Associated Press

DES MOINES, Iowa — The utility that provides water to 500,000 Des Moines customers says it's set a record in 2015 for the number of days it operated equipment to remove nitrate from water.

Des Moines Water Works said Monday its nitrate removal equipment ran 177 days last year, surpassing 106 days in 1999.

It cost the utility $1.5 million to run the equipment which keeps tap water below the federally mandated nitrate level of 10 milligrams per liter for drinking water.

Water Works CEO Bill Stowe says upstream farm drainage systems washing fertilizer from farm fields are the largest contributors to high nitrate levels.

Water Works has sued three upstream counties blaming them for the nitrate problem.

The counties say there's no proof farm field 200 miles upstream cause Des Moines' nitrate problem.