Tea and sunlight: Exploring how nature breaks down pollution in the St. Louis River

Water that forms the beginnings of the St. Louis River in northern Minnesota percolates through moldering plants in remote wetlands and bogs. All this peaty goodness turns the water brown, as if the mythical giant Paul Bunyan squeezed it through his tea bag.

Along its 190-mile journey from Seven Beaver Lake into Lake Superior, this dissolved organic matter in the water is diluted until, when the water reaches Lake Superior, the color clears and it more closely resembles the rain and groundwater sources that were its beginnings.

Researchers funded by Wisconsin Sea Grant have found that various amounts and types of dissolved organic matter (also known as organic carbon) combined with sunlight can break down different pollutants in the St. Louis River. The research team was headed by Christina Remucal, University of Wisconsin-Madison, and Kristine Wammer, University of St. Thomas.

They focused on four pollutants: the insect repellant DEET, the cholesterol-inhibitor atorvastatin, the antidepressant venlafaxine, and the anti-epileptic medicine carbamazepine.

They chose to study the St. Louis River because of the variety of organic matter along its course. Remucal explains, “The headwaters are full of wetlands and all the organic carbon is really terrestrial – coming straight from plants. Then as you move into the estuary itself, there’s more of an anthropogenic influence as well as wastewater effluent. Once you get out into Lake Superior, the quality of the organic matter is quite different.”

Energy from sunlight can also break down the pollutants on its own in a process called direct photolysis, or it can interact with the dissolved organic matter to break down the pollutants in a process called indirect photolysis. For this study, the researchers focused on indirect photolysis.

Their findings are detailed in the Sept. 11, 2019, issue of “Environmental Science and Technology.”

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