I think the biggest issue with nitrates in tap water is really with infants. They are much more susceptible to high nitrate than adults. The first site I found was for the state of Washington and their max level is 10ppm. Higher than that and they have to start notifying residents. Nitrate affects the red blood cell's ability to carry oxygen. Adults recover quickly without issues but babies can devlop serious problems leading to "blue baby" syndrone.
