Final report of the Peña Blanca natural analogue project [report]

Schön S. Levy, Steven Joel Goldstein, Amr I. Abdel-Fattah, Ronald S. Amato, Elizabeth Anthony, Paul Cook, Patrick F. Dobson, Mostafa Fayek, Diana French, Rodrigo de Garza, Teamrat Ghezzehei, Philip C. Goodell (+15 others)
2016 unpublished
5 EXECUTIVE SUMMARY The Peña Blanca region, 50 km north of Chihuahua City, Chihuahua, México, was a target of uranium exploration and mining by the Mexican government. After mining ceased in 1981, researchers became interested in this region as a study area for subsurface uranium migration with relevance to geologic disposal of nuclear waste. Many studies related to this concept were conducted at the Nopal I mine site located on a cuesta (hill) of the Sierra Peña Blanca. This site has geologic,
more » ... tectonic, hydrologic, and geochemical similarities to Yucca Mountain, Nevada, a formerly proposed site for a high-level nuclear-waste repository in the unsaturated zone. The U.S. Department of Energy (U.S. DOE), Office of Civilian Radioactive Waste Management (OCRWM), sponsored studies at Nopal I in the 1990s and supported the drilling of three research wells -PB1, PB2, and PB3 -at the site in 2003. Beginning in 2004, the Peña Blanca Natural Analogue Project was undertaken by U.S. DOE, OCRWM to develop a three-dimensional conceptual model of the transport of uranium and its radiogenic daughter products at the Nopal I site. SITE STRATIGRAPHY AND DEPOSITIONAL HISTORY Four main stratigraphic units were penetrated by the continuously cored PB1 well. These units comprise all of the unsaturated zone and about 25 m of the saturated zone. The lowest unit is Cretaceous limestone that represents carbonate deposits east (basinward) of the Cretaceous reef complex bordering the Chihuahua Trough. The Pozos Formation above the limestone is a continental molasse deposited on the margins of terrane uplifted during early to mid-Tertiary time. The age of the unit is constrained between 54 and 44.8 million years (Ma). The tuffaceous Coloradas Formation unconformably overlies the Pozos Formation. Constraints on the age of this unit are the same as for the Pozos Formation. The 44.8-Ma Nopal Formation, also a tuff, is the uppermost preserved stratigraphic unit of the cuesta. URANIUM DISTRIBUTION IN THE NOPAL WELLS The most complete information about uranium distribution in the subsurface rock comes from borehole natural gamma logs and hand-held scintillometer survey of the continuous core from PB1. Both surveys measured gross gamma counts. High gamma counts predominantly reflect the presence of uranium. High gamma counts associated with the main ore deposit are strongest in PB1, where values between 500 and 100,000 counts per second (cps) were recorded from the surface to about 113-m depth. The corresponding interval in PB2 is from the surface to 86-m depth (≤ 3,000 cps) and in PB3 from 17 to 119-m depth (≤ 4,500 cps). Deeper gamma anomalies (beneath the Nopal I ore body) are mostly within the Pozos conglomerate. The strongest anomalies are in the lower Pozos and, only in PB3, in Cretaceous limestone. Natural gamma values in the 125-m-deep well PB4, within limestone 1.3 km SE of Nopal I, do not exceed 175 cps. The hand-held gamma survey of PB1 drill core detected "hot Uranium concentrations and isotopic ratios were measured for shallow saturated-zone waters from Nopal I and regional wells. For most wells, samples were collected from 2003 to 2006. Uranium concentrations in the Nopal wells have decreased over time as the local effects of drilling contamination have diminished. During the last year of sampling (2005-2006), concentrations were between 23 and 61 ppb in PB1 and between 370 and 1077 ppb in PB3. Concentrations in the smaller sample set for PB2 are between 11.9 and 137.2 ppb. From 2003 to 2006, uranium varied between 0.1 and 15 ppb in PB4 and between 5.0 and 5.8 ppb at the Pozos Ranch well (~1.3 km ESE of PB1). Uranium in the Peña Blanca Ranch well (~4.4 km NW of PB1) varied from 9.5 to 10.0 ppb between 2003 and 2005. 