Drinking water, like every other substance, contains small amounts of bacteria. Most of these bacteria are common ones and they are generally not harmful. Chlorine is usually added to drinking water to prevent bacterial growth while the water streams through pipelines. This is why drinking water also contains minimal amounts of chlorine.
Water mostly consists of minerals and other inorganic compounds, such as calcium.
If you want to find out what substances your tap water consists of and whether it is totally safe to drink you can have a specialized agency check it out for you.
Drinking water can come from different resources. For one, it can be pumped from the ground through wells. This groundwater is than purified, so that it will contain no more contaminants and is suited to drink. Drinking water can also be prepared directly from surface water resources, such as rivers, lakes and streams. Usually surface water has to undergo many more purification steps than groundwater to become suited to drink. Preparing drinking water out of surface water is much more expensive due to this. Still 66% of all people are served by a water system that uses surface water.
Part of our drinking water is pumped from the ground, usually under sand dunes. In sand dunes water can also be infiltrated. As it sinks into the ground through the dunes it is naturally purified. This costs much less money than the purification of surface water. Part of our drinking water originates from dune water.