With the debate over sewering taking center stage on the Cape these days, many residents are aware that the biggest source of nitrogen pollution in local estuaries is wastewater that leaches out of septic tanks. Lawn fertilizers and road runoff are also contributors. But what do cars have to do with the problem?
That was one of the questions that Neil D. Bettez, of the Cary Institute for Ecosystem Studies in Millbrook, New York, spent five years as a doctoral student in Falmouth trying to answer. Working with Eric Davidson, a senior scientist at the Woods Hole Research Center, and colleagues at Cornell University, Dr. Bettez came across a local nitrogen source that is often overlooked: emissions from cars and trucks.
“We’d like to say it’s only the power plants in the Midwest [that are responsible], but the fact that a lot of cars driving on Cape contributes to local impacts. What we drive, and how much, really matters on a local level,” Dr. Bettez said.
‘The more you put in, the more you put out’
Those driving around down Route 28 between 2003 and 2007 may have noticed bottles with funnels on top sitting under trees at the Waquoit Bay National Estuarine Reserve (WBNERR) near the Mashpee-Falmouth town line, or on the lawn at the Woods Hole Research Center in Falmouth. These contraptions collected rainwater dripping from tree leaves, which Dr. Bettez analyzed for nitrogen content.
His results showed that areas of the forest within 30 feet of a road contained the highest amounts of nitrogen. He also found more nitrogen underground near the road, pointing to a nitrogen source that is leaching into the groundwater, and eventually into nearby water bodies.
“Understanding sources of nitrogen is a key first step in managing and mitigating nitrogen pollution,” he said. “The more you put in, the more you get out. Even kids know that.”
The research conducted by Drs. Davidson, Bettez and their colleagues suggests that road runoff may have been underestimated in the past, and it could contribute as much as 10 percent of the nitrogen load to some local water bodies.
According to Dr. Bettez, atmospheric nitrogen has been on the rise since the Industrial Revolution. Through fertilizer production and fossil fuel combustion, humans now release as much reactive nitrogen (NOx and NH3) as is created naturally from lightning and nitrogen-fixing plants, such as soybeans or blue-green algae.
Due to cleaner emissions standards, power plants in the US now produce 29 percent less NOx than they did 30 years ago; and despite better emissions standards for cars, Americans also drive twice as much as they did in the 1970s, contributing 33 percent more nitrogen, said Dr. Bettez.
A hidden killer
Excess nitrogen in waterways can lead to eutrophication, resulting in low-oxygen conditions that makes it difficult for marine animals to survive. Algae thrives on the nitrogen, but sudden algal blooms can lead to smelly die-offs, unpleasant swimming conditions, and degraded eelgrass, an important habitat for shellfish.
Because most of the nitrogen that settles on leaves is retained in the forest, Dr. Bettez said, wastewater is still the single largest source of nitrogen in the water. However, he added, the forest’s capacity to hold that nitrogen is limited. In large quantities, he said, NOx and NH3 in the atmosphere can lead to tree “die back” due to acidification, an effect similar to acid rain.
While nitrogen deposition on the Cape is still too little to impact local forests, it could reach a saturation point, and end up in waterways, Dr. Bettez said. “My research points out that it will continue to be a bigger problem as people drive more.”
According to Ivan Valiela, of the Marine Biological Laboratory’s Ecosystems Center, different parts of Waquoit Bay could receive an additional 31 to 79 percent nitrogen load from rain and snow falling directly on the water.
“There is an additional atmospheric deposition directly onto the surface of the water of the bay, and this can be large,” Dr. Valiela said.
“Management action such as sewering would not affect this direct atmospheric source… a consideration, given the current interest in installation of municipal sewers.”
In comparison to wastewater and atmospheric deposition, the nitrogen from road runoff is “trivial,” at about four percent of the total load to the watershed, Dr. Valiela said.
However, Dr. Davidson said that the nitrogen contained in road runoff is significant enough to warrant better management in coastal zones. Analyzing the runoff he sampled during storms from Woods Hole Road, Oyster Pond Road, and Quonset Road, Dr. Davidson found that even small residential roads contain significant amounts of nitrogen, from a combination of car exhaust, lawn fertilizers, and animal waste.
“Roads are conduits for moving all that nitrogen rapidly. Where roads lead to water bodies, they contribute directly to the nitrogen load,” he said.
To reduce road runoff, the town could install “wells” that shuttle water from storm drains through layers of soil, where microbes can break down nitrogen and other hazardous byproducts, Dr. Davidson said.
Global problem, local solution
Because nitrogen loading is a local problem, Dr. Bettez said that local solutions, such as driving less, or driving hybrid vehicles, will help reduce nitrogen emissions.
Another way to tackle nitrogen emissions would be to follow California’s lead in developing regulations that would limit ammonia, a compound that is created from NOx by highly efficient catalytic converters, said Dr. Davidson.
Dr. Bettez noted that NOx is responsible for smog and atmospheric ozone, while ammonia plays a role in creating haze. Ammonia is also deposited very close to its source, which means that emissions from cars can fall out onto nearby water bodies, Dr. Davidson said.
Both Dr. Bettez and Dr. Davidson added that dietary changes could make an impact on global nitrogen production, noting that it takes significant amounts of fertilizer to produce animal feed, and animals produce manure, another large source of nitrogen that is released into the environment
“If we’re eating meat three times a day, we have a bigger nitrogen footprint,” said Dr. Davidson. “For those who don’t want to be vegetarian, thinking about portion sizes, or whether they’re eating beef, or less nitrogen-demanding pork, chicken, or fish, makes a difference.”
Tags: car emissions, carbon footprint, Eric Davidson, Ivan Valiela, Marine Biological Laboratory, Neil Bettez, nitrogen deposition, nitrogen loading, NOx, UN Climate Conference, Woods Hole Research Center
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