Objective: To analyze the quality of groundwater potentially impacted by nitrate (NO3-), before and after the flooding of the Madeira River, in two urban areas of Rondônia on the Brazil / Bolivia border. Method: Data and water samples were collected according to Standard Methods for the Examination for Water and Wasterwater (APHA, 1998) together with the methodology proposed in the Water Sample Collection and Preservation Environmental Sanitation – CETESB. The place were georeferenced using the Global Positioning System (GPS). For nitrate measurement, the spectrophotometry method was used using the Spectrophotometer, brand Micronal B495, the chemical reagents used were made by Alfatecnoquímica and available in two vials called reagents 1 and 2 (nitrate reagent). Results: In the first large area of risk in 100% of samples, high levels of N nitrate (NO3-) were detected. > 10 mg / L. In the second area in 30% of the samples were detected NO3 content higher than 10mg / L. Conclusions: The population is consuming water with a high degree of nitrate contamination. It alerts itself to a public health issue.