Green Roof Nutrient Runoff & Future Environmental Compliance

by Dr. Anna Zakrisson on Tuesday, January 15, 2019


Nutrient runoff from green roofs

The Great Lakes and the Baltic Sea have such extreme algal blooms that they can be seen from space!
So what this has to do with nutrient retention on Green Roofs? A lot, it turns out!

Let me explain…

Raindrops on a succulent

A green roof is mainly an investment for stormwater management that is helping to reduce stormwater runoff volume, leading to less pollution. The latest developments in the green roof industry are detention -oriented because the real ROI of a green roof is found in the provided detention.

This detention will add to waterretention and help cities deal with ever-increasing extreme weather events. The weather is unfortunately changing, and it would be economically prudent to prepare for this.

There’s, however, one often forgotten aspect about what might be required by a future green roof: stringent nutrient runoff regulations.

My Ph.D. was on nutrient cycles in the Baltic Sea, and I have worked closely with regulators and scientists in this field. My prediction is that the regulations around green roof nutrient runoff will rapidly become stricter, particularly with regards to phosphorus.

Let me show you why!


Environmental standards

A green roof must comply with all sorts of environmental standards. Stormwater management isn’t the only item on that list. Runoff quality is also of importance and should not be forgotten.

There may be strict limitations for, particularly phosphate, in many states and nations, especially those bordering to important water bodies.

Sometimes, the nutrient regulations are stricter than for other types of green areas such as gardens or parks. These strict limitations might at times seem counterproductive and somewhat silly for people involved in the green roof industry simply because green roofs make up such a small portion of the total green areas. If youwanted to really make a difference in how much nutrients are leached from a city, you should focus on lawns. I agree. To a point. As I believe that the green roof industry will expand fast over the coming decades, we need to prepare for a future where green roofs make up a significant portion of a city's green areas. Also, if we don't prepare properly, this day may never arrive.

Algal bloom in a freshwater lake

The future ahead

I agree that the development of green areas in cities should be supported. We need green roofs for stormwater management , reduction of heat islands, improvement of air quality, and an increase in biodiversity. I see green roofs as an integral part of the future city.

The future ahead

However, let’s return to the Great Lakes and the Baltic Sea and take a look at some small water-dwelling organisms called cyanobacteria. Cyanobacteria are often bloom-forming and toxic. Blooms mainly tend to form when there is a lot of nutrients, especially phosphorus, combined with warm weather. These blooms are at times so massive that you can see them on satellite images.

These little buggers have a superpower: they can draw nitrogen from the air and fertilize themselves. They do, however, need phosphorus. Give them phosphorus and warm weather, and you’ll have a bloom in no time. Entire drinking water deposits might become infested within a couple of days. They truly are a nuisance.

I worked on these organisms during my Ph.D., and it seems as many of these blooms are increasing in both frequency and strength. You can understand why this might become a headache for many politicians. Not only do these blooms affect drinking water deposits, but these stinking mats of decaying bacteria have the potential to completely ruin tourism, something my home country Sweden is acutely aware of

Few things make politicians move faster than the potential destruction of a national industry.

Satellite image of phosphorus-fuelled cyanobacterial "algal" blooms in the Great Lakes

So, where do green roofs fit into this?

The main nutrient runoff issue for green roofs appears to be phosphate. Preliminary studies show that a newly built green roof leaches more phosphate than older roofs, but even so, older roofs still seem to be phosphorus sources.

This is quite unfortunate as this is the main nutrient-related headache for the regulators.
Hence, this is an issue that the green roof industry urgently must solve.

The regulations around phosphorus will not become more lacks, they will become harsher because of scientists like myself. The climate is changing rapidly and is becoming more extreme. Summers will be hotter, and with this development, the need will arise to continuously reduce land runoff of phosphates to save our drinking water, ecosystems, and tourism industries.

It would be judicious to have prepared for this yesterday.

Healthy sedums require nutrients, but balance is important. More is not always more.

The road ahead

A lot of exciting research is currently carried out trying to capture nitrate and phosphate runoff from green roofs. Many materials with adsorbent properties are being tested out. However, very few biologists seem to be involved in these ventures due to the engineered nature of the roof system.

Formal green roof standards seem to view soils mechanically as particles that manage water, and as a result, they tend to be unfriendly to soil biology. Notably, the soil media used on green roofs seem to have considerable potential for improvements from a biological perspective.

The currently used industry standard soils are often a one-size-fits-all-climates, which results in many locations having the wrong soils leading to poor soil retention, plant survival, and high nutrient runoff.

The goal should be to create roofs on which the hydrological and the organic layers form a sustainable and resilient ecosystem in which nutrients cycle efficiently. I’m convinced we can get there and that we can get there soon.

I also think that the best solutions are to be found in the collaboration between biology, engineering, and business.

I’m really looking forward to the years to come: green cities providing protection and refuge, as well as excellent financial incentives for the building owners to improve our cities, and our water bodies, even further.

Don’t hesitate to contact our experts for information, collaboration, and tips!

Reading tip: green roof nutrient runoff

Buffam, Ishi, and Mark E. Mitchell. "Nutrient cycling in green roof ecosystems."Green Roof Ecosystems. Springer, Cham, 2015. 107-137.