Ground Water at Yucca Mountain: How High Can It Rise? (1992)

In the ongoing national debate on nuclear power as a source of electricity, a key issue is the disposition of the high level radioactive wastes produced in the process. This waste consists primarily of spent nuclear fuel rods.

To resolve this issue, Congress designated the Department of Energy (DOE) to implement the Nuclear Waste Policy Act of 1982. DOE established the Office of Civilian Radioactive Waste Management (OCRWM) in 1983 to develop a mined geologic disposal system (MGDS) for the permanent disposal of the high level radioactive wastes from civilian nuclear power plants. To select a site for an MGDS, the DOE must study in detail the natural environment and the various natural processes to which a proposed deep geologic repository might be subject. For a site to be acceptable, these studies must demonstrate that the site could comply with regulations and guidelines established by the federal agencies to ensure the safety of the public.

Because radioactivity from spent nuclear fuel rods could most likely be released from an MGDS to the outside environment through water entering the repository and transporting the radionuclides into the ground-water system, it was considered that a repository located a considerable distance above the water table in an area with extremely low rainfall would limit that mode of release. Thus, after Yucca Mountain, in the desert of southern Nevada, had been identified as a potential site, DOE decided that if the site were found suitable, the MGDS would be located in the bedrock 300 meters above the water table.

During development of the site characterization plan (DOE, 1988), a report by a DOE staff scientist suggested that the ground water had periodically risen well above the level proposed for the MGDS onto the earth's surface in the geologically recent past and that such an event could happen again. This suggestion was based on his interpretations of field observations. The report proposed that changes in stress in the crust caused by nearby earthquakes had forced the water up to the surface, a mechanism known as “seismic pumping.” Two groups of reviewers, an internal peer review group, and an External Review Panel selected by both DOE and the staff scientist did not resolve the controversy that arose.

In response to a request from the DOE, the National Academy of Sciences ' National Research Council (NAS/NRC) established the Panel on Coupled Hydrologic/Tectonic/Hydrothermal Systems at Yucca Mountain, Nevada, under the auspices of the Board on Radioactive Waste Management, to evaluate (1) if the water table had been raised in the geologically recent past to the level of the proposed MGDS, and (2) if it is likely that it will happen in the manner described in the DOE staff scientist 's report within the 10,000-year period covered by the regulations. The report claimed that such flooding had repeatedly occurred in the past and could be expected to happen again. If that were so, and if engineered containments failed, the water could carry still-active radioactive isotopes into the biosphere, a possibility that would lead to serious questions concerning the acceptability of the site.

Evidence cited in the report of hydrothermal fluids having been driven to the surface by pressurization of ground water by earthquake or thermal processes were occurrences in the vicinity of Yucca Mountain of: (1) near-surface and sub-surface fracture fillings, or veins, composed of carbonate and silica, (2) breccias cemented by carbonate and silica, and (3) surficial, surface-parallel deposits of carbonate and silica,

To conduct the study, the Panel on Coupled Systems read the report and other pertinent literature, and interviewed or consulted with scientists involved in field and laboratory investigations of Yucca Mountain and the region for the DOE, the State of Nevada, independent scientists, and all five members of the External Review Panel. Because the thesis was based primarily on the staff scientist's interpretation of geological occurrences and relationships in the field, the panel spent several days in the field led by scientists on both sides of the controversy, visiting and studying carefully those sites that were purported to reveal evidence for upwelling ground water.

The panel regarded their task as not only evaluating the staff scientist 's thesis, but also assessing the likelihood that the ground water

level could rise to the height of the repository by any plausible geological process, or that such a rise had occurred in the past.

The field evidence evaluated to establish whether or not deep ground water had been forced up through faults and fractures and onto the earth's surface to produce the mineralized veins and surface deposits fell into six categories: (1) the character of soil development and geomorphic features; (2) hydrologic evidence from active and ancient springs; (3) morphologic and textural evidence from chemically precipitated mineral deposits; (4) the stratigraphic/textural/mineralogic character of carbonate-cemented breccias; (5) geochemical and mineralogical considerations; and (6) the isotopic composition of the ground water and mineral deposits.

The panel's overall conclusion was that none of the evidence cited as proof of ground-water upwelling in and around Yucca Mountain could be reasonably attributed to that process. The preponderance of features ascribed to ascending water clearly (1) were related to the much older (13-10 million years old (Ma)) volcanic eruptive process that produced the rocks (ash-flow tuffs) in which the features appear, (2) contained contradictions or inconsistencies that made an upwelling ground-water origin geologically impossible or unreasonable, or (3) were classic examples of arid soil characteristics recognized world-wide.

Surface-parallel layers of calcium carbonate (calcrete) are ubiquitous in the Yucca Mountain region, as is common in arid regions around the world. Thickness of the calcrete deposits within soils correlated well with the degree of development of the various geomorphic surfaces (alluvial fan, pediment, sand ramp, etc.), i.e., the older the surface, the thicker the calcrete. Calcareous root and stem casts were often seen throughout the calcrete zone, attesting to the fact that the deposits developed within the soil, not on the surface as suggested by the proponents of the ascending ground-water thesis. These features are developed typically where rain water infiltrates the soil and deposits chemicals as it evaporates and would not be expected from upwelling of ground water.

