Street tree pits as bioretention units: analysis of their performance in a residential area of Montreal, Canada

Urbanization, increased surface impermeability, and climate change have resulted in changes in the quantity, intensity and quality of urban stormwater runoff. Urban stormwater runoff has ecological impacts. Generally, these impacts are related to the total and peak flow volumes, and the presence of contaminants in runoff. Low impact development (LID) techniques are emerging as alternatives to traditional stormwater management systems to mitigate these impacts. One such technique is to combine soil, plants and infrastructure in a bioretention unit. This technique involves the use of multiple smaller units spread across an area; street tree pits may be suitable as bioretention units.The objective of this research was to analyze different designs of newly developed tree pits as bioretention units in the city of Montreal. These tree pits soil comprise the soil of the open part, where trees are planted, and the soil underneath the sidewalk. The two design factors were soil organic matter (SOM) content, and the permeability of the surrounding area (sidewalks and front lawns). A total of 24 tree pits were used in this study. The concentrations of trace metals and sodium were analyzed in soil solution and soil matrix. Using the estimated water flux mass flux of each contaminants was calculated. The mean contaminant concentration increased from the surface to the deep sampling depths (e.g. 46% for Ni, and 18% for Cu) but taking into account the accompanying decrease in water volumes, mass flux of contaminants decreased with the increase in depth (e.g. 72% for Ni, and 81% for Cu). In addition, tree pits with higher SOM content presented a higher reduction of mass flux of contaminants than tree pits with lower SOM content between surface and deep sampling depths. For example, tree pits with higher SOM content reduced the mass flux of Cr and Cu by 65%, and 86%, respectively, while tree pits with lower SOM content reduced the mass flux of Cr and Cu by 39% and 73% respectively.Tree pits with higher SOM content presented higher concentrations of Cr, Cu and Pb in the soil matrix. For instance, in the soil matrix of tree pits with higher SOM, Cr and Cu concentrations were 19.9 mg kg-1 and 15.3 mg kg-1, respectively, but were 17.4 mg kg-1 and 13.5 mg kg-1, respectively, in tree pits with lower SOM. This corroborates the observed effect on mass flux of Cr and Cu. In addition, an overall increase of contaminants was observed over time. For example, the concentration in soil of Cr increased from 15.9 mg kg-1 to 20.0 mg kg-1 and of Ni from 11.9 mg kg-1 to 14.4 mg kg-1 representing increases of 26% and 21% after about 18 months of monitoring. The soil matrix contained approximately one third of the maximum permitted concentration of contaminants stipulated by the Canadian Council of Ministers of the Environment for Cr and Ni in residential and park areas. High local permeability and high SOM bioretention units are recommended to mitigate the adverse effects of urbanization on runoff quality and quantity. In this study, tree pits with higher SOM retained contaminants better for all contaminants analyzed (except Pb). The increase in permeability of surrounding surfaces decreased the observed flux of water as well as the mass flux of contaminants observed in the open part of the tree pit. The reduced flux of water in the open part of the tree pit was likely a result of increased infiltration into the soil of the lawn and through the permeable sidewalk. The soil underneath the sidewalk is the same as, and contiguous with, the open area of the tree pit so it is designed to retain runoff contaminants. In this study, higher local permeability and higher SOM were both generally correlated with lower mass flux of contaminants and water. Tree pits can be used as bioretention units having their performance improved by increasing SOM and local permeability.

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