Yucca Flat Research Articles

Mesozoic deformation in the Nevada Test Site and vicinity: Implications for the structural framework of the Cordilleran Fold and thrust belt and tertiary extension north of Las Vegas Valley

Detailed studies in the CP Hills and Mine Mountain area of the Nevada Test Site (NTS), together with analysis of published maps and cross sections and a reconnaissance of regional structural relations, indicate that the CP thrust of Barnes and Poole [1968] actually comprises two separate, oppositely verging Mesozoic thrust systems: (1) the west vergent CP thrust, which is well exposed in the CP Hills and at Mine Mountain, and (2) the east vergent Belted Range thrust located northwest of Yucca Flat. Regional structural relations indicate that the CP thrust forms part of a narrow sigmoidal belt of west vergent folding and thrusting traceable for over 180 km along strike. The Belted Range thrust represents earlier Mesozoic deformation that was probably related to the Last Chance thrust system in southeastern California, as suggested by earlier workers. A reconstruction of the pre‐Tertiary geometry of the Cordilleran fold and thrust belt in the region between the NTS and the Las Vegas Range bears a close resemblance to other regions of the Cordillera and suggests that west vergent deformation developed in the hinterland of a part of the Sevier fold and thrust belt characterized by substantial structural relief. Reconstruction of the fold and thrust belt also suggests that previous estimates of upper crustal Tertiary extension north of the Las Vegas Valley shear zone (e.g., 80% [Guth, 1981]) are 2 or 3 times too large.

A comparison of regional phases from underground nuclear explosions at east Kazakh and Nevada test sites

Abstract Analysis of regional data from East Kazakh, USSR, and Nevada Test Site (NTS) underground nuclear explosions suggests possible mechanisms for the generation of both low- and high-frequency Lg . Spectral characteristics of Pn and Lg phases from 25 Soviet nuclear shots recorded at the Chinese Digital Seismic Network (CDSN) station WMQ are analyzed for their dependence on parameters such as m b (generally related to shot depth) and spatial location (Shagan versus Degelen test sites). The observed results are compared with those from the Nevada Test Site (NTS), where the near-source conditions are better known. The average amplitude ratio Pn / Lg is found to be stable with m b but to vary strongly with frequency. For both Shagan and Yucca Flat explosions of similar yield, the reduction in amplitude over the frequency range of about 1 to 5 Hz is considerably larger for Lg than for Pn . The low-frequency Lg appears to be generated by the scattering of explosion-generated Rg into S , especially for the Shagan River explosions for which good agreement between observation and theory is observed. This mechanism also provides a possible explanation for the stability of Lg as an estimator of explosive yields because the scattering should be expected to occur over a large region of the shallow crust. Another important result, supported by theory, is that the higher-frequency Lg is strongly dependent on the source medium velocity. At higher frequencies (3 to 7 Hz), the amplitude ratios Pn / Lg for explosions from Shagan, Degelen, Pahute Mesa, and Yucca Flat test sites show significant differences that appear related to differences in their source medium velocities. The observed large differences in the relative spectral content of Pn and Lg from East Kazakh and NTS explosions are mainly due to the vast differences in the crustal structure of the two test sites.

Paleomagnetic data bearing on the origin of arcuate structures in the French Peak-Massachusetts Mountain area of southern Nevada

The eastern margin of the Yucca Flat basin, in southern Nevada, is bounded by north-northwest-striking tilted fault blocks of the Halfpint Range whose strikes curve as much as 90° clockwise into east-northeast strikes in the French Peak-Massachusetts Mountain (FPMM) area. This pattern of arcuate structures has been attributed to clockwise drag along a postulated northwest-trending, right-lateral shear zone. The flexure model implies that rocks within the FPMM area were rotated strongly clockwise about a vertical axis. Directions of remanent magnetization of the middle Miocene Ammonia Tanks and Rainier Mesa Members of the Timber Mountain Tuff and of the Topopah Spring Member of the Paintbrush Tuff indicate no systematic vertical-axis rotation in the FPMM area and disprove the flexure model. After tilt correction, declinations of 29 site means obtained from the three ash-flow sheets in the FPMM area are not systematically different than declinations of 17 site means from the Halfpint Range or declinations of 16 site means from little-deformed mesa areas to the west. The paleomagnetic data thus indicate that structures in the FPMM area initiated with arcuate trends and were not originally straight elements that were rotated by right-lateral drag. The structures probably formed under the influence of spatially variable stress fields. The FPMM area lies in an accommodation zone between domains of oppositely tilted extensional fault blocks. Interaction between stress fields associated with propagating normal-fault zones may have been responsible for the arcuate structures in the FPMM area.

