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

B.2
biosphere. These earth material properties must be known in order to determine rates of
erosion. The properties of these materials that relate to erosional processes are the
strength, hardness, chemical composition, consistency, uniformity, and homogeneity.
Topography (land-surface configuration) has an economic impact because of its influence
on ease of access for materials and transportation, the amount of surface modification
required for construction of facilities such as buildings and railroads, and any unique
problems such as landslide potential or flash flooding. In addition, steep terrain fre-
quently indicates tectonic instability. In general, a relatively flat and open area with
low relief is considered desirable.
Weathering is the chemical and physical decomposition and/or transport of surface and
near-surface earth materials by surface erosional processes. It can decompose earth mate-
rials into smaller components that are more easily carried and deposited by other erosional
processes. The weathering process can break down earth materials as deep as several hundred
feet. In addition to climatic forces, the rate of weathering depends also on the resistance
of earth materials to chemical deterioration and physical pressures. The major chemical and
physical weathering processes are freezing and thawing, hydration, hydrolysis, oxidation,
carbonation, dissolution, and expansion caused by unloading, crystal growth, thermal difference,
and organic activity. All of these may remove material and thus decrease the depth to
the isolated waste.
Water (stream) erosion processes are a function of a base level (Office of Waste Isola-
tion 1977). Base level is a surface below which moving water cannot erode. The ultimate
base level for stream erosion is generally considered to be sea level. Base level can
change, however, over geologic time; for example, large fluctuations of sea level can occur
during glacial periods. The mechanisms of a stream erosion are acquisition of weathered
earth materials, abrasion of material through particle impact, transit abrasion of mate-
rials, and transport by the traction, suspension, or solution of weathered rock debris.
Erosional processes unaffected by a base level are those' related to ice, wind, and
gravity. These processes are important because of their potential for eroding below base
level.
Erosion by ice is caused by glaciation, and the continental type has the greatest
potential impact on depth of isolation (Office of Waste Isolation 1977). Glaciers are a
dynamic mass of recrystallized snow and ice, and the character and longevity of a glacier
depend on climatic factors. Glacial action alters the land surface and could reduce the
depth of a repository by 1) plowing or scraping earth materials from a site, 2) abrasion of
intact rock, and 3) assimilation of plowed and abraded material into the ice mass (Verhoogen
et al. 1970). The depth to the repository may be effectively reduced if fracturing or
faulting results from the loading and unloading of the ice on the land surface. Parameters
affecting glacial erosion are ice temperature and thickness, earth material and structure,
and topography. The depth below base level at which glaciers may erode can be substantial.
The lower depth of glacial erosion at a repository site can be predicted to some extent from
the glacial history.