This quality assurance project plan (QAPP) describes a study to determine the effectiveness of three high performance bioretention soil media (HPBSM) configurations in reducing or eliminating acute lethality of untreated stormwater to coho (Oncorhynchus kisutch) salmon (urban runoff mortality syndrome, or URMS) and its causes. The project is partially funded by the State of Washington Department of Ecology (Ecology). Both King County’s technical objectives and Ecology’s related project requirements are reflected in this QAPP.