Geochemist applying his broad-based geologic, environmental, and risk assessment/management/communication background to issues of national (nuclear waste disposal) and local (stormwater management and land use) importance.
Uranium and thorium decay series isotopes were measured in fluids and solids in the Coso geotherm... more Uranium and thorium decay series isotopes were measured in fluids and solids in the Coso geothermal system to assess the utility and constrain the limitations of the radioisotopic approach to the investigation of rock-water interaction. Fluid radioisotope measurements indicate substantial kilometer-scale variability in chemistry. Between 1988 and 1990, radium isotope activity ratios indicate temporal variability, which is exhibited by apparent mixing relationships observed as a function of time for single wells. Activity ratios of Ra-224/Ra-226 and Ra-228/Ra-226, and the processes that contribute and remove these radionuclide to and from the fluids, constrain residence times of fluids and may help constrain fluid velocities in the geothermal system. Activity ratios of Ra-224/Ra-226 > ten were measured. In groundwater and geothermal systems ratios of Ra-224/Ra-226 > ten are limited to zones of thermal upwelling or very young (days to weeks) waters in mountainous areas. Rn-222 results indicate that radon is also an effective tracer for steam velocities within the geothermal system. Analysis of carbon dioxide and Rn-222 data indicates that the residence time of steam (time since separation from the liquid) is short (probably less than four days). Estimates of fluid velocities derived from Rn-222 and radium isotopic measurements are within an order of magnitude of velocities derived from a fluorescein tracer test. Both Rn-222 and Ra-224 activities are higher in single-phase fluids in the northwest as compared to the southeast, indicating a higher rock-surface-area/water-volume ratio in the northwest. Thus, measurements of short-lived radioisotopes and gaseous phase constituents can constrain processes and characteristics of geothermal systems that are usually difficult to constrain (e.g., surface area/volume, residence times).
McKinley and Alexander (1996) criticized the model formulation of Ku et al. (1992) that described... more McKinley and Alexander (1996) criticized the model formulation of Ku et al. (1992) that described the use of naturally occurring U-Th series disequilibria in rock-water systems for evaluating the in situ retardation of radionuclide migration. Many of these criticisms appear to result from a misunderstanding of the model development and the formulation of the rockwater exchange terms. We present a clarification of the issues they have raised.
ABSTRACT Measurements of 222Rn, 226Ra, and 238U in formation waters derived from granitic rocks s... more ABSTRACT Measurements of 222Rn, 226Ra, and 238U in formation waters derived from granitic rocks surrounding the Cajon Pass borehole indicate concentrations of about 700, 1, and 0.01 dpm/1, respectively. The radon profile in the borehole indicates that inputs are localized and that several fracture zones must contribute significant amounts of inflow. Radon concentrations in fracture fluids indicate a minimum fracture width of 0.3 µm, and this width may be combined with the measured permeability to show that fracture porosity does not exceed 1.5 × 10−3. It is likely that the calculated fracture width is too small because incongruent weathering enriches fracture walls in 238U and 226Ra relative to the bulk rock. However, radon concentrations are lower than those in formation waters from other crystalline rocks, suggesting that the wall enrichments here are less pronounced and that fractures have experienced relatively limited fluid flow.
Concentrations of Ge(OH),, (denoted as Gei, or inorganic germanium) and Si(OH)4 were measured in ... more Concentrations of Ge(OH),, (denoted as Gei, or inorganic germanium) and Si(OH)4 were measured in water column and pore water samples from San Pedro and San Nicolas Basins in the Southern California Borderlands. There is a characteristic linear relationship between Gei and Si in the water column which implies that the distributions of germanium and silica are controlled by similar processes. In contrast, the pore water chemistry of germanium appears to differ from that of silica, particularly in the reducing, organic-rich sediments of San Pedro Basin. Silica concentrations in San Pedro Basin pore water asymptotically increase to 480 FM at 35-40 cm depth. Gei concentrations in the topmost 2 cm of pore water from San Pedro Basin are an order of magnitude greater than bottom water concentrations. Below 2 cm, Gei concentrations decrease exponentially with a half-distance of about 8 cm. Cycling of ferric oxyhydroxides at a redox boundary is the most likely cause of the Gei enrichment in San Pedro Basin pore water, while authigenic pyrite formation with the concomitant inclusion of germanium is a probable, but undocumented, cause of the downcore decrease in Gei. Pore water Gei and silica concentrations in sediments of San Nicolas Basin are generally greater than bottom water concentrations but vary irregularly with depth, probably due to irrigation.
