“Phosphorous is an essential element, and we find it in everything,” said Ashley Grundtner, a former graduate student at the University of Minnesota. However, too much phosphorous has been finding it’s way into lakes and rivers in the Midwest for years, and it’s causing a lot of problems.
Algae bloom is a natural consequence of too much phosphorous in water, and Lake Pepin on the Minnesota/Wisconsin border is a good example. The bloom threatens fish and can negatively affect recreational activities. For years, agricultural runoff was considered the only culprit, so researchers began studying other factors that may explain the increase.
“For 20 years or so, many folks have said Lake Pepin was filling up, and in 1988, they had a big fish kill there,” said Dr. Satish Gupta, the Raymond Allmaras Professor in the University of Minnesota Department of Soil, Water, and Climate. “That led to a whole series of studies by the Minnesota Pollution Control Agency (MPCA) and the Wisconsin Department of Natural Resources (DNR) that decided phosphorous was the problem.”
“I had been working on the source of sediment going back to 1994,” said Gupta. “That was the first year that MPCA came up with the report blaming agriculture for the sediment and phosphorous in the Minnesota River and then in Lake Pepin. Farmers were saying it’s not the agricultural land, it’s the banks along the river that was a bigger source.”
A sloughed tall bank along the Blue Earth River in Blue Earth County, Minnesota. (Photo taken by Satish Gupta)
He said, “When we started working on this, we took a look at the riverbanks and noticed they were bare. Every year, we get a new crop of dandelions, so we wondered why they weren’t growing on the banks. The answer is simple: they are very unstable and they come down every year. There isn’t enough time for vegetation growth.”
In 2005, Gupta and his students began a four-year study of several rivers in Blue Earth County to determine how much of the riverbanks were running off into the waters. “We found that 2.24 million cubic yards of soil was gone during that time,” said Gupta.
“The next question was how are bank materials reacting with the phosphorous in river waters,” said Gupta. “We knew when you apply fertilizer on the land, particularly phosphorous, it ties up pretty fast. There’s only a small amount for the crop to use, so they have to apply it again next year, and the next year after that.”
That’s what led to Ashley’s graduate thesis paper on the source of phosphorous in Lake Pepin. “If the riverbanks are absorbing the phosphorous we put on our farm fields, are they absorbing other sources of P too?” said Satish. “What is the role of sediments in the riverbank material in picking up phosphorous?”
Ashley Grundtner collecting sewage effluent sample along a drainage ditch in Blue Earth County, Minnesota. (Photo taken by Drew Kessler)
“We looked at historical records, and we found our rivers had been very polluted going all the way back to when Minnesota was a territory,” said Gupta. “Rivers were thought of as a conduit to carry waste away from the cities. They had virtually every kind of contaminant in the waters.”
Gupta said, “They didn’t have toilets to carry waste to a treatment plant back in that time period. Waste went directly into the river, and phosphorous is very common in sewage.”
South Saint Paul had a large meatpacking industry right on the Mississippi River. Gupta said, “It was set up on the river so they could push their waste in. Ice was also available in the wintertime to preserve meat.” He added, “Records show an ammonium phosphoric fertilizer plant was built along the Mississippi River south of St. Paul. We were not careful about what we put into our rivers.”
“Phosphorous is a basic element you’ll find everywhere in geology,” said Ashley Grundtner, who graduated with a Masters Degree in Water Resource Science in 2013. “We wanted to find out what were the natural levels of phosphorous in riverbank material. What happened to the material when it went through a river process? We then considered human input, like pollution, and what happened to it when it went into a river,” she said.
The group considered basic things like total P in the rivers and riverbanks, and how the riverbanks would absorb Phosphorous, and how much capacity the material had to store the element. From the different scenarios they worked on, they determined levels in Lake Pepin prior to and after 1850.
A tall sloughed bank along the Big Cobb River in Blue Earth County, Minnesota. (Photo taken by Drew Kessler.)
The results of their work, published in the Nov-Dec Issue of the Journal of Environmental Quality, showed that fine particles carrying P from eroded riverbanks was the main source of P in Lake Pepin before 1850. After 1850, the human element of pollution added to the levels of Phosphorous available for rivers to carry to Lake Pepin. “The rivers became increasingly polluted, and these riverbank materials are picking it up,” said Gupta. “River modification through locks and dams has added to it as well. Coarser materials are going to get stuck behind the dam, and there was more phosphorous in smaller amounts of fine particles going downstream.”
“Ashley’s paper has shown that most of the phosphorous in the Lake Pepin sediment is industrial and a sewage-phosphorous material, which was picked up by riverbank materials. That doesn’t mean there’s no other source, but that’s a big part of it,” said Dr. Gupta.