A Comparison of the performance of contrasting stormwater treatment systems, Ryelands Subdivision, Lincoln, Canterbury

Abstract

Urban stormwater runoff is a leading cause of contamination in urban surface water and soil. The quality of the stormwater discharges is increasingly important in the Canterbury region as the population grows, and land use becomes more intensive. I aimed to evaluate and compare the performance of two stormwater treatment systems (a wet and a dry system), in a residential subdivision, Lincoln, Canterbury. Stormwater samples were taken from inlets, outlets and along the systems during three rain events. Samples were analysed for a variety of water quality parameters including nutrients total phosphorus (total P), and nitrate nitrogen (NO₃ - N), suspended solids, E.coli and metals. Plant samples were collected from various locations down the stormwater systems and analysed for metals, and total P. Soil samples were also taken and analysed for metals, total P, total petroleum hydrocarbons, and polycyclic aromatic hydrocarbons. Laboratory results were analysed using Microsoft Excel statistical application to compare contaminant removal efficiency ratios, and event mean concentrations. The event mean concentrations of contaminants at the outlets were assessed against guidelines to determine the performance of the stormwater treatment systems. The results show that the dry stormwater treatment system’s performance was similar to the Wet system’s performance. In the Dry system, for all three rain events the suspended solid mean concentrations in the discharge were below the consent condition of 80 mg/L, and the NO₃- N mean concentrations were below the Australian New Zealand Environment and Conservation Council (ANZECC) 90 % guideline of 3.4 mg/L. Cu and Zn mean concentrations were below the ANZECC 90 % guidelines for Rain Event 1 and 2, but not below the guidelines in Rain Event 3. In the Wet system, the suspended solid mean concentrations in the discharge were the only contaminant concentrations below the guidelines for all three rain events. The mean concentrations of Cu and NO₃– N in the discharge were only below the guidelines in Rain Events 1 and 2. The Zn concentrations in the discharge were only below the guidelines in Rain Event 1. Neither system managed to remove total P in the discharge to below the ANZECC 90 % guideline of 33 µg/L in any of the rain events. In the three rain events sampled both the wet and dry stormwater treatment systems were performing to an adequate standard, which enabled the protection of the receiving environment. However, it is uncertain how the systems would perform if there were higher contaminant concentrations in the influent. The removal efficiency ratios for the contaminants were event specific. In rain events with positive contaminant removal efficiency ratios, the systems behaved as contaminant sinks. The Dry system was a sink for NO₃– N in Event 2. The Wet system was a sink for Cu and NO₃– N in Event 2. When there were negative contaminant removal efficiency ratios the systems behaved as contaminant sources. The Dry system was a source for Cu in Event 2, and P in Event 1. The Wet system was a source for Cu in Event 3, Zn in Event 2, P in Events 1 and 3, and NO₃– N in Event 1. Based on the current levels of contamination in the Wet system it will be at least 79 years before the first contaminant (Zinc) reaches a concentration in the soil where it poses a risk to human health and in the Dry system it will be 9,000 years.