- The construction of impervious parking lots results in increased stormwater runoff volume as well as higher contaminant contents. Such runoff usually flows untreated to adjacent water bodies and streams. On-site volume and contaminant reduction through best management practices incorporated within the design or retrofitted to existing designs reduces the impact of urban development. A bioretention area or rain garden is such a practice that uses plant and soil characteristics to reduce the stormwater volume through retention and improve water quality through bio-chemical processes. A bioretention area was constructed to treat parking lot stormwater runoff in Dallas, TX. Monitoring equipment to quantify the water balance were installed including monitoring wells, flumes and water level loggers. Grab samples were collected from the inflow, the outflow and the monitoring well to assess the performance of the bioretention area. The design and construction procedures were described in paper 084479 presented in 2008. This paper presents the water volume and quality data, as well as the analysis of the bioretention performance.