Need to know more about your backround; is this your first job out of college? Have you just done a course that highlighted this issue? Or are you trading down from a high risk position? From my perspective (as a possible customer), I trust your boss from historical evidence (unless there is a trend that has highlighted this as an issue to you). Possibly, you are as long in this game as I am, and possibly some new technology such as fracking has spooked you with regards the purity of your water. In my experience (or as the kids say, and to add some gravitas to this post, IMEX), put your findings in an email, bcc your own address, and let it go. If the Company have always done this, and no auditor has highlighted the issue, then maybe it isn't an issue (to be fair, I think it may be). Do some investigation on the quiet so that if it is ever raised as a non-conformance you have the info and the scientific evidence to back it up and close it out within 28 days. Don't let a single issue such as this blind you to the bigger picture of food quality. Take it from a bitter, bitter pro, these things can eat up the subconscious and blind you to the more obvious problems.
And hey, I'm Irish! What can ever go wrong with potatoes!