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

B.13
and the field measurement of hydrologic parameters enumerated will be needed. These data
will be input to hydrologic models of candidate areas for locating sites at which geologic
barriers are particularly effective.
Detailed evaluation of individual sites will be required in order to predict the complicated
interaction of a repository model and hydrologic regimes. An important field of
research, for instance, is the prediction of thermal effects on rock permeability near a
repository.
Site selection will probably avoid areas of known major aquifers. In areas other than
those with major aquifers, a preliminary ground-water characterization would certainly be a
factor to be considered in the early stages.
To characterize an area's ground-water supply and potential requires determination of
such factors as depth to producing zone(s), yield (usually determined from pumping tests)
and an estimate of the supply available. From available wells, porosity, permeability and
change in water level caused by pumping are measured. These aquifer properties and how they
change with distance are extrapolated over the area if other measurements are not available.
Usually other existing wells supply information to help determine the configuration of the
water table or artesian pressure surface, direction of flow and estimates of rates of movement.
These methods are generally applied to areas and rock units that yield water in
usable quantities, whether on a scale for cities or for a single dwelling (Walton 1970,
Davis and DeWiest 1966). They are not as applicable and have not been applied as widely to
areas where porosity, permeability and yield are very low--that is, where usable supply can-
not be obtained.
The areas of very low ground-water supply or flow are more favorable candidate areas
for waste repositories because of the smaller opportunity for moving water to contact the
waste and possibly transport it. Flow properties will need to be determined because of the
time periods associated with a repository. Flow rates and velocities of ground water that
are insignificant over a 50-yr period may be significant over hundreds to thousands of
years. Methods of evaluating free water and its movement in media of these areas are avail-
able (for example, laboratory determinations of porosity and permeability are made from
field core samples) but zones of fracture or joint flow are difficult to evaluate and
describe in laboratory tests. Field tests will be necessary to measure in-situ properties
after a potential site is chosen.
When possible, future climatic changes (for example, a change to a much wetter climate)
should be considered with the attendant possible effects such as change in ground-water
levels on a repository.
It seems reasonable to assume as one possibility that free water, over thousands of
years, may enter the repository even in shale and possibly salt. The effect of the water
will depend on the condition and state of the waste at the time, and transport of radionu-
clides will depend on the rate of water movement, if any, through the repository and the
physical-chemical properties of the repository medium.