Response of green roof performance to multiple hydrologic and design variables: a laboratory investigation

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.

The influence of extensive green roofs on rainwater runoff quality: a field-scale study in southwest China

Green roofs of young age (≤ 5 years old) have boomed in China since the Sponge City Construction initiative was implemented. To use green roofs for better urban stormwater management, it is necessary to investigate the runoff quality of field-scale young green roofs as well as to examine common plant-media combination in green roof projects of China. The influence of two Sedum-vegetated extensive green roofs of different designs at the early stage of operation on runoff water quality was investigated by a field-scale study in Chengdu, southwest China. The water quality parameters of pH, suspended solids (SS), chemical oxygen demand (COD), total phosphorus (TP), and total nitrogen (TN) of rainwater (that is, input water for roofs), runoff from the two green roofs, and runoff from a conventional concrete control roof were compared. The results indicate that both green roofs mainly act as pollutant sources with greater concentrations of SS, COD, and TP when compared with rainwater quality. When compared with runoff quality from the control roof, greater TP concentrations in runoff from one green roof with commercially available substrate were observed. Attention should be paid to TP leaching in runoff for retrofitted green roofs with imported commercial substrates in that region. Adoption of pre-cultivated S. lineare mats of low fertility and localized soils may reduce nutrient leaching in green roof runoff. A nitrogen-rich substrate is not recommended for a plant community of a single species. Investigation of the effect of green roofs on water quality involving various pollutants in the long run is recommended.

Phosphorus and Metals Leaching from Green Roof Substrates and Aggregates Used in Their Composition

Green roofs are constructions made of different layers, each serving a dedicated function. Substrates and materials used in their composition are essential from the point of view of rainwater retention and plant development, but they may have an adverse effect on runoff quality. Literature studies show that phosphorus and heavy metals are of main importance. The total roofs area covered with green increased in the last years in cities as they are efficient in retention of rainwater and delaying of the runoff, therefore, protecting the cities against floods. As green roofs filtrate a significant amount of rainwater, materials used in substrates composition should be carefully selected to protect urban receivers against pollution. The aim of this study was to assess phosphorus and heavy metals leaching from different green roof substrates and their components with the focus on green roof runoff quality. Both commercially made green roof substrates and often used compounds (construction aggregates) were tested in laboratory batch tests for P, Cu, Ni, Cd, and Zn content in extracts. Based on the results of this study, it could be emphasized that a large part of commonly used construction aggregates can be a source of phosphorus, some also can release elevated values of nickel. Therefore, the materials should be carefully tested before use in the green roof substrate composition, not only for their physical properties reflecting water retention capacity, but also for chemical composition.