The Des Moines Water Works board on Tuesday voted to spend another $650,000 on its federal lawsuit against three northwest Iowa counties over nitrate pollution from drainage districts. That is on top of the $700,000 authorized earlier. The lawsuit — slated for trial in June 2017 — contends the drainage districts should be regulated as “point sources” of excessive nitrate pollution in the Raccoon River, a key source of drinking water for a half-million Central Iowa customers of Water Works. The counties say they can’t control farm-related pollution, adding that the effect of the discharges would be minimal by the time the nitrate was diluted on the way to Des Moines. The lawsuit was filed in federal district court in Sioux City in March 2015.