Green roofs may be a source of pollution

Nov 12, 2013
Green roofs may be a source of pollution
The study roof with Dr Speak holding an anenometer

Green roofs could become a future source of water pollution, says a new study.

The research, published in the journal Environmental Pollution, looked at the differences between an established and a conventional roof surface.

Green roofs are known to be efficient at capturing air and until now it was thought that the polluting particles were retained in the soil layer and the vegetation.

This new study, conducted by researchers at the Universities of Manchester and Leicester, suggests that instead, the pollutants can leak out into rainwater as it runs off the roof.

'The rainwater runoff from the green roof was green and yellow in colour, 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.'

The raised lead levels, which were high for both the conventional roof and the green roof, led the team to look more deeply into the possible source.

Green roofs may be a source of pollution

'Part of the drainage was across a glass atrium which has some lead flashing and may have contributed to the levels. But we also took samples of soil, from the green roof, and lifted up slabs of the conventional roof to collect dust and found both were very high in lead,' says Speak.

Without any obvious significant inputs of lead nearby, the researchers think the lead is probably the legacy of leaded petrol cars which has collected over the years, since the roofs were constructed in 1970, long before the phase-out of leaded petrol.

Modern green roofs would have no significant input as we've moved towards unleaded petrol.

'The biggest discovery in this paper is that while green roofs do reduce , the pollution may accumulate and cause a problem in the future with reduced water quality of runoff,' explains Speak.

The researchers recommend that if planners intend to install a green roof, they must pay attention to its location.

'We shouldn't put green roofs next to busy motorways, or inner city motorways, basically anywhere where vehicle derived pollution could affect runoff in the future,' Speak says.

The team also suggest more research should be done into the substrates green roofs use; for example biochar – a type of charcoal – has been shown to reduce nutrients seeping out of soils, and could help mitigate the problem of finding their way into water systems.

Explore further: Classification system proposed for green roofs

More information: A.F. Speak, J.J. Rothwell, S.J. Lindley, C.L. Smith (2014) "Metal and nutrient dynamics on an aged intensive green roof," Environmental Pollution, Volume 184, Pages 33-43

Provided by PlanetEarth Online search and more info website

3.3 /5 (15 votes)

Related Stories

Classification system proposed for green roofs

Oct 22, 2013

Green roofs (or living roofs) are becoming a growing trend in North America – and have been long established in Europe – for their value in conserving energy, improving air quality, managing storm water ...

Turfgrass tested in shallow green roof substrates

Oct 21, 2013

Green roofs, rooftops covered with vegetation, provide multiple environmental and aesthetic benefits. These "living roofs" are increasing in popularity worldwide. As more cities invest in green roofs, planners are challenged ...

Recommended for you

New water balance calculation for the Dead Sea

16 hours ago

The drinking water resources on the eastern, Jordanian side of the Dead Sea could decline severe as a result of climate change than those on the western, Israeli and Palestinian side. This is the conclusion ...

Studying wetlands as a producer of greenhouse gases

22 hours ago

(Phys.org) —Wetlands are well known for their beneficial role in the environment. But UConn Honors student Emily McInerney '15 (CAHNR) is studying a less widely known role of wetlands – as a major producer ...

User comments : 16

Adjust slider to filter visible comments by rank

Display comments: newest first

Scottingham
3 / 5 (2) Nov 12, 2013
While it would increase the cost a bit, an activated charcoal filter would do well in this situation I would think...
rue
5 / 5 (3) Nov 12, 2013
If green roofs do not create pollution but collect it out of the atmosphere, then appropriately engineered systems located near pollution sources (e.g., next to busy highways) could be used to scrub our air.
nkalanaga
5 / 5 (2) Nov 12, 2013
Scottingham and Rue: Certainly. Just run the downspout through a filter system and the problem is solved. Cleaner air, clean water, and, maybe, enough metals to be worth recycling. Not today, probably, but in the future we could have bacteria tailored to sift out a particular metal, and then it could be reused.
ppnlppnl
3.9 / 5 (7) Nov 12, 2013

Yes but would a green roof produce more pollution than a natural grass lot? Should we kill all grass lands near air pollution sources?

Nonsense. Pollution exists. Capturing and concentrating it gives us options on how to deal with it. I say capture more.
LagomorphZero
1.7 / 5 (12) Nov 12, 2013
That pollution is coming out of the air eventually via rain anyway. It seems like the best idea is to increase green roofs near pollution, so we can capture it as quickly as possible. Leading to the filtering ideas presented by the other posters above.
NikFromNYC
1 / 5 (12) Nov 12, 2013
Did they do a control?
Urgelt
not rated yet Nov 12, 2013
Er... don't these guys know that burning coal pumps lead, mercury and other toxins into the atmosphere? And that this source of atmospheric toxins hasn't abated much since lead was phased out of 'petrol?'

