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Articles

Pocket wetlands as additions to stormwater treatment train systems: a case study from a restored stream in Brampton, ON, Canada

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Pages 321-334 | Received 09 Feb 2017, Accepted 19 Mar 2018, Published online: 03 May 2018
 

Abstract

Ontario urban development necessitates stream restoration projects for existing watercourses that flow through a property under development. This is part of the planning process, mandated by local conservation authorities to conserve the natural heritage and ecological function of natural areas identified under Ontario’s Ecological Land Classification. Stream restoration is commonly incorporated into larger stormwater management upgrades in the re-developed landscape. This study evaluates flow attenuation and water quality mitigation of a pocket wetland (PW), a shallow constructed wetland located at the outlet of a stormwater management system, within a major channel realignment and stream restoration project in Brampton, Ontario, Canada. The PW was incorporated into the floodplain of the natural channel design, between the outflow of the stormwater pond and a tributary of Churchville Creek. The latter contains habitat for redside dace (Clinostomus elongates), a designated species-at-risk under the Ontario Endangered Species Act. The addition of the PW to the stormwater treatment train was designed to mitigate the impacts that poor water quality has on local biota. Runoff, water temperature, total suspended solids and conductivity were monitored during 21 rainfall events between May and October 2014. Average PW event residence time was ~2 h and the overall changes in water quality between instream monitoring sites were negligible. Instream water temperature changes were minor (< 1°C) during flow events, with small increases in water temperature (< 0.5°C) observed during baseflow. The change in suspended sediment between the stream and PW flow was −22–31 mg/L. Suspended sediment inputs were greatest in the fall, which coincides with larger rainfall events. This study demonstrates PWs provide additional water storage time, and have added value in stormwater management. Although PWs are not formally included in current policy requirements, evidence from this study suggests the inclusion of PWs in future projects is worthwhile.

Le développement urbain nécessite souvent des projets de restauration des risseaux et cela est communément incorporé dans les initiatives d’améliorations de gestion des eaux pluviales dans le paysage re-développé. Cette étude a évalué l’efficacité à valeur ajoutée d’une marais construite (MC) intégrée à la fin d’un système de gestion des eaux pluviales dans un projet de realignment des canaux et restauration des ruisseaux à Brampton, Ontario, Canada. Le marais construite était situé entre les sorties de fonds de l’étang d’eaux pluviales et un affluent du ruisseau Churchville, un habitat important pour le méné long (Clinostumus elongatus), une espèce en péril désignée. Décharge, température de l’eau, et les charges dissoutes et suspendues ont été surveillé pendant 21 événements de pluies entre mai et octobre 2014. Temps de résidence moyen d’événement dans le MC était environ 2 heures et les changements globaux dans la qualité de l’eau entre les sites de surveillance dans le ruisseau étaient négligeables. Ruisseau changements de la temperature de l’eau étaient mineurs (moins de 1°C) pendant les événements de débit, avec petites augmentations (moins de 0,5°C) observés pendant du débit de base. Le changement dans les sédiments en suspension entre le flux du ruisseau et le flux de la MC était entre 22 et 31 mg/L. Les apports des sediments en suspension étaient plus grands à l’automne, coïncidant avec les plus grandes événements de pluies. Cette étude démontre que les MCs offrent de temps additionel de la stockage d’eau et ont une valeur ajoutée dans la gestion des eaux pluviales. Bien que les MCs ne sont pas officiellement inclus dans les exigences de la politique actuelle, l’évidence de cette étude suggère que l’inclusion des MCs dans les projets en future pourrait se révéler utile.

Acknowledgements

We are grateful to Dr. Andrea Bradford for providing comments on an earlier draft, and for the thorough reviews provided by two anonymous reviewers. Dorian Pomezanski, Samantha Feist and Jesse Van Patter provided able and enthusiastic assistance in the field and laboratory.

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