Nitrate levels in Lake Superior continue to rise
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an interview Nitrate levels in Lake Superior, which have been rising steadily over the past century, are about 2.7 percent of the way toward making the water unsafe to drink, according to a new study by University of Minnesota researchers. The complexity of the causes underlying the increase makes it difficult to predict when the water could become unhealthy.
A compound made from nitrogen and oxygen, nitrate is a component of agricultural fertilizer and is generated by fossil fuel combustion. Nitrate in Lake Superior has increased about five-fold since the earliest measurements in 1906. But surprisingly, the increase has been steady.
“It’s puzzling because it doesn’t reflect post-World War II increases in fertilizer and fossil fuel or the Clean Air Act of 1972,” says Robert Sterner, professor in the College of Biological Sciences and lead author of the study. “It’s much more complex than that.”
Sterner says factors include the vast size of Lake Superior, which means it registers change slowly, and conversion of other forms of nitrogen within the lake (in decaying plant matter and sewage) into nitrate.
Everyone is exposed to small, harmless amounts of nitrate from eating fruits and vegetables. But nitrate contamination of drinking water can expose people to harmful levels. Too much nitrate can reduce blood levels of oxygen, which poses a risk to infants and children or adults with lung or cardiovascular disease. Consuming excess nitrate over long periods of time is also suspected to cause cancer.
“We’re still a long way from drinking water advisories based on nitrate for Lake Superior, but it’s not too early to give this situation more attention, Sterner says. “We cannot easily or quickly reverse trends in this enormous lake.”
Sterner adds that in spite of two decades of legislation designed to address nitric oxide in acid rain, a source of nitrate pollution in water, nitrate in this body of water is still increasing, though perhaps at a slower rate.
The trend is a concern because Lake Superior contains 10 percent of the Earth's supply of surface fresh water. The new finding adds to recent news that global warming has decreased the level of water in Lake Superior.
The study is to be published online May 31 in Geophysical Research Letters.
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