In pond water that had been treated with diquat at 2.5 mg/litre (Grzenda et al., 1966), residues of 0.01 - 0.08 mg/litre were found, 7-9 days after applying the herbicide, and no residues could be determined after 14-30 days. The authors concluded that, compared with other herbicides, diquat appeared to have the greatest potential for use in sources of potable water.