By Marc Howe.

Green roofs have long been extolled as one of the most handy and convenient of sustainable building measures, making use of exposed surfaces which would otherwise sit idle and having them capture airborne pollutants in vegetation and soil while also providing much needed patches of natural scenery to urban environments.

A new study by  a team of scientists in the UK, however, claims green roofs may simply be passing the pollution buck from one environmental sphere to another.

Research conducted by scientists from the University of Manchester and the University of Leicester suggests that rainwater runoff from green roofs is taking the pollutants seized by urban vegetation from the atmosphere and transferring them to the surrounding environment.

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The study, the results of which were published in the journal Environmental Pollution, examined differences in the performance of green roofs and conventional roofing.

According to Dr. Andrew Speak, lead author of the study from the School of Environment and Development at the University of Manchester, the rainwater runoff from the green roof turned out to be green and yellow in appearance when collected.

An analysis of rain water samples from the green roof found that they contained a high concentration of heavy metals, some of which were in excess of environmental quality standards. These metals include copper, lead and zinc.

An analysis of samples of soil from the green roof found that they too contained high concentrations of lead.

Researchers believe these concentrations accumulated in the years prior to the gradual removal of leaded petrol from the market, given that the roofs were built in 1970. They noted that modern green roofs would not contain such large amounts of the toxic metal.

The paper nonetheless points to green roofs being a quid pro quo for the environment as a whole, with a reduction in air pollution achieved at the expense of the quality of water runoff.

The team has offered a number of recommendations for preventing green roofs from compromising water quality in urban settings.

These include refraining from installing the fixtures in locations where vehicular pollution is particularly intense, such as congested motorways, as well as the mixing of biochar – a type of charcoal – into the substrates of green roofs, in order to reduce the seepage of chemicals from the soil.

Speak also pointed to the potential development of new breed of “super plants,” which will possess an enhanced propensity for pollution removal and retention.

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