Two bioretention facilities with different designs were installed on the University of Maryland campus and were monitored from Summer 2003 through Fall 2004 to quantify water quality improvements to parking lot stormwater runoff. One cell was a standard design and the other had an anoxic sump. Twelve inflow/outflow water quality data sets were successfully collected and analyzed for total suspended solids (TSS), phosphorus, and zinc. Nine sets were collected for copper and lead, and three for nitrate. In two of the events, all of the runoff flow was attenuated by the bioretention media and no flow exited the cells, resulting in zero pollutant discharge. In all cases, the median pollutant output is lower than the input, indicating successful water-quality improvement through the bioretention media. Statistically insignificant differences were noted between the two cells for all pollutants examined. Median values for effluent event mean concentrations and percent removals based on combined data sets (both c...