Monitoring and Evaluating a Bioretention Filter Enhanced with Zero-Valent Iron and Biochar

Stormwater has become an increasing point of concern in water pollution. Non-point source pollution caused by runoff is one of the largest polluters to waterways, such as the Chesapeake Bay. The EPA and DEQ have created Total Maximum Daily Loads (TMDLs) for the Bay watershed to protect it from high pollutant levels. To comply with state and local TMDLs, increased runoff treatment is necessary. An existing bioretention system in Charlottesville, Virginia, was enhanced with zero-valent iron and biochar in the media, as well as an internal water storage layer. Flow data along with samples taken at the inlet and outlet of the BMP were used to evaluate the performance. Five post-enhancement storms were used to determine the overall effectiveness of the BMP under natural field conditions. The enhanced bioretention system attained significant volume and peak flow reductions, ultimately stemming from the internal water storage layer. These storms saw large removal rates of total suspended solids (TSS), as well. The nutrient analysis from the post-enhancement storms did not indicate that the enhanced bioretention system was significantly removing total nitrogen and total phosphorous. However, it is believed that the reason for poor percentage nutrient reduction is the low concentrations entering the system. These concentrations are below or approach irreducible concentrations. When inlet nutrient levels were artificially increased for a storm, excellent nutrient removal rates were observed. It is believed that the media enhancements and internal water storage (IWS) layer would be more suitable at a bioretention filter with higher inlet concentrations.