Аннотация:The quality of drinking water is determined by its chemical composition and is regulated by sanitary legislation. The Izhora groundwater Field, the largest in the Leningrad Region, is located to the south of St. Petersburg on the Izhora Plateau. The water-bearing rocks are composed of fractured and karst limestone and dolomite (O1–3). The priority substances for estimation of the quality of groundwater in the Ordovician aquifer were identified and analyzed: total hardness, Fe, Mn, Ba, and nitrates. A map of nitrate pollution of groundwater on the Izhora Plateau was constructed.