Green roofs are thought to capture harmful air pollution particles and store them in their soil layer and vegetation, but a new study from researchers at the Universities of Manchester and Leicester suggests these pollutants could be leaking out with rainwater that runs off the roof. The study was published in the Journal of Environmental Pollution and it compared the difference between a conventional roof and an established green one. The researchers found that heavy metals such as copper, lead and zinc were all found to be above acceptable levels in runoff from the green roof.
“The rainwater runoff from the green roof was green and yellow in color, so we sent samples for analysis of the heavy metal and nutrient concentrations,” says Andrew Speak, a PhD student at the University of Manchester and lead author of the study. “Some heavy metals were found to be quite high. Copper, lead and zinc all exceeded environmental quality standards.”
Despite the discovery, the researchers initially struggled to find a potential source. Their first guess was lead flashing from an atrium roof the water passed over, but eventually the team attributed the high levels of lead to a hangover from the 1970s when leaded petrol cars were still in use.
So while the research is in no way trying to mark green roofs as the next super pollutant, it does draw attention to the fact that they can act as sponges for pollution. “The biggest discovery in this paper is that while green roofs do reduce air pollution, the pollution may accumulate and cause a problem in the future with reduced water quality of runoff,” explains Speak.
Apart from pointing out the importance of location, the team also suggests more research should be done into the substrates green roofs use. One alternative is biochar, a type of charcoal which has been shown to reduce the amount of nutrients that seep out of soils. The material could help mitigate the problem of heavy metals finding their way into water systems.