Water purity is tested by a national standard for parts per million of a long list of minerals, bacteria and contaminants. If there is an excess, it gets into the papers and the municipality has to tell the people that the water is unfit to drink.
The standards are rather high. No water company, municipal or commercial, wants this sort of bad publicity.
Unless those doing the testing are perhaps bribed, there is little likelihood that the tests are inaccurate.
Commercial companies in search of profits have money and motive to bribe, the average water purification manager does not.
If you compare city water with all the other industries, you will find that there are many fewer infractions and scandals.