The processes of infiltration and migration through which groundwater is replaced are called water recharge. A rock that holds enough of water and transmits it rapidly enough to be useful as a water source is an aquifer. Many of the best aquifers are sandstones or other coarse, clastic sedimentary rocks, but any other type of rock may serve if it is sufficiently porous and permeable – a porous limestone, fractured basalt, etc. An aquiclude may be capable of absorbing water slowly, but it has extremely low permeability, so the water cannot flow rapidly enough and that’s why it cannot serve as a water source.

Major reservoirs of natural water are:
 atmosphere, the atmospheric moisture;
 underground layer which supports surface water in streams, lakes, ponds, the ocean, and the solid water as snow and ice;
 soil zone acting as a reservoir of soil moisture which is held against the pull of gravity and is thus made available for industrial consumption;
 groundwater reservoir;

Rainwater as it falls to the earth has ample opportunity to dissolve gasses from the air and also may dissolve particles of dust or other airborne materials. Ammonia and various nitrogen compounds are present in general. Dust particles are added locally in industrial areas, large population centers, and desert areas. The sources of these constituents are the oceans, fresh water and saline lakes, landmasses, vegetation, man-made industries and volcanic emanations.

Mineral waters – are waters different from fresh underground waters by their composition or by their temperature. Mineral water is water, in which the content of dissolved minerals or gasses is at least 1g/l. Thermal water is water with a temperature surpassing the mean annual air temperature near a thermal spring. The limit temperature varies within 20-30 degrees. Water quality

Majority of the water in the hydrosphere is in very salty oceans, and almost all of the remainder is tied up in glaciers. The rest of the water - very small amount – is surface or subsurface water for potential freshwater sources. Even the rainwater contains dissolved chemicals of various kinds, especially in industrialized areas with substantial air pollution. Once precipitation reaches the ground, it reacts with soil, rocks, and organic compounds, dissolving more and more chemicals naturally, aside from any pollution generated by human activities.
Certain dissolved substances that alter water properties may influence water quality.