A Synthesis of Bioretention Performance in the Pacific Northwest

Abstract

This presentation summarizes and discusses the data from five studies that have been implemented in the Puget Sound region to evaluate the hydraulic and water quality treatment performance of 19 various bioretention soil mixes (BSMs). Bioretention is currently classified in the Stormwater Management Manual for Western Washington (SWMMWW) as a basic (solids removal) and enhanced (copper and zinc removal) treatment best management practice (BMP). However, findings from three studies in 2010-2013 found that bioretention systems built with compost based medias were actually exporting copper as opposed to reducing it. These findings called into question bioretention’s enhanced classification and precipitated further studies of bioretention performance in the region and a look at alternate BSMs. In this synthesis we look at the hydraulic and water quality performance of BSMs classified into four categories 1. 60/40 (sand/compost), 2. 60/40 + Additives, 3. Loamy Sand, 4. Sand/Coir + Additives. The most commonly exported pollutants were copper, nitrogen, and phosphorus. All BSMs performed well for TPH, fecal coliform, and zinc removal. Of the 19 BSMs evaluated in this study, the 60/40 mix was, on average, the worst performing in terms of pollutant flushing and pollutant reduction. Conversely, on average, the best performing BSMs were those that contained Sand/Coir + Additives. Flushing results indicate that by one water year the majority of BSMs have completed their equilibration/flushing period. However, the BSMs with compost tend to export relatively high levels of total phosphorus, nitrate + nitrite, and dissolved copper for at least three water years. A high degree of dissolved pollutant removal is achievable at infiltration rates that exceed the current 2-12 in/hr requirement; however, solids removal suffers at the same high flow rates. It is recommended that compost not be used in BSMs with underdrains and that sand/coir mixes be considered instead