Are Stormwater Biofilters Causing Nutrient Impairments?

Biofilters can be used to treat stormwater runoff from urban landscapes, but stormwater professionals have discovered that biofilters containing compost can release phosphate and/or nitrate from the underdrains while other media mixes may have poor plant growth. To investigate the tradeoffs between nutrient release and plant growth, thirty outdoor biofiltration mesocosms comprising thirteen different media mixes were compared to clean washed sand for capture (or release) of phosphate, plant growth, and filtration rate. Media components were mixed at different volume or mass ratios with sand and included food residue compost, leaf compost, sphagnum or reed sedge peat, biochar mixed with leaf compost, spent lime mixed with leaf compost, iron filings mixed with leaf compost, sphagnum peat mixed with leaf compost, and leaf compost layered over iron filings. This presentation will share the final performance results from four rainy seasons of simulated runoff events including road-salt-laden events to simulate spring snowmelt in cold climates like MN. With the information from this study, stormwater practitioners will be able to design better biofiltration practices that capture phosphorus and support healthy vegetation.