"...nitrate in the water in 2016. It was not there in 2010. Usually this is the result of pesticides..."

Actually nitrites occur naturally in ground water and are not the result of pesticide application.

When nitrogen fertilizers are used to enrich soils, nitrates may be carried by rain, irrigation and other surface waters through the soil into ground water. Human and animal wastes can also contribute to nitrate contamination of ground water.  

Perhaps the locals should be a bit more discriminating about where they relieve themselves.  What's that old Army saying?  Don't defecate in your own mess kit...