Near-vertical veins of carbonates attributed by the proponents to deposition from upwelling ground water along faults, intersect the texturally similar surface deposits locally at Trench 14 and Busted

Butte. The slope-parallel soil carbonates on Busted Butte thin gently downslope and show no interruption in that gradient where the vertical carbonate vein, the purported source of ascending mineralized ground water, intersects them. If the fault were the source of the fluid, the surface deposits should be relatively thicker on the downslope side. The lack of any relationship between the thickness of the surface-parallel carbonates and the location of the vertical carbonate veins is compelling evidence that the latter cannot be a conduit for the hypothesized upwelling source fluids.

The panel concludes that the surface parallel carbonates formed from rainwater, and from pedogenic (soil-forming) and other surface processes.

Several features of active and ancient springs cast serious doubt on interpreting them as resulting from pressurized water from below. These include: (1) the absence of tufa, or travertine mounds, characteristic of hydrothermal springs near any of the purported past sources of ground water, (2) the occurrence of several active springs of minute discharge (0.01-0.1 liter/sec) emerging at the base of fractured ridges in the region at ambient temperatures, and (3) the isotopic compositions of these springs being similar to rain water.

Comparison of features of an unequivocal carbonate spring deposit in the southern Basin and Range region, at Travertine Point near Death Valley, with features in Trench 14 (excavated across the Bow Ridge fault east of Yucca Mountain) and Busted Butte shows clearly that the Yucca Mountain area veins cannot derive from upwelling CO ₂-charged spring waters. The Travertine Point occurrence is characterized by a well-exposed tufa mound, coarse-grained calcite from 6 mm to 1 cm, mirror-image symmetrical banding that results from chemicals precipitating simultaneously on opposite walls of a waterfilled fracture, and the absence of interlayered bands of amorphous silica. In contrast, veins in Trench 14 and other exposures in and around Yucca Mountain show no mounds, are composed of extremely fine grained calcite (less than 6 microns, or one thousandth the size of the Travertine Point calcites), have no symmetry, contain thin interlayered bands of low-temperature amorphous silica, and include soil material such as sand, clay, and volcanic ash. These features reflect low temperature, descending and evaporating water condi-

tions, and result from surficial processes, not upwelling hydrothermal water.

The widespread occurrences of breccias cemented by carbonate or carbonate/silica have been cited as evidence of explosive release of highly pressurized ascending fluids by proponents of this thesis. The characteristics of the breccias show unequivocally that the breccia origins range from formation related to the Tertiary high temperature volcanic processes 13-10 Ma to low temperature mechanical erosion processes which produced the talus at the base of weathering slopes in the geologically recent past and continuing to the present. These do not constitute evidence for any one process at any one time. All of the breccia types can be best explained by processes that do not involve ascension of deep fluids.

The panel's independent examination of the available stable and radiogenic isotope data show that vein carbonates in the subsurface above the present water table are generally isotopically consistent with the surface-parallel deposits. Moreover, vein carbonates from drill cores below the water table are isotopically consistent with ground water isotopic contents. Trench 14 and Busted Butte vein carbonates have isotopic contents within the range characteristic of soil carbonates in the region, showing the veins formed from rainwater and soil-forming processes.

The panel concludes from the geologic features observed in the field and geochemical data that there is no evidence to support the assertion that the water table has risen periodically hundreds of meters from deep within the crust. In fact, the evidence strongly supports a surface-process origin from rainwater for the vein and surface parallel carbonate and carbonate-silica deposits throughout the Yucca Mountain area.

Although available geological and geochemical evidence indicates that the water table has not risen to the proposed repository level in

the last 100,000 years, the possibility that it may do so in the future must be assessed. In considering the various mechanisms that could conceivably cause a significant rise in the water table in the Yucca Mountain area, the panel identified three possible processes that could result in a hydrologic response. The question was to determine how much of a response each of the mechanisms could generate. The three possible mechanisms were (1) an increase in rainfall, (2) a volcanic intrusion, and (3) an earthquake. In addition, the panel recognized that a steep hydraulic gradient less than 2 km north of Yucca Mountain has the potential to affect the water table of Yucca Mountain.

An evaluation of past climate changes led the panel to conclude that, even during the maximum glacial extent, the region did not experience more than a 40 percent increase in precipitation over the present approximately 158 mm (6 inches) per year. To evaluate how an increase in rainfall will affect the water table it is necessary to estimate the amount of that rainfall that will recharge the ground-water system. This is difficult to do, especially in arid regions. A model developed by scientists associated with DOE assumed a 100 percent increase in precipitation and a 15-fold increase in recharge. The panel considers these assumptions to be overly conservative. The model calculated a 140 meter rise in the water table under those assumed conditions.

