Influence of stormwater management ponds on chloride transport to urban headwater streams

Abstract

Sodium chloride (NaCl) is the most common de-icing agent used on roads and parking lots in North America. During the winter and spring, chloride (Cl⁻) is readily transported from paved surfaces to stormwater management facilities in a matter of hours to days. According to earlier studies, densimetric stratification in end-of-pipe stormwater management facilities such as wet stormwater management ponds (SWMPs) can result in latencies in Cl⁻ transport to receiving waters. As a result, wet SWMPs may generate Cl⁻ pulses in streams that exceed thresholds of acute toxicity to aquatic biota. This study identified the prevalence of this phenomenon at five headwater streams receiving discharge from wet SWMPs within the Greater Toronto Area over two years. All receiving streams in this study experienced exceedances of the chronic CWQG for Cl⁻ downstream of the SWMP outlet, while only some experienced exceedances of the acute CWQG for Cl⁻. For most of the salting season, SWMP contributions exacerbate downstream Cl⁻ concentrations, and occasionally are the primary driver of exceedances. Bottom-draw SWMPs were found to accumulate Cl⁻ and flush rapidly following a rain or melt event. Top-draw SWMPs accumulated Cl⁻ throughout the salting season and released diluted concentrations of Cl⁻ near the end of the season. Streams with large upstream catchments diluted SWMP contributions and ameliorated downstream Cl⁻ concentrations.