for National Geographic News
The crystal-clear springs that feed one of the most pristine rivers in the southern U.S. are becoming increasingly polluted, scientists say.
The pollution is threatening the health of the unspoiled Suwannee River, which runs for more than 200 miles (320 kilometers) through northern Florida to the Gulf of Mexico.
Researchers have detected increasing levels of so-called nutrient compounds, particularly nitrates, in the springs.
"In some of the springs, we have seen very high concentrations of nitrate, well above the maximum contaminant level for drinking water," said Brian Katz, a research hydrologist at the U.S. Geological Survey in Tallahassee, Florida, who studies the health of the springs.
Among the possible sources of nitrates are agricultural fertilizers, septic tank effluent, and animal waste.
Excessive levels of nitrates nourish a type of algae that blocks the light needed by important sea grass beds and consumes oxygen used by fish and other creatures.
Parts of the springs can already be seen covered by stringy blobs of algae.
There are human health concerns too.
Since 2002 dozens of swimmers in north Florida springs have reported allergic reactions to the algae. The most common symptoms are rashes and welts, though some people have experienced dizziness and respiratory problems.
Recently other pollutants have begun popping up, including pesticides, chemicals from personal care products, and traces of pharmaceuticals.
In some springs, Katz has found traces of DEET, a common ingredient in mosquito repellents.
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