Appendix A Natural Setting and Resources
As for any deep geologic facility, the natural setting of the Waste Isolation Pilot Plant (WIPP) is important in determining the facility's performance as a repository for radioactive waste. The natural setting consists of the geology, hydrology, geochemistry, climate, and natural resources of the region and local area immediately surrounding WIPP, some of which are discussed below.
For a large part of the early geologic history of the North American continent, the land now occupied by southeastern New Mexico was part of an ancient ocean south of the continent. For hundreds of millions of years, this region experienced almost uninterrupted deposition of marine shoreline sediments, primarily beach sands, and shallow water lime and clay muds. In the Pennsylvanian Period and later, coarser continental shelf and slope deposits occasionally slid into deeper basins. The relatively warm, calm seas promoted an abundance of marine life, which, upon death, accumulated in the muds and sands, altered to simple hydrocarbons, and eventually became the oil and gas found in some of these rock layers in recent years.
In pre-Permian periods, the collision of tectonic plates of the earth's mobile crust caused mountains to form to the southeast, north, and west of the WIPP site. The southeastern corner of New Mexico and western Texas remained relatively stable through the Permian Period of earth history (286 million to 245 million years ago [Ma])1, although some instability caused the earth's crust beneath this region to rise in some places and sink in others, in a belated response to the collision pressures. These fluctuations resulted in shallow areas on the sea floor ("shelves" or "platforms") and deeper areas ("basins") at the beginning of the Permian Period in southeastern New Mexico and western Texas (Figure A.1).
It was in the sediments accumulating in these deep basins that oil was generated, which moved into adjacent shelf environments to form the richest oil pools. One of these deep basins, the Delaware Basin, underlies WIPP and part of southeastern New Mexico. Most of these older rocks beneath WIPP are far too deep to bear on the performance of the repository. However, the presence of oil is considered a possible reason for inadvertent, or even deliberate, human intrusion into the repository in the future.
Late in the Permian, the warm and shallow seas encouraged the formation of reefs, similar to contemporary coral reefs in shallow tropical marine environments. As they grew, the reefs blocked off parts of the sea from time to time. With circulation of seawater restricted by the reef, the seawater began to evaporate, resulting in highly concentrated brines. As the brines continued to evaporate because of the warm temperature and restricted inflow of seawater, crystalline salts began to precipitate and accumulate on the bottom of the restricted basin. The salts varied in chemical composition, depending on the amount of water that had evaporated and on the concentrations of calcium (Ca), magnesium (Mg), sodium (Na), potassium (K), and other components of the common "evaporite" deposits.
As a result of these conditions, the WIPP area is underlain by a total accumulation of hundreds to thousands of meters of reef limestone (calcium carbonate, CaCO3); dolomite [calcium magnesium carbonate—CaMg(CO 3)2]; gypsum [a hydrous (water-bearing) calcium sulfate, CaSO4·2H2 O); halite rock salt, NaCl, the source of common table salt); smaller amounts of anhydrite (CaSO4); and potassium salts (collectively known as "potash," a commercially useful deposit).
Within part of the Permian, the Ochoan, during which the sea was blocked intermittently, the following four groups of rocks present at the WIPP site were formed successively:
- Castile Formation,
- Salado Formation,
- Rustler Formation, and
- Dewey Lake Red Beds.
The oldest and deepest of these rock units is the Castile Formation. Castile rocks are composed mainly of anhydrite with alternating thin limestone layers and a few thick layers of halite. Anhydrite and/or gypsum begins to precipitate from brines when 75 percent of the original amount of seawater has evaporated, continuing to be deposited until 90 percent has evaporated, at which point halite begins to precipitate (Kay and Colbert, 1965, p. 202). The alternating character of the Castile suggests that limited amounts of fresh seawater were able to enter the basin intermittently.
The Salado ("salty" in Spanish) Formation, in which the WIPP facility is located at 658 m below the surface, is composed predominantly of layers (beds) of rock salt, 200-400 m thick. It contains very thin layers (typically 0.1-1 m thick) of other materials (such as clay, anhydrite, and potash minerals) intercalated throughout the formation. Because these evaporite beds have been undisturbed by tectonic forces (large-scale movements of the earth's crust) for approximately 240 million years since their original accumulation, they are essentially horizontal and therefore can be traced continuously for great distances. The thin, distinctly identifiable nonhalite interbeds, called "marker beds" (MBs), have been numbered for identification from the lowest, at the top of the formation, to the highest, near the bottom. Marker beds have been used in the construction of the WIPP facility to keep the floor at a specific stratigraphic level, about 660 m below the surface. The floor of the facility is just above anhydrite MB 139 (Figure A.2).
