State regulators say Minnesota farm fields - and a fast-growing technique for draining them - carry millions of pounds of nitrates into waterways annually.
The study by the Minnesota Pollution Control Agency traced more than 70 percent of the nitrates in the state's rivers to farm fields. The Associated Press reports that in the Minnesota River, 95 percent of nitrates came from cropland.
Smaller sources of nitrates include wastewater treatment plants, forests, and the atmosphere.
High concentrations of nitrates have been found to be toxic to fish and other aquatic life. They also pose health risks to humans when they occur in drinking water.
In announcing the study's findings, MPCA Commissioner John Linc Stine said "...too much nitrate is ending up in streams and rivers. We have to do better."
MPR reports the study found that Minnesota sends 158 million pounds of nitrates downstream per year. Most of that winds up contributing to an oxygen-depleted dead zone in the Gulf of Mexico. An executive with the group Friends of the Mississippi River says the study shows Minnesota's current approach to cleaning its rivers is not working.
The MPCA study found that 30 percent of the nitrates in rivers arrive there after soaking into the groundwater and being carried underground to the nearest stream, But even more (37 percent) are deposited into rivers by a popular drainage technique known as drain tiling, in which underground tubes carry excess water away from cropland.
Last summer the Pioneer Press looked at the surge in the use of drain tiling on Minnesota farms. The method is credited with boosting yields at a time when crop prices are high. But, as the MPCA report underscores, there is an environmental consequence.
Similarly, the use of nitrogen-based fertilizers has raised farm output. One Dakota County farmer tells WCCO his yields are nearly double what they were thirty years ago. But Wayne Hallock says he's working on cutting his fertilizer use by 20 to 30 percent, partly to reduce input costs.