Underground Source of Drinking Water (USDW): An Underground Source of Drinking Water as defined in Title 40, Code of Federal Regulations (40 CFR) Section 144.3 is an aquifer or part of an aquifer which:
- supplies any public water system, or contains a sufficient quantity of ground water to supply a public water system and currently supplies drinking water for human consumption or contains fewer than 10,000 milligrams/liter of Total Dissolved Solids (TDS); and
- is not an exempted aquifer.
An "exempted aquifer" is part or all of an aquifer which meets the definition of a USDW but which has been exempted according to the criteria found in 40 CFR Section 146.4, which specifies that the aquifer does not currently serve as a source of drinking water, and the aquifer cannot now and will not in the future serve as a source of drinking water for one of the following reasons:
- It is mineral, hydrocarbon or geothermal energy producing, or can be demonstrated by a permit applicant as part of a permit application for a Class II or IIIoperation to contain minerals or hydrocarbons that considering their quantity and location are expected to be commercially producible;
- It is situated at a depth or location which makes recovery of water for drinking water purposes economically or technologically impractical;
- It is so contaminated that it would be economically or technologically impractical to render that water fit for human consumption;
- It is located over a Class III well mining area subject to subsidence or catastrophic collapse; or
- The total dissolved solids content of the ground water is more than 3,000 and less than 10,000 milligrams/liter and it is not reasonably expected to supply a public water system.