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971.
Yucca Mountain, the proposed site for the high-level nuclear waste repository, is located just south of where the present
water table begins a sharp rise in elevation. This large hydraulic gradient is a regional feature that extends for over 100 km.
Yucca Mountain and its vicinity are underlain by faulted and fractured tuffs with hydraulic conductivities controlled by flow
through the fractures. Close to and parallel with the region of large hydraulic gradient, and surrounding the core of the
Timber Mountain Caldera, there is a 10- to 20-km-wide zone containing few faults and thus, most likely, few open fractures.
Consequently, this zone should have a relatively low hydraulic conductivity, and this inference is supported by the available
conductivity measurements in wells near the large hydraulic gradient. Also, slug injection tests indicate significantly higher
pressures for fracture opening in wells located near the large hydraulic gradient compared to the opening pressures in wells
further to the south, hence implying that lower extensional stresses prevail to the north with consequently fewer open fractures
there. Analytical and numerical modeling shows that such a boundary between media of high and low conductivity can produce
the observed, large hydraulic gradient, with the high conductivity medium having a lower elevation than the water table. Further,
as fractures can close due to tectonic activity, the conductivity of the Yucca Mountain tuffs can be reduced to a value near
that for the hydraulic barrier due to strain release by a moderate earthquake. Under these conditions, simulations show that
the elevation of the steady-state water table could rise between 150 and 250 m at the repository site. This elevation rise
is due to the projected shift in the location of the large hydraulic gradient to the south in response to a moderate earthquake,
near magnitude 6, along one of the major normal faults adjacent to Yucca Mountain. As the proposed repository would only be
200–400 m above the present water table, this predicted rise in the water table indicates a potential hazard involving water
intrusion.
Received: 7 June 1996 / Accepted: 19 November 1996 相似文献
972.
The time evolution of stratospheric aerosol layer formed after a volcanic eruption is studied taking into account the aerosol
microphysical processes of growth, coagulation and sedimentation. Using a simple model we could explain the observed evolution
of the Pinatubo volcanic layer which decayed in about 3 years. The experimental data obtained by Nd:YAG backscatter lidar
over Ahmedabad further supports this finding. The data obtained after the El Chichon volcanic eruption also showed that the
El Chichon aerosol layer decayed in about 3 years time. Thus, though the amount of SO2 injected has been higher, in the case of Pinatubo, about two to three times more than El Chichon, it has resulted in the
production of larger aerosol particles due to faster growth and coagulation processes, and subsequently a faster removal rate,
to give more or less a similar background aerosol amount at the stratosphere in about 3 years time. 相似文献
973.
974.
Electrokinetic remediation of metal contaminated glacial tills 总被引:2,自引:0,他引:2
This paper presents the results of an experimental investigation which studied the feasibility of using the electrokinetic process to remediate contaminated clays of glacial origin, otherwise known as glacial tills. An overview of the electrokinetic phenomena, as well as previously performed laboratory and field investigations, is first presented. The methodology of the electrokinetic experiments which were conducted to investigate the removal of metals from a glacial till is then described. A total of 16 experiments were conducted using glacial till samples obtained from a project site near Chicago. Sodium and calcium were used as the surrogate cationic metallic contaminants. These experiments demonstrated that ion transport during the electrokinetic process occurs due to both electro-osmosis and electromigration, but that due to electromigration is significantly higher than that due to electro-osmosis. Unlike other clays such as kaolinite, the glacial till used for this investigation possessed high buffering capacity because of its high carbonate content which prevented the acid front migration from the anode to the cathode during the electrokinetic process. The ion removal efficiency of the electrokinetic process was found to increase when: (1) the voltage gradient applied to the soil was increased, (2) the initial concentration of the contaminants was increased, and (3) the duration of the treatment process was increased. The ion removal efficiency was also greater for smaller ions which possess less ionic charge and when the ions existed independently in the soil as compared to when they coexisted. This investigation suggests that the electrokinetic process has significant potential for remediating glacial tills contaminated with metals. However, the properties of Na and Ca are not representative of contaminants, such as heavy metals, so further investigations are needed. 相似文献
975.