The Nopal I uranium (U) deposit, in the Peiia Blanca District, Chihuahua, Mexico, has been identi... more The Nopal I uranium (U) deposit, in the Peiia Blanca District, Chihuahua, Mexico, has been identified as analogous in some regards to the candidate U.S. high-level waste (HLW) repository at Yucca Mountain, Nevada. Uranium transport at the Nopal I deposit has been studied to investigate mechanisms by which HLW components could be transported through silicic tuff over long time periods. This investigation focused on approximately 1400 m* of essentially continuous bedrock outcrop spanning the Nopal I deposit and surrounding host tuff. Data collected document: (i) the distributions of U within and around the Nopal I deposit, (ii) the distribution and characteristics of the fracture network within and surrounding the deposit. and (iii) the transport of U away from the deposit mainly along fracture paths. Uranium-series isotopic measurements indicate mobilization of U along the margin of the deposit within the last 1 Ma and significant U transport at about 54 Ka. Transport of U away from the Nopal I deposit along a few relatively continuous mesofractures achieved maximum distances at least 20 times greater than transport through the general fracture network composed of thousands of less continuous microfractures within and surrounding the deposit. Uranium transport away from the deposit appears to be largely independent of variations in the general fracture network pattern. Transport of U away from individual micro-and meso-fractures into homogeneous, unfractured tuff matrix appears limited to distances less than 1 mm. At the Nopal I deposit, matrix diffusion does not appear to have been an important factor for retardation of U. This analysis suggests a ranking for U retention: (i) microfracture network retention > > mesofracture retention, and (ii) individual microfracture retention > > matrix retention. Analogue Working Group Meeting and Pocos de Caldas projectfinal workshop (eds B. Come and N. A. Chapman), Final Reoort no. EUR 13014 EN: DD. 267-276. Commission of the European Communities,'Brussels. Neretnieks I. (1980) Diffusion in the rock matrix: An important factor in radionuclide retardation? J. Geophys. Res. 85, 4379-4397. Nordstrom D. K. (1982) Aqueous pyrite oxidation and the consequent formation of secondary iron minerals.
At the Nopal I uranium deposit, primary uraninite (nominally UO2+x) has altered almost completely... more At the Nopal I uranium deposit, primary uraninite (nominally UO2+x) has altered almost completely to a suite of secondary uranyl minerals. The deposit is located in a Basin and Range horst composed of welded silicic tuff; uranium mineralization presently occurs in a chemically oxidizing and hydrologically u,lsaturated zone of the structural block. These characteristics are similar to those of the proposed U.S. high-level nuclear waste (HLW) repository at Yucca Mountain, Nevada. Petrographic analyses indicate that residual Nopal I uraninite is fine grained (5-10/~m) and has a low trace element content (average about 3 wt%). These characteristics compare well with spent nuclear fuel. The oxidation and formation of secondary minerals from the uraninite have occurred in an environment dominated by components common in host rocks of the Nopal I system (e.g. Si, Ca, K, Na and I-I20) and also common to Yucca Mountain. In contrast, secondary phases in most other uranium deposits form from elements largely absent from spent fuel and from the Yucca Mountain environment (e.g. Pb, P and V). The oxidation of Nopal I uraninite and the sequence of alteration products, their intergrowths and morphologies are remarkably similar to those observed in reported corrosion experiments using spent fuel and unirradiated 002 under conditions intended to approximate those anticipated for the proposed Yucca Mountain repository. The end products of these reported laboratory experiments and the natural alteration of Nopal I uraninite are dominated by uranophane [nominally Ca(UO 2)2 Si207 "61-I2 O] with lesser amounts of soddyite [nominally (UO2)2SiO 4 • 2H20 ] and other uranyl minerals. These similarities in reaction product occurrence developed despite the differences in time and physical-chemical environment between Yucca Mountain-approximate laboratory experiments and Yucca Mountain-approximate uraninite alteration at Nopal I, suggesting that the results may reasonably represent phases likely to form during long-term alteration of spent fuel in a Yucca Mountain repository. From this analogy, it may be concluded that the likely compositional ranges of dominant spent fuel alteration phases in the Yucca Mountain environment may be relatively limited and may be insensitive to small variations in system conditions.
