Bioretention gardens for improved nutrient removal Mark T. Randall and Andrea Bradford ABSTRACT Bioretention gardens are stormwater management practices capable of providing numerous water quantity and quality benefits. However, previous studies have reported inconsistent removal of nitrogen and phosphorus in these systems. This study used ten, vegetated, mesoscale (0.20 m 3 ), bioretention cells in a field setting to provide a comparison of the nutrient removal capabilities of five, alternative bioretention designs. Applying a synthetic stormwater to the bioretention cells demonstrated that a sandy soil mix can provide a 75.5 and 53.4% reduction in concentrations of total phosphorus and total nitrogen, respectively. Phosphorus removal was found to be only slightly enhanced in bioretention cells where soil was amended with alum-based drinking water treatment residuals, a commercially available oxide-coated media, or a commercially available lanthanum- modified bentonite product. However, improvements in phosphorus removal were observed in some cells when elevated phosphorus loads were applied to evaluate longer term performance. In cells incorporating a permanently saturated zone containing shredded newspaper to promote denitrification, effluent concentrations of nitrate were reduced by >99%, however total nitrogen concentrations increased. Mark T. Randall (corresponding author) Computational Hydraulics International, Guelph, Ontario, N1H 4E9, Canada E-mail: mark@chiwater.com Andrea Bradford School of Engineering, University of Guelph, Guelph, Ontario, N1G 2W1, Canada Key words | anoxic, bioretention, low impact development, nitrogen, phosphorus, stormwater INTRODUCTION The bioretention garden is a low impact development (LID) stormwater management practice capable of mitigating runoff volumes, reducing peak flows, transforming and sequestering pollutants and providing aesthetic appeal in urban areas. Numerous studies have been conducted showing high removal efficiencies of pollutants including: nutrients, metals, oil and grease, and pathogens (Davis et al. ; Hsieh & Davis ; Davis ; Hsieh et al. a; Hath- away et al. ; Hunt et al. ). However, other studies have indicated leaching of phosphorus and/or nitrogen from bioretention systems (Dietz & Clausen ; Hunt et al. ; Toronto & Region Conservation Authority ; Denich ). Identifying bioretention designs which can reliably remove a high percentage of nutrients from storm- water is a necessity as eutrophication of surface waters remains a high priority water quality challenge. Hunt & Lord () specified a bioretention soil mix consisting of 85–88% sand, 8–12% fines (i.e. clay and silt) and 3–5% organics intended to provide adequate drainage, reduce pollutant levels and support plant growth. This soil specification is recommended in bioretention design guide- lines including those published by the Toronto and Region Conservation Authority and Credit Valley Conservation Authority () and has become commonly used in Southern Ontario. The low percentage of organic matter (OM) in this mix relative to previous mixes reduces the potential of nutrient leaching, however there is minimal data published on potential nutrient removal achieved in bioretention gardens using this mix. In addition to testing alternative bioretention mixes, some studies have investigated bioretention soil amend- ments capable of improving phosphorus retention. Some of the materials tested include: steel wool (Erickson et al. ), fly ash (Zhang et al. ) and alum-based drinking water treatment residuals (Al-WTRs) (Lucas & Greenway ). Bachand & Heyvaert () suggested that Phoslock, 372 © IWA Publishing 2013 Water Quality Research Journal of Canada | 48.4 | 2013 doi: 10.2166/wqrjc.2013.016 Downloaded from http://iwaponline.com/wqrj/article-pdf/48/4/372/379974/372.pdf by guest on 09 March 2024