You could say water from the depth of most petroleum reservoirs underground is naturally polluted. It can be more than just saline polluted. Pretty much anything that can be in the earth and can mix with water.

There are cases of shallower reservoirs where the petroleum is mixed with otherwise potable water. If it is within an agricultural area, like Bakersfield, it may be treated and used for irrigation. The treatment is mostly just multiple steps of skimming off petroleum.

While I don't necessarily disagree with your first statement, your second statement seems to put things in the wrong light.

> It can be more than just saline polluted. Pretty much anything that can be in the earth and can mix with water.

Most water we drink is rain filtered through many different layers of earth. It's the cleanest source of freshwater we have. Minerals only mix to a very small percentage with the water during this process and this "pollution" is actually wanted to a certain degree, i.e. mineral water.

Rock layers on the surface of the earth that have water running through them that are drinkable have had rain water removing the elements and chemicals harmful to life for, generally, millions of years. The best water often comes from rock that was actually created by life (limestones, dolomites, etc), so that it had almost no harmful chemicals in it to begin with.

The poster you are disagreeing with is correct. The deep earth is full of really toxic stuff. Arsenic, lead, sulfur, radon, etc. Then there is the deep oil and gas, salt domes, asbestos. Huge volcanic eruptions called flood basalts are likely the cause of many mass extinctions due to the chemistry of the eruptions, not the ash. The mother Earth we all love is just the little skin on top of a cauldron of death.

Except the earth includes minerals we probably don’t want to drink like asbestos, arsenic, radioactive isotopes, etc. The concentrations are going to vary wildly. Basically I’m sure deep well water and water from the surface will be exposed to vastly different things. I know, as a lifelong desert dweller who's almost exclusively lived off well water, that well water quality needs to be monitored and treated. My alma mater’s town had to stop using one well when arsenic tolerances were decreased, for instance.