MINNEAPOLIS, May 31 (UPI) -- Lake Superior's nitrate levels are rising and scientists say the lake is about 2.7 percent of the way toward having unsafe drinking water.
The University of Minnesota study found the complexity of the causes underlying the increase in nitrates that's been under way for more than a century makes it difficult to predict when the lake's water might become unhealthy.
Nitrate is a component of agricultural fertilizer and is generated by fossil fuel combustion. Lake Superior's nitrate level has increased about five-fold since the early 1900s, researchers said, and 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," said Professor Robert Sterner, lead author of the study. "It's much more complex than that."
Sterner says factors include the vast size of the lake and conversion of other forms of nitrogen within the lake in decaying plant matter and sewage into nitrate.
The research is available online in the journal Geophysical Research Letters.