Teleseismic P wave attenuation and nuclear explosion source functions inferred from Yellowknife Array data

We report here the results of a comprehensive seismic attenuation investigation along the paths connecting Canada's Yellowknife seismic array (YKA) with seven active nuclear explosion testing areas. The data consist of more than 600 explosion‐generated teleseismic P wave records. A dual time‐frequency averaging technique is used to take advantage of the array recording characteristics without the drawback of the conventional beam‐forming, excessive annihilation of high‐frequency signal energies. The dual averaging technique, deployed in conjunction with a multiwindow spectral analysis method, yields smooth amplitude spectra whose falloff at high frequencies suffers little from spectral leakage due to the familiar presence of a prominent low‐frequency plateau. Measured in terms of t*, the highest attenuation (0.66 s) is found along the path which originates from the Tuamotu test area; somewhat less attenuating are the two paths which depart from the Pahute Mesa (0.59 s) and Yucca Flat (0.50 s) nuclear test areas, both located within the U.S. Nevada Test Site. We find t* for these three paths to be substantially (up to 0.21 s) higher than recently published estimates (e.g., Der et al., 1985). We attribute these disparities largely to differences in spectral leakage control capability between the conventional single window and the improved multiwindow spectral analysis methods. The least attenuating paths all originate from the Soviet test areas: Novaya Zemlya (NZ), west Kazakhstan, Degelen Mountain (DM), and Shagan River (SR). The last two of these test areas, DM and SR, are both located in east Kazakhstan. The P wave signatures of the Soviet explosions are rich in high‐frequency (>4.5 Hz) energies, and the YKA data (0.5–8.0 Hz) support a frequency‐dependent t* whose value at high frequencies (>4.5 Hz) is as small as 0.17 s. To gain a grasp of the ramifications of the t* disparity between the multiple‐window and the single‐window results, we have compared explosion source time functions obtained by the multichannel deconvolution technique of Shumway and Der (1985) in order to assess their sensitivity to the input t* value. In our example involving the deconvolved source functions of five French Tuamotu explosions, we find that a 0.1‐s t* difference is large enough to cause clearly discernible signature differences, in terms of the signal frequency content as well as the extractability of a secondary arrival some 0.4 s behind the first P arrival. This secondary arrival is believed to be the depth phase pP, a seismic signature of importance in both yield estimation and earthquake/explosion source discrimination. The absorption band modeling (Minster, 1978a, b) of the French Tuamotu explosion data yields 1.08±0.05 and 0.079±0.008 s for t*0 and τm, respectively. The corresponding parameter estimates derived from the U.S. explosion data are somewhat smaller. The t*0 and τm estimates are the smallest along the paths which depart from the four Soviet test areas. For the NZ‐YKA path the t*0 and τm estimates are 0.56±0.08 and 0.061±0.013 s, respectively. Plagued by a strong trade‐off between the two model parameters, these estimates are not tightly constrained, however.

