ALTERNATIVE FLOW MODELS AT YUCCA MOUNTAIN, NEVADA; STATE OF NEVADA-FUNDED RESEARCH

Transcription

1 ALTERNATIVE FLOW MODELS AT YUCCA MOUNTAIN, NEVADA; STATE OF NEVADA-FUNDED RESEARCH ABSTRACT Linda Lehman and Tim P. Brown Technical & Regulatory Evaluations Group, (T-Reg), Inc Henning Circle NE * Prior Lake, MN Phone (612) Fax (612) In the hydrogeologic assessments of ground water pathways in complex systems such as exist at Yucca Mountain, Nevada, heat may be used as a flow path tracer. Using heat along with hydraulic head and chemistry measurements serves to constrain the results of flow path analyses that have non-unique solutions. Heat and chemical tracers can give a much more reliable answer to ground water flow directions in a complex system than the use of hydraulic head alone. It has been known for some time that the Yucca Mountain ground water system has a range of spatially distributed temperatures associated with it. It also has been hypothesized that certain features in the water table surface (embayments) are coincident with major faults. Unfortunately, the latest Total System Performance Assessment (TSPA) of Yucca Mountain (November 1998) did not include geologic structure, temperature, or chemistry data in their determination of saturated zone ground water flow paths. It is our contention that DOE must utilize all relevant data available to them in determining ground water flow paths and subsequent dose to potential receptors, not just selective data sets. A numerical model was constructed to evaluate flow paths to the accessible environment at Yucca Mountain, Nevada. The model is fully three-dimensional, evaluates thermal transport, and explicitly considers geologic structures as controls on the flow field. The latest model results indicate major differences exist in flow path direction and velocity when compared to the latest TSPA, which characterized the performance behavior of Yucca Mountain in the DOE Viability Assessment document. These conceptual differences are presented, along with supporting evidence from field data generated by the Yucca Mountain Project and Nye County studies. Results of calibrations against existing temperature and hydraulic head data will be shown. Recent chemistry analyses performed by the USGS also support a different flow path than analyzed in the TSPA or the DEIS. The conclusions of the DOE Viability Assessment and the Draft Environmental Impact Statement are questionable, since they have failed to analyze viable alternative models of the groundwater flow field; alternatives which may have adverse impacts to local populations.

2 INTRODUCTION In order to evaluate the conclusions of the Viability Assessment or the Draft Environmental Impact Statement (DEIS) for Yucca Mountain, the basic underlying assumptions in the latest Performance Assessment (PA) must be examined. The relative importance of these assumptions to the PA also needs to be evaluated. In conducting a performance assessment for Yucca Mountain, an accurate view of the groundwater flow field is essential. The velocity of the groundwater is one of the most important parameters in the transport equation. The direction of the groundwater pathway is important as it dictates the hydrologic and geochemical character of the pathway that influence sorption and other variables such as dilution in the saturated zone. It has been known for some time that temperatures in the Yucca Mountain ground water system show considerable spatial variability. It also has been observed that certain features in the water table surface (embayments) are coincident with major faults. In the hydrogeologic assessments of ground water pathways in complex systems such as exist at Yucca Mountain, heat may be used as a flow path tracer. Using heat along with hydraulic head and chemistry measurements serves to constrain the results of analyses of flow paths, which have non-unique solutions. Heat and chemical tracers can give a much more reliable answer to ground water flow directions in a complex system than the use of hydraulic head alone. Unfortunately, the latest Total System Performance Assessment (TSPA) of Yucca Mountain (1) did not include geologic structure, temperature, or chemistry data in their determination of saturated zone ground water flow paths. It is our contention that DOE must utilize all relevant data available to determine ground water flow paths to potential receptors. CONCEPTUAL MODEL A numerical model was constructed to evaluate flow paths to the accessible environment at Yucca Mountain, Nevada. The model is fully three-dimensional, accounts for thermal transport, and explicitly considers geologic structures as controls on the flow field. The latest model results indicate that major differences exist in groundwater flow vectors (velocity and direction) when compared to the latest TSPA. The latest TSPA characterized and calculated the performance behavior of Yucca Mountain in the DOE Viability Assessment, in terms of dose to the Critical Group. In order to evaluate an alternative conceptual model of saturated zone flow for the area around and including Yucca Mountain, a numerical model, based on previous work done by ourselves and other State of Nevada contractors, was assembled and tested. This conceptual model postulates that faults and fractures dominate the flow of groundwater through the volcanic tuffs underlying Yucca Mountain. The initial basis for this conceptual model lies in observations of the potentiometric surface as interpreted by the USGS and our own analysis of water table fluctuations and groundwater temperature measurements.

