July 27, 2026
Bioretention systems are widely used to control stormwater runoff, but their underlying soil media doesn’t always perform well in cold climates. Freeze-thaw cycles and the chemistry of winter runoff create conditions that standard soil mixes weren't designed to address.
Scientists from Cold Climate Stormwater Center of Excellence are testing a promising suite of cost-effective media amendments for compost replacement and phosphorus reduction, drawing on previous research conducted at the universities of Minnesota and New Hampshire.
Amendments under evaluation include compost, Turface, greensand, biochar, zeolite, vermiculite, expanded clay, expanded shale, pumice, AZOMITE®, perlite, and anthracite, as well as greensand recovered from iron reduction installations. The goal is to identify what type of biofilter media will retain enough moisture to support plant and microbial growth while not releasing pollutants like phosphorus.
To narrow the list, the team has completed a moisture study measuring how quickly water moves through each type of media. When the flow is too fast, the media does not filter pollutants effectively and starves plants of water; flow that's too slow floods the system and drowns plants. The most promising mixes will advance to outdoor mesocosm experiments with plants: a critical test of how these media perform under real-world, cold climate conditions.
Center scientists are also exploring whether innovative mixes, including water treatment residuals and hand warmer substrates, have the capacity to minimize phosphorus release, while retaining appropriate amounts of water in the soil matrix for plant health. The team recently completed laboratory batch and column studies using artificial stormwater in support of this goal; analysis of results is underway.