F-K analysis of NORESS array and single station data to identify sources of near-receiver and near-source scattering

Abstract F-k analyses of short-period recordings of both explosion and earthquake sources at the high-frequency NORESS array indicate secondary arrivals from a near-receiver source about 25 to 30 km southwest of the array. Use of residual seismograms, derived by subtracting the beamed record from each array channel, improved the identification of the same scatterer. F-k power difference plots, obtained by subtracting (after normalization) the f-k power versus slowness estimates for the initial P window from those for the latter windows, provided nearly identical results. The secondary seismic source appears to be short-period surface waves, Rg, generated by the scattering of incident P waves in the region of Lake Mjosa, 27 km southwest of NORESS, where there is about 1 km of relief from the bottom of the lake to the top of an adjacent hill. Polarization analysis of an explosion recorded on three-component elements of NORESS also suggested a secondary phase with polarization characteristic of Rg and arrival time and azimuth consistent with those derived from the f-k analysis. Near-source scattering is investigated by f-k analysis of data from reciprocal arrays formed by interchanging the roles of source and receiver. Application to closely spaced Yucca Flat (NTS) shots recorded at several stations provides evidence for Rg → P scattering near the northwestern and other basin boundaries of the Yucca Valley. It seems that deterministic locations of near-receiver P → Rg and near-source Rg → P scattering can be obtained by f-k techniques.

Proposed study of the Basin and Range from Death Valley to Yucca Flat

On March 1 and 2, 1990, Sandia National Laboratories, Albuquerque, N. Mex., organized a meeting of Earth scientists in Denver, Colo. This meeting set out to develop a research program that would embrace common goals of the Continental Scientific Drilling Program (CSDP) and the Yucca Mountain Project (YMP). The motivation for this meeting came from many directions, including Congress, which identified the solution of waste‐isolation problems as a potential contribution of the CSDP. Another impetus is that Yucca Mountain itself lies in a region of dramatic crustal extension and melting, which are crustal processes of great scientific interest to the CSDP program.

Yield estimation using regional mb(Pn)

Abstract Seismic yield estimation at regional distances will become increasingly important in monitoring a Low-Yield Threshold Test Ban (LYTTB). In order to investigate seismic yield estimation using regional data, we have examined regional mb(Pn) values for a large population of Nevada Test Site (NTS) underground nuclear explosions that occurred in widely varying geologic media with a range of yields from less than 1 kt to 300 kt. Magnitude-yield relationships were developed using data from a set of pre-1985 events (calibration set). To obtain an independent estimate of the uncertainty in estimating yields of events at NTS using mb(Pn) alone, the calibrated relationships were applied to a set of post-1984 events (prediction set). We found that the average uncertainty in the yield estimate, based on a weighted combination of the yield estimates from the four Livermore NTS Network (LNN) seismic stations, is about 1.8 (at the 2σ level). If data are available from all four LNN stations, the uncertainty is about 1.7 (at the 2σ level). By including a term for the gas-filled porosity of the rock near the working point in the magnitude-yield relationship, we were able to develop a single relationship valid for events both above and below the water table, as well as those in alluvium and tuff. We have found, however, systematic differences in the relationships for events in Yucca Flat, Pahute Mesa, and Rainier Mesa.

Effects of lateral velocity heterogeneity under the Nevada Test Site on short-periodP wave amplitudes and travel times

Short-period teleseismicP waves from the Nevada Test Site (NTS) show systematic variations in amplitudes and travel times, with low amplitudes corresponding to fast travel times, suggesting elastic focussing-defocussing effects. Also, the azimuthal amplitude and travel time patterns for events at the Pahute Mesa subsite are systematically different from those at the Yucca Flat subsite, indicating the presence of a near-source component in both the amplitude and travel-time variations. This component is isolated by removing the mean station pattern for all of NTS from the observations. A very-near-source component in the Pahute Mesa observations is also isolated by removing subsite station means from the measurements, whereas the Yucca Flat observations exhibited no coherent very-near-source component. These anomalies are back-projected through laterally homogeneous structure to form thin lens models at various depths. Travel-time delays are predicted from the amplitude variations using the equation for wavefront curvature. The long-wavelength components of the predicted and observed time delays correlate well, at depths of 25 km for the very-near-source component under Pahute Mesa and 160 km for the regional component under NTS. The time delay surfaces predicted by the amplitudes at these depths are mapped into warped velocity discontinuities suitable for the calculation of synthetic seismograms using the Kirchhoff-Helmholtz integral formulation. Both the intersite (near-source) and intrasite (very-near-source) differences in amplitudes are qualitatively predicted very well, although the range of variation is somewhat underpredicted. This deficiency is likely due to the destructive interference of anomalies inherent in back-projection to a single layer.

