MINNEAPOLIS (AP) — State regulators blamed farming for rising nitrate levels in southern Minnesota surface waters in a new report Wednesday, citing the increasing use of drainage tiling as a major reason.
The study by the Minnesota Pollution Control Agency found that more than 70 percent of the nitrates in surface water in intensively farmed southern Minnesota come from cropland, where anhydrous ammonia and other nitrogen compounds are commonly used as fertilizer. The rest comes from sources such as wastewater treatment plants, septic systems, urban and forest runoff, and the atmosphere.
"I believe Minnesota farmers are committed to conservation, stewardship and water quality protection," MPCA Commissioner John Linc Stine said in a statement. "But collectively, too much nitrate is ending up in streams and rivers. We have to do better."
Drain tiles — the perforated pipes that farmers install a few feet under the surface of their land to carry away excess precipitation — are the top pathway by which nitrates travel from cropland to streams, the study found.
Cropland sources account for as much as 95 percent of the nitrate load in the Minnesota, Missouri and Cedar rivers, and the Lower Mississippi River basin, the study said. The Minnesota River adds twice as much nitrate to the Mississippi River as the Upper Mississippi and St. Croix rivers, which flow through less intensively farmed areas to the north.
High nitrate levels can harm fish and aquatic life, and can pose health risks to humans when they pollute drinking water wells. Nitrates flowing down the Mississippi from Minnesota are blamed for contributing to the oxygen-starved "dead zone" in the Gulf of Mexico, which the MPCA said is currently the size of Massachusetts.
Farmers have become more efficient at using nitrogen fertilizers over the past two decades, the study said, but raising water quality significantly will be expensive.