3 The implications of this conceptual model on performance may be significant, because the potential for these fractured zones to transmit groundwater rapidly and transport contaminants with minimal dispersion or adsorption, is high. In addition, seismic activity in the region may result in unpredictable changes in the hydrogeologic system over time, because earthquakes may adjust the flow properties of conduits or create new fracture zones, which could cause major realignment of the system. The above information resulted in a different conceptualization of the flow field than that currently considered in the TSPA/VA or the DEIS. The proposed model is structurally controlled by fault and fracture zones. Fracture zone intersections play a key roll in the distribution of recharge, velocity fields and pathways. The proposed conceptual model is also dynamic rather than static, and has the potential to change rapidly due to tectonic movements. The proposed conceptual model postulates that some water movement occurs across the mountain block from east to west, primarily via discrete northwest trending fracture zones. The Solitario Canyon Fault zone creates a resistance to eastern flow but does not totally prevent it. A steep hydraulic gradient exists at the location of the Solitario Canyon Fault and is equal to about 35 meters of head difference over a lateral distance of about 1000 meters. Water movement across this fault probably occurs as a result of intersections with northwest trending shear zones and creates cascading flow to the next lower level of the water table. The proposed model of flow is shown in Figure 1. This figure shows the potentiometric surface and the proposed flow paths. Some colder flow also enters the Yucca Mountain block from the northwest across a very steep hydraulic gradient. This gradient is equal to over 300 meters of head change across 2500 meters distance. The faults in the Drill Hole Wash region no doubt play a role in the transport of water across this hydraulic barrier. Where the Drill Hole Wash Fault or those near it intersect the northern extension of the Solitario Canyon Fault, a potential breach may occur and allow the colder water north of the steep gradient to move down this fault zone and subsequently into the Ghost Dance Fault or Midway Valley Fault. The proposed model and the potentiometric surface suggested that another fault zone exists just to the south of the repository footprint. This fault zone was later identified when the C- Well tests were performed. This zone may also be transporting water from the Solitario Canyon side of the block toward the east.

4 Our conceptual model emphasizes fracture flow paths in the saturated and unsaturated zones. We hypothesize that fault and fracture zones are inter-connected and dominate saturated zone flow. The fracture conduits dominate flow paths in the proposed repository block, as well as important connections between this block and adjacent

