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

The late Tertiary geologic history of southwestern Nevada has been dominated by volcanism and the consequent deposition of volcanic flows and tuffaceous rocks. Yucca Mountain, like most surrounding ranges, is composed dominantly of a series of well-studied, Miocene ashflow tuff units and silicic volcanic rocks. The tuffs erupted between 17 and 7 Ma from multiple caldera complexes located throughout the region (Christiansen et al., 1977; Byers et al., 1976). The uppermost unit of these widespread eruptive tuff deposits exposed on Yucca Mountain is the Tiva Canyon member of the Timber Mountain caldera complex, dated at about 12.5 Ma.

Concurrent with the silicic episode, large-volume olivine basalt lava flows erupted between 11 and 8 Ma near and within some of the silicic centers in the Yucca Mountain region (Crowe et al., 1983a and 1986). In roughly the same time period (6-10 Ma), other basalt and basaltic andesite flows erupted that appear unrelated to the silicic centers (the “older rift basalts” of Crowe et al. (1986)). These basaltic flows probably mark the beginning of “modern” basin and range extension in this region of southern Nevada. Modern basin and range extension refers to the stress and faulting regime, still active today, which is responsible for the formation of the modern range and basin blocks. It is generally dated between 10 and 7 Ma throughout the northern Basin and Range region (see, e.g., Stewart, 1978; Zoback and Thompson, 1978; Zoback et al., 1981).

The next and last-recorded volcanism in the Yucca Mountain region involved flows and cinder cones of young basalt that erupted in Crater Flat, which borders Yucca Mountain on the west, between 3.7 and 0.13 Ma (the “younger rift basalts” of Crowe et al. (1986)). The alignment of cones suggests they were fed from dikes as is commonly the case elsewhere (see, e.g., Nakamura, 1977). These younger basalts interfinger with and are overlain by unconsolidated to semi-consolidated clastic (fragmental) material deposited in a variety of environments in response to extensional normal faulting.

We concur with the conclusion reached by previous researchers (Crowe et al., 1983b; DOE, 1988) that the possibility of future large silicic caldera-forming eruptions can be dismissed from consideration in the Yucca Mountain area. The Miocene caldera complexes formed because of their location in a continental intra-arc setting close to, and overlying, an active east-directed subduction zone along the western continental margin. The subsequent evolution of the San Andreas transform fault system (see, e.g., Atwater, 1970) replaced the subduction zone along much of the western margin and changed the tectonic regime in Nevada. The current southernmost boundary of active subduction below western North America is the Mendocino triple junction at approximately 40°N, more than 300 km north of the latitude of Yucca Mountain. The present day volcano-tectonic regime throughout the northern Basin and Range province, which includes most of Nevada and the western half of Utah, is extensional, characterized by low-volume basaltic eruptions, such as those in Crater Flat (see, e.g., Christiansen and Lipman, 1972).

The age of the basaltic volcanism in Crater Flat remains the subject of considerable debate. Isotopic ages between 3.8 and 0.3 Ma have been obtained from the cinder cone centers in Crater Flat by Turrin and Champion, (1991). However, Wells et al. (1990) have argued that a nearby basaltic center, the Lathrop Wells cone located about 20 km south of Yucca Mountain, may be as young as 20 ka based on geomorphic and pedogenic characteristics as well as on the scatter of isotopic ages. Wells et al. (1990) further suggest that this center contains at least three discrete and temporally separate eruptive events that may have occurred over time spans of 1–10 ka, based on mapping of stratigraphic relations of tephra (volcanic debris) units here and elsewhere in the Basin and Range (Crowe et al., 1989). In contrast, ⁴⁰Ar/³⁹Ar age dating of two separate flow units in the Lath-

rop Wells volcanic center yields arithmetic mean of ages of 183 ±21 and 144 ±35 ka (Turrin et al., 1991). On the basis of this dating and as yet unpublished K/Ar dates, Turrin et al. (1991) conclude that there were two eruptive events at Lathrop Wells, dated at 136 ±8 ka and 141 ±9 ka. They speculate that the time interval between flows may be less than 100 years because field mapping and paleomagnetic data indicate remanent magnetization directions only a few degrees apart for the two flow units. Differences in remanent magnetization directions can be accounted for by secular (temporal) variation of the earth 's magnetic field, the rates of which have been calibrated in other volcanic fields at approximately 4° per 100 years. Thus, they interpreted the nearly identical remanent directions in the two Lathrop Wells flow units to imply a short duration (<100 years) of eruptive activity. However, this interpretation of the paleomagnetic data is controversial. Because both remanent directions are very similar to the time-averaged geomagnetic field in the study area, these directions could represent equally well eruptions separated by 100 years, 10 ka, 100 ka, or 1 Ma.

