Nitrate – much of it from fertilizers applied to farmland – pollutes many Minnesota rivers and streams and contributes to the oxygen-depleted “dead zone” in the Gulf of Mexico, a new report from the state’s Pollution Control Agency concludes.
Excess nitrate is toxic to fish and the aquatic life food chain and potentially harmful to humans in drinking water.
The report, released Wednesday, June 26, was based on analysis of more than 50,000 water samples, and is the most exhaustive study ever conducted on nitrate pollution in Minnesota. Scientists and regulators have long known that nitrate pollution was a serious problem in lakes and streams and in groundwater.
The new report found a substantial number of streams in which nitrate levels exceeded the health limit set for drinking water. Between 2000 and 2010, the health standard — 10 milligrams of nitrate per liter of water – was exceeded in some sampling in 27 percent of the sites where water was tested.
Read an executive summary of the report and an accompanying fact sheet. Download the full 444-page report. View video of a news conference to release the report. Read a news release from the Minnesota Environmental Partnership praising the report and calling for state action to reduce the nitrate pollution.
Nitrate levels in northern Minnesota are relatively low, but much higher in the southern and southeastern parts of the states, according to the report.
The report estimated that cropland agriculture is the source of 70 percent of nitrate in the state’s surface waters. In intensively farmed areas of the state, including the Minnesota River Valley, cropland accounts for 89 percent to 95 percent of the nitrate loads.
In a news release, Minnesota Pollution Control Commissioner John Linc Stine said: “I believe Minnesota farmers are committed to conservation, stewardship and water quality protection, but collectively too much nitrate is ending up in streams and rivers. We have to do better.”
During a year with average rainfall and average river levels, about 158 million pounds of nitrate flow down the Mississippi to the Gulf.
The report estimated that 37 percent of the nitrogen flowing to surface waters across the state is routed through tile drainage systems, and another 30 percent goes from the land’s surface into groundwater that then is discharged to surface waters.
Point sources, such as sewage treatment plants, account for only 9 percent, and urban storm water makes up only 1 percent of the total, according to the estimate.
Can Minnesota substantially reduce the nitrate in its surface waters? Yes, but it won’t be easy, the report says.
The executive summary of the report summarizes the results of modeling on what it would take to achieve a 30 percent to 35 percent reduction in cropland losses of nitrate:
- Ninety percent of the state’s corn fields would be fertilized at optimum rates, with the fertilizer applied in the spring.
- Perennial plants would be grown in 100-foot-wide buffer strips along most streams.
- All tile drainage water would be reduced in volume or routed through wetlands or pits filled with wood chips to remove the nitrogen.
- Rye cover crops would be planted on most corn and soybean acreage to use nitrogen when the corn and soybeans are not growing.
- Row crops would be replaced by perennials on marginal land.
The net cost of all those changes in practices and infrastructure would be more than $1 billion per year, the report estimated.