11 Modeling Uranium-Isotopic Systematics of Seepage Water An analytical model was constructed based on the concept that intermittent seasonal infiltration and percolation in the shallow unsaturated zone leads to a linear relationship between reciprocal uranium concentration and 234 U/ 238 U ratio in percolating waters. Seepage waters from various locations in the adit define distinct linear trends based on wet-versus-dry-season collection time. The underlying cause of this difference is the accumulation of recoil-produced 234 U on rock-pore and fracture surfaces during the dry season, followed by preferential uptake of the surficial 234 U by percolating water during the wet season. Other factors contributing to the trends include rock-water interaction (or water-transit) time, uranium-dissolution rate, and the rate of recoil that supplies 234 U to the rock surfaces. Longer periods of low infiltration and lower uranium-dissolution rates lead to high 234 U/ 238 U ratios in the percolating water. RADIONUCLIDE TRANSPORT IN SOIL AT THE NOPAL CUESTA Surficial weathering of the uranium ore body exposed at the surface of the Nopal cuesta has potential importance as a source of uranium transported into the unsaturated zone. Aspects of this process could be relevant analogues to extreme nuclear-waste release scenarios involving exposure or deposition of contaminated material on the ground surface. At Nopal I, the natural soil was removed during the mining operation. A site with natural soil was studied close to where blocks of uranium ore were stockpiled in the 1980s. One residual ore block was selected to study the migration of uranium and its daughters into the underlying soil over a period of about twenty-five years. Samples from the ore block and underlying soil profile were analyzed by gamma spectroscopy to identify radionuclide peaks from the 238 U-decay series. Gross-gamma counts on two incomplete soil-sample suites show the radioactivity generally decreasing with depth to low levels below six to seven centimeters. These findings are consistent with similar studies elsewhere. The ore-block sample was relatively close to secular equilibrium for daughter/parent pairs 230 Th/ 234 U and 226 Ra/ 230 Th, with activity ratios of 1.222 and 1.206, respectively. A majority of the soil samples have non-equilibrium 230 Th/ 234 U activity ratios in the range of 1.80 to 2.18. The two shallowest samples from two soil suites have activity ratios between 1.18 and 1.50, probably due to incorporation of ore-block fragments into the soil. All soil samples had non-equilibrium 226 Ra/ 230 Th activity ratios in the range of 1.70 to 2.71. Data for a complete eight-interval suite from below the ore block show that background radiation levels are reached at depths of about 10 to 15 cm. PERFORMANCE ASSESSMENT MODELING The simulation of radionuclide transport at Nopal I was conducted by the Yucca Mountain Project, a former repository program. A numerical model was used to analyze the mobilization and groundwater transport of radionuclides released from the ore 12 deposit. The goal of the investigation was to estimate whether further investigations at the Nopal I site would provide a credible natural-analogue comparison with the expected performance of the Yucca Mountain site. The numerical analysis used as input a combination of Yucca Mountain process models and Nopal I site-specific data such as the vertical section of rock units in the unsaturated zone, porosity and permeability data, the inferred eastward direction of regional groundwater flow, and the dimensions of the ore body. Estimates of the uranium content of the original ore deposit and the inventory of uranium species and daughter products were based on published studies. The model assumed that uranium oxide (uraninite) is analogous to spent nuclear fuel. The model simulated meteoric-water infiltration into the unsaturated zone, downward water percolation and dissolution of the uraninite, fluid mixing in the unsaturated and saturated zones, and eastward transport in the saturated zone. Radionuclide concentrations were captured for the saturated zone 150, 600, and 1,300 m downgradient from the ore body. The only corroborating data available were water analyses for well PB4, 1,300 m SE of Nopal I. A base-case analysis was run, along with sensitivity analyses, to investigate the effects of reduced infiltration, variations in the solubility of the ore body, and variations in sorption coefficients. The study concluded that, even with strong sorption, uranium could be transported from the vicinity of Nopal I in amounts that would be detectable in PB4 water. The simulations projected concentrations of 99 Tc in the groundwater derived predominantly from natural fission of 238 U . The pertechnetate anion behaves as a conservative non-sorbing species in water, unlike uranium. Model results suggested that 99 Tc