The geologic record thus indicates that the only likely style of intrusion in the Yucca Mountain region in the lifetime of the repository is a low-volume basaltic dike intrusion. To place constraints on the size and geometry of such intrusions we can look at the occurrence and distribution of other Quaternary basaltic dikes, flows, and cinder cones throughout the northern Basin and Range province. Most of this young basaltic activity occurs within the modern basins, with many of the eruptive centers occurring along basin margins, particularly localized near range-bounding faults (Smith et al., 1990). In virtually all cases, including the basaltic centers in Crater Flat, the alignment of cinder cones, fissures, and other eruptive centers trends N to NNE. This is the direction of the regional maximum horizontal principal stress throughout the Quaternary, as established by earthquake focal mechanisms and fault striation studies (Zoback, 1989). The N to NNE trend thus appears to coincide with a principal stress plane, as expected theoretically (Nakamura, 1977; Nakamura et al., 1978). There is little or no shear stress across these planes. Models in which basaltic volcanism is localized along (or controlled by) major crustal shear zones have little or no corroborating analogs, either theoretically or in field examples.

Some basalt intrusion has occurred within what are now modern range blocks. A dike exposed along the Solitario Canyon normal fault where it cuts into the Yucca Mountain range block NW of the proposed respository site has been dated at 10 Ma (C. Fridrich, pers. comm., 1991). This dike is probably part of the the “older rift basalt”

sequence that erupted in the early phases of modern basin and range extension. Models that fit ground magnetic highs located further to the south adjacent to Solitario Canyon include a vertical zone of increased magnetization which could be interpreted as a basaltic dike (Bath and Jahren, 1985); however, there is no surface expression of such a dike in this region. Bath and Jahren (1985) did report a personal communication from R. B. Scott, USGS, that weathered basalt fragments had been found at a lower elevation and about 0.8 km south of the magnetic anomaly. However, despite excellent and abundant outcrops (especially scarp slopes of ridges) and core from five continuously-cored drill holes, no dikes younger than 10 Ma have been found within the Yucca Mountain block. (See also R.F. Marvin, written comm., 1980 in Carr et al. (1986)). However, the drill holes were vertical as are most of the dikes.

Dikes produced during the most recent episode of basaltic volcanism near Yucca Mountain are typically 2 m or less in width (Bruce Crowe, pers. comm. in Carrigan et al., 1990). The maximum width of the 10 Ma dike exposed along the Solitario Canyon fault mentioned above was also 2 m (Chris Fridrich, pers. comm., 1991). Theoretical consideration of elastic strains in surrounding rock in response to normal stresses caused by dike widening constrains dike dimensions (Davis, 1983). A brittle crust 10-15 km thick should limit dike width in the upper crust to 2-4 m (Carrigan et al., 1990). Thus, theoretical considerations, as well as structural and geologic evidence from the Yucca Mountain region and the surrounding Basin and Range province, suggest a likely geometry for basaltic dike intrusion in the upper crust in the vicinity of the repository to be a 2-4 m wide (or less) near-vertical zone of intrusion of magma initially at about 1200°C.

A recent analysis of seismicity in regions of young volcanism cited the Crater Flat area as one example of a number of young basaltic centers characterized by a paucity of background seismicity and low rates of recent normal faulting (Parsons and Thompson, 1991). Parsons and Thompson suggest that ongoing regional extensional strain accumulation is released in these areas primarily by expansion of the crust due to dike intrusion, rather than by normal faulting. Thus, the present-day lack of seismicity down to very small magnitude ranges (M≃0) in the vicinity of Yucca Mountain (Gomberg, 1991) may be an indication of either active dike intrusion at depth or the fact that the regional strain accumulation was largely relieved by the Quaternary basaltic volcanic events. Possible evidence for molten rock below Crater Flat is discussed later in this chapter.

Estimating the accumulated extensional strain since the last basaltic episode (20 ka or 140 ka), using an average Quaternary extension-

al rate of 0.01 mm/yr (based on estimates of Quaternary offset of faults in the Yucca Mountain region, (Scott, 1990), results in an accumulated extension on the order of 0.2 m (for a 20 ka event) or 1.4 m (for a 140 ka event). This strain could be relieved either by extensional faulting or by dike intrusion. A 1.4 m dike width is consistent with the other estimates of basaltic dike widths in the Yucca Mountain region as discussed above. It should be noted, however, that not all dike intrusions accommodating regional strain necessarily reach the surface or the near-surface. Dikes tend to propagate subhorizontally in the seismogenic (brittle, elastic) crust in the depth range of about 2-10 km (Halls and Fahrig, 1987; Rubin and Pollard, 1987).

