The following violation numbers were obtained from the EPA's "Drinking water and ground water statistics 2011"   

There were 17,510 violations from Water Supply Systems NOT conducting the required water testing or NOT submitting their test results.

Why would a Water Supply System not submit the test results to the EPA? are they hiding something?

There was also 8,500 Health Based Violations in 2011. This means the people drinking that water were subjected to water that the EPA does not feel is safe to drink.

The EPA places the following NOTICE on every Safe Drinking Water Information System Violation Report available to the public:
“NOTICE: EPA is aware of inaccuracies and under reporting of some data in the Safe Drinking Water Information System. We are working with the states to improve the quality of the data.”
- - - - They know some systems are UNDER REPORTING their findings - - -

In review:
8,522 Health Based Violations for Water Supply Systems in one year
17,519 violations for Water Supply Systems did not conduct the required monitoring for contaminants in the water or did not submit their test results
And the EPA states they know other Water Supply Systems are "under reporting" their test result findings.