The prediction of radionuclide retardation and transport in geologic media requires a quantitativ... more The prediction of radionuclide retardation and transport in geologic media requires a quantitative knowledge of the sorption behavior of radioactive species, including the effects of groundwater chemistry and the water/rock ratio. In this study, experiments were conducted to determine the effects of solution pH and the sorbent-surface area on the sorption behavior of uranium (+6). The sorbent phase used was α-alumina (National Institute of Standards and Technology Reference Materials 8005, 8006, and 8007) with reported surface areas of 2.09, 0.229, and 0.0686 m 2 /g, respectively. Experimental results indicate that uranium (+6) sorption on α-alumina occurs at near-neutral pH where U0 2 (0H)5 is the predominant aqueous species, and that the total uranium sorbed increases as the sorbent-surface area/ solution-volume ratio increases. Results also show that as the sorbent-surface area/solution-volume ratio increases the "adsorption edge" shifts to more acidic pH, whereas the "desorption edge" shifts to more alkaline pH. Surface complexation modeling using the diffuse-layer model reproduces the sorptive behavior of uranium (+6) on the α-alumina relatively well.
The Nopal I uranium deposit at Pefla Blanca, Mexico is being studied as a natural analog of the p... more The Nopal I uranium deposit at Pefla Blanca, Mexico is being studied as a natural analog of the proposed high-level nuclear waste repository at Yucca Mountain. Identification of secondary uranium phases at Nopal I, and the sequence of their formation after uraninite oxidation, provides insight into the source term for uranium, and suggests that uranophane may control uranium release and transport in a silicic, tuffaceous, chemically oxidizing, and hydrologically unsaturated environment. Possible constraints on contaminant transport at Nopal I are derived from the spatial distribution of uranium and from measurements of 238 U decay-series isotopes. The analyses indicate that flow of U-bearing fluids was influenced strongly by fracture density, but that the flow of these fluids was not restricted to fractures. Gamma spectroscopic measurements of 238 U decayseries isotopes indicate secular equilibrium, which suggests undetectable U transport under present conditions.
Solid phase and pore water profiles of compounds containing iron and sulfur have been determined ... more Solid phase and pore water profiles of compounds containing iron and sulfur have been determined by wet chemical, magnetic, and X‐ray diffraction techniques in three California continental borderland basins. The observed profiles have been fit by simple reaction‐diffusion models in order to determine reaction rates and constrain budgets for iron and sulfur. More than 95% of the solid phase reduced sulfur is pyrite, and down core profiles are well fit by a model in which net sulfate reduction rates decrease exponentially with depth. Net sulfate reduction rates determined from models fit to solid phase reduced sulfur measurements and pore water sulfate profiles yield results that are consistent. Depth integrated sulfate reduction rates for modern sediments in San Pedro, Santa Catalina, and San Nicolas Basins are, 11.4, 6.3, and 6.3–8.8 (μmol cm‐2yr‐1), respectively. Measurements of solid phase iron species indicate that surficial sediments are enriched in easily‐reducible ferric oxyhydroxides. The enrichment is maintained by a combination of oxidation of Fe2+ diffusing upward from underlying anoxic sediments, as well as input of fresh sediment enriched in ferric oxyhydroxides. The three primary sources for iron converted to pyrite and the sequence in which they are utilized are: ferric oxyhydroxides, magnetite and other crystalline oxides, and “exchangeable” iron in phyllosilicates. The majority (50–80%) of the iron converted to pyrite is from the silicates, and budgetary calculations indicate the amount of iron released from San Pedro basin silicates agrees within 35% with the amount of magnesium removed from pore water to solid phases. Fe2+ is enriched in near‐surface pore waters because rates of dissolved iron production by oxyhydroxide reduction exceed rates of sulfate reduction and pyrite formation. At depth, pore waters are sulfidic because rates of sulfate reduction exceed rates of iron release from silicates. Sulfide produced at depth diffuses upward until it reaches sediments with available iron, causing a step‐like increase in solid phase sulfur concentration. Over 90% of the magnetite present in surficial sediments is dissolved at depth due to reaction with H2S. A model is developed to predict the depth at which magnetite dissolution should occur, based on sulfate reduction rates and the flux of ferric oxyhydroxides. The results of this model predict the onset of dissolution at depths of 5–40 cm in different basins and agree well with the observed depths of magnetite dissolution.