Two independent types of models of water level changes associated with dike intrusion have been investigated for Yucca Mountain. Both models assume intrusion along a vertical zone beneath the repository with the top of the intrusion located 250-500 m below the water table. Kuiper (1991) computed the potential water table rise due to the increased temperature effect of the intrusion using both scoping calculations and a two-phase (steam and water) flow model. In an alternate approach, Carrigan et al. (1990) focused solely on the elastic strain effect of a dike intrusion that results in a region of dilation just above the level of dike intrusion and a region of contractional strain centered just below the dike top. Pore-water pressure enhancement was computed from the volumetric strain, and the excess pore-water pressure was used to drive the flow.

The computed response of the water table for these two very different models was less than 25 m for the thermal model of Kuiper and generally 4-6 m for the poro-elastic model of Carrigan et al. Kuiper assumed a 10 km long, 100 m wide vertical disk-shaped zone of temperature rise (related to dike intrusion) with a 1-hour period of temperature increase of 300°C, and included the effects of subsequent cooling. His maximum increase in water table level of less than 25 m was strongly dependent on both the assumed width and the magnitude of the temperature increase related to the zone of intrusion. Thin basaltic dikes intrude into the near surface at approximately 1200°C and cool rapidly. Unfortunately discussion of Kuiper's model was available only in an abstract in which the exact intrusion dimensions assumed to produce the 100 m wide zone of a 300°C temperature increase were not given.

Carrigan et al. (1990) investigated the effect on the water table of

intrusion of both a 2 m and a 4 m wide dike beneath the repository. Using uniform permeability values, they found a maximum rise of the water table of 4 m for a 2 m wide dike (6 m for the 4 m wide dike) over a 1 km wide region centered on the dike, occurring 8.6 days after the intrusion. For an assumed vertical permeability that was everywhere 10 times greater than the horizontal permeability, they found a peak of 6.5 m in less than one day for the 2 m dike. They also examined the effect of a tenfold to a thousandfold increase in the vertical permeability (10¹ to 10⁴) of a local 100 m wide zone centered on the dike, such as might result from cracking induced by the intrusion. Sharp peaks in the water table rise were predicted for the large vertical permeability enhancement. The maximum water table rise of 12 m for the 2 m wide dike (14 m for the 4 m dike) was associated with a 10³ increase in vertical permeability. Further increases in vertical permeability resulted in no further increase in the water table level, suggesting that any further rise was limited by the volume of ground water available to be channeled into the zone of enhanced permeability.

Analysis of far-traveled earthquake waves (P-waves) passing nearly vertically through the crust and upper mantle beneath Yucca Mountain and surrounding regions (Evans and Smith, 1992) shows no evidence of a low velocity feature that would suggest a volume of molten rock (or magma chamber) beneath Yucca Mountain. The minimum size of a feature identifiable in that study is 4 km across. However, a volume of slightly lower velocity material extending upward from the Moho (the boundary between the earth's crust and mantle) to within 12 km of the surface was found directly to the west, beneath Crater Flat. The seismic P-wave velocity in this anomalous volume beneath Crater Flat is about three percent lower than material beneath Yucca Mountain in the same depth range. The standard errors of this type of velocity analysis are about 0.4-0.5 percent; the minimum believable velocity change is therefore on the order of 1 percent.

The three percent lower velocity anomaly identified under Crater Flat is considerably smaller in magnitude than velocity anomalies associated with known large-scale high silica magma chambers, such as Long Valley, where velocity decreases of six to eight percent are typically found (Iyer, 1988). The possibility that molten rock is present beneath Crater Flat deserves further study.

To establish the presence (or absence) of molten rock currently beneath Crater Flat, a broader and higher resolution analysis would be required. Seismic reflection profiling techniques tuned for identifying the presence of “fluids” should also be included. Such a study would need to include the determination of the velocity of shear (S) waves, which are much more sensitive to the presence of fluids or melts, in addition to P-waves, as well as attenuation studies, which investigate the rate of change of the amplitude of the waves, and can detect the presence of fluids or melts.

Several assessments of the probability of a basaltic dike intrusion at Yucca Mountain are available. All rely on dividing the probability assessment into two steps: (1) P₁, the probability of basaltic activity in the general region of the proposed repository, including Lathrop Wells and Crater Flat (called the Crater Flat Volcanic Zone by Crowe), and (2) P₂, the conditional probability that, given volcanic activity in the area, the assumed basaltic dike would be close enough to affect the proposed repository. The total probability of volcanic activity affecting Yucca Mountain is calculated as the product of the two.

Crowe and his co-workers (1983, 1986, 1991) have estimated P1 to be on the order of 10⁻⁶ per year, based on four clusters of events in 3.7 Ma. If the seven identified events in the region are used, the rate is twice this. If volcanic activity in the region is mature and will wane in the future, as suggested by Crowe (1991), the assumption of a stationary rate of occurrence is conservative for future occurrences.

