Implications of Snowmelt on Urban Water Quantity Quality: Experience from the City of Toronto

Snowfall and snow melt are major processes of hydrology in areas located on northern latitudes and/or at high altitudes. In certain Canadian drainage basins, one third of the annual runoff is the direct result of snow and ice melt, and therefore plays a very important role in the hydrologic cycle. However, impact of snow on water quantity and quality has not been given appropriate consideration in designing hydrologic structures and implementing pollution prevention practices in these cold climates. This is partly due the fact that snow is one of the most complex components in hydrology. However, accuracy of snow simulation has a significant impact on their overall performance.

Snow is a form of precipitation but at the same time it acts as storage until it melts depending on the energy availability. Due to this storage effect, sudden meltdown could cause considerable flow and pollutant loading in streams. Even slow melt rates could create saturated soil conditions promoting higher runoff due to subsequent rainfall or further snowmelt. Water quality issue is more severe because snow metamorphism concentrates pollutants on the snow cover surface creating a “first flush” phenomenon during initial snowmelt.

Difficulties encountered by hydrologists in snow hydrology are exacerbated in urban areas. Snow hydrology in urban areas is significantly influenced by anthropogenic activities. This is more evident in winter and spring hydrological processes. Ploughing and snow removal from roadways affect the snow distribution. Urban heat island effect due to buildings influences the precipitation and snowmelt rates. In addition, shades due to urban structures alter the spatial variability of snow cover energy fluxes. Change in albedo due to pollutants in snow also affects energy availability. Another important factor impacting urban snow hydrology and water quality is de-icing operations.
This paper presents preliminary results from the monitoring program undertaken at several streams within the City of Toronto analyzed with Environment Canada weather and stream flow data. Approximately 250,000 tonnes of road salt is applied in Toronto and the monitoring program was developed to evaluate how much road salt is entering the streams and to find out effectiveness of the current management practices. The monitoring program has generated a wealth of continuous data on road salt impact on streams within the City. This paper discusses the impact of road salt application on stream flow in addition to the contribution from snowmelt due to heat/energy input and rain-on-snow. These stream flows are compared with rain-storm induced summer flows. Water quality impact in terms of chloride due to road salt application is also presented. The contrast of impacts of highly developed and less urbanized areas in Toronto on water quantity and quality are also presented in this paper.