Road-deposited material that is not removed by street sweeping enters stormwater channels and sumps where it releases contaminants to stormwater. The understanding of these processes is often based on laboratory leachate tests that are not representative of the stormwater system. This study subjected swept material to simulated channel flow and sump drainage over eleven weeks of fortnightly wetting, measuring chemical release, mass loss, and changes in the particle size and composition of the material.
Both systems continued releasing nitrogen, organic carbon and suspended solids after 11 weeks with no sign of completion, indicating that retained material continues to deliver pollutants to receiving waters, not just in the first flush. Sumps released significantly higher loads than channels for all major analytes and lost three to five times more material, attributable to the longer inundation. Phosphorus release was consistent from the channels but reduced in the sumps after the system that lost its fine fraction.
Channels and sumps delivered pollutants differently. Channels exported buoyant coarse material while retaining the fine, metal-rich fraction, so that traffic-derived metals became concentrated in the residual. Sumps flushed the fine fraction out, depleting those metals. Some metal concentrations in the channel residues were elevated above sediment quality guideline values that the original material did not exceed. Soil-derived elements were largely retained, confirming that treatment mobilised the organic and soluble fractions rather than the mineral matrix.
Drainage type therefore determines when and which pollutants are exported, but not whether export occurs. Neither channels nor sumps retained the pollutant load: it was either held in place as a continuing source or flushed onward to receiving waters. These experiments confirm why material should be removed from roads before it reaches the stormwater system and provides evidence for the value of street sweeping and guidance on frequency of sweeping required.