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.
The kind of major and expensive efforts that have helped to improve rivers in many ways are needed in agriculture to reverse the nitrogen and sediment trends.
Better technology that helps farmers target fertilizer applications has helped reduce overall usage, but continued reductions will be more limited going forward.
Use of low- and no-till farming, in which crop residue is left on fields after harvest, also helps in reducing nitrogen loss — as well as wind and water erosion — but even more use of tillage conservation won't make a big dent in nitrogen loss to lakes and rivers.
There have been promising technologies developed to tackle the root of the problem of nitrogen and sediment loss in farm drainage. While varying in design, the systems either collect water from fields in holding areas to be released more slowly, and/or shut off the tile drainage lines in fields when the surface of the field is dry enough — holding the remaining water and nitrogen in the subsurface. That not only keeps the nitrogen out of the waterways, but keeps it in the fields where crops can use it for growth.
The systems are more expensive and require more attention. Incentives should be given for part of the costs for farmers who upgrade their systems, just as some state and federal assistance was given to cities for upgrading treatment plants. But just as urban residents pay significantly higher water and sewer fees to cover much of the cost of those upgraded systems, ag operators will need to bear a burden for pollution caused by their businesses' drainage systems.
A failure to act will inevitably lead to a federal crackdown on Minnesota. If and when that happens, the costs and regulatory burden of reversing the trend will be much higher than by acting now.