The 1900–1700 Ma Waterberg Group in the main Waterberg fault-bounded basin consists of dominantly coarse siliciclastic red
beds with minor volcanic rocks. The sedimentary rocks were deposited mainly by alluvial fans, fluvial braidplains and transgressive
shallow marine environments, with lesser lacustrine and aeolian settings. Uplifted, largely granitic source areas were located
along the Thabazimbi-Murchison lineament (TML) fault system in the south, and along the Palala shear zone in the northeast.
Palaeoplacer titanomagnetite-ilmenite-zircon heavy mineral deposits, best developed in the Cleremont Formation in the centre
of the basin, reflect initial fluvial reworking and subsequent littoral marine concentration. Coarse alluvial cassiterite
placer deposits are found in the Gatkop area in the southwest of the basin, and appear to have been derived from stanniferous
Bushveld Complex lithologies south of the TML. Hydrothermal zinc and U-Cu mineralisation in the Alma lithologies in the same
area appears to be related to the TML fault system. Small manganese deposits and anomalous tungsten values occur in the south
of the basin, where they are again closely spatially associated with the TML. Copper-barium mineralisation is found associated
with dolerite dykes, and in stratigraphically controlled, inferred syngenetic settings. The most interesting of these apparently
syngenetic occurrences is found within green coloured reduced mudrocks and inferred volcanic rocks, at an unconformity developed
within the overall red bed sequence of the Waterberg Group, adjacent to the TML in the southwest of the basin. The most important
potential mineralisation in the main Waterberg basin thus encompasses shoreline placer Ti and the possibility of substantial
sediment-hosted copper deposits.
Received: 31 May 1996 / Accepted: 17 February 1997 相似文献
976.
Diffusion of tetravalent cations in zircon 总被引:18,自引:1,他引:18
D. J. Cherniak J. M. Hanchar E. B. Watson 《Contributions to Mineralogy and Petrology》1997,127(4):383-390
Diffusion rates for the three tetravalent cations U, Th and Hf have been measured in synthetic zircon. Diffusant sources
included oxide powders and ground pre-synthesized silicates. Rutherford backscattering spectrometry (RBS) was used to measure
depth profiles. Over the temperature range 1400–1650 °C, the following Arrhenius relations were obtained (diffusion coefficients
in m2sec−1):
log D
Th = (1.936 ± 0.9820) + (− 792 ± 34 kJ mol−1 /2.303 RT)
log D
U = (0.212 ± 2.440) + (− 726 ± 83 kJ mol−1 /2.303 RT)
log D
Hf = (3.206 ± 1.592) + (− 812 ± 54 kJ mol−1 /2.303 RT)
The data show a systematic increase in diffusivity with decreasing ionic radius (i.e., faster diffusion rates for Hf than
for U or Th), a trend also observed in our earlier study of rare earth diffusion in zircon. Diffusive fractionation may be
a factor in the Lu-Hf system given the much slower diffusion rates of tetravalent cations when compared with the trivalent
rare earths. The very slow diffusion rates measured for these tetravalent cations suggest that they are essentially immobile
under most geologic conditions, permitting the preservation of fine-scale chemical zoning and isotopic signatures of inherited
cores.
Received: 12 July 1996 / Accepted: 2 December 1996 相似文献
977.
Effect of tunnel depth on modulus of deformation of rock mass 总被引:1,自引:2,他引:1
M. Verman B. Singh M. N. Viladkar J. L. Jethwa 《Rock Mechanics and Rock Engineering》1997,30(3):121-127
Summary Deformability of rock mass significantly influences its behaviour and is, therefore, an important consideration for the design of underground openings. The modulus of deformation of rock mass is, however, normally obtained from expensive and time-consuming uniaxial jacking tests, whose results often have a large scatter. An empirical correlation has, therefore, been proposed for a quick and inexpensive preliminary estimation of the modulus of deformation of rock mass on the basis of field instrumentation carried out in tunnels in India. 相似文献
978.