The panel did not model this problem. However, until the DOE model assumptions are tested or better constrained by more complete hydrologic data, and the techniques for estimating recharge in the region better developed, the panel considers an increase in precipitation due to a climate change a possible means of raising the water table significantly.

In considering the long and complex volcanic history of the region, the possibility of a recurrence of the highly explosive volcanism of the Tertiary was dismissed because the subduction zone origin of the activity has been replaced by extensional tectonics that has resulted in the basaltic volcanism of more recent geologic time. The progressive decline in volume of these eruptions convinces the panel that the only likely volcanic intrusion in the region during the lifetime of a proposed repository is a low-volume basaltic dike. Theo-

retical considerations of elastic extensional strains suggest a near-vertical dike of 2-4 m width. Models of water table response to a dike intrusion resulted in rises of 10-15 meters, far below that necessary to pose a threat to a repository 300 meters above the present water table. Calculating the probabilities of occurrence of a dike intrusion close to Yucca Mountain results in a very small number, 10⁻⁸ per year. Although there may be considerable uncertainty in the probability values, the panel considers that the small effect a basaltic dike intrusion would have on the water table and the low probability of a dike forming close to Yucca Mountain mean that volcanic intrusions can be discounted as potentially disruptive events with respect to water table stability.

The panel considered the experience of historic earthquakes and the results of different types of modeling of earthquake responses. Historically, the severest earthquakes in North America for which there is hydrologic information indicate that water table changes are on the order of tens of meters. The nature of the response differs with different types of faulting movements involved, and the reasons for the responses are not yet well understood.

Various types of modeling, using credible assumptions about the elastic properties of the earth and the aquifers, also indicate a response of the water table to be of the same magnitude as those of the historic earthquakes, that is, a rise of tens of meters. This indicates that the seismic pumping mechanism is inadequate to raise the water table significantly.

In addition, the probabilities of occurrence of earthquakes of significant size near enough to Yucca Mountain to affect the water table, such as up to a few kilometers away, are quite small unless a wider area, several tens of kilometers, is considered.

The panel concludes, given the experience from historic earthquakes, the small modeled response of the water table to earthquakes consistent with the historic experience, the low strain rates and low seismicity both in magnitude and frequency of occurrence of the Yucca Mountain area, that significant water table excursions to the design level of the repository are unlikely.

It is important to keep in mind, however, that all the foregoing modelling results and probability estimates involve very large uncertainties because of the limits of the information presently available. The panel, therefore, supports continued site characterization ef-

forts to obtain the critical information necessary for more definitive assessments of the future behavior of the natural systems in the Yucca Mountain region.

It must be emphasized here, however, that these conclusions should not be interpreted as the panel's evaluation of Yucca Mountain as a repository site. The panel was not asked to, and did not, take a position on the suitability of Yucca Mountain for the MGDS. There is more work to be done, however, as there are few data to constrain the complex hydrologic system acting in the vicinity of the proposed repository. The panel supports initiation of the studies needed to develop the necessary information with which to evaluate the site.

Foremost of the goals of the studies to characterize the site is understanding the local hydrologic system and particularly the nature and source of the steep east-west hydrologic gradient north of the proposed repository site. This gradient is the expression of a rapid 300 m decline in the elevation of the water table starting just north of Yucca Mountain, which results in the local unsaturated condition of the proposed repository level. Too little is known about the characteristics of the geohydrologic system in general to explain the cause of this gradient. The panel recommends a comprehensive program of drilling, scientific testing and logging and core and fluid analysis in and close to this gradient.

Results of regional elastic strain modeling to determine the water table response to earthquakes indicated that information on the deep carbonate aquifer is essential to more realistic modelling. The elastic and hydrologic properties of this important feature of the Yucca Mountain area are essential to predictions of water table behavior in the event of a significant earthquake.

The panel recommends that several deep holes be drilled well into this aquifer to obtain the relevant information to determine its extent, its properties, and its communication with the Tertiary aquifer. These have been identified as essential to predictions of water table behavior.

Throughout the text, the panel has recommended studies to enhance the depth and breadth of the information essential to characterization of the area to increase confidence in the knowledge and parameters necessary to understand the complex interactive systems, hydrological, tectonic, and hydrothermal. However, during the course of the panel 's study, it became clear that there was a significant lack of communication among project scientists in different disciplines, especially between those of the hydrological and solid earth sciences, and among the different scientific organizations involved in the study, such as governmental agencies and national laboratories. Even among the geologists and geophysicists there seemed to be little integration of their individual spheres of knowledge and data. Because this important site characterization program is large and complex, strong scientific leadership must be provided to the participants and adequate attention must be paid to the continuing coordination and syntheses of scientific results. No large scale multidisciplinary study of this type known to the panel has been undertaken without a strong scientific leader coordinating and integrating the on-going efforts of the various parts of the project.

To that end, the panel strongly recommends that DOE appoint a scientist as site characterization science coordinator. Such a person should not be currently associated with any of the participating organizations. That scientist should have a reputation for independence and excellence, as well as the experience in managing and integrating interdisciplinary programs, and would therefore lend further credibility to these investigations and their results.