The lakes of Midwestern states, such as Minnesota, Wisconsin and Illinois, all have a reputation for being beautiful. This makes them a major draw to nature lovers, fishers and boating enthusiasts – a fact that underscores the importance of maintaining the lakes' health for the sake of the environment and the local economies. Residents and companies near the lake have to be sure to refrain from dumping certain chemical solutions into the water.
One source of environmental pollution that has drawn a lot of attention is agriculture. The large volume of crops grown and livestock raised have attracted attention because of the use of inorganic fertilizers and pesticides. Nutrients such as nitrogen and phosphorus have been known to wash away in runoff and contaminate watersheds. However, when it comes to Lake Pepin, which sits on the border of Minnesota and Wisconsin, researchers from the University of Minnesota concluded that historic industrial use may be more to blame for water pollution than agriculture, as published in the Journal of Environmental Quality.
Why is phosphorus so bad for water?
Similar to nitrogen, phosphorus is a mineral that is often found in agricultural fertilizers. When it ends up in streams, rivers and lakes, it is likely to lead to algal blooms. Any explosion in algae populations will prevent sunlight from nourishing other plants in the waters, which will disrupt the local food chain. Additionally, when the algae die, the decomposition process depletes the oxygen supply in the water. This creates areas known as "dead zones," which are unable to support aquatic life.
In addition to harming the local food chain, algae will also produce toxins that can significantly contaminate drinking water supplies. According to the U.S. Environmental Protection Agency, people who consume algal toxins may develop rashes, gastrointestinal distress, neurological problems and respiratory issues.
Sewage, industry are to blame
The authors of the new study noted that algal blooms have hurt fish and recreational activities in Lake Pepin. They noted that phosphorus seemed to be the biggest factor driving algae growth in the lake. To better understand the source of phosphorus pollution, they investigated the sediments upstream of Lake Pepin. Ultimately, they discovered that, before 1850, selective water transport of finer sediments over heavier sediments – the former of which had more phosphorus – was responsible for most of the phosphorus in the lake. After 1850, local businesses played a major factor, and none of them were agricultural.
"Our research shows that [for Lake Pepin] most sediment phosphorus is likely the sewage and industrial phosphorus that was picked up by riverbank sediments," study co-author Satish Gupta, a University of Minnesota professor, said in a statement.
Instead of agriculture, meat processing was the likely source of phosphorus contamination, and several plants were built in the area after 1850. Additionally, the lack of sewage treatment plants before the 1930s meant that "all domestic raw sewage and industrial waste was dumped in the rivers upstream of Lake Pepin," Gupta said.
In the interest of reducing phosphorus pollution in the lake, the study authors recommended upgrading sewage treatment plants, which can be a major contributor to contamination. Specifically, plants need to be upgraded near the Twin Cities area, and all along the Minnesota and Mississippi river basins upstream of the lake.
Part of what is complicating matters is the fact that there are numeric water quality standards for phosphorus and algae, but not nitrogen, when it comes to lakes in Minnesota, according to the Minnesota Center for Environmental Advocacy. Furthermore, no such policies exist to help protect rivers and streams in the state.
