Material is directly quoted from bestfunfacts.com, but the general idea is well documented in a variety of casual and research literature:
“EPA standards for tap water are much stricter than FDA standards for bottled water. Result: tap water is cleaner and cheaper.
Two independent studies confirmed the above statement.
The first study at Case Western Reserve University tested 39 samples of bottled water and found that 15 of the samples contained about twice as much bacteria as Cleveland tap water.
Another study by the Natural Resources Defense Council found that about 18% of bottled water brands contained more bacteria than allowable, besides other chemicals found in the bottled water.
Also about 1/3 of bottled water sold in the US is tap water. Aquafina (Pepsi product) and Dasani (Coke product) are nothing but tap water.”