Nitrates in water can be very harmful especially in rural areas. The water company stopped using the bore holes in our village, because of the nitrates, since we have a very free draining subsoil which lets the farm chemicals through. However you have to be a little wary of "clusters" as evidence, since it is in the nature of random chance, that when you are dealing with things like human populations, which are measured in millions, there are bound to be some very concentrated clusters, just by pure random distribution.