(a) The representative volume is the volume of groundwater that would be withdrawn annually from an aquifer containing less than 10,000 milligrams of total dissolved solids per liter of water to supply a given water demand. DOE must project the concentration of radionuclides released from the Yucca Mountain disposal system that will be in the representative volume. DOE must use the projected concentrations to demonstrate a reasonable expectation that the Yucca Mountain disposal system complies with § 63.331. The DOE must make the following assumptions concerning the representative volume:

1. (1) It includes the highest concentration level in the plume of contamination in the accessible environment;
2. (2) Its position and dimensions in the aquifer are determined using average hydrologic characteristics which have cautious, but reasonable, values representative of the aquifers along the radionuclide migration path from the Yucca Mountain repository to the accessible environment as determined by site characterization; and
3. (3) It contains 3,000 acre-feet of water (about 3,714,450,000 liters or 977,486,000 gallons).

(b) DOE must use one of two alternative methods for determining the dimensions of the representative volume. The DOE must propose its chosen method, and any underlying assumptions, to NRC for approval.

1. (1) DOE may calculate the dimensions as a well-capture zone. If DOE uses this approach, it must assume that the:

   1. (i) Water supply well(s) has (have) characteristics consistent with public water supply wells in the Town of Amargosa Valley, Nevada, for example, well-bore size and length of the screened intervals;
   2. (ii) Screened interval(s) include(s) the highest concentration in the plume of contamination in the accessible environment; and
   3. (iii) Pumping rates and the placement of the well(s) must be set to produce an annual withdrawal equal to the representative volume and to tap the highest concentration within the plume of contamination.

2. (2) DOE may calculate the dimensions as a slice of the plume. If DOE uses this approach, it must:

   1. (i) Propose, for approval, where the location of the edge of the plume of contamination occurs. For example, the place where the concentration of radionuclides reaches 0.1% of the level of the highest concentration in the accessible environment;
   2. (ii) Assume that the slice of the plume is perpendicular to the prevalent direction of flow of the aquifer; and
   3. (iii) Assume that the volume of groundwater contained within the slice of the plume equals the representative volume.