The rock magnetic properties of marine sediments from the California continental borderland (San ... more The rock magnetic properties of marine sediments from the California continental borderland (San Pedro, Santa Catalina, and San Nicolas basins) have been studied in order to quantitatively assess the effects of sediment diagenesis on magnetic minerals. Previous studies have noted that the natural remanent magnetization in these sediments, primarily carried by detrital magnetite, decays to 10% or less of its surface value soon after deposition. This decrease is caused by magnetite dissolution related to sediment diagenesis and is unrelated to paleoclimatic variations or changes in the regional influx of detrital magnetic material. Detailed rock magnetic measurements show that shifts to softer remanent coercivity and differences in the rate and degree of magnetic intensity loss with depth can be related to the dissolution process. The shift to softer remanent coercivity is related to a coarsening of the magnetic mineral grain sizes with depth due to preferential dissolution of the finest‐grained magnetic material. The intensity decreases, which are linearly proportional to magnetite concentration decreases, indicate that dissolution occurs with rate constants ranging from 0.3 to 1.6 kyr−1. The rate constants, sulfide concentrations, and magnetite grain size estimates from the borderland are consistant with previous studies of magnetite dissolution. Our results demonstrate the importance of both sulfide and magnetite surface area in the dissolution process. Anomalous peaks in viscous remanence within the sediments suggest the authigenic growth of greigite and its subsequent transformation to pyrite.
This is an annual status report on the results of research conducted on behalf of the US NRC by t... more This is an annual status report on the results of research conducted on behalf of the US NRC by the Center for Nuclear Waste Regulatory Analyses in support of activities under the Nuclear Waste Policy Act, as Amended. Nine specific projects are underway; eight of which are reported here. The Geochemistry project is using laboratory methods and computer calculations to assess key geochemical constraints and to evaluate sorptive properties of zeolites present at the proposed repository site. The Thermohydrology project has as its focus improved understanding of heat and fluid flow in unsaturated media. Laboratory, field, and calculational studies are combined in the Seismic Rock Mechanics project to examine the effects of repeated seismic loadings on the rock-mechanical and hydrological responses of rock masses. The Integrated Waste Package Experiments have been initiated to evaluate degradation modes of candidate waste container alloys. Three-dimensional computer analysis techniques are being used to investigate spatial variability of flow and transport in variably saturated fractured porous media in the Stochastic Flow and Transport project. The recently initiated Geochemical Analogs project seeks to investigate the role of such analogs in the licensing process, and is currently focused on characterizing and evaluating a potential site for investigation. The Sorption Modeling Project has as its objective the evaluation and eventual selection of model(s) of sorption processes which are deemed technically acceptable in the context of repository licensing. Finally, the Performance Assessment project is directed toward developing and evaluating methodologies for evaluation of the long-term performance of the proposed repository.
ABSTRACTSeveral radionuclides of the naturally occurring 238U, 235U and 232Th decay series have b... more ABSTRACTSeveral radionuclides of the naturally occurring 238U, 235U and 232Th decay series have been measured in high-temperature (300–350 °C) brines from several flow tests of the Sal ton Sea Scientific Drilling Project (SSSDP) well. Activities (dpm/kg) at the initial flow test of the well were 222Rn (2130), 226Ra (2200), 228Ra (1120), 224Ra (1010), 223Ra (6), 210Pb (3260), and 212Pb (2250). In contrast, activities of U and Th isotopes were much lower (< 1 dpm/kg). Following the first flow test (12/85), the well was deepened and sampled again during a short flow test (3/86). Radium and uranium concentrations were about two times lower during the second flow test, while other elements had similar concentrations. During a subsequent 20 day flow test of the well (6/88), flow rates were regulated at the wellhead to investigate reservoir characteristics, and 226Ra and 210Pb activities were found to be negatively correlated with the flow rate and positively correlated with the wellhead pressure. The 226Ra activity varied by a factor of five, roughly the variation in total flow rate, while well-head pressure varied only by 25% and 210Pb activity varied by a factor of two. Ratios of short-lived daughters to longer-lived parents increased in these flow tests. Results from mass balance calculations incorporating reaction kinetics suggest that differences in the degree of radium adsorption, rates of precipitation and dissolution of reservoir minerals exist in different flow zones.
