The first policy is that if a use is an "existing" use for a waterbody, then the waterbody must have that use in its designated uses (sometimes called use classifications). Remember, as noted previously, the term "existing use" has a special meaning in the context of water quality standards.
The second rule is simply a reflection of the CWA's "fishable/swimmable" goal (protection and propagation of fish, shellfish, and wildlife and recreation in and on the water), as articulated in EPA's regulations, which say that these uses should be designated for all waters, unless it is demonstrated that it is impractical to meet them. Only in those cases where the "downgrading" process has been followed (see next slide) can these uses be excluded from the DUs for a waterbody.
The third rule is that "waste transport" is not an acceptable DU, because in passing the 1972 CWA, Congress said that our nation's surface waters should no longer be used as waste conveyances or treatment systems.
The fourth rule has been covered in the WQS: Use Classification slide. When a waterbody has been classified for more than one DU, as is usually the case, regulatory activities and other programs are "driven" by the DU that requires the cleanest water. This is simply because if one DU requires a concentration of pollutant "x" of 50 mg/L or less and a second DU requires 25 mg/L, then meeting the second DU (and the corresponding WQC of 25 mg/L) automatically results in meeting the first DU and its corresponding WQC.
The last key rule regarding the setting of DUs is that economic and social factors can be considered, although this is not required. More specifics about this will be presented in the next slide, which deals with changing DUs.