Leaching and Bioavailability of Stormwater Contaminants in Compost-Amended Bioretention Systems.

Bioretention systems are best management practices that remove contaminants from stormwater. Past studies, however, have showed that bioretention systems can export excess nitrogen, phosphorus, copper, and dissolved organic carbon when amended with compost. The objectives of this study were (1) to quantify the effects of two types of compost materials on removal of nitrates, phosphorus, copper, and dissolved organic carbon, and (2) to investigate the role of dissolved organic carbon on the bioavailable copper in leachate. Simulated bioretention systems were irrigated with stormwater at two-weeks interval for seven storms. During each storm, leachates were collected and analyzed for basic constituents, nutrients, copper, and dissolved organic carbon. Results showed that simulated bioretention systems were a source of nitrates, phosphorus, copper, and dissolved organic carbon.  Nitrate and phosphorus amounts were elevated up to three orders of magnitude in the leachate compared to the stormwater itself. Chemical speciation modeling showed that higher amounts of dissolved organic carbon and high pH reduces the bioavailable copper, thus decreasing the risk for copper toxicity to aquatic life.