Your neighbor own a body of water, and decides to contaminate it by dumping industrial waste into the river, upstream from your property.
The poluter should be held personally liable, but the EPA is an intermediary that you have to appeal to for justice. Pre-EPA, companies that polluted areas could be held liable in civil and criminal court, with the creation of the EPA that was no longer the case. The EPA became the arbiter of Environmental justice, even though there were already mechanisms in place to where you could file suit against whom ever was the causing damages the environment
This is a total layman's perspective, and I will update if I can find the source for the legal precedents pre-EPA. Essentially the EPA is a Bureaucratic Construct, that can be lobbied by wealthy corporations to be more lenient in cases against the company... just like any other bureaucratic body.
Not saying the EPA is all bad, and u should go shyt in a river, but it ain't all good either