About midway between the bottom and top of the Salado Formation is a notably thick series of beds, known as the McNutt member of the Salado, which contains economically valuable amounts of potash. The McNutt member, stratigraphically above the salt in which WIPP is located (Figure A.2), is about 120 m thick (Griswold, 1995) but is limited in a real extent within the evaporite basin (Figure A.1). The potash minerals, however, are of sufficient quantity for mining and extraction to have been a source of major economic activity in the area for many years.
Overlying the Salado Formation is the Rustler Formation, a thinner-bedded series of strata consisting of five distinct subunits (such sequences are called "members" of a formation). At the bottom of the formation are nonevaporite beds derived from clay muds and sands alternating with layers of halite, anhydrite, limestone and dolomite, and gypsum. The total thickness of the Rustler is about 100 m. The presence of nonevaporite materials such as the clay, silt, and sand (called "clastic deposits" or "clastics") indicates that periodic fluctuations in basin water levels and intermittent inflow of river and marine waters carried clastics into the basin material that sometimes formed separate layers without evaporites, such as sandstones and shales, or settled with the evaporites. The Culebra Dolomite Member of the Rustler Formation, which lies directly above the lowest Rustler clastic strata, is only about 7-8 m thick. The chemical, mineral, and hydrogeologic characteristics of the Culebra are important to a ground-water flow model for the WIPP area.
Dewey Lake Red Beds
The youngest (uppermost) Permian strata in the WIPP vicinity are thin, reddish beds of clay, silt, and sandy sediments produced in a restricted marine environment. The red color derives from the oxidized iron dispersed throughout the clay and sand, suggesting semiarid conditions during deposition of the sediments.
An assortment of rock types and ages younger than Permian is present in different locales around the region. Some discontinuous surface deposits were carried by high-energy streams that formed from the melting glaciers to the north during the Ice Age (the Pleistocene, from about 1.6 Ma to 10,000 years ago). One of the Pleistocene stream channels is still present west of the WIPP area as a broad, shallow valley, called Nash Draw, that is wider than 6 km in places (Figure A.3). The valley formed when the high-energy streams eroded down through the rock layers and exposed parts of the Rustler Formation, which are now visible in the walls of the valley. Nash Draw plays a role in determining the regional and local hydrology in the vicinity of WIPP.
Hydrologic Setting Of WIPP
Because flowing ground-water is one of the most likely means by which contaminants could reach the accessible human environment, it is important to understand the hydrogeologic character of the region.
Most of the hydrologic studies of the WIPP site and surrounding region have focused on the Rustler Formation in general, and on the Culebra Dolomite Member in particular. Because the latter is a water-bearing unit that is exposed in Nash Draw, it is a major concern in assessing the capabilities of WIPP to isolate radioactive wastes. However, the other major formations also play a role in WIPP hydrology for varying reasons.
The top of the Castile Formation lies about 200 m below the level of the Salado Formation at which the WIPP facility is located (Figure A.2). Under natural conditions, there is no flow of ground-water between the Castile and Salado Formations. However, during exploratory drilling to locate a site for WIPP, project scientists found large, highly permeable zones (pockets) at depths within the Castile, filled with brine under very high pressure (from the weight of hundreds of meters of overlying rocks). This is considered significant in evaluating the WIPP facility because brine in pressurized pockets, once penetrated by drilling, flows out to the surface rapidly and could continue to flow for hours or days, depending on the depth of the pocket, the volume, and pressure of the brine.
Hydrogeology of the Salado Formation
The mechanical behavior of salt, especially massively bedded salt, provides the basis for the inference that water cannot flow continuously through salt along interconnected pathways as it does in many other types of geologic materials. As a weak solid, salt flows under low pressure after a period of time, much as glacial ice flows.
Both are solids, but under pressure such as its own weight or the weight of material above, the crystal structure of the individual minerals that make up the rock salt deforms, causing movement within the minerals (intracrystalline gliding) and movement between crystals (intercrystalline slip) over time. This type of movement, termed ''creep," is a very slow process in human terms, in which the actual movement of the salt cannot be seen, although the results of creep are readily observable in room closure and the deformation of boreholes.
