to find out what happened exactly, who dumped what where, if it was done on purpose by somebody to spoil your water?
A government has more resources than a single individual. As well as being able to pay for things like trained investigators (essential in pretty much any kind of criminal investigation, from road accidents to murders) and labs to do chemical analysis, they could levy special investigative and punitive rights. For example, a criminal court can award search warrants for police to execute in order to collect evidence, and then impose a custodial penalty upon successful conviction. By ensuring that amateurs are not involved in the investigation and prosecution, this branch of government (theoretically) adheres to standards high enough to warrant them being given the kind of rights that allow them to suspend the freedom of those who break laws.
The government would also be able to address these problems without requiring private individuals to drop everything they're doing and start acting as investigators and lawyers. Even small civil cases take a LONG time to litigate (never mind investigate), especially if you aren't a specialist. As a software engineer, do you really expect me to drop my day job while I poke around a neighbour's private property to collect evidence of mercury poisoning I had to pay an expensive lab to uncover? I regard that kind of act to be a crime against society (like murder or drunk driving) and thus deem the onus of investigation/prosecution to be on society's collective representative - the executive and judicial branches of government.