Ukraine is a country rich in underground waters. Fresh waters widely used for water supply are abundant, as well as almost all types of mineral waters (among them unique which no country in the world has, e.g., Naftusia), brines, and thermal waters.

Physiographic, geological, tectonic, hydrological, and biological factors are main natural factors of forming the composition of underground waters and their distribution. Among the most important of them is the climatic factor that determines not only the replenishment of aquiferous strata due to atmospheric precipitation but also the intensity of rock transformation (weathering). The influence of the hydrological factor on subterranean waters depends on the peculiarities of the hydrographic network that plays an important part in the drainage of aquifers. The relief is of decisive importance in the formation of the surface flow and influences greatly the water exchange in aquiferous horizons. The influence of the biological factor shows in the change of the composition of water and rocks under the influence of bacteria action and products of organic matter transformation.

The geological structure, conditions of bedding, genesis, and dynamic regime of aquifers, their interaction and connection with surface waters, mineral composition, and organic matter of rocks have great influence on the formation of the underground water composition.

In studies of natural conditions of Ukraine’s territory, the object of typification is natural complexes and systems that show their individual character through a set of characteristics on each level of typification. For that, such terms as water-pressure system, geohydrodynamic system, and global water cycle or geosystem are used in hydrogeology.

At present, two types of zoning are used – based on component parts of regions (structural or uniform regions) and on unity and trend of processes taken into account in zoning (functional zoning). At every level of zoning they can be complexed with the differentiation of basic and auxiliary principles.

The structural hydrogeological zoning is based on the principle of the uniformity of conditions of water exchange formation in natural conditions (uniform zoning). There are three taxonomic units of zoning: 1 – megaregion, 2 – region, and 3 – district.

According to the geological structure and major characteristics of water exchange in the boundaries of Ukraine, 5 megaregions are distinguished: I – the Ukrainian Shield; II – the Russian Plate; III – the Donets’ folded structure; IV – the Carpathian folded structure; V – the Crimean folded structure.

Within these megaregions, regions are defined, and within regions – districts. For example, the megaregion of the Russian Plate comprises four regions – artesian basins: Volyn’-Podillia, Dnipro, Donets’, and the Black Sea.

The distribution of underground water in Ukraine’s territory is determined by the geological structure of hydrogeological regions (see the map Structural Hydrogeological Zoning) that differ in age, composition, and conditions of bedding of components of their deposits, as well as the complex of major natural factors that determine mechanisms of formation and distribution of underground waters.

Within the Ukrainian crystalline shield (UCS) underground waters are found in the fissure zone of crystalline rocks and products of their ruination, as well as in the sedimentary rocks of the Cretaceous, Palaeogene, Neogene, and Anthropogene, water saturation of which is very diverse. The greatest fissuring and watering of crystalline rocks are observed at the depth of 70–150 m, below the fissuring substantially decreases, concentrating in individual zones of tectonic dislocations. The aquiferous horizon of the fissure zone of crystalline rocks almost on the entire territory of the UCS is widely used for water supply, despite the fact that some stratigraphic complexes of rocks have no high water saturation. In the formation of the chemical composition of the underground waters of the Ukrainian crystalline shield the decisive part belongs to its geotectonic peculiarities of a positive structure as well as colloid-chemical processes of exchange absorption of cations. There is certain regularity in the distribution of waters in the fissure zone of crystalline rocks both by the value of general mineralization and by the proportion of some chemical components. Hydrocarbonate calcium or hydrocarbonate-calcium-magnesium waters with the mineralization to up 0.5 g/dm³ are found mainly in the northwestern part of the UCS. Highly mineralized waters (mostly chloride-sulphate and sodium chloride) occur in the submersed parts of the UCS. In general, exploitational groundwater resources within the UCS are small and irregularly distributed. Waterbearing horizons of the sedimentary deposits have practical importance in limited sections.

