Hanford Site National Environmental Policy Act (NEPA) - Pacific ...

Clastic dikes typically occur in swarms and in plan view may appear as regularly shaped
polygonal patterns, irregularly shaped polygonal patterns, or as random occurrences. Regular
polygonal networks resemble four- to eight-sided polygons. Individual clastic dikes are known to
extend vertically from as little as 30 cm (1 ft) to greater than 55 m (180 ft) deep. Clastic dikes have
been reported at several single-shell tank farms, the Environmental Restoration and Disposal Facility,
the Integrated Disposal Facility, and throughout the Cold Creek Valley (Fecht et al. 1999; Reidel and
Chamness 2007).
Clastic dikes have the potential to act as preferential pathways and/or barriers to the movement of
soil moisture in the vadose zone. Infiltration studies, ongoing for several years, are examining the
influence these structures have on water flow in the vadose zone. A recent finding indicates that at
low water fluxes typical of vegetated areas on the Hanford Site, flow is dominated by the relatively
finer-grained dikes. At high input fluxes, the coarser-grained host sediments dominate flow
suggesting clastic dikes containing fine sediment may actually retard vertical flow rather than act as
conduits for fluids through the vadose zone (Ward et al. 2004). It also suggests that such features may
act as cutoff walls, limiting the lateral spread of fluids, which otherwise could move significant
distances laterally.
4.3.4 Structure and Tectonics
The Yakima Fold Belt subprovince covers about 14,000 km 2 (5,410 mi 2 ) of the western Columbia
Basin (extending into the Pasco Basin) and formed as basalt flows and interbedded sediments were
folded and faulted under north-south compression. Most of the present structural relief in the
Columbia Basin has developed within about the last 10.5 million years when the last massive
outpouring of lava buried much of the central Columbia Basin. Almost all of the present structural
relief is post-basalt.
The Yakima Fold Belt consists of a series of narrow, asymmetrical anticlinal ridges separated by
broad synclines or basins that, in many cases, contain thick accumulations of Pliocene to Quaternaryage
sediments (5.3 million years to present). The northern limbs of the generally east-west trending
anticlines dip steeply to the north or are vertical. The southern limbs generally dip at relatively
shallow angles to the south. Thrust faults, or high-angle reverse faults with fault planes that strike
parallel or subparallel to the axes of the folds, are principally found on the north sides of these
anticlines (Figure 4.3.2 and Figure 4.3.7). The amount of vertical stratigraphic offset associated with
these faults varies but commonly exceeds hundreds of meters.
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