Inversion of P coda for isotropic scatterers at the Yucca Flat test site

Although many studies have addressed the statistical properties of P coda, there have been few attempts to analyze coda in a deterministic framework. In this analysis, 1050 teleseismic P-wave seismograms from 32 Yucca Flat explosions are inverted for the locations of any isotropic point scatterers in the source region that contribute coherent arrivals to the coda. A rectangular grid of potential scattering locations is constructed around the source array and the semiblance (a measure of waveform ensemble coherence) for each point is computed by slant-stacking the full set of seismograms at various move-out velocities. The potential resolution , or sensitivity, of the method for this data set is tested by constructing a corresponding suite of synthetic seismograms with direct arrivals plus arrivals from various point scatters and calculating the semiblance at each point on the grid. The particular source-station geometry produces better sensitivity near the array. Sensitivity is enhanced parallel to the trend of the elongated source array and diminished in the orthogonal direction. Stacking the data for a velocity of 2.5 km/sec yields relatively high semiblance values in the central portion of the test site as well as about 10 km to the ENE, and moderately high semiblance valuesmore » about 10 km to the WNW.« less

Network-averaged teleseismicP-wave spectra for underground explosions. Part I. Definitions and examples

AbstractA new procedure is described for estimating network-averaged P-wave spectra from short-period, teleseismic recordings of explosions. This procedure is then applied to a large sample of approximately 1000 digitized Nevada Test Site (NTS) explosion seismograms to simultaneously derive estimates of frequency-dependent station correction factors and network-averaged P-wave spectra for each of the 50 selected explosions. An analysis of the station correction factors is then presented which demonstrates that the Veith-Clawson (1972) B(Δ) curve for time domain mb adequately describes the teleseismic distance dependence of the spectral amplitude data over the entire short-period band extending from 0.50 to 2.25 Hz. A simplified scaling analysis of the network-averaged spectra is described and used to confirm the fact that the P-wave source coupling characteristics below the water table at the Yucca Flat and Pahute Mesa testing areas of NTS are essentially identical. We conclude that this new spectral measure can provide robust means for comparing different explosion testing areas.

Body-wave magnitude variation at Yucca Flat, Nevada

Abstract Teleseismic body-wave magnitudes due to nuclear tests at Yucca Flat, Nevada, display a fluctuation of approximately ±0.2 m b units. The magnitude anomaly correlates well with geologic structure at this test site, and it is hypothesized that a seismic response model of the local structure could account for most of the variance in the magnitude observations. A detailed model of the geophysical structure along a profile near the center of Yucca Flat has been constructed from independent geologic and geophysical data. The seismic response of this model has been calculated for a global network of six observing stations. The theoretical m b anomaly compares favorably with the observed anomaly. Corrections to the observed magnitudes based on the model would result in an order of magnitude decrease in the variance of the observations.

Observations of teleseismicP wave coda for underground explosions

The earlyP wave coda (5–15 sec after the first arrival) of underground explosions at the Nevada Test Site is studied in the time domain using 2082 teleseismic short-period recordings, with the intent of identifying near-source contributions to the signals in the frequency range 0.2–2.0 Hz. Smaller magnitude events tend to have relatively high coda levels in the 0.4–0.8 Hz frequency band for both Yucca Flat and Pahute Mesa explosions. Coda complexity in this low-frequency passband is negatively correlated with burial depth for Pahute Mesa events but is only weakly correlated with depth for Yucca Flat events. Enhanced excitation of relatively long-period scattered waves for smaller, less deeply buried events is required to explain this behavior. Coda complexity in the 0.8–1.1 Hz band is positively correlated with magnitude and depth for Pahute Mesa events, but has no such dependence for Yucca Flat events. This may result from systematic variations between the spectra of direct signals and coda arrivals caused bypP interference for the largest events, all of which were detonated at Pahute Mesa. Another possible explanation is a frequency-dependent propagation effect on the direct signals of the larger events, most of which were located in the center of the mesa overlying strong lateral velocity gradients in the crust and upper mantle. Event average complexity varies spatially for both test sites, particularly in the 0.8–1.1 Hz band, providing evidence for frequency-dependent focussing or scattering by near-source structure. Both the direct arrivals and the early coda have strong azimuthal amplitude patterns that are produced by defocussing by mantle heterogeneity. The direct arrivals have stronger coherent azimuthal patterns than the early coda for Pahute Mesa events, indicating more pronounced deep crustal and shallow mantle defocussing for the direct signals. However, for Yucca Flat events the direct arrivals have less coherent azimuthal patterns than the coda, suggesting that a highly variable component of near-source scattering preferentially affecting the downgoing energy is superimposed on a pattern produced by mantle heterogeneity that affects the entire signal. This complicated behavior of the direct arrivals may be the result of triplications and caustics produced by the complex basement structure known to underlie the Yucca Flat test site. The presence of strong azimuthal patterns in the early coda indicates that source strength estimates based on early coda are subject to biases similar to those affecting estimates based on direct arrivals.

