Stormwater Treatment Effectiveness of Established Lined Bioretention Facilities in Portland, Oregon

Abstract

Bioretention systems are commonly used to treat and detain stormwater runoff and help mitigate many negative effects of urbanization. The goal of this project is to assess the pollution reduction effectiveness of lined bioretention facilities that have been in use and functioning for four to eight years. Lined bioretention facilities are installed with an impermeable liner below the facility to prevent exfiltration of stormwater into the surrounding soil. Grab samples were taken from seven facilities in Portland, Oregon, over a two-year period to measure the quality of stormwater flowing into and out of the facilities. Results showed decreased concentrations of total suspended solids (TSS; 94%), ammonia (85%), total copper (59%), total zinc (80%), and dissolved zinc (41%). However, the facilities were found to increase levels of nitrate (2,070%) and orthophosphate (141%) in the outflow. Based on this study, effective TSS removal by bioretention facilities does not necessarily equate to equally effective treatment of other pollutants (especially orthophosphate and nitrate). Additional research is necessary to determine the significance of the observed increase in nutrients, understand the underlying mechanisms, and test possible design modifications to improve nitrate and orthophosphate removal.