5 structurally separated blocks. Further, these fracture conduits have hydrologic properties that are sensitive to seismic events, especially fracture apertures. Two-dimensional non-isothermal modeling efforts conducted in 1994 (2), by the State of Nevada, concluded that upward flow from the carbonates was important and must be accounted for by Yucca Mountain flow models, to be credible. This idea was also independently developed by John Bredehoeft (3) in his work for Inyo County, California. At well number UE-25 p#1, Bredehoeft calculated upward flux rates and fault permeabilities derived from earth tide calculations. At the UE-25p#1 location, head in the carbonates was approximately 20 meters higher than in the volcanics. (This location remains the only measurement of temperature and head in the Paleozoic carbonate aquifer in the area of Yucca Mountain.) Bredehoeft concluded that the upward movement of hot water was coincident with major extensional faults, such as the Bow Ridge in Midway Valley, which breach a very tight confining unit. This work was published in January of 1998, though performed much earlier. Farrell et al. (4) and Painter and Armstrong (5) state that buoyancy resulting from thermal instability may also contribute to the observed heat effects. Large temperature and hydraulic gradients also exist across the mountain block. These observations and those mentioned above led to the conclusion that three-dimensional non-isothermal models would be required to accurately model the flow field at Yucca Mountain. Stratigraphy and Hydrostratigraphic Units The stratigraphic section utilized by the 3D model is assumed to be as described by Dudley (6). In this conceptualization, flow in the carbonates is upward and discharges to the south of the Yucca Mountain block. Flow in the volcanics is downward in the northern reaches of the block and also discharges south of Yucca Mountain where the volcanics are pinched out by the carbonates. This hydrostratigraphic section was modeled as three layers. The first layer represents the aquifer series of upper Miocene tuffs. Layer 2 represents the lower Miocene volcanic confining units. The Eleana Formation is not known to be present under Yucca Mountain. Since its properties would also be that of a confining unit, it has not been specifically singled out in these analyses. Layer 3 represents the Paleozoic carbonate aquifer. In this modeling exercise Layer 3 is implicitly included through the use of boundary conditions. Structure The conceptual model is that of a fracture flow system where flow paths and velocities are controlled by the existing fracture networks. The two dimensional analyses done in 1994, (2) indicated that a significant upward component of flow would be possible on the west side of Yucca Mountain due to the high temperatures noted at the water table at WT-10 and WT-7 along the alignment of the Solitario Canyon Fault indicating potential vertical permeability. The Solitario Canyon Fault also appears to be influencing the

6 hydrology in a lateral sense by creating a medium hydraulic gradient from west to east near the repository block and further south, possibly ponding water behind it. Measured heads range from about 775 meters to the west of the fault and drop abruptly to about 730 meters on the eastern side. The Solitario Canyon Fault is a major north-striking scissors fault, which to the south is down thrown on its western side and to the north is down thrown on the eastern side (8) and may exhibit differing hydraulic properties along its length. Northwest trending strike-slip faults exhibiting right lateral movement also seem to play a role in influencing the hydraulic potentials at the site. Strike-slip faulting may provide vertical conduits to flow and in some cases barriers to horizontal flow and have been linked to tectonic activity in the Walker Lane Belt, a large northwest-trending structural zone that parallels most of the southwest border of Nevada (7). For example, note the potentiometric surface contours at the Drill Hole Wash fault, Sun Dance fault and the southerly fault location, as shown earlier on Figure 1. This figure is a potentiometric surface map drawn by Lehman and Brown (2). Areas of fault intersections are important and may act as drains in the northern regions and conduits for upwelling in the southern regions of the mountain block. These fault and structural related flow properties are different from those assumed by the US DOE in their analyses of the performance of Yucca Mountain in both the DEIS and in the Viability Assessment. The analyses performed by the US DOE do not utilize heat, chemistry or structures such as faults and fractures. Rather, potentiometric surface maps utilized by the US DOE show very smooth contour intervals, which fail to show embayments and ignore their consequences. Failure to include these data may lead to erroneously conservative estimates of repository performance and ultimately, dose consequences to the citizens of Nevada. Modeling Objectives The primary objective was to calculate a steady state flow field in which the measured head and temperature measurements will be matched as closely as possible. In addition to calculating the steady state velocity field, one objective of this exercise was to ascertain which faults exhibit control on the flow field and which do not. Model Grid There are three layers each possessing 396 cells in an 18 x 22 grid. In this layering, a few of the faults are explicitly included and specific hydrologic properties have been individually assigned to them. The key, which defines the hydrologic properties assigned to each grid zonation, is given as Table I.

