Nitrates, are the result of groundwater contamination mainly occured by reckless use of nitrogenous agricultural fertilizers. 
Nitrates that are not assimilated by plants are washed away by watering or rain, thus contaminating the groundwater aquifer and then rivers and lakes.
Nitrates are converted to nitrites, which  can be converted to one of the most potent carcinogenic families of chemicals known as nitrosamines.
Here  you can see the results of water analysis (area: Dafnoudi, Serres) before the installation of  Veluda Reverse osmosis system and here  after its installation.
We observe that the nitrates from 19.3 mg / L after the installation of the Veluda reverse osmosis system were reduced to 0.045 mg / L.
The limit of safety content of nitrate in the drinking water of the American Environmental Service is 10 mg / l, while the Greek legislation has a limit of 50mg / lt.