A Milwaukee biofilter captured sediment and phosphorus while exporting some dissolved pollutants

Three years of paired inlet–outlet monitoring found strong runoff, sediment and particulate-pollutant retention, but dissolved phosphorus, sodium and some metals did not follow the same pattern.

Wild Lands Ecological Intelligence Editor
Wild Lands Institute · 28 September 2026

Evidence: Government technical report / three-year field monitoring
Geography: Milwaukee, Wisconsin, USA
System: Urban stormwater biofiltration

WHAT THE EVIDENCE SAYS

USGS monitored an urban biofiltration system from 2022–2024. The system reduced runoff volume by about 86%, peak flow by 92%, suspended sediment by 99% and total phosphorus by 86%, with strong retention of several particulate metals. Dissolved phosphorus and some dissolved metals showed net export, and road-salt sodium also moved through the system differently from particulate pollutants.

WHY IT MATTERS / OUR READ

Green infrastructure is not one treatment effect. A biofilter can perform exceptionally well for hydraulic control and particulate pollutants while exporting or weakly retaining dissolved constituents, so post-occupancy ecological performance should be assessed by pollutant form and season rather than a single 'stormwater treated' label.

CAVEATS / UNCERTAINTY

This is one monitored facility and performance depends on media, age, inflow chemistry, maintenance and climate. Loads and concentrations can tell different stories, and the study does not establish that every biofilter will reproduce these efficiencies or export patterns.

SOURCE

U.S. Geological Survey
View original evidence ↗