Fluoride concentration was determined by potentiometric method in rainwater and in groundwater at Rio Grande city. In rain water, the range of fluoride concentration was 0.05 – 4,40 ppm, and in ground water it was 0,08 – 2,20 ppm. It was concluded that fluoride concentration in rainwater has relation to wind direction and rain parameters (pH and conductivity). Up until distant places from the factories (10 km), the fluoride concentration in rainwater can be several times higher than those found in seawater, showing a great evidence of the fertilizer factories effect. Groundwater fluoride concentrations demonstrated a tendence of decreasing while the distance from the factories increased.