Pesticides in Surface Drinking Water Supplies of the Northern Great Plains.
2007. David B. Donald, Allan J. Cessna et al.

Tested for 45 pesticides.
Detected 2 insecticides and 27 herbicides in reservoir water.
Drinking water contained 3-15 herbicides (average 6.4) at levels below respective guidelines of the 7 that have guidelines.

http://www.ehponline.org/docs/2007/9435/abstract.html