Numerical-analytical interfacing in two dimensions with applications to modeling NTS seismograms

A new method for interfacing numerical and integral techniques allows greater flexibility in seismic modeling. Specifically, numerical calculations in laterally varying structure are interfaced with analytic methods that enable propagation to great distances. Such modeling is important for studying situations containing localized complex regions not easily handled by analytic means. The calculations involved are entirely two-dimensional, but the use of an appropriate source in combination with a filter applied to the resulting seismograms produces synthetic seismograms which are point-source responses in three dimensions. The integral technique is called two-dimensional Kirchhoff because its form is similar to the classical three-dimensional Kirchhoff. Data from Yucca Flat at the Nevada Test Site are modeled as a demonstration of the usefulness of the new method. In this application, both local and teleseismic records are modeled simultaneously from the same model with the same finite-difference run. This application indicates the importance of locally scattered Rayleigh waves in the production of teleseismic body-wave complexity and coda.

Models of the Bouguer gravity and geologic structure at Yucca Flat, Nevada

The Bouguer gravity anomaly at Yucca Flat, Nevada, has been modeled by two different techniques: the Cordell‐Henderson and Parker‐Oldenburg methods. The three‐dimensional model has incorporated known density and structural information where possible. These models predict the structural relief on the Cenozoic‐Paleozoic contact to within 150 m or about 15 percent of the actual depth. The three‐dimensional Parker‐Oldenburg method has been found to be efficient in an application involving a large (9000 sample) data base. Numerical stability was ensured by the application of a consistent regularization (a low‐pass filter tuned to suppress the noise‐dominated portion of the data spectrum) of the downward continuation operator. The use of a single regularizing filter for the entire model is not completely satisfactory due to the oversmoothing of shallow regions of the basin. The model is useful in the delineation of the geologic history of the area. Structural features in the model support the hypothesis that regional stress fields rotated significantly during the Tertiary. Major structural elements of the basin are well defined on the Cenozoic‐Paleozoic interface. The principal basin‐bounding fault is the large‐throw Carpetbag fault on the west. This fault was most active during the earliest phases of subsidence. The Yucca fault is seen to be a much smaller feature in the model presented here. The basin is rotated down to the west, with normal hinge faults on the eastern margin.

Effects of local geologic structure on Yucca Flats, Nevada Test Site, explosion waveforms: Two-dimensional linear finite-difference simulations

Abstract Two-dimensional linear elastic finite-difference calculations were performed for a two-dimensional geologic model of Yucca Flats, Nevada Test Site, Nevada. The calculations were used to produce synthetic teleseismic P -wave seismograms for explosive line sources in Yucca Flats. P -wave coda (in the first 5 sec) is observed to be highly dependent on take-off angle for the teleseismic synthetics. P -wave coda also varies with the position of the source in the valley structure and may produce variations in the individual station teleseismic P -wave m b magnitude of up to 0.3 magnitude units. However, these magnitude variations should be substantially reduced by averaging over stations at multiple azimuths. The reverberant coda appears to rise from scattered modal waves that are initially excited in the low-velocity near-surface structures of the Yucca Flats deposits of alluvium and tuff. Scattering of the waves occurs at offsets in the basement structure and at the sides of the valley. The combined effects of scattering, source function, intrinsic attenuation, and instrument response serve to obscure the P+pP spectral scalloping that is expected from a linear model. This loss of spectral resolution is the product of P -coda filling in the P+pP interference notches and the lengthening of the initial P -wave source time function by a convolution of the source time function, intrinsic attenuation operator, and instrument response. Therefore, short time windows (1 to 2 sec) that do not include P -coda energy do not have sufficient resolution to reliably detect the P+pP interference notches.