The Dnipro artesian basin is Ukraine’s largest. It comprises a system of aquiferous horizons in Palaeozoic, Mesozoic, and Cainozoic deposits the general thickness of which reaches several kilometres. A characteristic feature of the Dnipro artesian basin is the high hypsometric position of its northeastern wing as compared with the southwestern and a deep subsidence of aquifers in its central part. The Cainozoic and partly Mesozoic (Palaeogene, Cenomanian-Lower Cretaceous, and Jurassic) deposits, except for the central subsided part of the basin, have a thick zone of high-quality fresh water. The thickest (300–800 m) fresh water zone is found in the marginal parts of the basin along the Ukrainian and Voronezh crystalline massifs. As water-bearing rocks in the central part of the artesian basin subside, the conditions for water exchange become gradually complicated, and as a result at great depths in all Palaeozoic deposits and in some parts of Mesozoic mineralized waters and brines prevail. The zone of intensive water exchange is confined in the main by the depth of the local drainage network and the conditions of the formation of underground waters in it are associated mainly with the physiographic factors, but the main role is played by rocks leaching. The zone of the considerable water exchange is separated from the first one by regional water resistance of Kyiv clays. The formation of the chemical composition of underground waters goes on under the conditions of the well-washed-out deposits on the slopes of the basin and brackish and saline waters in its central part. Zones of difficult and very difficult water exchange conditions are found in the central part of the basin and are separated from each other by a stratum of Permian clays and salt beds. Thus, depending on the conditions of the formation of the groundwater chemical composition, the Dnipro artesian basin has a vertical hydrochemical zoning, according to which three zones are distinguished in the profile of the sediment strata: 1) the zone of hydrocarbonate calcium waters; 2) the zone of hydrocarbonate-chloride sodium waters; and 3) the zone of chloride-sodium and sodium-calcium brackish, salt, and brine waters. The hydrochemical zoning is broken in the areas of salt-dome structures and within the peripheral parts of the basin.

The Volyn’-Podillia artesian basin is situated in the western part of Ukraine, where the sedimentary complex of deposits from the Proterozoic, Lower Palaeozoic and Mesozoic lies on the folded crystalline basement. In the places of the shallow deposition of the basement, mainly in the eastern part of the basin to the depth of 300–350 m spreads the zone of fresh water in the Proterozoic, Palaeozoic (except for the Carboniferous), Upper Cretaceous, Neogene, and Anthropogene deposits. Some parts of the basin differ in the character of aquiferous rocks, conditions of recharge, interrelation and the groundwater discharge of the entire mass of sedimentary deposits. Between the ground waters of these aquifers the hydraulical connection exists. In the central and western parts of the Volyn’-Podillia artesian basin, within which the crystalline basement occurs at the depth of 5 km, the freshwater zone considerably decreases and is found only in the upper fractured Cenomanian-Turonian strata to the depth of 100 m, as well as in Neogene and Anthropogene deposits. Almost all aquifers developed within the borders of the Volyn’-Podillia artesian basin are used for water supply. It should be also mentioned that in the territory of the Volyn’-Podillia artesian basin in the Cretaceous, Jurassic, and Carboniferous deposits at the depth of 400–800 m are found nitric-carbonated, hydrogen sulfide, and sodium chloride with the mineralization of 50–100 g/dm³ thermal waters with the temperature of 20–30°C. At the greater depth – 1 000–3 000 m in Devonian deposits are found waters with the temperature 35–70°C of the sulphate-calcium, sodium, and chloride-sodium composition with the mineralization of 1.1–4.3 to 31–184 g/dm³. These waters are characterized by increased content of bromine, iodine, and boron.

The Black Sea artesian basin takes up the southern slope of the Ukrainian crystalline shield (UCS) and the Black Sea depression. Hydrogeological conditions of the Black Sea artesian basin are rather complicated which can be explained by a great variety of lithological composition of sediments that constitute it. The heterogeneity of deposits in the profile and a frequent alternation of water-bearing and impervious strata defined the formation of numerous isolated aquifers. Main aquifers are developed in the sediments of the Cretaceous, Palaeogene, Neogene, and Anthropogene systems. The groundwaters of the basin are characterized by the variability of their mineralization – prevalent are brackish and salt water. The freshwater zone in the territory of the basin is not continuous. In general, the territory of the Black Sea artesian basin is poorly supplied with underground water and the problem of water supply is solved here with great difficulties. However, on the entire territory of the Black Sea artesian basin, in the complex of Cretaceous deposits at the depth of 200 m (in the north) to 2 480 m (in the south) are found thermal waters with the temperature of 27–93.6°C and mineralization of 1.6–1.8 g/dm³ that have biologically active components – radon, bromine, iodine, iron, strontium, lithium – and are widely used in balneology.