The continuous creep of rock salt causes opening and closing of pore spaces that trap droplets of water and prevent the continuous flow of ground-water through the interconnected pore spaces in the rock from one location to another. Creep also closes fractures and "heals" them as salt crystals flow into them and interconnect with other salt crystals. For this reason, the permeability of salt (the degree of interconnected pore spaces that allows flow of ground-water through the rock matrix or fractures) is generally regarded as extremely low to zero. Pressure tests done on the Salado Formation at various times, both from wells drilled at the surface and from underground tests after the facility was constructed, have indicated that the salt permeabilities are very low, close to the sensitivity limits of the instruments (Mercer, 1987; Beauheim et al., 1991, McTigue, 1993).
During the construction phase of the WIPP facility, brine was observed flowing or seeping into the space created whenever a new opening was made in the salt, whether in a narrow hole or in a large room. These observations led to consideration of the possibility that the disposal rooms could be flooded over the time required for isolation of the long-lived radionuclides
and that the consequent chemical reaction of the brine with metal containers would produce large volumes of hydrogen gas (Brush, 1994). These considerations assumed that brine was flowing in from the far field and had a continuous source. However, tests to monitor and measure the inflow of brine over a period of years have shown that the rate of inflow is greatest immediately following the disturbance of the salt to produce a new opening, then declines rapidly, and finally tapers to almost immeasurable amounts (Deal et al., 1991a, b).
These observations have led WIPP project researchers to consider that brine inflow into a newly created opening results when the salt is disturbed by drilling or mining and fracturing occurs in a zone around the new opening (disturbed rock zone, or DRZ), thus releasing brine from entrapped pore spaces (Stormont et al., 1991). Further details and discussion of this issue can be found in Chapter 3 and Appendixes C and D.
The only likely source of measurable Darcy flow in the Salado is the nonhalite interbeds, especially the anhydrite interbeds, which show varying degrees of fracturing. Fracture permeability can provide interconnected pathways for ground-water.
Hydrogeology of the Culebra Dolomite
The geological units comprising the Rustler Formation are illustrated in the stratigraphic column shown in Figure A.4. Note that the lithologies represented in this figure are generalized representations for the region and do not necessarily depict the exact distribution of rock types encountered in the Rustler at the WIPP site itself. As illustrated in Figure A.4, the Rustler outcrops are in the Nash Draw area several miles west of the WIPP site, but the formation lies at a depth of about 200 m below the land surface at WIPP.
Ground-water flow in the Rustler Formation is restricted mostly to the Rustler-Salado contact zone, the Culebra Dolomite Member, and the Magenta Dolomite Member (Brinster, 1991). Of these, the Culebra is the most transmissive. Although it is relatively thin (the thickness of the Culebra is generally between 7 and 8 m), it is also extensive in area.
The Rustler Formation, especially the Culebra Dolomite Member, has been the principal focus of hydrogeologic characterization of the WIPP vicinity (Lappin, 1988). The ground-water system in the Rustler is generally characterized by relatively low permeability (or transmissivity) and relatively poor water quality (i.e., too salty to be potable). The Rustler contains a predominance of secondary permeability features, such as fractures, bedding planes, and dissolution features; these may dominate flow in parts of the system, complicate analysis, and render quantitative descriptions difficult. At the very least, these features imply that the ground-water flow system is three-dimensional in a heterogeneous framework that is a challenge to model and understand properly. Both observation and water-supply wells are relatively scarce, contributing to the difficulty and expense of characterization. Although much excellent work has been done during the past 20 years in an attempt to characterize the relatively complex hydrogeology of the area near the WIPP site, some large uncertainties remain in the understanding of the subsurface system above the Salado. The recent state of knowledge has been summarized by Brinster (1991).
A number of water-level measurements and observations of drawdown and/or recovery during hydraulic tests have been used to measure the hydraulic heads and transmissivities in the Culebra. The changes in head over distance determine the hydraulic gradient, Transmissivity, which can vary significantly in value from point to point, is a measure of the ability of the rock to let water flow through it under a given hydraulic gradient.
However, some inconsistencies remain in the interpretation of the long-term flow history of the Culebra based on hydrologic evidence, compared to that inferred on the basis of geochemical and isotopic indicators. These inconsistencies, which reflect some degree of lack of understanding of present or past flow regimes within the Culebra, mean that there is uncertainty in the conceptual model that underlies the performance assessment (PA) model.