7 Table I. Parameters used for the VTOUGH Saturated Zone Model Material Porosity X Permeability (m 2 ) Y Permeability (m 2 ) Z Permeability (m 2 ) Wet Conductivity (W/m- o C) Specific Hear Capacity (J/kg- o C) TUFF E E E TRANS E E E TIGHT E E E TUFF E E E FRAC E E E FRAC E E E FRAC E E E FZON E E E FZON E E E FZON E E E CONF E E E CARB E E E Initially, all permeabilities, vertical and horizontal were set equal, as there is no information available to indicate differences. The initial permeabilities of the tuff units were assigned the average value as measured by the USGS and the DOE. The permeabilities in the fault zones were then adjusted arbitrarily until the potentiometric surface, including the embayments were matched. Layer 1 represents the volcanic aquifer. Thickness of this unit generally decreases from north to south. In this modeling exercise, the layer is set uniformly at 500 meters in thickness. At UE-25-p#1 the thickness is approximately 800 meters. While this difference in thickness may influence the outcomes somewhat, we felt that for this very simple model, it was more important to assess the effects of structure. Permeability increases generally from north to south. The Tight Unit comprises the northern boundary and creates the large hydraulic gradient conditions. The Transitional Unit (TRANS) represents a permeability transition to the generally more transmissive tuff properties (TUFF1 & 2). This unit was necessary to maintain numerical stability and to simulate the observed potentiometric surface, although in terms of numerical stability, smaller grid cell size may have accomplished the same result. Fault hydraulic properties range from less permeable than or more permeable than the tuffs, in some cases causing barriers and in others conduits. Explicitly included in Layer 1 are the Solitario Canyon, the Ghost Dance and the Bow Ridge faults. Three northwest trending strike-slip faults are included as Drill Hole Wash, Sun Dance and a third unknown fault zone (name unknown to us at the time of this document) just south of the repository horizon. Boundary conditions are head and temperature on the northern boundary and head and temperature at two positions along the southern boundary at WT 11 and WT-12. In a few simulations, points for head and temperature were also placed along Solitario Canyon to the west of the mountain, near WT-7 and WT-10. The eastern boundary represents the

8 Forty Mile Wash. It is represented as a no flow boundary condition, but no pressures or temperatures were assigned along this boundary. Layer 2 has the same number of elements (396) as does Layer 1. This layer represents the lower volcanic confining unit. This unit is about 400 meters in thickness at UE-25- p#1. In these simulations it is held constant at 500 meters in thickness. It is represented by the Lithic Ridge and older tuffs down to the Tertiary/Paleozoic contact. The volcanic system appears to be acting as an aquitard at its lower boundary with the carbonates, as interpreted from UE - 25p#1 data by Bredehoeft, (3) The head increases slightly in the Lithic Ridge Tuff and significantly increases in the Older Tuffs beneath the Lithic Ridge, to a potential of meters ASL. At UE - 25p#1 this aquitard lies at a depth of 873 meters. The Tertiary/Paleozoic contact is at a depth of approximately 1,200 meters. Rock properties assigned to this layer are the same for the fault zones in Layer 1, but the rest of the grid is assigned the lowest permeability of m2 (TIGHT). Bredehoeft, (3) indicates that this confining unit must be quite tight in order to sustain earth tides. This permeability was adjusted to best match measured head and temperature values in the lower volcanic confining units. Upward vertical gradients are apparent in a number of the 10 wells that have measured data with depth. No boundary conditions are assigned to this unit and it is allowed to float, or equilibrate naturally. Layer 3 is the carbonate aquifer, which apparently discharges in Ash Meadows, and generally to the south, southeast and possibly southwest of the Yucca Mountain block. As mentioned earlier, it is simulated via constant temperature and head boundary nodes of 752 meters and 57 degrees C, and does not actually exist as a separate layer. Flow will therefore be upward from the carbonates into the tuffs. In the southeastern section of the grid heads are 20 meters above those in the volcanics. Data Sets In 1994, the USGS published a report entitled Revised Potentiometric Surface Map, Yucca Mountain and Vicinity, Nevada (9). In this report, the USGS has undertaken to correct the earlier water level measurements by re-surveying the elevations of the wellheads and by correcting for temperature and density. The earlier potentiometric surface map showed an embayment in the vicinity of Drill Hole Wash. The USGS explains that they had removed the embayments by selectively discarding the revised water level data, because they saw no physical reason for the hydraulic lows. Fortunately, this report contained the actual revised data, so we were able to re-plot the potentiometric surface utilizing all the new data and the embayments are clearly visible again. There are three major embayments, which appear to be coincident with the NW-SE trending strike-slip Drill Hole Wash Fault, Sundance Fault and an unnamed fault just south of the repository footprint. We believe that this is the correct potentiometric surface and we have attempted to reproduce this surface in our numerical model. Other data are available in terms of hydraulic head on which to calibrate model outputs. There are 10 sites that have been measured at multiple depth intervals. These sites and

