This is a useful reference from Purdue Extension on water that goes into the spray tank. From the guide:

"Water often comprises ninety-five percent (or more) of the spray solution. What affect might it have on product performance? Research clearly shows that the quality of water used for spraying can affect how pesticides perform. Its effect on product efficacy is reflected in the success of your spray operation ...

Time spent addressing the quality of water used in the spray tank can pay big dividends. This publication provides an overview of water quality and related factors known to affect pesticide performance; testing methods and options to improve the quality of the water used are discussed."

I learned of this document when I read Megan Kennelly's post on nozzles and water quality.