Characterization of nutrient leaching of non-amended and amended bioretention cells

To mitigate nutrient leaching of bioretention systems, especially phosphorus (P), a practical solution is to use amendments. In this paper, six selected amendments, including three waste materials (at 10% in volume) and three commercial amendments (at 5%), were examined using bioretention cells in a field environment for over 2 years. The results demonstrated that all the amendments can consistently mitigate P leaching in 2 years, with the water treatment residuals outperforming all the other amendments. However, sorbtive media and activated aluminum (two commercial amendments) would be superior if mixed at the same percentage, as they had a higher adsorption capacity than other amendments. On the other hand, these amendments, except eggshells, which released nitrogen (N), did not generally affect the N leaching. In addition, irrespective of whether the cells were amended or not, the observed P discharge patterns, namely, discharged at approximately constant rate and the absence of the first-flush release, suggest that P leaching might be governed by the decomposition of the compost in the growing media that had been implemented. But the amendment type significantly affected the degree of the P discharge. Different from the P discharge, the consistent decrease in the N discharge and the first-flush release in the first year (except the cell amended by eggshells), along with the cumulative inflow being the significant explanatory variable and the good performance of the nutrient leaching model, prove that the majority of leachable N was available at the beginning, and the N leaching process can be described by the pollutant wash-off concept. However, the inflow and amendment type would also play significant roles when the leachable N in the media is low.