Surface- and borehole water
The earth is covered for more than 70% with water. The rest exist out of land. Water is the prerequisite to the existence of all known forms of life.
Groundwater plays a very important role in the water cycle. Mostly this water comes from precipitation which, after that it falls on earth, directly or indirectly (after falling into lakes or rivers) vanishes in the earth.
Another source of groundwater is water which appears when metamorphic reactions take place in the deeper layers of the earth crust or when water is compensated by upcoming magma.
Generally groundwater is more pure than surface water. This doesn’t mean that groundwater cannot be contaminated. There are many possible contaminations, chemical as well as natural; like calcium, fluoride, arsenic, lead, copper, mercury, etc.