The only real explanation for this variance are the political considerations that went into the promulgation of each of those standards. This is a dangerous way to run the public health railroad. Finally, the political determination of regulatory standards explicitly encourages a utilitarian approach to environmental protection. And there is no way to remedy these fundamental problems. Electoral strength comes and goes.
More regulation simply compounds the problem of inevitably sub-optimal regulation.
Let me suggest that we address not the symptoms, but the disease. And that disease is the politicization of the environmental commons. The fundamental premise of environmentalism is that it is the legislature's role to determine to what extent this trespass should be allowed and it is the executive branch's job to enforce those trespass limitations. Yet there is no reason why environmental resources cannot be owned by private parties. For example, the legal mechanics of private groundwater rights are conceptually no more difficult that the existing legal mechanics protecting private oil field rights.