According to the Colorado State University website, there are four possible ways to remove nitrates from contaminated drinking water: distillation, reverse osmosis, ion-exchange and blending. Although it is certainly possible to remove nitrates from groundwater, nitrate extraction treatments can be costly, laborious, and even wholly ineffective.
Preventing the nitrate contamination of drinking water is the most reliable way to ensure that it is fully clean and drinkable. Distillation is an option for purifying drinking water that has already been contaminated by nitrates. To do this, the water must be boiled and the steam caught and condensed on a cool surface. Regular boiling of water contaminated with nitrates is ineffective as the water evaporates and leaves the nitrates behind, which actually increases the concentration of nitrates in the water.
Reverse osmosis works by forcing the water through a special membrane filter and discarding the removed water. Ion-exchange cleans the contaminated water by exchanging the nitrate ions for another type of ions by introducing charged beads into the water. As the water comes into contact with the beads, the nitrate will be exchanged for the other ion (usually chloride) and the bead solution, now concentrated with nitrate, will be discarded. Blending reduces nitrate concentration by mixing clean water with contaminated water.