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

Abstract

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.

Global climate-driven trade-offs between the water retention and cooling benefits of urban greening

“…Urban greening strategies have previously been associated with a wide range of potential environmental, economic and social ecosystem services 13 , 14 and while our results show that they may be effective in contributing to urban development and renewal, they are not a panacea. For example, despite potentially being able to reduce the intensity of the urban stream syndrome (USS) through their favorable effects on stream hydrology, green roofs potentially exacerbate the USS through pollution 32 , 33 . Hence, while the trade-offs we focus on here between retention and cooling may be important but manageable aspects of sustainable urban development and renewal programs, other environmental trade-offs need to be considered on a case-by-case basis.…”

Impact of Green Roofs and Vertical Greenery Systems on Surface Runoff Quality

“…Based on a recent study by Karczmarczyk et al [51], a list of 29 GR substrate components were tested regarding their potential for leaching of contaminants to urban receivers. Out of the materials tested, those of natural origin, including sands and gravels, showed a very high release of Ni and Zn into runoff.…”

“…In the group of human-made materials, crushed red brick, expanded clay, and ash were strongly polluted with Ni, Zn, and Cu. Regarding Ni emissions, four of the tested mineral materials were characterized as materials associated with high risks in application [51].…”

Phosphorus leaching from substrates commonly used in rain gardens

“…Sands and gravel are marked on the list of the risky green roof substrates components as materials with high or very high risk of application connected with phosphorus release and potential eutrophication of urban receivers. Limestone was characterised as a material with low risk of application [4]. Base on construction projects of analyzed rain gardens it was possible to calculate the mass of materials used for filling up functional layers.…”

“…Rain gardens, similary to green roofs, filter rainwater causing its purification or pollution. Thus materials used in their construction should be careffully tested before filling up the system [4]. The aim of the study was (1) to analyze potential phosphorus leaching from construction materials sampled from two rain gardens, and (2) to assess the quality of effluent from rain garden in container.…”