Influence of Bioretention Media Compost Content and Composition on Water Quality Function: Replicated Laboratory Column Studies Inform Bioretention Soil Guidance

Abstract

While bioretention cells have been shown to ameliorate nutrient pollution in urban stormwater, previous studies show newly installed bioretention soil media (BSM) may leach nutrients. This leaching has been attributed to use of compost in BSM. Compost is often recommended as a component of BSM to maintain plant health. Stormwater guidance documents define wide ranges of acceptable compost amounts in BSM, and few include information on compost feedstock type and age considerations. Limited studies exist exploring how compost amount and type affect bioretention cell performance and for how long. The water quality effects of compost in BSM were studied in a replicated column study by simulating one typical year of rainfall for the Columbus, Ohio region. Seven BSM mixes constituting four different compost amounts (none, low, medium, high) and two different types (woody waste, yard waste) were evaluated. Yard waste effluent contained the highest concentrations of nitrogen species. Nitrogen species were generally sensitive to both type and amount of compost. Orthophosphate (OP) and total phosphorus (TP) leaching was significantly higher from yard waste BSM in high compost mixes; for low compost mixes, yard waste TP concentrations were significantly higher than woody waste. Total suspended solids (TSS) concentrations were unaffected by compost amount but were significantly higher in yard waste effluent compared to woody waste. Alarming levels of NO2-3, total nitrogen, TP, and TSS were detected in effluent from early storms and tapered asymptotically over the 1-year of rainfall to relatively consistent effluent concentrations. Elevated leaching levels for most studied pollutants persisted only for the first few storms. This study demonstrates compost inclusion, type, and amount in BSM substantially affect leaching potential and duration. Therefore, stormwater guidance documents should be updated to recommend woody waste compost in lieu of yard waste compost and limit the amount of compost acceptable in BSM.

Keywords: Stormwater, filter media, nitrogen, phosphorus, TSS, organic matter