Chemical composition of the ground water in the Karelian drainage area is noted for high diversity: the mineralisation classes represented vary from ultrafresh (less than 100 mg/l) to brackish (over 1000 mg/l). According to the Kurnakov-Valyashko classification the waters belong to the carbonate, sulphate and chloride types. Fig.3 is the map showing the chemical composition of ground waters in Karelia, and fig.4 is the map of rated water intakes (headworks) in Karelia and mineral water sources. The principles and methods of compiling 1:500 000 hydro-geochemical maps were used when creating the maps. The main materials used in compiling the maps were the data of the fifth geological department, results of boring water reconnaissance test wells by the Northwestern geological department and some other organizations obtained in 1974-1995. A schematic map was compiled to identify the main distribution patterns of ground waters in Karelia differing in the salt-ion composition and mineralisation degree in various geological structures. This information may also be of interest for solving applied tasks related to the economic uses of the water in water supply and balneology. Ground waters in most of the Karelian territory are confined to sandy quaternary deposits and the upper fissured zone of crystalline rocks. Only in the south of the republic there are small areas on the margins of the Leningrad and Moscow artesian basins. Mineralisation of the ground waters varies from 0.03 to 1.0 g/l. Waters with high mineralisation are developed locally notably in the Gdov and Kotlin horizons of the Leningrad artesian basin. Water with high mineralisation have been found along the Ladoga lake shore in the deposits of the Salma and Pitkaranta suites, in the rocks of the Zaonezhje-Suisar' complex of the Onego structure, and in the White Sea coastal zone. As mineralisation increases the salt-ion composition of ground water changes from the bicarbonate-calcium-magnesium to bicarbonate-chloride-sodium. Medicinal waters with the mineralisation up to 1 g/l contain biologically active components - radon and iron. Radon-rich waters with radon concentration of more than 185 Bk/l are found to the north of Lake Ladoga, east and west of Lake Onego and in central Karelia. Ferrugineous waters with iron concentrations over 190 mg/l have also been recorded in different areas of Karelia. Most well-known are the ferrugineous mineral waters of the "Martial spa" ("Marcial Waters") resort.
The schematic map of exploitable ground water resources (Fig.4) is based on the calculations of the headworks yields using the values of the modules of the natural ground water resources and averaged hydro-geological indices of various aquiferous rock complexes. The latter were defined using the data obtained from statistical treatment of over 700 wells. The study is interesting from the point of view of assessing the opportunities of supplying Karelian population with ground water, which is better protected from pollution. The high water quality category in the suggested map includes all oligohumous (colour up to 40 grad.) and oligotrophic (Pt up to 12 (g/l, Chl"a" up to 3 (g/l) waterbodies with Fe content up to 0.2 mg/l, total ions 20-100 mg/l, pH 6.5-8.0 and oxygen content 80-105%. The good water quality category includes mesohumous and mesopolyhumous (colour 30-120 grad.), oligo- and mesotrophic (Pt 8-25 (g/l, Chl"a" up to 10 (g/l) with the pH value between 6.2 and 8.5, Fe content 0.1-0.5 (0.75 mg/l permissible at low content of >Pt and colour less than 120 grad.), total ions 15-50 mg/l, oxygen content 60-120%. The satisfactory quality category includes all waterbodies with low-acidic reaction of the environment (pH 5.5-6.2), as well as eutrophic waterbodies (306.5, iron content from 0.5 to 1.5 mg/l and colour up to 200 grad. Poor quality includes all waterbodies with acidic reaction of the environment (pH<5.5) irrespective of all other indices and polyhumous waterbodies (colour over 160 grad., pH<6.2) with iron content over 0.7 mg/l, as well as lakes with high eutrophication degree for Karelia (Pt>40, Chl"A">30 (g/l). Classed as contaminated are all waterbodies (or areas) where sewage is discharged or those within the impact zone of air-borne pollution or agricultural facilities. These waterbodies are characterised by exceeding the background indices (Pt, BOD, the content of organic matter, oil product, etc.)
The given classification takes no account of the total water mineralisation. This is due to the fact that according to this parameter all non-contaminated surface waters in Karelia are ultrafresh (Su<100 mg/l), and no relationship has been found between water mineralisation and its quality. It can be mentioned that high-colour and acidic waterbodies have the lowest value of total ions, normally less than 15 mg/l. The spatial distribution of the waters according to mineralisation is the following. In most of the territory the mineralisation degree is less than 25 mg/l and hardness 0.2-0.4 mg-e/l. These waters are found within the White Sea lowland, in the Kem, Segezha, Suna river basins and throughout the western part of Karelia from lake Kuito to Janisjarvi. Up to 30% of the territory is occupied by the water with the mineralisation 25-40 mg/l and hardness 0.5-1.0 mg/l. These include the lake-river systems of Kovda, Vyg, Vodla, White Sea-Onego and Onego-Ladoga water divides, lakes Onego, Sjamozero, basins of the rivers Niukhcha, Letnaja (Belomorskaya). A small area is occupied by lakes and rivers with the mineralisation 40-100 mg/l. They occur to the west of Lake Ladoga, north of lake Onego (Putkozero, Vangozero), in the lower reaches of the Shuja (Konchezero lake group) and Suna (Pjalozero, l. Sandal) river basins. This group includes also the waters of Lake Ladoga. Among the studied lakes there are no more than 10 (e.g., Padmozero, Valgozero, Mjagrozero - in the Zaonezhje selka area; Dolgoje, Pelgozero, Belozero - in eastern Prionezhje) with the mineralisation over 100 mg/l.
Of high quality is the water in large deep lakes (most of the Onego, lakes Segozero, Kuito, Kovdozero, Topozero, Keret, etc.), and in small waterbodies with slow water exchange (lakes of the Konchezero, Nizhnaja Suna groups, etc.).
Water of good quality is found in most of the Kem, Suna, Vodla river basins, and in lakes Ladoga, Sjamozero, Vodlozero, etc.
Waters of satisfactory quality include most waterbodies with eutrophied catchment area (basin of the river Nizhny Vyg, Onezhskaja Shuja, Koitajoki, etc.), small lakes with atmospheric nutrition (Lizhmenskoje, Kaskesnavolok, Langozero, etc.) and eutrophic lakes.
Most promising from the point of view of the quality and amount are the aquifers in sand of the quaternary cover more than 10 m thick. They allow building headworks yielding 200-300 m3/day. Tectonic fault zone may become a source of ground water with the headworks yield of up to 1500-2000 m3/day (Sortavala, Kem, Meliorativny municipalities). Sandy deposits of the Gdov and Kotlin horizons are promising sources of ground water their resources e.g. near Petrozavodsk and Olonets estimated at 10 000 m3/day.