DES MOINES, Iowa (AP) — Iowa farmers efforts to control the spread of nitrates in recent years appears to be helping but research shows that levels of the fertilizer byproduct have soared in the state’s major rivers. The Des Moines Register reports  that nitrate levels have more than tripled since the 1950s to more than 7 milligrams per liter in 2010.

Researcher Keith Schilling says the upward trend in nitrate levels is unmistakable. Schilling studies nitrate levels for the Iowa Geological Survey at the University of Iowa. Last month, the Des Moines Water Works sued Buena Vista, Calhoun and Sac counties over high nitrate levels. The utility has invested in costly measures to control nitrates in drinking water.