The calculation of P₂ depends on the geometry of the proposed repository with respect to the volcanic field, and on the spatial characteristics governing the location of vents and dikes (the size and ratio of length to width of the volcanic field). The trend of the major axis of vents or feeders of contemporaneous eruptive centers within a volcanic field can be estimated from paleovolcanic events; this trend is NNE for the two main eruptive centers in the Crater Flat Volcanic Zone. The trend of dikes is generally perpendicular to the direction of least principal horizontal stress, which is WNW-ESE in the region. Sheridan (1990) developed an illustrative calculational model of these effects and estimated P₂ at about 8 × 10⁻³. Crowe (1991) estimated P₂ at about 10⁻².

Combining these two probabilities (P₁ and P₂) leads to a total probability of about 10⁻⁸ per year for the occurrence of a basaltic dike that would affect the proposed repository at Yucca Mountain. This is the thresh-

old probability for consideration of events that might affect a waste repository in EPA regulations, and is indeed a small number.

Some researchers (e.g., Smith et al., 1990; Trapp, 1989) use other areas within which to define probabilities of occurrence, based either on geometrical areas (Smith et al.) with some consideration of the present regional stress regime or on interpretation of aeromagnetic data (Trapp). Any associated calculations that assume that future volcanic vents are unrelated in space to existing vents have less credibility, in the panel's view. Furthermore, where probabilities are assessed only qualitatively (Smith et al., 1990), comparison to quantitative calculations is not possible. While the uncertainty may be several orders of magnitude in the probability values described here, the panel concludes that the probability of volcanic intrusion into the proposed repository is low.

Geologic observations indicate that the form of intrusion most likely to affect the repository is a near-vertical dike of basaltic composition, with a width of 2-4 m. Modeling of the pore-elastic effects of such a dike intruded directly beneath the repository (and with a top less than 1 km below the bottom of the repository) results in maximum water table rises of less than 10 m in both an isotropic case, in which rock properties are the same in all directions, and in a case where the vertical permeability is 10 times the horizontal permeability. Concentrating the water rise in a 100 m wide vertical highly permeable zone directly above the dike resulted in a maximum rise of 12-14 m when vertical permeabilities were enhanced 10³-10⁴ within the vertical zone. Thus, the panel concludes that the elastic effect of dike intrusion would result in raising the water table no more than a few tens of meters.

Models of the thermal effects on water table rise are highly dependent on the total amount of heat energy released as a result of the intrusion. The modeled 100 m wide zone of 300°C temperature increase produced a water table rise of less than 25 m; however, the number and exact configuration of dikes within this zone was not specified. For a single dike of 2-4 m width, the modeled 300°C temperature rise over a width of 100 m is clearly an overestimate of the heat energy. As an upper bound, considering energy equivalence, the maximum amount of energy available in a 4 m wide dike intruded at an effective temperature of 1400°C (which includes effects of the latent heat of crystallization) is approximately equivalent to a temperature rise of about 60°C, not 300°C, over a 100 m wide

zone. In the opinion of the panel, therefore, a 25 m rise in water table is clearly a conservative upper bound estimate for the expected form of intrusion in the Yucca Mountain region.

It is the panel's view that the low probability of a dike forming close enough to affect the proposed repository, combined with the small effect that a basaltic dike intrusion at Yucca Mountain would have on the water table, means that the threat of volcanic effects is sufficiently low that volcanic intrusions can be discounted as potentially disruptive events.

Unfortunately, modeling of the coupled thermal and pore-elastic problem of the effects of dike intrusion on the water table has not been addressed. While the thermal effects may dominate, the panel notes that this modeling could establish the maximum water table excursion due to dike intrusion.

The study plan for the characterization of volcanic features calls for drilling core holes to investigate aeromagnetic anomalies that may represent either buried volcanic centers or intrusive rocks (probably basaltic in composition). No mention is made of studying the core to determine the extent, if any, of hydrothermal activity (temperature variations in adjacent rocks, fluid inclusion studies, hydrothermal alteration products) that may have been induced by intrusion or extrusion of a body of volcanic rock. The panel recommends this type of investigation be added to the work plan.

Further teleseismic studies for better definition of the low velocity zone beneath Crater Flat should be carried out. A combination of P- and S-wave velocity and attenuation studies should constrain both the nature and source (e.g., possible fraction of melt if present) of this velocity anomaly. Although the panel does not consider it likely that either a larger volume or a different style of igneous intrusion will be found that is inconsistent with the recent geologic record, it would be prudent to follow up with an evaluation of this anomaly to ascertain its significance.

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