concentrations would be very low, but detectable (~10 -8 mg/L or ~10 -5 ppb). The numerical model was updated as the Peña Blanca Natural Analogue Model (PBNAM). The results were calibrated to uranium concentrations reported for 2003 water samples from the shallow saturated zone in boreholes PB1, PB2, and PB3. Examples of model results include a base-case simulation for 238 U transport for 100 realizations of the uncertain dissolution parameters, but not including sorption. The observed uranium concentrations in Nopal well waters are bracketed by the range of results obtained in the simulations and within the uncertainty of the source-dissolution parameters. This result remains generally valid even with the trend of gradually diminishing uranium content shown by more recent water samples. The updated PBNAM predicted a 99 Tc concentration of 2.8 × 10 -2 ppb in groundwater directly beneath the ore body. However, analytical results of Nopal well waters from two laboratories determined that no 99 Tc was observed above a detection limit of about 6 × 10 -5 ppb. After the PB wells were drilled at Nopal I, elevated uranium concentrations in the wells might have provided an opportunity for an informal saturated-zone tracer test in which higher uranium concentrations might be detected in well PB4. The three-year record of water chemistry for PB4 contains no clear evidence of water with elevated uranium 13 content. The record of decreasing uranium concentrations and the concentrations of short-lived uranium-series daughters in the Nopal wells were used to estimate upper-limit groundwater velocities of 5 to 15 m/y, so that any tracer plume could take hundreds of years to reach PB4. CONCLUSIONS The Peña Blanca Natural Analogue Project studies reported here will support the development of hypotheses regarding unsaturated-zone and saturated-zone uranium transport away from the Nopal I ore body as an analogue for a nuclear waste repository in the unsaturated zone. The drilling of three boreholes, one continuously cored, at Nopal I provided core samples for stratigraphic, mineralogic, and radiometric analyses. Borehole geophysical logs documented vertical and lateral variations in bedrock radioactivity. Analyses of the multiyear sample sets of saturated-zone waters from the Nopal wells, along with samples from regional wells, supported estimates of groundwater travel time, water mixing, saturated-zone retardation factors, and the relative contributions of various uranium reservoirs to the groundwater. These data also contributed to conceptual models of uranium dissolution and transport. Percolating waters collected from a shallow adit below ground surface generally show differences in uranium content and 234 U/ 238 U ratios between waters traversing the ore body and waters moving through fractures in unmineralized rock. Waters from the ore body tend to have higher uranium contents and 234 U/ 238 U ratios close to unity. Percolation from the unmineralized rock has lower uranium content and higher 234 U/ 238 U ratios. The first radiometric dating of uranium minerals established an age of 32 ± 8 Ma for the primary uranium mineralization. Alteration and secondary-uranium-mineral deposition at 3.1 ± 0.5 Ma may mark the beginning of a transition from a reducing environment under saturated conditions to an oxidizing environment under unsaturated conditions. Local reducing environments have survived, indicating that after three million years, the rock units have not fully equilibrated with the mostly oxidizing environment. Initial uranium enrichment in the Pozos conglomerate may date from a period of early diagenesis. Authigenic uraninite less than one million years old in the Pozos documents recent water-rock interaction involving the redistribution of uranium. Most of the uranium redistribution in the unsaturated zone appears to have diminished prior to about 200 ka. The updated numerical transport model, dubbed the Peña Blanca Natural Analogue Model, included results calibrated to uranium concentrations for 2005 Nopal waters. Within the uncertainty of the source-dissolution parameters, the results are generally valid even with the trend of diminishing uranium content shown by more recent samples. A 99 Tc concentration of 2.8 × 10 -2 ppb predicted by the model in groundwater directly beneath the ore body was not confirmed: no 99 Tc was observed above a detection limit of about 6 × 10 -5 ppb.
doi:10.2172/1329043 fatcat:rxfzlpyaavb7rjx4oqxj7xc2vu