Clogging mechanism of stormwater filter media by NaCl as a deicing salt

The effect of NaCl deicing salt on compost-included stormwater filtration media (SFM) has not been studied so far. This article presents results of an experimental and analytical study conducted to understand the mechanism of permeability reduction in two SFM caused by the application of a common deicing salt (NaCl). The two SFM used were made up of clayey silty sand with and without added compost (15% by mass). Constant-head flow experiments were conducted applying NaCl at two different concentrations. Permeability of SFM without compost reduced by an average of 19.1%, while the permeability of SFM with compost reduced by an average of 93.7%. Mercury intrusion porosimetry (MIP) tests and chemical analyses were conducted on representative samples of the media, both before and after salt application. MIP results showed that median pore sizes enlarged, while permeability reduced. This is counterintuitive, as we usually expect that permeability reduction would be caused by pore-size reduction. MIP results (enlarged pore sizes) suggest that deicing salts caused flocculation of aggregates. Based on analysis of our experimental results and comparison with similar results in the literature, we conclude that permeability reduction of SFM was due to blocking of pore throats by dispersed particles and biofilms. Permeability reduction of the SFM with compost was larger, because compost-salt interactions were comparatively greater than the soil-salt interactions. Results of this study suggest that salt-laden snow should not be piled up near the bioretention facilities built with compost-included SFM.