Of the total value of prognostic exploitational resources of underground waters of the Black Sea artesian basin about 60% belong to waters with the mineralization of more than 1 g/dm³, mainly 1–3 g/dm³.

The Donets’ artesian basin is situated in the east of Ukraine. It has relatively simple hydrogeological conditions peculiar to geostructures of the platform type. All sedimentary deposits have water-bearing strata and complexes that divide conventionally into two floors. The hydraulically connected upper aquifers of the Quaternary to Lower Jurassic age are situated in the zone of active water exchange. Within its borders, the richest in water is the aquifer of Upper Cretaceous deposits. The lower floor – aquifers of the Carboniferous, Triassic, Jurassic, and Lower Cretaceous deposits – is characterized by difficult water exchange.

The Donets’ folded structure is located in the southeast. The richest here are water-bearing strata and complexes of Carboniferous, Permian, Jurassic, Cretaceous, and Quaternary deposits. In the upper part of the geological profile, to the depth of about 500 m, are found predominantly fresh and weakly brackish waters of different composition – from hydrocarbonate calcium to chloride sodium. Lower lie salt waters, and at the great depth (more than 1 000–1 500 m) in the northwestern part, highly mineralized chloride sodium brines with the mineralization to 300 g/dm³, with the increased content of iodine and bromine and the temperature of 50–77°C. It should be noted that this region has great potential for prospecting and use of thermal water. Especially promising are the southeastern and southwestern wings of the Kal’mius-Torets’ depression, where at the depth of about 500 m lie chloride-sulphate-hydrocarbonate waters with the temperature of 29–30°C and the mineralization of 2–4 g/dm³. In the eastern part of the Donets’ basin at the depth to 3 000 m circulate waters of the chloride-sodium composition with the mineralization of 222 g/dm³ and the temperature of 77°C.

The megaregion of the Carpathian folded structure is a complex region in terms of its geology and hydrogeology. Here, practically the only aquiferous rocks are terrigenous flysch deposits, highly dislocated, creased into numerous folds, often fractured, overturned, and complicated by thrusts, hence there are no mature aquifers. The peculiarity of the Precarpathian artesian basin lies in the fact that almost all its underground waters are highly mineralized or they are brines, and only the aquifer of Anthropogene deposits is the main source of water supply to inhabited localities. The folded mountainous Carpathians is a region of the development of fresh underground waters in the fissures of flysch formations of the Palaeogene and Cretaceous. In the Transcarpathian foredeep filled with thick strata (to 2 000 m) of the Neogene and Anthropogene deposits, aquifers of waters of different mineralization (from 1.0 to 25.0 g/dm³) are developed.

The underground waters of the Crimean folded structure, with its peculiar relief of the mountain terrain, features of the geological structure and climate that determine major mechanisms of the distribution and formation of underground waters, are found mainly in upper parts of the geological profile and are widespread in the Anthropogenic and Upper Jurassic deposits. Rocks of the Taurian formation, Cretaceous, and Palaeogene that take part in the geological structure of the region are poorly supplied with water. The water saturation of fissured and karsted limestones of the Upper Jurassic is evidenced by numerous springs, predominantly of fresh water widely used for water supply of the population.

The distribution of exploitational resources of underground waters in Ukraine is rather nonuniform due to the peculiarities of the geological structure of its different parts that represent individual hydrogeological regions which differ considerably in the level of water content of deposits that comprise them.

The richest resources of underground waters are found in the Dnipro-Donets’ artesian basin within which there is more than a half of exploitational resources of all Ukraine with the average module of 1.67 l/s per 1 km². The second place belongs to the Volyn’-Podillia artesian basin (about 20% of all resources with the average module of 1 l/s per 1 km²). The regions situated in the southeastern part of the UCS, in the Carpathians, Donbas, and the southwestern part of the Black Sea artesian basin, as well as the Kerch Peninsula, are poorly supplied with exploitational groundwater resources. Here modules do not exceed 0.1 l/s per 1 km², and these resources can be used only through individual wells that have a discharge most often to 1 l/s, rarely to 10 l/s.