A study of P waves from Nevada Test Site explosions: Near-source information from teleseismic observations?

Spectral analyses of short-period P arrivals on teleseismic recordings of nuclear explosions at Yucca Flats and Pahute Mesa regions of the Nevada Test Site seem to provide explosion source medium information. The spectral ratio P/P coda on short-period vertical component records at NORSAR shows significant correlation with explosion medium velocity. For a given yield, spectrum of the initial P waves from an explosion within a high-velocity medium is richer in high frequencies as compared to that for P waves from an explosion within a low-velocity medium. The spectra of P codas, on the other hand, are considerably less sensitive to the near-source velocity differences from one shot to another so that the spectral ratio P/P coda is diagnostic of the shot medium velocity. Relative insensitivity of the P -coda spectra and amplitudes to the overburden velocity is perhaps due to several factors, including an effective source function controlled by average properties of rock within a fairly large nonlinear region around the shot point. The study provides valuable insight into effects of the shot medium on the spectra and amplitudes of compressional arrivals and offers the exciting possibility of extracting near-source information from teleseismic observations.

Variation of physical properties of alluvium in an arid basin

We have used geologic and geophysical data from 139 drill holes in Yucca Flat, an alluvium-filled basin within the Nevada Test Site, to study the ways in which differing sources and transport processes of the alluvium are reflected in variations in physical parameters. Bulk density, grain density, pore water, and carbon dioxide are routinely measured in the subsurface alluvium for each drill hole, and porosity is calculated from the first three of these parameters. The alluvium is derived principally from three source rocks exposed along the perimeter of the basin: Plleozoic elastic and carbonate sedimentary rocks, and Tertiary volcanic tuff. The alluvium of Yucca Flat is subdivided into an older volcanic-rich basal tuffaceous alluvium and an overlying younger mixed alluvium; the latter is composed of both Paleozoic and Tertiary fragments, except near isolated source terranes, where it may be derived from only one of these rock types. The mixing of different sources where individual fans merge and coalesce at the center of the basin is reflected by progressive changes in the physical properties. Alluvium consisting of dominantly Paleozoic debritus has high density and relatively low water content and porosity, whereas alluvium composed of dominantly volcanic debris has low density and high porosity and water content. Mixing of these deposits near the center of the basin is reflected by intermediate densities. Water content and porosity are greatest at the center of the basin, where the alluvial sediments have undergone the greatest amount of distal fining and sorting.

Three‐dimensional crust and upper mantle structure at the Nevada Test Site

The three‐dimensional crust and upper mantle structure at the Nevada Test Site (NTS) is derived by combining teleseismic P wave travel time residuals with Pn source time terms. The NTS time terms and relative teleseismic residuals are calculated by treating the explosions as a network of ‘receivers’ which record ‘shots’ located at the surrounding stations. Utilization of the Pn time terms allows for better crustal resolution than is possible from teleseismic information alone. Average relative teleseismic P wave residuals show a consistent progression of positive (late arrivals) to negative residuals from east to west across the NTS. However, Pn time terms beneath Rainier Mesa are at least 0.3 and 0.5 s less than those beneath Pahute Mesa and Yucca Flat, respectively, indicating the presence of high‐velocity crustal material or crustal thinning beneath Rainier Mesa. The time terms at Pahute Mesa are surprisingly uniform, and the largest time terms and residuals are observed in the northwest and southern parts of Yucca Flat. The Pn time terms show a slight correlation with the working‐point velocity at the shot point for Pahute Mesa and Yucca Flat, indicating that part of the observed lateral variations are caused by shallow effects of the upper crust. Three‐dimensional inversion of the travel time residuals suggests that Yucca Flat is characterized by low‐velocity anomalies confined to the upper crust, Rainer Mesa by very high velocities in the upper and middle crust, and Pahute Mesa by a high‐velocity anomaly extending through the crust and into the upper mantle. Relatively low velocities are observed in the lower crust beneath the Timber Mountain caldera south of Pahute Mesa with no expression in the upper mantle. These observed differences in velocity beneath the Tertiary Silent Canyon and Timber Mountain calderas may be related to their magma volume and mode of enrichment from a mantle‐derived magma source.

