Présentations Colloques

Increasing impermeable surfaces in urban settings and installation of storm sewers and other urban infrastructure can affect groundwater flow patterns in urbanized watersheds. Although interaction between storm sewers and the groundwater system is widely recognized in the literature, there are few studies describing quantitatively how the storm sewer system ‘karstifies’ an aquitard groundwater system. This paper presents a unique study of an urban community in North America that has proven a wide-scale interaction through a groundwater pathway in the storm sewer bedding material between storm sewers and the foundation drainage collection (FDC) system that were designed to operate as hydraulically separate entities. The work was part of a larger study to assess the cause of community wide basement water infiltration for the City of Mississauga, Ontario, Canada.**The main problem of the study was to identify the mechanism for FDC surcharging under storm conditions that caused clear water to infiltrate into basements. The study team designed a series of storm sewer leakage tests based on the integration of water level and temperature data from groundwater monitoring wells, the FDC system, storm sewer and surface water gauges, footage from FDC CCTV and the original engineering designs. The storm sewer leakage tests comprised blocking and filling of storm sewers and the use of classical karst dye tracing methods to track the pathway of the leaking waters. Clear breakthroughs were recorded in the FDCs only hours after dye release. The coarse granular fill material within cross-connecting sewer trenches was identified as the flow path, with the surrounding till aquitard acting to focus flow within the utility trench. The data obtained, including measured estimates of storm sewer leakage, were a key input to the system modelling to better understand and inform the overall community storm sewer system behaviour and ultimately solving the problem.**