A prime reason Yucca Mountain has been thought acceptable as a site to store radioactive waste is because of the dryness and deep depth of groundwater at the site. Yucca Mountain was one of the few sites where the repository could be placed substantially above the water table.
Some of the groundwater issues at Yucca Mountain were already discussed in Chapter 11 on geologist Jerry Syzmanski. However, Szymanski's theories tend to bias the discussion of groundwater issues toward rather extreme hypotheses. There are a number of traditional hydrological reasons the site is considered unique and worthy of study:
1) Historically dry climate.
2) Extremely low water table.
3) The presence of minerals called zeolytes that would slow transport of radionucleides through the rock.
4) Isolation from the groundwater of the rest of Nevada.
5) The fact that the surface tends to shed water rather than absorb it.
6) The absence of a thermal source near the earth's surface (magma) to cause thermal upwelling.
Some concerns still exist over surface and groundwater issues, but they are unrelated to the Szymanski theories. The Syzmanski theory that thermal upwelling of deep waters could cause the inundation of the site has generally been dismissed by his peers as poorly conceived. More important concerns include:
1) The effect of increased rainwater over the next ten thousand years
2) The possibility of focused recharge (focused water infiltration).
3) The heat pipe effect causing thermal vaporization / condensation cycles.
The historical fact of many millenia of relatively dry climate has been established in a number of ways. One is the paleobiology of the area, i.e., the traces of ancient plant and animal life in the ground that are indicative of dry or wet climates. Rat middens, the nests left by rats over many thousands of years, have proven a valuable insight from the fossilized plant remains, crystallized urine and droppings left there. The presence of
Another method of dating the underlying groundwater is by analysis of the isotopes of various elements including strontium, oxygen-13, oxygen-18, carbon-13, etc.. Also, by testing the ratio of strontium-87 to strontium-86, it is possible to compare the originating sources of calcium deposits. The importance of this has been that sensitive areas, such as Ash Meadows (where resides the endangered pupfish) have been checked to determine whether water from the Yucca Mountain drainage system, invades this aquifer. Fortunately, it does not.
Other relative rather than absolute means of aging groundwater process stem from the ordering and style of layering of various calcite deposits. The cumulative data from all the above data presently does not support the theory that Yucca Mountain has within recent millenia experienced a significantly wetter climate capable of raising the localwater table, though certainly somewhat wetter periods have existed.
POSSIBLE GROUNDWATER INTRUSION MECHANISMS
Yucca Mountain is made of many layers of volcanic ash of various consistencies. The repository itself is to be sited in one of these layers called the Topopah Spring tuff. This layer ranges from about 800 to 1300 above the water table and up to 1500 feet below the top of the mountain.
There are a number of conceivable (though unlikely) mechanisms for water to rise to the level of the repository: earthquakes, volcanic dikes and increased rainfall. The National Research Council, in its report "Groundwater at Yucca Mountain: How High Can It Rise" considered a number of these and other scenarios and offered some observations:
In considering the long and complex 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 repository is a low-volume basaltic dike. . . . .
Calculating the probabilities 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. ["Ground Water At Yucca Mountain: How High Can It Rise", National Academy Press, 1992, p6]
The National Research Council also commented on the earthquake problem in regard to groundwater:
The panel concludes, given the experience from historic earthquakes, the small modeled response of the water table to earthquakes consistent with 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.["Ground Water At Yucca Mountain: How High Can It Rise", National Academy Press, 1992, p6]
Further backing the NRC's conclusions on the minimal effects of earthquakes on the Yucca Mountain groundwater is the report, Earthquake-Induced Water-level Fluctuations at Yucca Mountain, Nevada, April 1992 issued by the U.S. Geologic Survey. This presents the results of measurements taken at wells near Yucca Mountain during a series of earthquakes that rattled the California and Nevada area in March and April of 1992:
This report presents earthquake-induced water-level and fluid-pressure data for well USW H-5 during April 1992. Well USW H-5 is located in the Yucca Mountain area, Nevada. On April 22, 1992 a 6.2-magnitude earthquake occured in southern California which caused a maximum fluid-pressure change of approximately 50 centimeters (approximately 1.6 feet) in well USW H-5. Within 18 hours on April 25-26, 1992, three major earthquakes occurred in northern California. The water-level responses to these earthquakes were detected in well USW H-5. The maximum water-level fluctuation from the northern California earthquakes was in excess of 52.5 cm. [Earthquake-Induced Water-level Fluctuations at Yucca Mountain, Nevada, U.S. Geologic Survey, Open File Report 92-137, April 1992, p1]
Water fluctuations due to earthquakes, hydroseisms, are generally small. The USGS report continues:
Hydroseisms, or water-level fluctuations in response to earthquakes, are relatively common phenomena observed in wells penetrating confined aquifers. For example, the Anchorage, Alaska earthquake of 1964, the largest North American earthquake thus far in the 20th century, caused water-level fluctuations throughout the world.; the largest peak-to-trough range was about 7.0 meters (22 feet) in a well in South Dakota. Hydroseisms are more commonly in the centimeters to meter range and typically are observed for minutes to tens of minutes. [U.S. Geologic Survey, Open File Report 92-137, April 1992, p1]
Although the State of Nevada makes much of the earthquake and volcanic activity at Yucca Mountain affecting the level of the water table, in reality these are peripheral issues. The most difficult problem now challenging the scientists at Yucca Mountain is verifying that long term changes in rainfall cannot sufficiently affect the levels of the groundwater, at least not enough to endanger the repository.
