During this summer’s drought, the Minnesota River has fallen to near-historically low levels, exposing riverbanks and sandbars.
But these low flows also created optimal conditions – last seen in 2012 – for researchers to collect data on phosphorus from wastewater treatment plants and its effects on the health of the river.
“The good thing about drought or low flow conditions is that we are actually able to do this study, and it doesn’t happen very often – on the order of every 10 to 15 years,” Lee said. Engel, responsible for water monitoring supervisor for the Minnesota Pollution Control Agency.
Phosphorus promotes the growth of algae, which eventually decomposes and robs the water of the oxygen needed to support fish, insects and other aquatic life.
It comes from runoff from agricultural fields and urban areas, but also from sewage treatment plants in towns that discharge treated wastewater into the river.
In a typical August, when the river is flowing at 3,000 cubic feet per second, it is difficult to determine the different sources of phosphorus that could be causing problems in the river, Engel said.
But in August, the river’s flow dropped to 540 cubic feet per second, with almost no runoff coming from the landscape.
“When we effectively suppress this surface runoff and only deal with what is in the stream, it is easier to quantify the contribution of water from sewage treatment facilities entering,” Engel said. “And it’s easier for us to sort out what we think is going on.”
In early August, MPCA personnel placed instruments in the river along a 35 km stretch from Shakopee to St. Paul.
They measured the amount of dissolved oxygen in the river, which is important for aquatic life.
During a drought, fish, insects and other aquatic species are already stressed, Engel said. This situation can be even worse if there are high concentrations of phosphorus entering the river, promoting the growth of algae that will further deprive the water of oxygen, he said.
âThis basically makes a predicament for biology even more difficult,â Engel said.
With less water to absorb solar radiation, lower rivers tend to be warmer, which also promotes algae growth.
This year’s study is an opportunity to assess progress in efforts to limit phosphorus pollution from wastewater treatment plants and through better land management practices, Engel said.
In 2004, the MPCA demanded that dozens of wastewater treatment plants that flow into the Minnesota River Basin adopt more stringent phosphorus limits. A 2012 study found that the boundaries helped improve conditions in the river.
âThe goal is to reduce phosphorus entering the river,â Engel said. âThe question is, can we tell if we are making a difference at this point? “
He hopes the data collected from the study will be available in early November.
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