Bioretention as a Sustainable Stormwater Management Option in Cold Climates
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Two pilot size bioretention boxes were constructed for field investigations at the Risvollan Urban Hydrological Research Station in Trondheim. The seasonal pollutant retention, hydraulic lag times, and rainfall runoff versus snowmelt chemo dynamics have been studied with respect to zinc, copper, and lead. The field investigations were divided into four parts; a long term continuous hydrologic performance, heavy metal retention of rainfall runoff during different seasons, and heavy metal retention from roadside snowmelt. The chemo dynamic pathways through the system were investigated for the warm versus the cold season, and rainfall runoff versus snowmelt. Overall the results showed consistent high retention of particles and total metals with respect to concentrations and mass removal, with more than 90% mass removal of total zinc and more than 85% mass retention of lead, while copper retention varied from 46% to 86% by mass. However increases in dissolved fractions through the system for all events in the case of copper and for the snowmelt events in the case of zinc could lead to an increase of bioavailable dissolved metals in the outflow which is not desirable. The top mulch layer was identified as the largest sink of metals and particles, which helped avoid clogging the soil due to high particle concentrations in the inflow. The plants did show some ability to retain and absorb metals in the roots and shoot, however this was less than 5% of the total metal retention. The plants had a more important function in improving root zone infiltration, and rejuvenating the system in the spring every year, making it a valuable green space in the urban landscape. Snow storage was also considered and it was found that snow storage, dependent on annual snow volume, quickly became a deciding design parameter with respect to sizing.