Nutrient leaching and copper speciation in compost-amended bioretention systems

Bioretention systems are designed to remove contaminants from stormwater; however, studies have shown that bioretention systems can export excess nitrogen, phosphorus, and copper when amended with compost. The objectives of this study were (1) to quantify removal of nitrates, phosphorus, copper, and dissolved organic matter (DOM) from compost-amended bioretention systems, and (2) to investigate the role of DOM on the leaching of copper. Simulated bioretention systems were irrigated with stormwater for seven storms in two-weeks intervals. Leachates were analyzed for nutrients, copper, and DOM. Visual MINTEQ was used to determine the speciation of copper and to quantify interactions of copper with DOM. Results showed that compost-amended bioretention systems were a source of nitrates, phosphorus, and DOM. Nitrate and phosphorus amounts were elevated up to three orders of magnitude in the leachate compared to the stormwater itself. Bioretention systems were a source for copper during the first 3-5 storms, but during later storms, they were a sink for copper. Copper speciation modeling indicated that the majority of dissolved copper was complexed with DOM. Dissolved organic matter thus helps to mobilize copper from the compost, particularly in the first few storms after compost application. However, since copper-DOM complexes are usually much less toxic than free copper ions, we expect that compost amendments may reduce harmful effects of copper on aquatic organisms.