This work describes the approach of a situation of potential risk to health due to nitrate contamination in water intended for human consumption, found in a study conducted in 88 municipalities in the Northeast region of the State of São Paulo (Brazil). 4,347 public water supply samples were analysed for 21 physico-chemical and 2 microbiological parameters. A municipality was distinguished from the others with regard to higher nitrate levels, while the principal components analysis showed that it belonged to a group of 14 municipalities (nitrate group). The Sanitary Surveillance Group was then contacted and, after confirmatory tests in new public water supply samples, analyses were performed on water samples from 16 wells in the county. The groundwater contamination was found, and technical meetings were held. One of the deliberations was the repetition of water analyses of the 16 wells. Consolidated results showed that the 5 wells had nitrate levels above the maximum value permitted by legislation. To mitigate nitrate groundwater contamination of that municipality actions are being taken as a result of 3 technical meetings already held. The other municipalities of the nitrate group are being monitored.