9 values are given in Luckey et al (7). Many of these wells exhibit increasing head with depth, especially when the lower volcanic confining unit is reached. Additional information on the pathways in which groundwater may be moving is obtained from the temperature distribution. The data set utilized for this work is that of Sass, et al. (10). The temperature distribution is shown as Figure 2. This temperature distribution was utilized to calibrate the flow model. Zell Peterman of the USGS has developed plots of major ion and isotope chemistry. These plots were distributed at the NWTRB January 1998 meeting in Amargosa. These plots should also be helpful in delineating flow paths. Models should be consistent with these data, to the extent possible. More recently, the USGS has analyzed split samples obtained by Nye County Early Warnng Drilling Program (11), which also supports a southerly flow path. Model Results Numerous simulations were carried out as part of this modeling exercise in the calibration and sensitivity analyses phases. We have selected two simulation outputs as our final product and have examined the resultant velocity fields. These model results are believed to be representative of the flow system at Yucca Mountain. The model, while simple, allowed us to examine the relationship of the head distribution to the position of major faults. It is our conclusion that the major faults included in this model significantly affect the observed head distributions. The head and temperature distributions for the immediate vicinity of the repository on the Yucca Mountain block are believed to be correct. Figure 3 represents the Layer 1 potentiometric surface. We believe that the general shape of the surface matches quite well with Figure 1. This plot has reproduced the observed embayments by adjusting the permeability of the northwest trending shear zones. One feature that is significant is the flattening of the meter contour line. As can be seen, this flattened area coincides with the Ghost Dance Fault nodes and allows for the southerly movement of water along the fault. Figure 4 is the temperature distribution in Layer 1. We believe that we have matched to shape of the Sass distribution. One could argue that the 30-degree contour is a little far to the south, and that this position should represent the 32-degree contour line. However, we felt that given the limited data for this site, coming within a degree or two of the actual distribution was an acceptable error. Where we have had difficulty with these simulations is in the higher than observed temperatures in the southeastern quadrant, just west of the Forty Mile Wash region. The authors believe that this condition could be corrected by adding additional cold recharge to the area. For our final runs we added 10 cm/yr recharge along the top of the mountain

