I'm one step ahead here I think. Obviously the EPA is empowered to do the study. That's part of why they exist. However, my understanding is that if the level that the EPA sets is not precisely prescribed by law then that gives an opening to anybody to challenge that limit in the courts because Congress didn't set a definitive level and therefore the EPA overstepped its authority.
That is not a correct understanding. Congress simply needs to have charged the EPA with ensuring water is safe and setting limits.