This paper describes the detailed laboratory tests of biofiltration media that are being considered for use in engineered stormwater treatment systems recently proposed for a large field site in the southwestern United States. These stormwater treatment systems were designed to treat 90% of the long-term runoff volume from drainage areas ranging from 5 to 60 acres at the site. The main pollutants of interest for the project include cadmium, copper, lead, and dioxins, and the effluent concentrations had to meet design criteria that are based on numeric effluent limits that are applied to stormwater discharges through the site's NPDES permit. An additional feature of the project is that existing runoff concentrations for the pollutants of interest are generally below levels typically seen in urban and industrial stormwater runoff, therefore the tests needed to simulate site-specific conditions by adjusting raw influent samples to representative levels, where possible. The purpose of this study is to determine optimal biofiltration media combinations and contact times (based on achievement of permit limits in treated effluent), hydraulic properties, and clogging/breakthrough frequency for design purposes and maintenance planning.