Rainwater in the Yucca Mountain area now ranges around 6 inches per year. There is evidence in the area and the Vegas valley that the climate was wetter in the past. The presence of sediment along the Vegas valley floor and the bones of horses, camels and even mastodons argues for a more lush climate. How wet the climate has to be to cause an increase in height of the water table is still a matter of debate, however:
. . . Analysis of the paleoecological and paleoclimatic information of the area suggests that even at the last glacial maximum during the Pleistocene Wisconsin 18 ka (thousand years before present) the Yucca Mountain area experienced no more than a 40 percent increase in rainfall over the present. ["Ground Water At Yucca Mountain: How High Can It Rise", National Academy Press, 1992, p141]
One of the important considerations is where groundwater drains from the Yucca Mountain area. Unlike areas surrounding the site, this area drains into the hydrological deadend of the Amargosa River and Death Valley rather than into the river systems that flow towards the Colorado River and drinking supplies. One of the purposes of the isotopic studies being conducted at the Mountain is to verify that all the water in that area does indeed flow towards Death Valley and not into other drainage systems that enter the ocean.
A common misconception among the public is that contamination of the groundwater at Yucca Mountain would somehow quickly contaminate the Las Vegas water supply. There are two reasons this is absurd, the first being the already cited observation that Las Vegas is part of the Colorado River drainage system and Yucca Mountain of the Amargosa River, an underground river which empties into Death Valley. The second is that the movement of radionuclides through the rock, even in water saturated rock, is slow. It would require thousands? of years for radioactive waste to migrate towards the surface through groundwater flow.
An environmental concern often expressed is that the radioactive materials threaten the habitat of the endangered pupfish that reside in isolated pools in an area called Ash Meadows. The pupfish are goldfish-sized creatures that are vestiges of a lake that existed in the valley west of Yucca Mountain about 40,000 years ago. The evaporation of that lake left the pupfish isolated in a few deep pools balanced precariously at the mercy of stable groundwater levels. Isotopic analysis of the water in Ash Meadows shows, however, that it derives from the nearby Spring Mountains and is not from the Yucca Mountain area. In any event, water levels are being monitored in these pools and the dangers to the pupfish from Yucca Mountain seem less than from intrusions from overanxious biologists.
APOCALYPTIC GROUNDWATER THEORIES
Nevada NWPO has raised a number of groundwater issues regarding Yucca Mountain. Unfortunately, because of their concentration on anti-repository sensationalism, some of these theories have been extremist and not in agreement with empirical evidence.
For example, at the heart of geologist Jerry Szymanski's theory that water had risen to the Yucca Mountain surface was the presence of calcite veins in Trench 14, a trench dug into the side of the mountain to expose its history (see Figure 11). When Szymanski viewed this trench, only part of the calcite veins were exposed and they certainly appeared to point to the possible presence of springs in the area. Further trenching at the site however, showed a different story. The calcite veins tapered as they reached further below the surface showing this to be a surface water phenomena rather than the result of massive rises in the water table.
Another argument expressed by the state is that fractures within the rock cannot be adequately modeled and that water could seap quickly along these fractures. Kristin Shrader-Frechette, the science philosopher whose ethical theories we analyzed in an earlier chapter, claims expertise in hydrology, among other fields. In a work published in the Dutch philosophy journal, Synthese, she dismissed the entire basis of the field of hydrology, using a one dimensional form of D'Arcy's Law [Shrader-Frechette, Kristin; "Idealized Laws, Anti-Realism, and Applied Science: A Case In Hydrogeology", Synthese, vol. 81, 1989, p329]. Shrader-Frechette seems to believe thousands of experimental and analytical hydrologists have not already considered the problems she associates with the use of D'Arcy's Law. As someone with a background in fluid dynamics, this author's experience is that rather than ignoring these effects as Shrader-Frechette claims, hydrologists and fluid dynamics professionals are in fact consumed with these issues.