Nutrients Removal in Constructed Wetlands in a Cold Temperate Region
Creators: Liang, Shuntian
Publisher: name: University of Alberta Library
Year: 2022

<p>Constructed stormwater wetlands have become an effective management practice to control urban stormwater, which contributes to eutrophication in urban aquatic ecosystems. This study examined the effects on nutrient control of two created wetlands, the Rocky Ridge wetland (RR) and the Royal Oak wetland (RO), in Calgary, Alberta, Canada, in a cold temperate region. Field measurements were conducted from May to November 2018 and April to November 2019. The considered meteorology parameters and flow data were collected, and nutrient concentrations were measured from the inlets to the outlets. The water budget analysis was done to guarantee the reliability of flow data. The concentration reduction and load reduction of nutrients were analyzed to assess the performance of these two stormwater wetlands. The effect of weather characteristics and hydraulic parameters on removal efficiency was also examined.<br>
The volumes of direct rainfall on the wetlands and evaporation were minor components of the event budgets, but evaporation had a certain important effect on the water budget in these two wetlands annually, especially in 2018 with less annual rainfall depths, reaching 63.9% in the Rocky Ridge wetland and 23.6% in the Royal Oak wetland of annual outflow volume, respectively.<br>
The loading of total nitrogen (TN) into the RR wetland was retained at approximately 59.7% in 2018 and 70.1% in 2019. Nevertheless, the RO wetland trapped 48.6% of TN inflow loadings in 2018 and only trapped 10.9% in 2019. Both wetlands can effectively remove total phosphorus (TP) from stormwater, given the annual TP reductions were 47.0% in 2018 and 66.8% in 2019 by the RR wetland and 84.1% in 2018 and 83.1% in 2019 by the RO wetland, respectively. Both wetlands can have an excellent annual reduction rate of TN and TP in a wet year. The RR can have better nitrogen performance while the RO can have better performance on phosphorus.<br>
Results of nutrient measurements also indicate that the event removal efficiency (RE) of TN ranged from -68.6% to 98.4% of the RR wetland and from -104.1% to 93.8% of the RO wetland. The median and mean RE-TN of these two wetlands were close, they were 41.6% and 34.1% of the RR wetland, 44.2% and 31.5% of the RO wetland, respectively. The RE-TP ranged from 29.9% to 97.2% of the RO wetland and from -160% to 80.3% of the RR wetland. The RO wetland had better phosphorus reductions than the RR wetland, comparing the median and mean RE of TP and total reactive phosphorus (TRP). Both wetlands’ capability to remove nutrients exhibited better in the wet year 2019. The event REs of TN and TP only exhibited the same fluctuations as the dry year&#39;s hydraulic loading rate in both wetlands, while the event RE-TN positively correlated with the inflow concentrations in both wetlands. <br>
Specifically, nutrient removal can be attributed to the suspended solid reductions in the RR wetland, which were influenced by air temperature, evaporation rate, hydraulic loading rate and retention time. The major processes of nutrient retention in this wetland should be vegetation uptake and accompanying sedimentation. In the RO wetland, rainfall characteristics and evaporation rate played important roles in nitrogen removal efficiency. The sedimentation forebay acted as a nitrate/nitrite internal source, while organic decomposition and nitrification were assumed to be the reasons for poor annual nitrogen removal. In comparison, the evaporation rate exhibited a contrary correlation with the event RE of TN and TP in these two wetlands. Additionally, small and clean rainfall events which have low flow rates can reduce the reliability of phosphorus removal assessment by the RR wetland. The interference from large rainfall events on nitrogen removal by the RO wetland needs to be explored in the future.</p>