The layer of water-bearing sandstone (PR₂ V kt₁) named Lower-Kotlin aquifer is widespread in the northwestern part of the Russian platform, within the Moscow Artesian Basin. It is mostly confined. The aquifer lies directly on the surface of the crystalline basement throughout the basin. The water mineralization rises from north to south together with the depth of the aquifer. The mineralization growth is mainly caused by the increase of sodium and chloride contents. Three consequent vertical zones can be marked out: fresh-water zones (<1 g/dm³), brackish-water zone (1-35 g/dm³) and brine zone (>35 g/dm³). Each of these zones has specific regularities of variation of chemical composition. The role of each component is different for waters of each zone. The aquifer is actively exploited-for public water supply and bottling in the north and for mineral water treatment in the south. Within St. Petersburg, this water is used for technical purposes of industry. There are some health resorts and spas in St. Petersburg Region, such as Sestroretsk, Peter-hof, Luga, which use the mineral water of the Lower Kotlin aquifer. A number of bottling plants produce bottled mineral water. Wide regional location and peculiarities of chemical composition of the Lower-Kotlin aquifer cause the possibility of the organization of many other spas and health resorts near St. Petersburg.