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

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Hydrologic Technology<br />

5.17<br />

The role <strong>of</strong> hydrologic studies in site exploration can be separated into three overlap-<br />

ping areas: 1) two-dimensional characterization <strong>of</strong> the surface and ground-water systems for<br />

the region or hydrologic basin in which the site is located, 2) three-dimensional character-<br />

ization <strong>of</strong> ground-water conditions at candidate sites, and 3) the potential effects <strong>of</strong> the<br />

repository, the climate, or other perturbations <strong>of</strong> the ground-water system.<br />

Because it is believed that hydrologic transport will be the principal mode <strong>of</strong> translo-<br />

cation <strong>of</strong> radionuclides, a considerable amount <strong>of</strong> field and test data will be acquired to<br />

assess the hydrologic system. The techniques for obtaining most <strong>of</strong> the data are currently<br />

available; others, including improved techniques for ground-water dating, fracture-flow<br />

modeling, and permeability determinations for low permeability rocks, need development (Barr<br />

et al, 1978 and Bredehoeft et al. 1978). Hydrologic models combined with geochemical<br />

studies are used to estimate the likely composition and concentration <strong>of</strong> any and all radio-<br />

nuclides at any given point and time relative to a site's regional aquifer system.<br />

Data from hydrologic testing are combined with geologic interpretations <strong>of</strong> a site and<br />

region to produce a detailed three-dimensional model <strong>of</strong> the near-field (see p. K.4) hydrolo-<br />

gic flow system which includes the fracture-flow conditions. This is then integrated with<br />

thermal and mechanical models to calculate the near-field disposition <strong>of</strong> the wastes should<br />

they escape containment. The near-field models determine the source terms for regional two-<br />

dimensional flow models <strong>of</strong> a subject hydrologic basin. These regional models are used to<br />

calculate the isolation potential <strong>of</strong> the far-field natural system. Retardation mechanisms<br />

(e.g., sorption, precipitation and diffusion into the rock matrix) and radioactive decay<br />

chains for the radionuclides will be factored into both near- and far-field models <strong>of</strong> the<br />

isolation system. Conservative assumptions regarding potential changes in the hydrologic<br />

system that may be caused by climatic and tectonic changes will be used to develop scenarios<br />

for modifying models <strong>of</strong> present ground-water flow conditions.<br />

Permeability, effective porosity, and rock compressibility can be determined by pump<br />

or injection tests in wells at the depth intervals <strong>of</strong> interest. Hydraulic properties are<br />

routinely measured for laboratory specimens <strong>of</strong> core or other rock samples obtained from the<br />

site (Lin 1978). Using appropriately spaced wells, hydraulic communication between them can<br />

be established during pump or injection tests (Davis et al. 1966) to provide reliable calcu-<br />

lations <strong>of</strong> in-situ ground-water velocities.<br />

Isotopic dating <strong>of</strong> ground water (Barr et al. 1978 and Bredehoeft et al. 1978) provides<br />

an alternative reference for evaluating calculated velocities. Water can be sampled for<br />

dating from selected discharge points and well locations throughout the ground water basin<br />

considered likely to be influenced by a repository. Differences in water ages among sam-<br />

pling points are used to calculate natural velocities.<br />

The identification and analysis <strong>of</strong> hydrologic conditions in nearly impermeable rocks<br />

is necessary to establish the degree <strong>of</strong> impermeability possessed by the host rock unit

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