The system of study of the groundwater flow is based on dynamic peculiarities of objects singled out on the principle of the uniformity of groundwater flow from the regions of the flow formation to its main discharge controlled by a common closed balance of underground waters. This approach is widely used for non-uniformly scaled hydrogeological zoning of Ukraine’s territory. Within the boundaries of hydrogeological structures are distinguished water exchange basins of the first order characterized by common or near trends of regional water exchange. The existence of principal regional “flows” of underground waters is connected with the presence of powerful regional drains – seas and large rivers, and of extended regions of maximum hydraulic heads, which in general correspond to watersheds situated on local elevations. They influence the formation of groundwater heads, direction and velocity of their movement.

Thus, the hierarchy of zoning of the formation of the groundwater flow and natural resources of underground waters – zones of intensive and considerable water exchange (depths in different regions reach hundreds – thousands metres) is analogous in many respects with hydrogeological principles of zoning of surface runoff. In particular, distinguished are basins of the sea runoff (regions) and main rivers that flow into seas (provinces). The next level of zoning takes into consideration peculiarities of hydrogeological conditions and close trends of lateral water exchange, which permits to include basins of the underground flow of small rivers into the framework of unified units of zoning (districts).

Hence, in the territory of Ukraine, according to the functional zoning, two regions are defined: A – Baltic, B – Black Sea-Azovian.

Within the regions, provinces are determined, and within provinces – districts that correspond to water exchange basins of different ranks. Under more detailed large-scale zoning smaller units of zoning are distinguished.

Ukraine has rich resources of mineral waters, diverse in their chemical composition and balneologic effect on humun organism.

Mineral waters whose balneologic effect on organism is determined by the content of specific biologically active components are singled out as mineral waters with specific components. According to the classification of Ukrainian mineral waters (2001) 9 types of mineral waters with specific components are distinguished: 1) carbonic acid waters; 2) sulfide waters; 3) waters enriched with organic matter; 4) boric waters; 5) siliceous waters; 6) bromine and iodid waters; 7) chalybeate waters; 8) radon waters; 9) polymetallic waters.

In Ukraine’s territory, mineral waters are developed at different depths in heterochronous deposits. On the Ukrainian Shield, they are found at the depth no more than 120 m from the surface, while in artesian basins, at the depths of over 3 000 m.

Mineral waters whose peculiar balneologic effect on organism is defined by the composition and ratio of main water components (they include sodium, calcium, magnesium, sulfate, hydrogen carbonate, and chlorine ions) and their quantitative composition (mineralization) are called mineral waters without specific components. According to the classification of mineral waters of Ukraine (2001) there are 25 types of mineral waters without specific components, which belong to 11 different classes (by anions), 7 subclasses (by cations), and 5 groups by mineralization (from weakly mineralized of Myrhorods’ka type to strong brine of the Morshyns’ka type). At that, the definition of the mineral water types was based on their direct use in medical practice.

The geological structure, lithologic composition of rocks, hydrogeological and palaeogeological conditions determine main features of the composition of mineral waters without specific components in different regions. According to these main conditions, on the map Mineral Waters of Ukraine without Specific Components 19 regions of distribution of mineral waters with certain peculiarities of the composition of main components are defined within major hydrogeological regions analogous to those on the map of mineral waters with specific components.

In Ukraine, thermal waters are widespread in the Transcarpathian, Peredkarpattia (Forecarpathians), Black Sea, and Dnipro artesian basins. At present, because of the great depth of occurrence of aquifers with thermal waters, their considerable mineralization, and ecological problems connected with their discharge after utilization, the geological survey of underground thermal waters and their wide use are restrained by all these factors, despite the fact that under certain conditions underground thermal waters can be an alternative to traditional kinds of sources of obtaining heat energy.

The map Thermal Waters on the grounds of specialized explorations held to date presents two most promising basins of thermal waters – the Black Sea and Transcarpathian, within which are represented probable depths of wells, expected yield and temperatures and explored territories with already proven reserves of thermal waters.