Stormwater Quality Improvement through Bioretention in a Continental, Cold Climate

  • Author / Creator
    Kratky, Hannah, H
  • Bioretention is a widely used best management practice of low impact development. It uses an engineered soil media with amendments and vegetation to manage stormwater runoff near its source through volume reduction, peak flow reduction, and physical, chemical, and biological contaminant treatment. Bioretention has been well studied in warm climates but gaps of knowledge exist in bioretention implementation and performance in cold regions with clay rich soils like in Edmonton, Alberta. Here, annual precipitation is on average 456 mm and freezing temperatures typically last 7 months. Therefore, this research was initiated to better understand the hydraulic and water quality performance of bioretention designed for and operated under local conditions. Four large columns of 0.36-meter diameter were filled with approximately 0.90 meters of bioretention media. Two columns contain loam soil (soil media “A”) and two columns contained sandy loam soil (soil media “B”). All four columns contained a surface layer of mulch, a top layer of compost/soil mixture, and two switch grass plants. One column containing loam soil and one column containing sandy loam soil each also contained nutrient amendment layers intended to encourage nutrient removal or degradation. One layer consisted of steel wool to enhance phosphate adsorption/precipitation and another layer was submerged at room temperature and contained woodchips for a supplemental carbon source, both of which promote denitrification. All four columns underwent five stages of laboratory operation equating to 1.6 years of precipitation volume in Edmonton. These stages consisted of synthetic stormwater application for 1st summer operation, winter operation, spring runoff, 2nd summer operation, and slightly larger rainfall events (i.e. a single 1:5 year and a single 1:10 year event). All stages except for winter operation and spring runoff were simulated in a room temperature laboratory. For winter and spring runoff, all four large columns were moved into a cold room and frozen to -20℃ and thawed to approximately 1-3℃ repeatedly. Design rainfall events were applied during this intermittent thawing to simulate Edmonton’s intermittent warming periods throughout winter in which the snowpack may melt multiple times.This research investigated the change in concentration from the influent to the effluent of each column of total suspended solids (TSS), phosphorus (phosphate and total phosphorus), nitrogen (ammonium, nitrate, and total nitrogen), chloride, organics as measured by chemical oxygen demand (COD), and heavy metals (copper, zinc, lead, and cadmium). Excellent removal of TSS, phosphate, and ammonium (i.e. ≥ 90% average concentration reduction) were observed in both the loam and sandy loam columns without the nutrient amendment layers during summer operation of 1:2 year events and the 1:5 and 1:10 year events conducted. The columns containing the nutrient amendment layers also reduced TSS, ammonium, and phosphate, but the removal efficiency was decreased for TSS in the 1st summer and for ammonium in all conditions as compared to the columns without the nutrient amendment layer.TSS was physically removed via the processes of sedimentation and filtration, phosphate was dominantly removed via precipitation/adsorption and subsequent filtration, and ammonium was quickly and dominantly removed via adsorption to bioretention media and likely degraded to nitrate via nitrification during subsequent intermittent drying between applied storm events. This research discovered that even during winter and spring runoff, if infiltration of snowmelt can eventually occur, the physical removal of TSS, phosphate, and ammonium can still be achieved, even only days after the complete freezing (and subsequent thawing) of bioretention media at -20℃.After an initial maturation, nitrate was well removed (≥ 59.8% average concentration reduction) via denitrification when in the presence of woodchips and anoxic conditions. This removal occurred in the columns with both loam and sandy loam soil with the nutrient amendment layers during summer operation of 1:2 year events and the 1:5 and 1:10 year events conducted. Typically, in all other conditions, nitrate leached substantially from every column. Chloride did not appear to accumulate in any columns in this research and simply leached from each system without causing an obvious negative impact to the columns functionality. Organic matter continued to leach considerably from each column, but with a decreasing trend over time. Heavy metals appeared to be well removed by each column likely due to the high cation exchange capacity of the bioretention media utilized in this research.

  • Subjects / Keywords
  • Graduation date
    Spring 2019
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
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