You would be forgiven for thinking that if you buy water in a bottle it is in some way better for you than the water you get out of your tap. However that depends on the water you buy and where you live. For example I drink bottled water because even the filtered water here in Pakistan gives me very violent stomach upsets. I must say in my defense that I usually boil tap water and filtered water which is equally good, less expensive and less of an environmental problem.
There are several types of bottled water and you need to read the label to find out what is actually in the bottle you are thinking of buying. The sparkling water is natural water which contained carbon dioxide when it sprang from its underground source, this may have been removed but topped up later so that it is more or less as natural as it was before being bottled.
Mineral water should come from bore holes or a spring and a protected (both geologically and physically) source, such as an artesian well or aquifer. It should not have added minerals in it.
These are the main types of water, but there is also purified water which has been subjected to distillation, deionization or reverse osmosis and so on. It has had its natural minerals removed.
There have been several studies undertaken which have discovered that bottled water is only tap water, and these have led the public to be skeptical about buying bottled water. One of these studies was published in 1999 by the natural Resources Defense Council (USA) and the results showed that of the 1,000 samples 25% was the same as tap water, although sometimes this had been further treated. However, we should expect more than purified tap water when we pay for a bottle of water which is advertised as being natural spring water, for example.
Rather than buy bottled water, which is problematic as the bottles are plastic and not easily biodegradable, we should consider installing solid block carbon filters in our home water system. These provide us with the best purified water as most of the harmful substances, such as lead from out ancient pipes, or fluoride, which is added by the local authority which controls the water supply, as well as volatile organic compounds (VOCs). Water filters can retain trace minerals but remove chlorine, VOCs and the protozoa which can cause gastrointestinal problems.
If you are not satisfied with the water that comes out of your tap then installing a solid block carbon filter will probably solve the problem. You will not have to pay for bottled water which may, or may not be as natural as it claims to be. You may also be interested in this quotation from the US Environmental Protection Agency in their booklet "Water on Tap: What You need To Know" published in December, 2009: -
"... although required to meet the same safety standards as public water supplies, bottled water does not undergo the same testing and reporting as water from a treatment facility."
So perhaps it is safer to drink filtered tap water after all.