Once you have listed likely sources for the evidence you need, you have to decide what will count as proof for your hypothesis or an adequate answer for your question. Will it be enough to prove that the water plant now does fewer quality checks than it used to do? Or do you also need to find out what the consequences of fewer checks were? The best investigative reporters do not only assemble evidence that supports their hypothesis, but also evidence that contradicts it. For instance, a government official who is already very rich may be unlikely to waste his time performing a service for a $10,000 bribe. Considering contradictory evidence is the best way to avoid the ‘wishful thinking’ trap. Keep questioning yourself at various stages about reliable and complete evidence, types of sources, number of sources, and what could invalidate or disprove your evidence. Which pieces of evidence will require the most careful and detailed checking? Can you deal with this during your research?
Be careful with the notion of ‘proof’. It is very rare to find absolute proof of something – the ‘smoking gun’, as it is called. Sometimes you may be able to assemble enough evidence to only speculate that your hypothesis is correct. This is very similar to how criminal charges require proof ‘beyond reasonable doubt’, whereas civil charges only require a ‘balance of probabilities’ weighed toward one side of a case. As long as your final story makes it clear whether you are presenting proof or probability, you may still have a story, even without watertight proof. But you will need to be very careful how you write it.