A Retrospective Comparison of Water Quality Treatment in a Bioretention Cell 16 Years Following Initial Analysis

Abstract

One of the most popular stormwater practices in (sub-)urban North Carolina is bioretention. While bioretention has been researched intensively to determine the most efficient designs, few long-term studies have attempted to assess the performance of older bioretention. However, previous research and design guidance for bioretention has predicted long-term water quality treatment. This study compared discharged concentrations and loads of nitrogen and phosphorus from a bioretention cell (1) post-construction and (2) following 17 years of treatment. A conventionally-drained bioretention cell with lateral underdrains in Chapel Hill, North Carolina, USA, was first monitored post-construction for 10-months from 2002–2003 and, again following continuous use, for 14 months from 2017–2018. Estimated mass load reductions during the initial monitoring period were 40% for total nitrogen (TN) and 65% for total phosphorus (TP). Mass load reductions were increased 17 years after construction, with reductions of 72% and 79% for TN and TP, respectively. Plant growth, death, and decay over the 17-year life of the bioretention cell are hypothesized to have contributed additional nitrogen assimilation and carbon to the fill media, serving as a catalyst for nitrogen treatment. Phosphorus removal remained relatively unchanged between the two monitoring periods. Filter media samples indicated the top 20 cm of filter media were nearing phosphorus saturation, but with 1.2 m of filter media, lower depths would most likely continue to provide treatment. If designed, built, and maintained correctly, bioretention appears to provide sustained treatment of stormwater runoff for nitrogen and phosphorus for nearly two decades, and likely longer.

A Review of Nitrogen Removal for Urban Stormwater Runoff in Bioretention System

“…High nitrate removal rates are observed with soil media containing higher organic matter [116]. In fact, the use of a carbon source has been proven to be successful in various systems [110,111,117,118], however, reducing the amount of carbon source additives in the soil media is also recommended to avoid N leaching [5,119], because excessive use of carbon source additives can sometimes be the source of nutrients leaching [35,101].…”

Urban stormwater characterization, control, and treatment

“…The TN load reductions increased from 40% to 72%, while TP load reductions increased from 65% to 79%. The top 20 cm of the bioretention media was close to phosphorous saturation (Johnson & Hunt, 2019). Carbon stocks measured in 25 bioretention cells in Australia showed that the most C accumulation occurred in the upper 20 cm of the system, with around 32% of all C in the system limited to the top 5 cm (Kavehei, Jenkins, Lemckert, & Adame, 2019).…”

Hydrologic and soil properties of mature bioretention cells in Ontario, Canada

“…Bioretention is a small depressed area consisting of engineered soil media and vegetation which allows stormwater runoff to percolate slowly through (Liu et al 2014). Bioretention has been shown to reduce significant amounts of surface water runoff (Hatt et al 2009;Davis et al 2012;Khan et al 2012;Winston et al 2016;Johnson & Hunt 2019), which improves water quality largely by reducing the load of contaminants into surface water bodies. Bioretention has many co-benefits beyond stormwater management such as carbon sequestration in plants and trees (Kavehei et al 2018), evaporative and shading cooling to reduce urban heat island effect ( Jamei & Tapper 2019), and improved quality of life and land values (Foster et al 2011).…”