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  Наука > Геология
Lankomumo reitingas Версия для печати Spausdinti
Geology Of Ward Valley

By Howard Wilshire and Jane Nielson

Ward Valley has a long geologic history, but many of the valleys geological facets relevant to nuclear waste dump siting are of quite recent origin. About 10 to 20 million years ago, as the San Andreas Fault began to move and volcanoes erupted in the Berkeley Hills, tectonic forces pulled the earths crust apart in a belt that parallels the lower Colorado River. This event (known to geologists as "extension") shaped the present landscape. The lower part of the extending crust was relatively hot and therefore stretched (ductile deformation), to double its previous width. The cold and rigid upper crust shattered along near-vertical faults, allowing lava to erupt. The stretched lower plate and the brittle, fractured upper plate were separated by shallow sloping and undulating "detachment faults."

During extension the fault-bounded upper plate blocks tilted; gaps between them formed basins in which sediment and volcanic rock collected. After extension, the lower plate rebounded and exposed both detachment faults and stretched rocks in areas east of Ward Valley, such as the Sacramento Mountains. These detachment faults slope gently westward under the north end of Ward Valley at a depth of a few hundred feet, then rise to the surface in the eastern Piute Mountains. Tilted upper plate blocks and basin deposits fringe the west side of the Sacramento Mountains and form the highly faulted Mopah Range and Turtle Mountains at the southeastern margin of Ward Valley.

The choice of Ward Valley as a radioactive waste disposal site ignored many important aspects of the geologic setting. For example, alluvium (decomposed rock transported by seasonal streams and gravity) derived from Cretaceous granitic rocks in the Piute Mountains is especially well-suited as desert tortoise habitat. A more dangerous assumption was that the valley fill consists of homogeneous, unfractured alluvial sediments, at least 800 feet deep. Assuming homogeneity allowed the physical properties of alluvium at the site to be mathematically modeled on the basis of very few measurements. The assumption that the materials are unfractured leads to a secondary assumption: that all water moving through the ground percolates evenly through the sediment. Most disposal sites for hazardous material have been evaluated using these same assumptions; however, at waste repository sites around the world human-produced contaminants are being discovered at much greater depths than models like the one used for Ward Valley would predict. Recent studies have focused on "preferred pathways" such as fractures, because water is transmitted much more rapidly through fractures when the ground is saturated.

Contrary to the assumptions used to model the site, actual geologic study of Ward Valley suggests that the alluvial fill is much less than 100 feet thick; construction of a pipeline a few miles south of the proposed dump site revealed granitic bedrock less than five feet down. Fractured sand, gravel, and volcanic rock, which accumulated in a basin in the upper plate during extension, are exposed near the proposed waste site in both the Little Piute and Sacramento Mountains. Seismic records obtained by the applicant suggest that such highly fractured rocks lie just under the proposed site; if so, the fractures could funnel water and contaminants to the water table under saturated conditions.

These saturated conditions now occur only locally during high rainfall, on an undetermined cycle: perhaps as short as decades, perhaps as long as centuries. Periods of high rainfall were not factored into the site assessment; instead the average annual rainfall at Needles was used to calculate water flow through the "unsaturated zone" (layers of soil and sediment above the water table). Annual precipitation at Ward Valley is almost certainly greater than at Needles: the town is further from the Pacific and lies in a potential rainshadow of the Sacramento Mountain.Further, the rate of recharge of ground water in arid areas depends on the size of individual storms, not on the annual average.

Another important factor at the site is the flow of surface water to Ward Valley from drainages in nearby Lanfair Valley to the northwest. A low divide separates the south-flowing Ward Valley drainage from Sacramento Wash, which discharges surface water from Lanfair Valley eastward into the Colorado River valley. Examination of aerial photographs shows sediment from Lanfair Valley extending in plumes south of the present divide. To change the course of some Lanfair Valley drainages from Sacramento Wash to Ward Valley would take nothing more than a shovel and a few hours of diligent digging. Lanfair Valley drainages now going into Sacramento Wash could flow toward Ward Valley in a period of high rainfall. This would increase the catchment area used to calculate flood risk by a factor of about 25; a wider area catches more water.

More ominously, subsurface connections may exist between Ward Valley ground water and the Colorado River. Fractured and tilted upper plate rocks and detachment faults may lie at shallow depth beneath Ward Valley. Such rocks are exposed between Ward Valley and the Colorado River, a strong indication that they lie close to the surface in the Valley itself. If they do, contaminants in Ward Valleys ground water could flow into the Colorado River. Whether a groundwater connection to the Colorado River constitutes a significant risk depends on the actual (and highly disputed) composition of the waste intended for Ward Valley.

            

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