Determining residence time of a pocket wetland and evaluating urban runoff quality through a pocket wetland in Brampton, Ontario.

English Abstract:

Stormwater treatment trains improve urban runoff quantity and quality by remediating water quality and mitigating peak flow. A pocket wetland (PW) at the outlet of a stormwater pond is designed to increase runoff detention, while increased residence time within the PW provides, a ‘polishing’ step with respect to water quality. A PW in Brampton, ON was monitored to assess effluent discharge and water quality. Discharge, temperature and electrical conductivity (EC) were monitored upstream, downstream and at the inlet of the PW from May – Oct 2014, while Suspended sediment concentrations (SSC) were determined through discrete and automated pump samples. The changes between up and downstream monitoring points were used to monitor PW effluent discharge and quality. The PW attenuated event flows on average for 2 ± 0.8 h during rainfall events. The maximum change in discharge between up and downstream was 0.55 mm due to 31 mm rainfall suggests a slow release of stormwater from the PW. A change in storage capacity after an intense rainfall (17.5 mm/hr) event in early August resulted in poor correlation between influent and effluent PW discharge (R² = 0.2). A difference of means t-test suggested there was no change in water quality due to the PW, but in-depth event analyses suggest a minor change in EC and SSC at ‘first flush’. The PW remediated water quality well through rainfall events, but incised channels downstream of the monitoring area would continuously flow from the PW. Small volumes of warm surface water with an increase in EC and SSC resulted in increased temperatures (0.1 - 0.5°C), EC and SSC. Water quality parameters remained within the provincial water quality levels. The PW provided flow attenuation while increased residence time ensured good water quality. Further improvements in residence time may improve water quality and further justify the use of stormwater treatment trains for best management practice in urban stormwater management systems.