In 2012, the City of Fremont installed two Tree Well Filters (TWFs) on Osgood Road to improve city aesthetics and treat urban runoff. Two distinct TWFs configurations were designed and built side-by-side so that they could be tested against one another for efficacy of pollutant removal and maintenance costs.
The San Francisco Estuary Institute monitored both Tree Well Filters over a series of storms in order to:
- Qualitatively assess whether the TWFs were treating stormwater runoff at rainfall rates up to 0.2 in/hr, estimated to be equivalent to 80% of the total annual rainfall;
- Quantitatively assess whether the TWFs reduced pollutant concentrations in water entering the storm drain system;
- Compare the effluent concentrations exiting the TWFs to findings from other studies of Low Impact Development performance.
Results from this study show that TWFs likely meet the flow and volume reduction targets, while reducing the concentrations of a range of pollutants, even though some nutrients appeared to be sourced from the filters.