Underground fluid injection can have disastrous consequences for drinking water and, in turn, for human health. Injected fluid is hard to trace once it enters the ground, and polluted aquifers are hard to remediate. Congress' cautious "preventive" approach requires permit applicants to show that their injections will not harm underground sources of drinking water. It presumes, until an applicant shows otherwise, that injections will contaminate an USDW. Although this approach may result in forbidding some injections that would not contaminate an USDW, it is a valid exercise of Congress' authority.