Estimation of groundwater evaporation and salt flux from Owens Lake, California, USA 总被引:3,自引:0,他引:3
S. W. Tyler S. Kranz M. B. Parlange J. Albertson G. G. Katul G. F. Cochran B. A. Lyles G. Holder 《Journal of Hydrology》1997,200(1-4):110-135
Groundwater evaporation and subsequent precipitation of soluble salts at Owens Lake in eastern California have created one of the single largest sources of airborne dust in the USA, yet the evaporation and salt flux have not been fully quantified. In this study, we compare eddy correlation, microlysimeters and solute profiling methods to determine their validity and sensitivity in playa environments. These techniques are often used to estimate evaporative losses, yet have not been critically compared at one field site to judge their relative effectiveness and accuracy. Results suggest that eddy correlation methods are the most widely applicable for the variety of conditions found on large playa lakes. Chloride profiling is shown to be highly sensitive to thermal and density-driven fluxes in the near surface and, as a result, appears to underestimate yearly groundwater evaporation. Yearly mean groundwater evaporation from the playa surface estimated from the three study areas was found to range from 88 to 104 mm year−1, whereas mean evaporation from the brine-covered areas was 872 mm year−1. Uncertainties on these mean rates were estimated to be ±25%, based on comparisons between eddy correlation and lysimeter estimates. On a yearly basis, evaporation accounts for approximately 47 × 106 m3 of water loss from the playa surface and open-water areas of the lake. Over the playa area, as much as 7.5 × 108 kg (7.5 × 105 t) of salt are annually concentrated by evaporation at or near the playa surface, much of which appears to be lost during dust storms in area. 相似文献
979.
Nils-Otto Kitterrød Lars Gottschalk 《Stochastic Environmental Research and Risk Assessment (SERRA)》1997,11(6):459-482
Simulation of multigaussian stochastic fields can be made after a Karhunen-Loéve expansion of a given covariance function.
This method is also called simulation by Empirical Orthogonal Functions. The simulations are made by drawing stochastic coefficients
from a random generator. These numbers are multiplied with eigenfunctions and eigenvalues derived from the predefined covariance
model. The number of eigenfunctions necessary to reproduce the stochastic process within a predefined variance error, turns
out to be a cardinal question. Some ordinary analytical covariance functions are used to evaluate how quickly the series of
eigenfunctions can be truncated. This analysis demonstrates extremely quick convergence to 99.5% of total variance for the
2nd order exponential (‘gaussian’) covariance function, while the opposite is true for the 1st order exponential covariance
function. Due to these convergence characteristics, the Karhunen-Loéve method is most suitable for simulating smooth fields
with ‘gaussian’ shaped covariance functions. Practical applications of Karhunen-Loéve simulations can be improved by spatial
interpolation of the eigenfunctions. In this paper, we suggest interpolation by kriging and limits for reproduction of the
predefined covariance functions are evaluated. 相似文献
980.
The separation of the influence of nutrients and climate on the varve time-series of baldeggersee, Switzerland 总被引:7,自引:2,他引:7
The varve data-set from a freeze-core taken in the deepest part of Baldeggersee was subjected to different multivariate statistical
analyses in order to estimate the amount of variance in the varve thickness measures explicable by past climate and by the
trophic state of the lake. A comparison of two different time-periods (1902 to1992 versus 1920 to 1980) revealed that the
lake restoration programme since 1982 has had a significant impact on the formation of the seasonal layers. Results of the
partitioning of the variance in the varve thickness measures showed that about two thirds of the variance are unexplained
by a climate and trophic state model and that trophic state explains 6%, whereas climate accounts for about 28% of the variance
before the effect of lake restoration had a strong impact on the varves. Among the climate parameters the amount of annual
precipitation is a strong predictor for explaining the thickness of both dark layer and total couplet thickness, whereas summer
precipitation is important for the thickness of the light layer. 相似文献