Adapting bioretention for stormwater treatment in xeric climates

Bioretention as a green approach to urban stormwater management has gathered a great deal of attention by researchers over the last 10 years, and implementation is becoming more widespread. However, much of the research and implementation of bioretention has occurred in mesic sites that receive over 100 cm of annual precipitation (such as Prince George's County, MD, and North Carolina) and a need has been identified to study these systems in other climates. To address this need, a new bioretention design is proposed for stormwater treatment facilities in water-limited climates based on research that describes biogeochemical processes such as vegetation evapotranspiration (ET) and nutrient cycling in water-limited ecosystems. The nutrient removal of this design was tested over 1 year and compared to the performance of the more commonly implemented wetland bioretention design and a media filter with no plants. Also, a 15N isotopic label was added to each treatment to verify that plants directly participate in the removal of nitrogen from stormwater inputs. The results of these studies demonstrate that a bioretention system designed to closely match arid ecosystem hydrology that includes the use of upland plants does remove more total nitrogen and phosphorus than the no-plant media filter, but the more commonly used wetland community removed the most total nitrogen and phosphorus of the tested designs. The added nitrogen label was identified in all vegetation in both the upland and wetland communities. Forty-six percent of the added 15N label was recovered in the effluent of the control cell within 1 month of the addition of the label; 21% and 7% of the added label was recovered in the same time period from the upland and wetland treatments, respectively. In conclusion, the proposed design does protect receiving waters from nutrient loading associated with stormwater runoff from urban landforms, but increasing planting densities by two or three times and expanding the palate of vegetation used may improve this performance.