Nutrient leaching behavior of green roofs: Laboratory and field investigations

Despite the benefits of green roofs in managing stormwater quality, green roofs especially at their early age might leach nutrients. Research in this regard is still very limited. Therefore, this paper conducted both the laboratory and field observations to characterize and model the leaching of nutrients including nitrogen (N) and phosphorus (P) and to examine the discrepancy in knowledge produced from these two settings. The experiment revealed that the higher the initial nutrient contents of media were, the higher the degree of nutrient leaching was. The nutrient leaching from both the laboratory cells and the field green roof declined temporally, which was largely explained by the cumulative inflow. The semi-physically based nutrient leaching model generally captured the nutrient leaching from both the laboratory cells (R² in the range of 0.87-0.98) and the field green roof (R² in the range of 0.28-0.86). The mass balance analysis for the laboratory cells demonstrated that the masses of nutrients leached in outflow were 85-112% of the nutrients reduced in media in general (except P of two laboratory cells). The analysis and modeling results supported that media was the primary source for nutrients leached and the pattern of nutrient leaching was consistent with wash-off being the dominant process. The results also revealed the difference in the P leaching between the laboratory cells and the field green roof. Apart from the wash-off, other chemical and biological processes and/or nutrient sources might play non-negligible roles on the P leaching of the field green roof, implied by the relatively low performance of the models (R² of approximately 0.30 in both the regression analysis and the nutrient leaching model). The difference observed between the laboratory experiment and the field observation also calls into attention when translating knowledge derived from laboratory experiments into real practice.