The large uncertainties in many of the published dates for volcanic rocks in the Yucca Mountain r... more The large uncertainties in many of the published dates for volcanic rocks in the Yucca Mountain region (YMR) strongly affect probability and consequence models of volcanism, especially models focusing on Quaternary volcanism. Dating techniques for post-10 Ma basaltic rocks in the YMR have inherent limitations and uncertainties, which are rarely discussed in detail. Dates produced from the most widely available techniques have uncertainties that generally increase with decreasing age of the rock. Independent evaluation of most published dates is difficult due to a lack of information on analytical techniques, sample characteristics, and sources of error. Neogene basaltic volcanoes in the YMR have reported dates that generally reflect the precision and accuracy of the analytical technique, although the number of samples dated for each volcanic center only ranges from one to three. Dates for Quaternary volcanoes in the YMR have relatively large reported uncertainties and yield averages with large errors when reported uncertainty is propagated through statistical calculations. Using available data, estimates of the average ages of the Quaternary YMR volcanoes are 1.2±0.4 Ma for Crater Flat, 0.3±0.2 Ma for Sleeping Butte, and 0.1+0.05 Ma for Lathrop Wells. These dates generally do not represent the best dates possible with currently available geochronological techniques. Uncertainty in the age of Lathrop Wells is relatively small and does not affect current probability models significantly. However, the relatively large uncertainties in the ages of Crater Flat volcanoes strongly affect probability models.
Uranium and thorium decay series isotopes were measured in fluids and solids in the Coso geotherm... more Uranium and thorium decay series isotopes were measured in fluids and solids in the Coso geothermal system to assess the utility and constrain the limitations of the radioisotopic approach to the investigation of rock-water interaction. Fluid radioisotope measurements indicate substantial kilometer-scale variability in chemistry. Between 1988 and 1990, radium isotope activity ratios indicate temporal variability, which is exhibited by apparent mixing relationships observed as a function of time for single wells. Activity ratios of Ra-224/Ra-226 and Ra-228/Ra-226, and the processes that contribute and remove these radionuclide to and from the fluids, constrain residence times of fluids and may help constrain fluid velocities in the geothermal system. Activity ratios of Ra-224/Ra-226 > ten were measured. In groundwater and geothermal systems ratios of Ra-224/Ra-226 > ten are limited to zones of thermal upwelling or very young (days to weeks) waters in mountainous areas. Rn-222 results indicate that radon is also an effective tracer for steam velocities within the geothermal system. Analysis of carbon dioxide and Rn-222 data indicates that the residence time of steam (time since separation from the liquid) is short (probably less than four days). Estimates of fluid velocities derived from Rn-222 and radium isotopic measurements are within an order of magnitude of velocities derived from a fluorescein tracer test. Both Rn-222 and Ra-224 activities are higher in single-phase fluids in the northwest as compared to the southeast, indicating a higher rock-surface-area/water-volume ratio in the northwest. Thus, measurements of short-lived radioisotopes and gaseous phase constituents can constrain processes and characteristics of geothermal systems that are usually difficult to constrain (e.g., surface area/volume, residence times).
McKinley and Alexander (1996) criticized the model formulation of Ku et al. (1992) that described... more McKinley and Alexander (1996) criticized the model formulation of Ku et al. (1992) that described the use of naturally occurring U-Th series disequilibria in rock-water systems for evaluating the in situ retardation of radionuclide migration. Many of these criticisms appear to result from a misunderstanding of the model development and the formulation of the rockwater exchange terms. We present a clarification of the issues they have raised.
ABSTRACT Measurements of 222Rn, 226Ra, and 238U in formation waters derived from granitic rocks s... more ABSTRACT Measurements of 222Rn, 226Ra, and 238U in formation waters derived from granitic rocks surrounding the Cajon Pass borehole indicate concentrations of about 700, 1, and 0.01 dpm/1, respectively. The radon profile in the borehole indicates that inputs are localized and that several fracture zones must contribute significant amounts of inflow. Radon concentrations in fracture fluids indicate a minimum fracture width of 0.3 µm, and this width may be combined with the measured permeability to show that fracture porosity does not exceed 1.5 × 10−3. It is likely that the calculated fracture width is too small because incongruent weathering enriches fracture walls in 238U and 226Ra relative to the bulk rock. However, radon concentrations are lower than those in formation waters from other crystalline rocks, suggesting that the wall enrichments here are less pronounced and that fractures have experienced relatively limited fluid flow.
