After you have sorted out the sources to contact, you must decide what counts as proof for your hypothesis. Is it enough to determine that the water plant now conducts fewer quality checks than it used to? Or would you also need to find out what the consequences of fewer checks are? The best reporters not only evaluate evidence supporting their hypothesis, but also those that contradict it. For instance, a government official who is already very rich is unlikely to take up a $10,000 bribe – could such ‘evidence’ be wishful thinking on the part of your source? Again, ask yourself regularly about the reliability of the evidence you have: how complete it is, where it comes from, and if multiple sources can corroborate it.
You should note it is rare to find absolute proof of something. Sometimes, the most you can muster is an assemblage of evidence that points to an outcome more likely than another. Not everything can be proven beyond reasonable doubt. That said, as long as your final story makes it clear, you might still have a story, even it is not watertight. But you will need to be very careful about how you phrase certain things.