A surprising source of phosphorus in cities?

The atmosphere seems to be contributing more phosphorus to urban ecosystems than we thought. Planting urban trees above soils rather than pavement could help to mitigate the problem.

By Stephen Decina, Pamela Templer, and Lucy Hutyra | Boston University

Is it raining phosphorus in cities? Conventional wisdom says that there should be very little phosphorus coming down in rain or snow, but if this assumption is incorrect, cities could have a surprising source of pollution to worry about as they clean up their lakes and rivers.

Most people probably know about phosphorus from that bag of fertilizer in the garage. It’s the “P” in the very common N-P-K fertilizer blend. Though phosphorus helps garden plants thrive and lawns grow lush, too much phosphorus is unhealthy for ecosystems, particularly lakes and rivers. Dump too much phosphorus into a lake, and the water will go from a healthy blue to a toxic green, as algae growth explodes. When these algae die, they are consumed by bacteria in the water, which use up all the oxygen in the water, essentially suffocating the lake’s fish and other organisms. This process is called eutrophication, and it’s one of the main reasons that the U.S. Environmental Protection Agency (EPA) is concerned with the amount of phosphorus entering waterways. In one instance, the U.S. EPA required more than 200 municipalities bordering Massachusetts’ Charles River to remove more than 50% of the phosphorus washing into the river. These kinds of efforts cost millions of dollars.

To clean phosphorus up, cities first need to know how much there is and where it comes from. In urban areas, we expect to find phosphorus on ground surfaces from sources like fertilizers and pet waste. We also expect phosphorus to be in treated sewage — sewage that has been pumped to a treatment plant, processed to remove excess nutrients like phosphorus and nitrogen, and then released into a nearby waterway. In fact, in developed countries, treated sewage is generally the largest point source of phosphorus to freshwater sources like rivers. The atmosphere, on the other hand, is not expected to be a big source of phosphorus, because gases containing phosphorus are uncommon and combustion of fossil fuels (aside from coal) don’t produce a large amount of particles that contain phosphorus. Those who study phosphorus in rain find that it generally comes from agriculture. For these reasons, many people interested in urban air pollution don’t look for or measure phosphorus in rain.

However, when we measured phosphorus in rain around the Boston area in 2015, in a paper published recently in Earth’s Future, we found that the conventional thinking about sources of phosphorus might be incomplete in cities. We found a big surprise: Boston’s rainwater has about twice as much phosphorus as rainwater in rural areas. An even bigger surprise — when we extrapolate our measurements over the the City of Boston for the period of the study, we found that rainwater was a larger source of phosphorus than treated sewage, which is generally thought to be the largest point source of phosphorus to urban waterways.

We also found a big difference between the rainwater passing through the tree canopy (also known as “throughfall”) and the rainwater that fell directly from the atmosphere to the ground. We found that rainwater that passed through the tree canopy deposits six times as much phosphorus to the ground than rainwater that doesn’t pass through vegetation. The extra phosphorus beneath trees likely comes from a host of factors: dust settles on tree leaves and is washed off, trees produce biological matter like pollen that contains phosphorus, and birds and insects gather in trees and produce waste that contains phosphorus.

These two findings, that there is a lot of phosphorus in urban rainwater and that phosphorus in rainwater is elevated beneath urban trees, lead to two distinct conclusions that may help cities deal with phosphorus pollution.

First, as cities attempt to clean the phosphorus from their waterways, they should also consider how to monitor and mitigate phosphorus coming from rainwater, which could be as large as or larger than the sources already commonly measured, including fertilizers and sewage.

Second, and perhaps a more practical consideration, is that we encourage cities to rethink where they’re planting trees. Though many urban trees are located in yards and parks, there are also many trees that line streets and sidewalks. Rainwater falling through a tree in a park or yard can move into the soil below, where plant roots and soil microorganisms can grab on to phosphorus in the rainwater, reducing the likelihood that it will be washed into nearby waterways. But phosphorus that falls beneath tree canopies onto impervious pavement can move directly into nearby waterways without being taken up by plant roots and microbes. Planting urban trees above soils or other surfaces such as permeable pavement might be a sensible solution to this problem, maximizing the many benefits of urban trees while minimizing phosphorus pollution in waterways and saving cities money that would otherwise be needed to remove phosphorus from their waterways.

As new cities develop, and cities like Boston continue to improve their infrastructure, understanding and mitigating sources of urban pollution is an essential effort to creating the sustainable cities of the future.

For additional commentary by Boston University experts, follow us on Twitter at @BUexperts and on Instagram at @buexperts.

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