Concentrations of Ge(OH),, (denoted as Gei, or inorganic germanium) and Si(OH)4 were measured in ... more Concentrations of Ge(OH),, (denoted as Gei, or inorganic germanium) and Si(OH)4 were measured in water column and pore water samples from San Pedro and San Nicolas Basins in the Southern California Borderlands. There is a characteristic linear relationship between Gei and Si in the water column which implies that the distributions of germanium and silica are controlled by similar processes. In contrast, the pore water chemistry of germanium appears to differ from that of silica, particularly in the reducing, organic-rich sediments of San Pedro Basin. Silica concentrations in San Pedro Basin pore water asymptotically increase to 480 FM at 35-40 cm depth. Gei concentrations in the topmost 2 cm of pore water from San Pedro Basin are an order of magnitude greater than bottom water concentrations. Below 2 cm, Gei concentrations decrease exponentially with a half-distance of about 8 cm. Cycling of ferric oxyhydroxides at a redox boundary is the most likely cause of the Gei enrichment in San Pedro Basin pore water, while authigenic pyrite formation with the concomitant inclusion of germanium is a probable, but undocumented, cause of the downcore decrease in Gei. Pore water Gei and silica concentrations in sediments of San Nicolas Basin are generally greater than bottom water concentrations but vary irregularly with depth, probably due to irrigation.
The Nopal I uranium (U) deposit, in the Peiia Blanca District, Chihuahua, Mexico, has been identi... more The Nopal I uranium (U) deposit, in the Peiia Blanca District, Chihuahua, Mexico, has been identified as analogous in some regards to the candidate U.S. high-level waste (HLW) repository at Yucca Mountain, Nevada. Uranium transport at the Nopal I deposit has been studied to investigate mechanisms by which HLW components could be transported through silicic tuff over long time periods. This investigation focused on approximately 1400 m* of essentially continuous bedrock outcrop spanning the Nopal I deposit and surrounding host tuff. Data collected document: (i) the distributions of U within and around the Nopal I deposit, (ii) the distribution and characteristics of the fracture network within and surrounding the deposit. and (iii) the transport of U away from the deposit mainly along fracture paths. Uranium-series isotopic measurements indicate mobilization of U along the margin of the deposit within the last 1 Ma and significant U transport at about 54 Ka. Transport of U away from the Nopal I deposit along a few relatively continuous mesofractures achieved maximum distances at least 20 times greater than transport through the general fracture network composed of thousands of less continuous microfractures within and surrounding the deposit. Uranium transport away from the deposit appears to be largely independent of variations in the general fracture network pattern. Transport of U away from individual micro-and meso-fractures into homogeneous, unfractured tuff matrix appears limited to distances less than 1 mm. At the Nopal I deposit, matrix diffusion does not appear to have been an important factor for retardation of U. This analysis suggests a ranking for U retention: (i) microfracture network retention > > mesofracture retention, and (ii) individual microfracture retention > > matrix retention. Analogue Working Group Meeting and Pocos de Caldas projectfinal workshop (eds B. Come and N. A. Chapman), Final Reoort no. EUR 13014 EN: DD. 267-276. Commission of the European Communities,'Brussels. Neretnieks I. (1980) Diffusion in the rock matrix: An important factor in radionuclide retardation? J. Geophys. Res. 85, 4379-4397. Nordstrom D. K. (1982) Aqueous pyrite oxidation and the consequent formation of secondary iron minerals.