Activity Diagrams for Calcium/Hydrogen, Sodium/Hydrogen, and Potassium/Hydrogen, and H4SiO4and Their Relation to Reactions in Systems Containing Radioactive Waste Forms, Cement, and Rock in the Presence of Water

ABSTRACTIn order to identify reactions which can occur in systems containing nuclear waste forms, cement, and repository rock in the presence of water, activity diagrams were calculated from free energies for aluminosilicates and calcium silicates. Log (calcium activity)/(hydrogen activity)2was plotted against log (H4SiO4activity) for reactions among calcium aluminum silicates for which free energies are available in the literature. Similar diagrams were made for sodium/hydrogen activity, and potassium/hydrogen activity. For calcium an activity diagram was constructed for the silicates.Groundwater compositions from candidate repository sites in the Palo Duro Basin of Texas, the Delaware Basin of New Mexico, and the Nevada Test Site were plotted on these diagrams. Essentially all of these are shown to be in the calcium zeolite field as shown on the diagram for calcium in the absence of other cations. When considerable Mg and Fe are present in the water, other phases may replace the calcium zeolite in the pertinent region of the diagram. Chlorite is shown to be stable in this region at the Mg and pH level of the Ogallala if the chlorite is high in iron, and at the Mg and pH level of the Wolfcamp low- or high-Fe chlorites are stable. Potassium and sodium mineralrelationships fall in two categories, dilute waters and saline waters. Of the dilute waters, that from the Ogallala aquifer in the Palo Duro Basin, and most of the Nevada Test Site waters, including that from the proposed radwaste repository horizon, are in equilibrium with Na-beidellite and kaolinite. Boreholes at Yucca Flat and Mercury Valley at the Nevada Test Site, and shallow ground water from the Rolling Plains north and east of the Palo Duro Basin are in equilibrium with kaolinite. Of the saline waters, that from the Castile (deep in the Delaware Basin) is a kaolinite water. The brines from the Salado and Rustler formations are in equilibrium with kaolinite and possibly also with sodium-potassium zeolite and illite.Leachates of cement and water, and cement, waste, and water were plotted on the calcium silicate activity diagram. These solutions are in equilibrium with calcium silicate hydrate hydrolysis reactions, with grossular and possibly with Ca-zeolites. Among the calcium silicates, calciumsilicate-hydrate gel (C-S-H gel) and tobermorite are the most likely candidates, but the thermodynamic data are not adequate to distinguish all the possibilities. The underlying assumptions are discussed.

Radioactive Waste Disposal in Thick Unsaturated Zones

Portions of the Great Basin are undergoing crustal extension and have unsaturated zones as much as 600 meters thick. These areas contain multiple natural barriers capable of isolating solidified toxic wastes from the biosphere for tens of thousands to perhaps hundreds of thousands of years. An example of the potential utilization of such arid zone environments for toxic waste isolatic is the burial of transuranic radioactive wastes at relatively shallow depths (15 to 100 meters) in Sedan Crater, Yucca Flat, Nevada. The volume of this man-made crater is several times that of the projected volume of such wastes to the year 2000. Disposal in Sedan Crater could be accomplished at a savings on the order of $0.5 billion, in comparison with current schemes for burial of such wastes in mined repositories at depths of 600 to 900 meters, and with an apparently equal likelihood of waste isolation from the biosphere.