Ground-Water Flow Directions
In general, the direction of ground-water flow in the Culebra Dolomite in the WIPP area is from north to
south. Because the salinity of the ground-water is high and variable, the fluid density also varies significantly. Davies (1989) analyzed variable density flow in the Culebra and concluded that, at least in areas south of WIPP, flow directions can be calculated accurately only if variations in fluid density also are evaluated. Potentiometric data in the Magenta Dolomite indicate that the flow direction is predominantly from east to west. Differences in flow directions within the Rustler Formation reflect the complexity of the hydrogeologic system and may be influenced by vertical components of flow.
A fair degree of uncertainty remains about the amounts and locations of recharge to and discharge from the Culebra. In general, Brinster (1991) notes that recharge occurs north of the WIPP site, although some recharge may also occur east of WIPP because of vertical leakage from the Magenta. Natural discharge is south of WIPP, with some discharge to the Pecos River likely occurring at Malaga Bend.
Transmissivity estimates for the Culebra Dolomite in the WIPP area and in Nash Draw range over more than six orders of magnitude. Transmissivity is lowest east of WIPP (˜10-10 m2/s) and tends to increase to the west (˜10-3 m2/s). However, the transmissivity is highly variable and spatially correlated (e.g., LaVenue et al., 1995). In some areas, the transmissivity is controlled largely by fracture permeability, whereas in other areas it is governed by primary intergranular properties.
The time it takes for a parcel of water in the Culebra Dolomite to travel from the center of the WIPP site to its southern boundary is an important characteristic of the system controlling the transport of any contaminants released into the Culebra. This travel time depends on the transmissivity, hydraulic gradient, and effective porosity. Analyses assuming that the effective porosity governing transport is a value characteristic of the rock matrix (about 0.16) indicate that the mean travel time would be of the order of 14,000 years (LaVenue et al., 1990). However, if one assumes that the effective porosity is characteristic of fracture porosity (˜0.0015), then the travel time is reduced to only about 200 years (Davies et al., 1991).
Hydrogeology of the Dewey Lake Red Beds
The Dewey Lake Red Beds are believed to be less permeable than the Culebra Dolomite. However, very few hydraulic tests of this unit have been completed, and few observation wells are available to characterize the flow system. Brinster (1991) reports that although a continuous saturated zone has not been found, some localized zones of relatively high permeability have been identified. Potable water has been reported in some parts of Dewey Lake, perhaps because it is sufficiently shallow to receive direct recharge from precipitation events.
The geologic evolution of this region has resulted in the presence of economically valuable natural resources. The late Permian evaporating basin accumulated a large deposit of potash. The region in earlier Permian time can be characterized as a marine shoreline and some deeper offshore basins accumulating sands and limestone, clastics, and organic debris that resulted in accumulations of economically recoverable oil and gas in rock strata below the evaporites. Thus, the two major activities to extract mineral wealth from the WIPP region are the mining of potash and the drilling and extracting of oil and gas.
Potash, the commercial name for potassium-bearing minerals, is mined for its potassium, one of the three main plant nutrients in fertilizer. The potassium minerals are concentrated in a middle zone of the Salado Formation, the McNutt Member, which is about 140 m thick (Figure A.2). The Carlsbad Potash District contains the largest potash reserves in the United States (Barker and Austin, 1995; see also Figure A.1). The district is bounded on the west by the dissolution of the shallow Salado strata caused by circulating ground-water in the Pecos River drainage area to the west and south of Nash Draw.
Potash mining has been going on in the WIPP vicinity for more than 60 years. Potash was discovered in southeastern New Mexico in 1925. The first mine shaft to the potash 324 m (1,062 ft) below the surface was completed in 1930, and the first commercial shipment took place in 1931. Over the next 30 years, development and production advanced steadily as a large number of companies grew and merged. By the 1940s, New Mexico was the largest domestic potash producer, supplying 85 percent of the country's consumption.
This growth came to a halt and started to decline when potash was discovered, produced, and exported to the United States from Canada in the early 1960s at a lower cost than was economically feasible for U.S. producers. By 1970, Canadian imports exceeded domestic production. The decline continued despite a favorable ruling of dumping against Canadian producers by the International Trade Commission in 1987. Shortly thereafter, the former Soviet Union countries began exporting potash to the United States, further increasing competition and depressing both price and demand. Because of the foregoing and other economic and declining resource factors, the potash industry in the WIPP region is now much depressed, and very few companies are still active (Barker and Austin, 1995).