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13 block and along the Drill Hole Wash area eastward to Midway Valley. The additional recharge in this area is justified as potential recharge could be coming from higher elevations at Yucca Mountain, or along Forty Mile Wash. The amount of recharge is consistent with average estimated recharge at Yucca Mountain. We believe that our latest 3D runs have matched the Yucca Mountain and Forty Mile Wash temperatures fairly accurately. However, the very much higher heads and temperatures utilized as boundary conditions in the carbonates dominate all attempts to lower the temperature through varying the vertical permeability tensor. The only way to cool the upper layers was to add more advective flux or recharge. In a comparison of measured temperatures and heads with modeled temperatures and heads, the heads at WT 7 and WT 10 are much to low, 740 meters ASL as opposed to the actual of 775 meters. The general trend of the modeled heads, i.e., upward or downward appears to be accurate over the mountain block with a few exceptions. As mentioned previously, the western side of the modeled region has several instances of heads being too low. Another instance is with H-5. This could indicate that there are areas of ponding, such as west of Solitario Canyon in lower Crater flat, or in the case of H-5, upward head has been underestimated for the head boundary condition in the carbonates (750 meters as opposed to the measured 775 meters) The temperature distribution in the lower Crater Flat area is also too low, running near 30 degrees rather than the observed 38 degrees. One reason for this is that the actual well placement is right in the Solitario Canyon Fault, while our simulations place the Solitario Canyon Fault three grid cells to the east. Also the Solitario Canyon Fault dips to the west, where in our simulations the fault is vertical. The higher temperatures arise in this simulation coincident with the simulated Solitario Canyon Fault and come within two degrees of those measured at WT-7 and WT-10. CONCLUSIONS In summary, we believe that our simple 3-D non-isothermal model of Yucca Mountain has been instructive in ascertaining areas where more data are needed to sort out alternative flow models and alternative flow paths. The conclusion of this research and reviews conducted for the State of Nevada indicate that major deficiencies exist in the current TSPA and those utilized in the DEIS and the Viability Assessment. Therefore, no confidence can be placed in conclusions of these documents unless all data are analyzed in the determination of potential groundwater flow pathways and radionuclide transport to potential receptors. Reasonable alternative flow paths must be analyzed to ascertain significance to outcomes. While the US DOE acknowledges that reasonable alternatives exist, they have not analyzed their significance in the latest performance assessments.

14 REFERENCES 1. USDOE, Viability Assessment for Yucca Mountain and Total Systems Performance Assessment, Washington, D.C. November (1998). 2. Lehman L.L. and T.P. Brown, Alternate Conceptual Models in the Saturated Zone at Yucca Mountain, Presentation to the Nuclear Waste Technical Review Board, Reno, NV, April (1994). 3. Bredehoeft, J.B, Water Resources Research, January (1998). 4. Farrell et al., Structural Controls on Groundwater Flow in the Yucca Mountain Region, Center for Nuclear Waste Regulatory Analyses, SRI, San Antonio, TX (1999). 5. Painter, S. and Armstrong, A., On the Origin of the Groundwater Temperature Variations Near Yucca Mountain, Nevada, Proc. AGU Fall Meeting, Vol. 80, No.46, (1999). 6. Dudley, A.L., R.R.Peters, J.H. Gauthier, M.L. Wilson, M.S. Tierney and E.A. Klavetter, Total System Performance Assessment Code (TOSPAC) Volume 1: Physical and Mathematical Bases. Sandia National Laboratories, SAND , Albuquerque, New Mexico. (1988). 7. Luckey, R, P. Tucci, C. Faunt, E. Ervin, W. Steinkampt, F. D Agnese and G. Patterson. status of Understanding of the Saturated-Zone Ground-Water Flow System at Yucca Mountain, Nevada, as of 1995, USGS WRI Report , (1996) 8. Scott, R.B. and J. Bonk, Preliminary Geologic Map of Yucca Mountain, Nye County, Nevada, with Geologic Sections, USGS Open File Report (1984). 9. Ervin, E.M., R.R. Luckey and D.J. Burkhardt, Revised Potentiometric Surface Map, Yucca Mountain and Vicinity, Nevada, USGS, Water Resources Investigations Report , (1994). 10. Sass, J.H., A.H. Lachenbruch, W.W. Dudley Jr., S.S. Priest, and R.J. Munroe, Temperature, Thermal Conductivity and Heat Flow Near Yucca Mountain, Nevada: Some Tectonic and Hydrologic Implications, USGS Open File Report , (1988). 11. Stellavato, Nick, Transmittal letter for the Nye County Early Warning Drilling Program (EWDP) Phase I Data Report, Pahrump, NV, April 20, (1999).