Coal-fired electricity production remains so ubiquitous, there is literally nowhere on Earth where 'green roofs' won't be grabbing toxins and pumping them into run-off.
thatsitalright
1 / 5 (7) Nov 12, 2013
"Green roofs may be a source of pollution"

No. The amount of pollution from a green roof is no more than from a conventional roof, and is almost solely due to the lead flashings which are ubiquitous in roof construction. Absorbing pollution from the environment does not equal being a source of pollution.

Zinc accumulation can be traced to galvanized steel flashings which are also ubiquitous, and copper is more rarely used but is common in coastal regions where it replaces aluminum, since aluminum does not stand up to corrosion as well from the salt.

"Modern green roofs would have no significant lead input as we've moved towards unleaded petrol."

Not a very scientific conclusion. Perhaps the researchers should test a modern green roof, or better yet, a large sample of them.

thatsitalright
1 / 5 (7) Nov 12, 2013
"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,"

The output of pollution can never exceed the input of pollution, so although pollution from past times (such as leaded petrol) may take longer to work their way out, the roofs themselves are not specifically causing a problem.

You could argue that older roofs should be replaced due to their accumulation of pollutants that will continue to run off into the environment long after they have reached peak absorption, but in the end the whole thing ends up in a landfill, so there's not much benefit to be realized.
thatsitalright
1 / 5 (7) Nov 12, 2013
"'We shouldn't put green roofs next to busy motorways, or inner city motorways, basically anywhere where vehicle derived pollution could affect runoff in the future,' Speak says"

Why, because it might trap pollutants from the environment and slowly release them back into our waterways for years after? This is not a roof problem, and it's not a green roof problem.

From a pollution standpoint a green roof is still better than a conventional roof since a green roof at least reduces air pollution. A conventional roof will experience the same exposure to outside contaminants but will simply flush them through faster rather than retaining them.
thatsitalright
1 / 5 (7) Nov 12, 2013
"The team also suggest more research should be done into the substrates green roofs use; for example biochar – a type of charcoal – has been shown to reduce nutrients seeping out of soils, and could help mitigate the problem of heavy metals finding their way into water systems."

Charcoal might help to retain external pollution that has absorbed into the substrate, but it will not prevent pollution from lead or copper flashing which reside beneath the substrate as they typically do.

For those suggesting a filter in the downspout, that will never work. A roof takes in water from a very large area and drains it into a small pipe. It's trouble enough keeping the a loose grating free of debris, never mind a charcoal filter. Charcoal dispersed throughput the substrate may capture more pollutants without releasing them, but even then the charcoal has to have some saturation point. Talking about roof from the 70's, that's 40 years of pollution. Water charcoal filters average 6 months tops.
On secondthoughtthinkagain
1.4 / 5 (9) Nov 13, 2013
The studies conclusion are very left field. And my initial reaction is the same as most people above.

It doesn't matter what colour the roof - the pollution will still fall onto it and get washed out. And a roof with vegetation on it cannot be worse than one without unless the dirt or plants used to cover the roof brought the pollution with them from their source.

The study did not conclude that the plants somehow made the roof worse so how can they conclude that having them on the roof near pollution would somehow be worse than not having them?

Unless some idiot thinks that just because the roof is "green" then the water that comes off it should somehow be better than otherwise? If that is their thinking then they need to remind people that an organic roof is just the same as a non organic roof. And if you can get up there and use it for something then it is a damn sight better just don't drink the runoff.
wwqq
not rated yet Nov 16, 2013
While it would increase the cost a bit, an activated charcoal filter would do well in this situation I would think...


Charcoal captures ORGANIC chemicals and things like chlorine, it emphatically does not capture metal ions or salts.
kochevnik
1 / 5 (3) Nov 16, 2013
Everywhere I go in USA Americans are busy KILLING trees. Any green at all is welcome relief from the burning, patch black tar and concrete lifeless deserts Americans prefer
Straw_Cat
not rated yet Nov 22, 2013
Read the Greenroofs.com article in response to this piece.

http://www.greenr...s-may-b/
Straw_Cat
not rated yet Nov 22, 2013
1: pollutants will fall on green roofs.
2: Some of them will be broken up or bound to the plants on the roofs.
3: Other ones will move through the roof environment just as they would move through any other environment. To avoid that, it is better to reduce or eliminate the sources of those pollutants.
4: the more green roofs (and living walls) there are, and the more energy they save by reducing heating and cooling needs, along with delaying run-off time for rainwater and snow melt, the less energy will need to be produced to power those processes. In this circumstance, the lower energy demands can lead to a reduction in the amount of pollutants created by power plants that run off coal, etc.

Given enough green roofs and living walls, a reduction in maximum storm run-off and retention of water can lead to the need for a smaller water treatment system than would be needed otherwise. Smaller water treatment plant(s) will require less energy to operate at peak times.