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Water

According to scientific estimation, 97.5 percent of all water reserves on Earth is salt water of seas and oceans, leaving only 2.5 percent as fresh water.

Considering that 75 percent of fresh water is frozen in mountain glaciers and ice caps, plus 24 percent lies in deep underground aquifers as groundwater, and yet 0.5 percent is "dispersed" as soil moisture, it turns out that hardly over 0.01 percent of the world’s water resources are left as the most accessible and cheapest water sources – these are rivers, lakes and other surface water bodies. Taking into account the importance of water for human life and activities, and for the whole biosphere on Earth, the specified numbers clearly confirm the truth of water resources being the most precious ones.

Despite its utterly simple (as one would think) chemical composition, water is one of the most mysterious and “abnormal” substances on Earth. It will suffice to mention that it is the only chemical substance on this planet that exists in three common states of matter simultaneously: gaseous (water vapour), liquid, and solid. However, the existence of life is only possibly owing to these "oddities" of water still not fully explored.

Water cycle

As we all might remember from our nature studies at school, water moves perpetually. Through evaporation from the surface of water reservoirs, soil, and plants, water is accumulated in the atmosphere and, sooner or later, it falls out as precipitation, replenishing water storage in oceans, rivers, lakes, etc. Thus, the total amount of water on Earth stays the same; water changes its forms only – this is what we call the hydrologic cycle. 80 percent of all precipitation falls directly to ocean. We are mostly interested in the remaining 20 percent falling to land, since most water springs for human use are replenished through this kind of precipitation. Put simply, water falling to land has two ways. Either through gathering into streams and rivers it flows into lakes and water storage basins, the so called open (or surface) water withdrawal points. Or it replenishes groundwater storage by penetrating the soil and subsurface layers. Surface water and groundwater constitute two main water supply sources. Both of these water resources are interdependent, having their advantages and disadvantages as a source of drinking water.

Groundwater

A considerable part of falling rainwater, as well as meltwater penetrates the soil, where it dissolves organic substances contained in the soil layer, and is oxygenated. Sand, clay, and limestone layers lie deeper. Organic substances are mostly filtered there, but water gets filled with salts and microelements. In general, several factors have impact on the quality of groundwater:

1) the quality of rainwater (acidity, the concentration of salts, etc.).

2) the quality of water in subsurface reservoir. The age of such water can reach tens of thousands of years.

3) the character of layers that water comes through.

4) the geological nature of an aquifer.

Most commonly, groundwater would contain such mineral substances as calcium, magnesium, sodium, potassium, and iron in greatest amount, and manganese (cations) in a lesser degree. Together with common anions like carbonates, bicarbonates, sulphates, and chlorides, they form salts. The concentration of salts depends on the depth. Salts in the most "ancient" deep waters have such a great concentration that it gives a clearly salty taste to the water. The majority of well-known mineral waters belong to this type. Water of upmost quality is obtained from limestone layers, but those may be positioned quite deeply requiring high drilling costs. Groundwater is characterized by rather high mineral content, hardness, low presence of organic substances, and almost free from microorganisms