vehicle movements and new residential housing. These factors may be a reason for the low concentrations of contaminants, relative to other studies, noted in the influent to both facilities.

The results of this investigation indicate that treatment wetlands may not be well suited to treating stormwater with low concentrations of contaminants. While these devices provide great amenity value, flood retention and stormwater attenuation, they may not provide significant and reliable treatment in catchments with low influent contaminant concentrations.