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

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B.4 HYDROLOGIC CONSIDERATIONS<br />

B.11<br />

The hydrologic regime (the surface water and ground-water systems) at the repository<br />

site must be favorable to geologic isolation (Office <strong>of</strong> <strong>Waste</strong> Isolation 1977).<br />

Surface hydrology includes the distribution and occurrence <strong>of</strong> water at the surface <strong>of</strong><br />

the area. Large rivers and lakes represent collection areas for surface water from sur-<br />

rounding regions and may be areas where underground water is moving to the surface. Such<br />

areas will probably be avoided because <strong>of</strong> the risks <strong>of</strong> flooding and entrance <strong>of</strong> water into<br />

the repository workings.<br />

reasons:<br />

Ground water is an important consideration in;geologic site selection for two main<br />

1. It is a valuable and widely used resource and a repository should not be located<br />

where it will affect the quality or availability to an unacceptable level.<br />

2. Ground water is generally considered to be the most likely agent for transporting<br />

radioactivity away from the repository during its expected lifetime.<br />

Ground water is present in varying degrees <strong>of</strong> saturation in nearly all subsurface earth<br />

materials. Also, all rock units have some permeability (although it may be small in some<br />

cases), and have hydraulic conductivity varying from relatively high to very low. Ground<br />

water can dissolve and transport radionuclides. <strong>Waste</strong> isolation requires that the prop-<br />

erties <strong>of</strong> the host rock minimize transport <strong>of</strong> the waste and that the host rock be isolated<br />

from more permeable media. The ability <strong>of</strong> a disposal media to isolate radionuclides within<br />

a hydrologic regime is determined from the factors that govern hydrologic transport via the<br />

local and regional flow patterns.<br />

The local flow regime <strong>of</strong> a repository site can be characterized by the geohydrologic<br />

properties <strong>of</strong> the host rock and <strong>of</strong> the hydraulic gradients (inducement to flow). Evalua-<br />

tion <strong>of</strong> the isolation potential <strong>of</strong> these components requires geologic studies, hydrologic<br />

testing, and analysis <strong>of</strong> water characteristics (de Marsily et al. 1977; ERDA 1976).<br />

The geohydrologic character <strong>of</strong> the repository medium is concerned with intergranular<br />

fluid properties (Walton 1970). A rock substance is composed <strong>of</strong> minerals compacted and<br />

cemented or crystallized together into a matrix. Spaces between grains and cementation<br />

material (called pore space) can contain fluid. The percent <strong>of</strong> pore space in the total<br />

matrix is the porosity. The volume <strong>of</strong> fluid a repository medium can contain is described in<br />

terms <strong>of</strong> percent water saturation and porosity (secondary rock discontinuities also contri-<br />

bute to its fluid volume capacity). Pore space is an important property in determining:<br />

1) the ability <strong>of</strong> a fluid to flow through a medium, 2) the volume <strong>of</strong> fluid flow and 3) the<br />

rate <strong>of</strong> flow. The evaluation <strong>of</strong> porosity for the repository medium includes the in-situ<br />

condition and the effect <strong>of</strong> radioactive waste-induced alteration, e.g., precipitation and/or<br />

solution. Porosity alone does not determine the permeability <strong>of</strong> a medium. For example, a<br />

shale has high porosity because <strong>of</strong> the clay size particles but is essentially impermeable<br />

because the pores are not interconnected or are so small that capillary forces dominate.

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