The highly toxic carcinogenic potential of nitrites, resulting under certain conditions due to the reduction of nitrate, is well known and continuously studied. The main aim of this study was to exactly establish the amount of pure nitrite NO₂^- expressed in mg/L in eight different unknown groundwater sources by a new visible spectrophotometric method. Nitrate NO₃^- from unknown water samples was quantitatively reduced into NO₂^- nitrite anions using 10%-15% NH₂-OH hydroxylamine solution. The obtained nitrite NO₂^- was completely transformed into a diazonium salt by the existing β-naphthol in a 0.2% alcoholic solution, in a strongly acidic medium (HCl, 15%-20%), and in cold storage at 1º -7 ºC for 25 minutes. Then, the synthetized diazonium salt was quantitatively coupled with free β-naphthol in excess at double concentration in the solution, which led to the formation of an orange–intense orange azo dye that presented an absorption maximum at λ = 478 nm and was obtained in a quantitative proportion perfectly equivalent to the concentration of pure NO₂^- nitrite in the water samples. Through the spectrophotometric dosing of the orange–intense orange azoic dye formed at λ = 478 nm in relation to the absolute ethanol as a blank, the pure nitrite NO₂ ^- in the unknown water samples was directly determined. The eight water samples studied showed high nitrite concentrations (2.2092 mg/L; 3.0669 mg/L; 3.6109 mg/L; 3.6736 mg/L; 3.9038 mg/L; 3.7155 mg/L; 4.2385 mg/L; 5.1589 mg/L), which exceeded the maximum allowed official limit of nitrites in drinking water, at 0.5 mg/L, by approximately four to ten times. All of thewater samples studied cannot be intended, as such, for domestic consumption. The method was then subjected to complete statistical validation. The linearity, the limit of detection (LOD), the limit of quantitation (LOQ), the method and system's precision, and the robustness and accuracy of the analysis were calculated.