In recent decades, Minnesota cities have dramatically slashed the pollutants they put into rivers and lakes. Under state requirements, cities such as Mankato and those up and down the Minnesota River have upgraded, or in most cases replaced water treatment facilities, dramatically cutting phosphorus output by 60-70 percent, as well as reducing other pollutants from going in the river. Homeowners in Minnesota no longer can use lawn fertilizers containing phosphorus. Cities have installed more holding ponds that collect and hold storm runoff so contaminants can settle out before going into rivers.
But such success stories are countered by two other major water pollution problems that have gotten worse — both driven primarily by agriculture. Extensive farm field drainage systems have, according to a consensus of studies, caused too much water to flow to rivers too quickly, eroding ravines and river banks and sending dirt into the rivers. The problem on the Minnesota River is rapidly filling Lake Peppin with sediment.
Last week, the state released a report on nitrogen loads in rivers and lakes. The Minnesota Pollution Control Agency said that 41 percent of the streams and lakes in southern Minnesota have excessive nitrogen, which can be toxic to fish and other forms of aquatic life, cause algae blooms that foul lakes, poison pets who drink from lakes, and is a major cause of the Dead Zone in the Gulf of Mexico.
Three fourths of the nutrient comes from fertilizers used in agriculture, most of that coming through tile drainage that sends contaminated water from farm fields directly into ditches and streams. About 9 percent comes from wastewater treatment plants, and 1 percent from urban runoff. In the Minnesota River, more than 90 percent of nitrogen comes from farm land.
And the trend is worsening. In areas of the Mississippi River next to and below farming regions, nitrogen loads from 1976 to 2010 increased between 87 percent and 268 percent. Those are years in which highly efficient farm drainage systems were installed and nitrogen loss from urban landscapes was being reduced.