With the expansion of Seattle comes more cars on the roads. The fact that transportation results in pollution is widely known, but School of Environmental and Forest Sciences‘ Phil Levin, Ian Davies and Mathis Messager, in partnership with Boeing and The Nature Conservancy, pinpointed the exact locations in Puget Sound where pollution has accumulated in a paper published in Science Direct.

Q: Where are the hot spots for pollution in the Puget Sound?

A: One of the major causes of pollution in the Puget Sound is the runoff of chemicals off our roads. 75% of that comes from cars and transportation. So we wanted to ask the question, where are the hotspots of pollution? Can we pinpoint where that pollution is coming from? Once we figure out where that’s coming from, we know how to must effectively and efficiently treat that pollution.

We sought out to map where pollution was coming off of cars, and identify those hotspots. What we found was that if you want to reduce the pollution in Puget Sound by 50%, you can target 3.3% of the area around the Puget Sound. What that means is that if you were to target that 3.3%, you will be able to cut pollution by half.

Q: Can you tell us a little bit about how you went about that study?

A: What we did in order to identify 3.3% of the area that turns out to be a hotspot for pollution is to first develop a model prediction of where we thought that pollution would be. That comes from how many cars there are, how fast they’re going, how often they’re braking…this is all information that’s publicly available. Then, we had to make a prediction, and we wanted to test that prediction to validate our computer model.

In order to do that, we realized that we can use moss, which is just growing along the roadsides all around Puget Sound and test for metals in that moss. We know that moss basically absorbs chemicals out of the air. They don’t have roots, they grow on trees, and so anything that passes by, they absorb into their tissues and stay for about a year. Moss integrate over a year, so if we can find a way to measure quickly and inexpensively for those metals, that would serve as a proxy for where there’s pollution coming from cars. Particularly, if we focus on zinc, it is very characteristic of pollutants that come from cars (especially tires from cars). Using an x-ray fluorescence gun, we literally went out in the field and shoot this gun at the moss, it reflects back and it tells us within 30 seconds how much zinc is in that particular place. We did this across an area, and we were able to validate that our model was an accurate prediction of how much pollution from cars is in a specific place. Then, we’re able to scale it up from our region to the entire U.S. At a resolution of five meters across North America, we can tell you where and the amount of pollution from cars.

Q: What can we do with this information?

A: The idea is, once you know where the pollution is, you can treat it. One of the most powerful ways to treat it in our area is called green infrastructure. These are rain gardens that capture rain coming off your roof before it goes into the road or indentations by the road that capture rainwater before it comes off the road. If we can catch that rainwater before it enters our surface streets and before it goes down into the Sound, it will get absorbed into the soil and be neutralized. What we want is to make the city act like a sponge, act like a forest like it used to, before we paved it over. If we can identify those places where we want that toxic rainwater to enter the soils and be purified, and do that in an efficient way, we can save money and solve the problem.

Q: Why did you focus on measuring zinc and copper?

A: We measured zinc and copper and other metals, but focused on zinc and copper because those are characteristic of pollution coming from transportation. Transportation creates all sorts of health issues for people, ranging from asthma to increased cancer risk and so forth. It wasn’t zinc and copper, necessarily, that we were worried about, but in this case they serve as an indicator for all pollution coming from transportation. Where we know there’s a lot of zinc, we know that it was a hotspot of transportation-generated pollution.

Q: Are there any impacts aside from human health?

A: Transportation-related stormwater runoff is bad for people and nature. It’s incredibly bad for the health of our watersheds. In fact, if we go around Puget Sound, the areas that are getting runoff with (for example) 40 percent pavement, those areas have almost no survival of salmon in those streams. By identifying these hotspots of pollution, we’re making the world healthier for people and saving salmon.