It is not natural for water to be contaminated, however, all natural water is contaminated because water in essence is a particle carrier and particles are contaminants.
Pollution from power plants, littering, or vehicles.
The water table is the term for how deep underground you have to dig to find ground water.
Surface water is on top of the ground where as ground water is below ground.
groundwater can be fresh, or can have various salts and/or minerals dissolved in it