Quantifying chloride retention and release in urban stormwater management ponds using a mass balance approach

Chloride (Cl-) in urban waterways largely originates from runoff containing deicing salts. Cl(-)is retained in watersheds after deicing ends, resulting in deleterious effects on aquatic biota. Stormwater management ponds (SWMPs), designed to mitigate 'flashy' urban runoff response, are known to impact pollutant transport. However, there is little information on what role SWMPs play in the timing and magnitude of Cl(-)transport over different timescales. This study quantifies the mass of Cl(-)retained in two SWMPs over varying timescales. Both ponds are in an urbanizing watershed in south-central Ontario; one drains a commercial area, the other, a residential area. High frequency measurements of water level and specific conductivity, from which flow and Cl(-)concentration were derived, were taken with sensors at pond inlets and outlets. For one SWMP, data were also collected upstream and downstream of the confluence of the pond outflow and the receiving creek to quantify the in-stream response to Cl--laden pond outflows. The findings suggest that SWMPs likely play a role in watershed-scale Cl(-)retention; one SWMP consistently retained Cl(-)while the other had variable retention and release of Cl-. In the receiving creek, Cl(-)concentrations downstream of the pond exceeded the acute toxicity threshold for aquatic organisms twice as often as concentrations upstream of the pond, and Cl(-)pulses corresponded to Cl(-)release events from the pond. The results of this study suggest that SWMPs concentrate spatially distributed salt inputs and modify the timing and magnitude of their release to receiving streams. Stream reaches that receive water inputs from SWMPs may be more vulnerable to Cl(-)toxicity than reaches that do not receive flow via SWMPs. The results of this study will help parameterize the role of SWMPs in watershed-scale Cl(-)transport models and geospatial models of salt vulnerable areas.