This report summarizes the data from five studies in the Puget Sound region that evaluated the hydraulic and water quality treatment performance of various bioretention soil mixes (BSMs). This study was conducted to help ascertain the duration and magnitude of metals and nutrient flushing from bioretention systems using the 60 percent sand, 40 percent compost (60/40) mix, to determine which media components contributed to pollutant export, and to establish whether other media compositions would perform better than the 60/40 Mix in bioretention systems. Of the 19 BSMs reviewed the 60/40 Mix was the best performer for total suspended solids removal, but the worst for total phosphorus, nitrate+nitrite, and dissolved copper (which were exported in all cases). The compost component within the 60/40 Mix appeared to be the source of the export. Of the 19 BSMs reviewed mixes which contained sand, coconut coir, and some combination of biochar, activated alumina, or iron filings, tended to perform the best for all pollutants of concern. The study concludes with a set of recommendations for further study.