Tracing the course of phosphorus pollution in Lake Pepin

In recent years, many lakes in the upper Midwest have been experiencing unprecedented algae blooms. These blooms threaten fish and affect recreational activities. A key culprit implicated in overgrowth of algae in lakes is phosphorus (P). Lake Pepin, located on the Minnesota/Wisconsin border, has seen increasing P concentrations over time. Researchers are now trying to identify upstream factors that could explain the increase.

researcher collecting sample from river bank

Much of the P is bound to sediment (particulate P) and moves from soil to water. “Historically, erosion from agricultural lands has been—and still is—the main source of particulate P for surface bodies, such as rivers and lakes,” says Satish Gupta, the Raymond Allmaras Professor in the department of soil, water, and climate at the University of Minnesota.

In addition to agricultural sources, processes like riverbank erosion also contribute to sediment load and transport of particulate P. However, these processes have not been studied extensively, according to Ashley Grundtner, lead author of a paper published by Gupta’s group in the November-December issue of the Journal of Environmental Quality.

Therefore, their new study aimed to determine the role that riverbank materials play as sources and carriers of P to Lake Pepin. The study had three goals:

  1. To assess how riverbank sediments acted as a source of P to the Minnesota River and Lake Pepin.
  2. To determine whether riverbank soil could adsorb P from river waters and then carry it to the lake.
  3. To identify factors that could explain increasing P concentrations in Lake Pepin over time.

Previous studies that analyzed P concentration in Lake Pepin suggested that the continuous increase in P concentrations since 1850 were due to settlement of the area by European immigrants. Specifically, the studies implicated agricultural activity and fertilizer use as the main sources of high P.

However, Gupta notes, “In the1850s, there was no large-scale agricultural production in Minnesota.” Factors other than agriculture were likely contributing to the increased P, and some of these non-agricultural sources have been pinpointed.

In the 1880s, a series of meat-processing plants began operation along the Mississippi river upstream of Lake Pepin. Prior to the 1930s, there were also no sewage treatment plants. As a result, “all domestic raw sewage and industrial waste was dumped in the rivers upstream of Lake Pepin,” Grundtner says. In the 1940s, detergents used in washing machines became a key source of P. Finally, by 1970, concerted efforts began to address reducing P in rivers and lakes. These efforts included reducing P in detergents and upgrading sewage treatment plants.

In their new study, Gupta and his colleagues wanted to consider all of these possible sources of P. To do so, they looked at several features of the river and riverbanks including total P, particle size distribution, and the ability of the riverbank materials to absorb P. From these measurements, they estimated P concentrations in Lake Pepin for scenarios prior to and after 1850.

Minnesota river bank

The results showed that selective transport of fine particles eroded from the riverbanks was the main source of P in Lake Pepin sediment before 1850. In this process, heavier particles such as sand (with lower P concentrations) remain behind in the river basin. Fine particles such as silt and clay (with higher P concentrations) are transported to downstream lakes. After 1850, the riverbanks absorbed P from the polluted river water providing additional P that could be transported downstream into Lake Pepin. 

These results contradict the previously held view that agriculture was the biggest contributor of P in Lake Pepin, as it is in some other lakes and rivers. “Our research shows that [for Lake Pepin] most sediment P is likely the sewage and industrial P that was picked up by riverbank sediments,” says Gupta.

Finding a solution to this problem will prove challenging. “To achieve a substantial reduction in total P loads to Lake Pepin,” says Grundtner, “the major pathway is to eliminate bank sloughing.” However, since sloughing is largely due to natural causes—like freeze-thaw cycles and heavy rainfall—reversing it will be expensive and difficult.

The best solution will be to control P pollution upstream of Lake Pepin, thus controlling the amount of P that could be adsorbed by the river banks. “Upgrading of sewage treatment plants should continue, not only near the Twin Cities, but in the rest of the Minnesota and Mississippi river basins upstream,” concludes Gupta.

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