The <--?1-->stormwater treatment efficiency of a swirl reactor—a common best management practice—was determined by evaluating the fate and transport of two classes of primarily anthropogenic contaminants: polycyclic aromatic hydrocarbons (PAH) and heavy metals (copper, zinc). In addition, total dissolved and total suspended solid (TDS and TSS, respectively) treatment was evaluated. For this study, treatment efficiency was defined as the percent difference between the swirl reactor influent and effluent concentration measured during one particular storm event. The results of this study indicate that TSS was removed efficiently (between 64.7 percent and 85 percent). However, possible resuspension of sediments during high flow conditions was observed. The swirl reactor did not remove TDS. Treatment of copper was 13 percent efficient, whereas zinc was not attenuated. The maximum dissolved PAH (= total of 10 PAH compounds) treatment efficiency was 16.8 percent initially, but decreased to 8.4 percent by the end of the storm (average: 13.1 percent). Significant differences in removal effectiveness existed between dissolved low–molecular weight (LMW) and high–molecular weight (HMW) PAHs. While LMW PAH removal was moderately effective, HMW PAH concentration was not affected by the swirl reactor. About 19.9 percent of PAHs associated with sediments that settle were removed. Similar to dissolved PAH removal, LMW PAH-loaded sediment effectively settled out of the wastewater stream, whereas HMW PAHs were found to migrate through the reactor unhindered.