Management of Commercially Generated Radioactive Waste - U.S. ...

B.12
The flow potential of a repository medium depends on the interconnection of pore space
(permeability) and the pressure differential. Geologic materials may be grouped in order
of flow potential into aquifers and aquitards (confining units). The definition and deline-
ation of these units require knowledge of the geologic stratigraphy and matrix and rock mass
hydrologic properties of the medium. Conventional subsurface geologic techniques are used
to define the lithologic and horizontal and vertical distribution of a flow unit. Fluid
chemistry and core analysis of porosity and permeability allow the estimation of volume of
fluid available for flow within a unit and of the hydrologic characteristics of the reposi-
tory medium. Hydrologic field testing of the individual flow units completes the delinea-
tion of these units.
A flow unit may contain a large volume of fluid and a high permeability but require an
inducement to flow (Davis and DeWiest 1966). A difference in hydraulic head (gradient) is
necessary before a fluid will flow through a porous medium. A repository site contains
local gradients, both vertical and horizontal, and a regional gradient for given flow poten-
tial units. A hydrologic gradient could exist because of elevation differences between the
surface point where fluid enters the unit (recharge area) to the repository medium. Hydrau-
lic gradient across the repository medium is generally determined by finding the difference
in fluid level (potentiometric head) between wells of known depth.
The regional geohydrology is important to waste isolation in terms of conditions that
may affect the local hydrologic regime. Possible effects include changes in hydraulic gra-
dient from water usage or climatic changes.
Regional hydraulic parameters significant in maintaining isolation are recharge and
discharge conditions. Recharge is of particular interest in establishing the volume of
fluid available to an aquifer. Tectonic movements have the potential to significantly
alter the hydraulic regime.
Hydrologic considerations enter into each stage of the site selection process. In the
early stages the broad regional characteristics of surface and subsurface water flow are
examined for compatibility with waste isolation. Regions may be eliminated from considera-
tion on the basis of unfavorable characteristics, for example, high regional flow gradients,
presence of aquifers near the proposed repository depth, or alteration of hydrologic regime
from future climatic changes or tectonic events.
Other hydrologic characteristics may be of overriding importance to site selection.
For example, interior drainage (surface runoff that does not drain to the ocean) is particu-
larly well developed in the Great Basin of Nevada and Utah. The characteristics of such a
hydrologic regime offer longer flow paths and greater travel times than does surface water
flow to the migration of radionuclide wastes beyond the boundaries of the system. Other
examples of favorable regional hydrologic conditions include arid climate and low hydraulic
gradients (vertical and horizontal) in the surface and subsurface regimes.
Further consideration of hydrology involves more detailed characterization of the
regional regimes that have passed the first phase. The collection of rock properties data