Monitoring of a Rain Garden in Cold Climate: Case Study of a Parking Lot near Montréal

Poor stormwater management is one of the primary factors behind the environmental degradation of water bodies. As a result, best management practices (BMP) and low-impact development (LID) have been implemented and promoted in many cities. This study presents some interesting results from a 1-year monitoring program for a rain garden located at a car park in Longueuil, Quebec, Canada (near Montréal). The design was specific in nature as it combines a perforated underdrain, an internal water storage zone of 0.97 m, an almost impervious surrounding soil, and a flow regulator at the outlet. A monitoring program was conducted to assess the performance of the rain garden in warm and cold conditions. Inflow volumes were estimated with an SCS method using precipitation data, but runoff resulting from snowmelt was not monitored. Outflow rates were directly measured. Results from this monitoring program showed that the bioretention system retained $35.0\%\pm 11.6\%$ of the total runoff during the cold season, whereas $59.7\%\pm 3.3\%$ of the total runoff was retained during the warm season. The event’s hydraulic retention efficiency depends highly on the rain garden water content at the beginning of the event and on the time since the previous precipitation event. The average event hydraulic retention efficiency reached 54.8% in warm conditions and $-188.8\%$ in cold conditions only for events producing outflows. The use of a vortex flow regulator at the outlet efficiently limited flow rates, but generated very long runoff events, which made the system sensitive to consecutive rainfall events. The water-quality monitoring activities showed that concentrations in Escherichia coli, fecal coliforms, polycyclic aromatic hydrocarbons, petroleum hydrocarbons, lead, and zinc were effectively reduced during the study period. The total suspended solids (TSS) concentrations were also well reduced by the bioretention, as average TSS concentrations in effluents were $4.1\mathrm{mg}/\mathrm{L}$ even in winter. However, removal of copper and nickel was not efficient and heavy metals concentrations were sometimes higher in the outlet than in the inlet. The same leaching phenomenon was seen for nutrients (total Kjeldahl nitrogen and phosphorous). The chlorides’ concentration significantly increased in effluent during the cold season ($p=0.0001$) because de-icing salt was spread on the parking lot during winter. No clear evidence was observed that the wintry conditions had any effect on any other contaminants during this study (with a 95% confidence level).