|Fig. 1: Image of the Waste Isolation Pilot Plant (WIPP) in New Mexico (Source: Wikimedia Commons ).|
The Waste Isolation Pilot Plant (WIPP) is a deep geologic depository for transuranic waste, which is defined by Congress in The Waste Isolation Pilot Plant Land Withdrawal Act as material containing more than 100 nanocuries of alpha- emitting transuranic isotopes per gram of waste, with half-lives greater than 20 years.  "Transuranic" describes elements such as plutonium whose atomic numbers are higher than uranium.  Over the past 15 years, the WIPP has become storage site for over 91,000 cubic meters of transuranic waste, which consists of laboratory equipment and clothing and residual chemical waste from U.S. nuclear weapons projects.  The majority of transuranic contaminants are long-lived plutonium isotopes (Pu-239 whose half-life is 24,100 years and plutonium-240 whose half-life is 6,560 years) and shorter-lived Am and Cm isotopes.  Alongside engineered barriers, the WIPP incorporates a number of natural geologic barriers to prevent any stored transuranic waste from leaking into the surrounding biosphere.
Located 42 km from Carlsbad, New Mexico, the WIPP storage facilities are 655 m underground in 600-meter- thick bedded halite of the Permian Salado Formation in the Delaware Basin.  The transuranic waste is stored in plastic-lined steel drums within these underground storage sites.  This bedded salt formation is the major geologic barrier for radionuclide leakage at the WIPP site. The halite layers have exceptionally low permeability with hydraulic conductivity of 10-11 to 10-16 m/s depending on the purity of the halite layer.  Despite the low permeability of the salt layer, it is not completely dry; it is estimated that the brine comprises 1 to 2% of the Salado formation by weight and up to 25% by weight of the clay seams within the layers.  Fluids such as brine pose a significant danger to effective storage of transuranic waste because if brine seeps into the transuranic waste storage sites, the radionuclides can enter the migrating fluid and eventually enter groundwater or leak onto the surface.
Although the low permeability of the halite layers is estimated to prevent leakage of transuranic-waste- contaminated brine from reaching the surface for the next ten thousand years, the process of excavating the storage rooms within the salt bed can induce slightly increased permeability of the local halite layers.  When a new storage site is first mined from the salt bed, volumetric expansion and inelastic dilation of the surrounding salt mass occurs as it expands into the newly excavated space. In terms of percolation theory, fluid flow through a mass occurs when an interconnected pore network of sufficient size has formed.  Under undisturbed conditions, the halite layer may have low enough porosity that an interconnected pore network will not form. However, even miniscule levels of volume dilation (>0.05%) can generate micro-fractures that generate an interconnected pore network of sufficient size for the flow of contaminated fluid to the surface, even when such a network was not present prior to excavation. 
Portions of the WIPP storage sites lie within the Castile formation, which is a 385 m thick salt bed consisting of three major anhydrite units with intermittent halite layers.  While the permeability of the layers is sufficiently low enough to consider it impermeable, large reservoirs of brine exist in the region, remnants of ancient Permian seas.  The largest brine reservoir near the WIPP is estimated to be 985 m below the surface and contain 2.7 × 106 m3 of brine. The hydrostatic pressure within the reservoir is estimated to be 12.7 MPa, which is slightly higher than the hydrostatic pressure (11.1 MPa) that is expected for a column of brine at that depth. This hydrostatic pressure is high enough to propel brine onto the surface through a bore-hole drilled into the reservoir. 
Salt deposits such as the Castile and Salado formation are associated with mineral and energy resources like hydrocarbons and potassium salts.  Indeed, the Delaware basin has been exploited for its oil and gas resources since the 1920s.  Near the WIPP, sandstones and limestones at depths between 3600 to 4600 m produce natural gas. Sandstone and carbonate reservoirs in the Bone Spring, Brushy Canyon, and Cherry Canyon formations produce oil at depths from 1680 to 3360 m below the surface.  While drilling within the 41.4 km2 region enclosing the WIPP is prohibited, drilling is currently underway in most of the surrounding regions. As knowledge of the WIPP is lost over the next couple thousand years, it becomes increasing likely that drilling in the region may spread into the protected zone. In calculating the probability that future drilling in the region may accidentally puncture a brine reservoir, the EPA has estimated 67.3 boreholes per km2 over the next 10,000 years based on averaging the number of boreholes made per year over the past 100 years.  However, drilling near the WIPP has rapidly increased the past few years. Advances in hydraulic-fracturing techniques have allowed companies to access new regions of hydrocarbon-bearing rocks, making the Permian Basin one of the largest hydrocarbon-rich areas in the U.S. Based on drilling rates in the region from 2002 to 2012, it is estimated that there will actually be closer to 150 boreholes per km2 over the next 10,000 years, which is more than double the projected EPA values.  As more and more boreholes are drilled in the nearby region, it increases the likelihood that one of them may lead to leakage of the stored transuranic waste.
The WIPP is the United States' only transuranic waste depository and plays a key role in managing the waste generated from nuclear weapon defense projects. While the geological barriers for transuranic waste leakage in the Delaware basin have been rigorously studied and characterized since the 1970s, human intrusion into the region through oil drilling and potash mining can easily compromise the integrity of these natural barriers. Furthermore, the current regulatory period is short compared to the 24,100 year half-life of Pu-239. Intrusion into the transuranic waste storage sites over the next couple millennia could release radionuclide contamination into the surrounding biosphere.
© Jeremy Uang. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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