Understanding and Assessing Impacts
Stream health depends on various factors, including hydrology, temperature, geomorphology, habitat structure, and water quality. Water temperature is a critical parameter as it regulates both biotic and abiotic processes in streams. The water temperature regime in streams is influenced by the source of discharge (e.g. groundwater, surface runoff), shading, solar radiation, and anthropogenic stresses. A longitudinal gradient of cooler upstream and warmer downstream waters can be observed during the summer months. Near the headwaters, the streams are well shaded from solar radiation and primarily fed by cool groundwater, which averages approximately 8°C in southern Ontario.
The maximum pond effluents in Greater Toronto Area ponds typically range between 26 and 31⁰C, with an observed inlet to outlet temperature increase of between 4 and 11⁰C during the summer months. Variations in discharge temperatures among ponds can be explained by several factors, especially detention time and the elevation of the outlet below the permanent pool. In some contexts, the thermal pollution from stormwater ponds can offset the cool water temperatures of streams beyond the threshold of the inhabiting species.
The Storm Water Management (SWM) pond used in this study is located at 60 Upperlinks Drive in Brampton, Ontario. It treats runoff from a medium-density residential catchment. It has a surface area of ~3000 m2 and a permanent pool depth of ~2 m. During a storm event, the pond level rises as stormwater runoff from the subdivision enters the pond from the pond inlet.
The project team assembled models to predict the pond’s thermal load from the previous six years (2012-2018). The 5-min aggregated rainfall data for the outflow flow rate model was available from a nearby TRCA weather station. Ambient temperature data was collected from Environment Canada via weatherstats.ca. The most significant thermal load, peak and seasonal, occurred during 2016 and 2018. This was primarily caused by the higher ambient temperatures in these years.