The mining front is now close to the WIPP site boundary and has reached the edge of the study area on the southwest side of the WIPP site (Figure A.5). In the future, the southwest or north side of the WIPP boundary may be the next target area for mining (Griswold, 1995).
In assessing potential future impacts of potash mining on the WIPP facility, the adoption of solution mining of potash minerals, a technique now being used in Canada, has been mentioned as a possible development. However, of the two dominant potash minerals being mined in the Carlsbad Potash District, sylvite and langbeinite, only sylvite is sufficiently soluble to make this mining process economically and practically feasible. According to specialists familiar with potash mining in this area, the beds containing sylvite are considered too thin for solution mining to be practical, but mine owners have not discounted this process as a future development in the area (Griswold, 1995).
If the potash resources overlying the repository were mined in the future, the resulting subsidence could significantly increase the transmissivity of the Culebra Dolomite, located approximately 200 m above the potash (Figure A.2). This could result in faster release of radionuclides to the environment following a human intrusion event.
Oil and Gas Resources
After the strata below the evaporites of the Delaware Basin had been bypassed for many years as unlikely to be producers of economically recoverable oil resources in the WIPP area, oil was discovered in the late 1980s and early 1990s. Indications in earlier exploration had suggested that mostly water was present in these strata. New techniques in analyzing the geophysical logs led to identification of several commercial oil pools in the Delaware Mountain Group of rocks at and below about 2,500 to 2,700 m in depth.
Major oil drilling activity began in the Delaware Basin following the discovery. This is now one of the most active areas of oil exploration and extraction in the United States (Broadhead et al., 1995, p. XI-10; Figure A.6). In the vicinity of WIPP, four major pools have been discovered, at least one within 1 km of the site boundary.
New technologies in oil drilling have improved recovery, safety, and cost efficiency. Improvements in well productivity result from advances in directional drilling, completion technologies, and stimulation techniques (Hareland, 1995). These advances and projected future oil and gas production were used to calculate the estimated value of oil and gas underneath the WIPP land withdrawal area. The estimated total primary and secondary recovery of probable resources in oil reservoirs under WIPP is about 54 million barrels of oil and gas condensate (Broadhead et al., 1995).
The secondary oil recovery technique known as waterflooding has been used in some areas around WIPP. A concern has been raised that such a process could introduce large amounts of water into the Salado Formation that could find its way to the WIPP facility by flow through marker beds, a process that has been observed in some oil fields in the region. In 1991, a flow of up to 1,200 barrels per hour of brine inflow was encountered in a hole being drilled for petroleum
production. At the time of inflow, the hole was approximately 100 m above the base of the Salado. The inflow was attributed to a waterflooding operation in the Rhodes Yates Field in which a large number of old wells were simultaneously injected. These injections occurred in the Yates Formation some 100 m below the base of the Salado, and 2 miles (3 km) from the well where the inflow occurred. This has raised concerns that similar inflows may occur due to waterflooding operations near WIPP and could compromise the integrity of the repository. This is further discussed in Chapter 3 (Box 3.2). See Figure A.7. A full discussion of waterflooding is presented in Broadhead et al. (1995).
A key issue is whether a future society would be aware of WIPP and its hazardous contents. The presumption that the existence or significance of WIPP would be lost is subjective and unquantifiable and therefore beyond the realm of rigorous scientific prediction. A second issue is whether such a society would elect to drill through it if knowledge of WIPP's existence were retained.
If future societies retain a knowledge of the existence of WIPP, yet still want to explore for or recover petroleum resources below WIPP, then directional drilling is feasible and likely. With present technology, a drilling operation could avoid damage to the repository if it were operated from the surface beyond the WIPP site boundary, and directed to avoid the waste horizon.
Manmade site markers, to be emplaced after closure, are required under the EPA standards. The reason for this requirement is that such markers, if they persist over time and are understood, could reduce the likelihood of inadvertent intrusion. The intent of erecting markers is to lead drillers to reconsider vertical boreholes that would penetrate the repository, and to choose instead alternative methods such as directional drilling techniques.
The assumption that the WIPP area would be drilled by today's methods for the next 10,000 years is also questionable. The recent advances in directional drilling techniques show that technology evolves in time.