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If your neighbourhood stream has turned brown in recent years, don’t be alarmed. It’s actually a sign the water is getting significantly cleaner.
Streams in central and eastern Canada, the northeastern U.S., Scandinavia and Britain are returning to a natural brown after years of running clear because there’s less acid rain polluting them, a new study has found.
The colour, similar to that of weak tea, comes from dissolved organic matter and is “indicative of a return to a more natural, pre-industrial state,” the study’s British, Canadian and American authors say in the science journal Nature.
Mostly, the brown stain comes from organic carbon. It’s harmless, but after living with clear water for decades, many people don’t like it.
“A huge amount of carbon is stored in the form of organic deposits in soils, and particularly in the peatlands that surround many of our remote surface waters,” said Don Monteith of University College London’s ecology centre.
“In the past two decades, an increasing amount of this carbon has been dissolving into our rivers and lakes, turning the water brown.”
At first, no one knew why this soil matter was suddenly entering the water.
The British researchers and others from the U.S. Environmental Protection Agency kicked around theories ranging from climate change to urban development and farming practices, but none of these fit the pattern.
Then they started looking at acidity.
When acid rain first became a big issue in North America in the 1970s, it was hard to persuade the public that lakes and rivers were in trouble because they looked so clear from the acidity.
Acid rain levels have fallen since then in most of North America and northern Europe, usually through improvements in coal-burning furnaces and smelters.
“As acidity and pollutant concentrations in the soil fall, carbon becomes more soluble, which means more of it moves into our lakes and rivers,” said John Stoddard of the EPA, the American partner in the research.
The team assessed data from 522 lakes and streams from 1990 to 2004. Data for the study came from national freshwater monitoring programs in Canada, Britain, the United States, Norway, Sweden and Finland. In Canada, all the streams were in Ontario, Quebec and the Atlantic provinces.
Ottawa naturalist Dan Brunton called the study “most encouraging”.
“Shrinking ozone layer gap, improved acidic rain conditions—we can improve overall environmental quality without the economic catastrophe that the industrial apologists always seem to suggest. That’s a great story for a gloomy November,” he said.