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

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.