I worked with a chemical company for several years. My specialty with the company was water treatment. In the argument of which is better Tap water or Bottled water, there really are two sides to that coin. Let's start with Bottled water.
There is a lot of money to be made in bottled water. The consumers must really trust that the bottled water is clean, safe and of the highest quality. The consumer believes in most cases that Bottled water comes from some magical stream, or some pure source better than the water they are drinking out of the faucet. In the majority of cases this is not true. Bottled water producers take the water from the very same aquifer that the municipalities do. The difference is that the bottled water producers typically run that same water through either a carbon filter, or a Reverse Osmosis filtration system. So then It would seem that bottled water is the way to go right? Well it is a bit more complicated than that. It all relies on the testing of the water, and if the bottled water producer understands his filtration equipment.
First let's talk about water purity before we move on to filtration. Water purity is measured using Conductivity measured as uhmos. Pure water does not conduct electricity, it is the suspended and dissolved solids that do this. This is why salt water is more conductive and, since corrosion is an electrical process of leaching electrons out of metal, it is more corrosive. Let us say that the Municipalities water that the bottler is taking from comes in with a conductivity of 248uhmos. Which is very good quality water. Now we give this already high quality water to the bottler.
Let's look at the filtration. Activated carbon filtration has a limited life cycle in which the adsorption properties of the filtration media degrades over time and needs to be replaced. This is expensive, and will cause a shutdown of production. In order to tell it the media is still performing one must test the input against the output to see the efficacy.
Reverse osmosis filtration membranes use pressure to force water through a membrane that filters the particulates suspended and the majority of dissolved particulates as well. In order to tell if the filtration is at optimal efficacy one must record the pressure differential of the input versus the downstream throughput. Record these numbers and when a significant difference is noted this is because the membranes have started to be compromised with particulates that is severely reducing the flow of water to process.
Now if both of these filtration devices are operating at maximum efficacy the end result will be water that has a conductivity of ~0uhmos. Sounds impressive right? The only problem with water this pure is that on an electron level it is unnatural. You will never find pure water in nature as it seeks ionic neutrality. So what happens if you drink water that has ~0uhmos? You will become violently ill as the pure water strips electrons from your body, calcium from your bones, iron from your blood, sodium from the lining of your stomach. All the water seeks is ionic neutrality and since you are the only available source of ions, it will rip them out of you.
Mister bottler knows this to be true so in order to combat this he allows the majority of the water he sourced from the Municipality to pass through treating only a small portion of the water which he then adds back to the bulk. So our already high quality 248uhmos city water when mixed with his filtered portion now comes in at about 232uhmos. That is pretty much the same water quality the city provides you. You would need a lab full of sophisticated equipment to tell the difference.
Now let's look at the Municipality water since, honestly 95% of all drinking water AND bottled water comes from this source. Don't be fooled by labels that say they come from a spring. It may be true a portion of the municipality water comes from that spring, and since they source from the municipality, they can call it spring water. Tricky huh?
Municipality water comes from either a River, Lake, or Aquifer. It is important to know the quality of the water in the city you live in. If you already know these data and you live in a place with high quality, don't waste your precious money on bottled water. It is a fools errand. If you live in a city with a lot of ups and downs in water quality, you might want to think about reading the back of the bottled water you want to buy and make sure it came from someplace out of state, which then you much research their water quality before you make your purchase.
Wherever they bottled it, that's the municipality they are poaching from.
On to testing and accountability. The EPA tests the water quality for municipalities, they have in stream electronic testing apparatuses that alert the municipalites if any tollerances are being compromised so that they can take corrective actions. Even in "low" quality city drinking water, the water is safe to drink in the US.
The FDA makes yearly inspections of bottling plants, more if warranted. Often times bottlers use such low amounts of filtered water that their filtration devices become fouled with bacteria and impart bacteria and particulates to the water they are treating. So the 248uhmos water from the Municipality hits the bottle at 263uhmos. So on this day they didn't do you any favors.
All that to say Drink which ever one you want. The smart money is on tap water. But on any given day if an engineer is having an off day at the plant in the city or the bottling plant, who knows what you are drinking? But don't worry the most likely worst outcome for you is spending a little extra time on the toilet. Gross but true.