At the Nopal I uranium deposit, primary uraninite (nominally UO2+x) has altered almost completely... more At the Nopal I uranium deposit, primary uraninite (nominally UO2+x) has altered almost completely to a suite of secondary uranyl minerals. The deposit is located in a Basin and Range horst composed of welded silicic tuff; uranium mineralization presently occurs in a chemically oxidizing and hydrologically u,lsaturated zone of the structural block. These characteristics are similar to those of the proposed U.S. high-level nuclear waste (HLW) repository at Yucca Mountain, Nevada. Petrographic analyses indicate that residual Nopal I uraninite is fine grained (5-10/~m) and has a low trace element content (average about 3 wt%). These characteristics compare well with spent nuclear fuel. The oxidation and formation of secondary minerals from the uraninite have occurred in an environment dominated by components common in host rocks of the Nopal I system (e.g. Si, Ca, K, Na and I-I20) and also common to Yucca Mountain. In contrast, secondary phases in most other uranium deposits form from elements largely absent from spent fuel and from the Yucca Mountain environment (e.g. Pb, P and V). The oxidation of Nopal I uraninite and the sequence of alteration products, their intergrowths and morphologies are remarkably similar to those observed in reported corrosion experiments using spent fuel and unirradiated 002 under conditions intended to approximate those anticipated for the proposed Yucca Mountain repository. The end products of these reported laboratory experiments and the natural alteration of Nopal I uraninite are dominated by uranophane [nominally Ca(UO 2)2 Si207 "61-I2 O] with lesser amounts of soddyite [nominally (UO2)2SiO 4 • 2H20 ] and other uranyl minerals. These similarities in reaction product occurrence developed despite the differences in time and physical-chemical environment between Yucca Mountain-approximate laboratory experiments and Yucca Mountain-approximate uraninite alteration at Nopal I, suggesting that the results may reasonably represent phases likely to form during long-term alteration of spent fuel in a Yucca Mountain repository. From this analogy, it may be concluded that the likely compositional ranges of dominant spent fuel alteration phases in the Yucca Mountain environment may be relatively limited and may be insensitive to small variations in system conditions.
The prediction of radionuclide retardation and transport in geologic media requires a quantitativ... more The prediction of radionuclide retardation and transport in geologic media requires a quantitative knowledge of the sorption behavior of radioactive species, including the effects of groundwater chemistry and the water/rock ratio. In this study, experiments were conducted to determine the effects of solution pH and the sorbent-surface area on the sorption behavior of uranium (+6). The sorbent phase used was α-alumina (National Institute of Standards and Technology Reference Materials 8005, 8006, and 8007) with reported surface areas of 2.09, 0.229, and 0.0686 m 2 /g, respectively. Experimental results indicate that uranium (+6) sorption on α-alumina occurs at near-neutral pH where U0 2 (0H)5 is the predominant aqueous species, and that the total uranium sorbed increases as the sorbent-surface area/ solution-volume ratio increases. Results also show that as the sorbent-surface area/solution-volume ratio increases the "adsorption edge" shifts to more acidic pH, whereas the "desorption edge" shifts to more alkaline pH. Surface complexation modeling using the diffuse-layer model reproduces the sorptive behavior of uranium (+6) on the α-alumina relatively well.
The Nopal I uranium deposit at Pefla Blanca, Mexico is being studied as a natural analog of the p... more The Nopal I uranium deposit at Pefla Blanca, Mexico is being studied as a natural analog of the proposed high-level nuclear waste repository at Yucca Mountain. Identification of secondary uranium phases at Nopal I, and the sequence of their formation after uraninite oxidation, provides insight into the source term for uranium, and suggests that uranophane may control uranium release and transport in a silicic, tuffaceous, chemically oxidizing, and hydrologically unsaturated environment. Possible constraints on contaminant transport at Nopal I are derived from the spatial distribution of uranium and from measurements of 238 U decay-series isotopes. The analyses indicate that flow of U-bearing fluids was influenced strongly by fracture density, but that the flow of these fluids was not restricted to fractures. Gamma spectroscopic measurements of 238 U decayseries isotopes indicate secular equilibrium, which suggests undetectable U transport under present conditions.
Solid phase and pore water profiles of compounds containing iron and sulfur have been determined ... more Solid phase and pore water profiles of compounds containing iron and sulfur have been determined by wet chemical, magnetic, and X‐ray diffraction techniques in three California continental borderland basins. The observed profiles have been fit by simple reaction‐diffusion models in order to determine reaction rates and constrain budgets for iron and sulfur. More than 95% of the solid phase reduced sulfur is pyrite, and down core profiles are well fit by a model in which net sulfate reduction rates decrease exponentially with depth. Net sulfate reduction rates determined from models fit to solid phase reduced sulfur measurements and pore water sulfate profiles yield results that are consistent. Depth integrated sulfate reduction rates for modern sediments in San Pedro, Santa Catalina, and San Nicolas Basins are, 11.4, 6.3, and 6.3–8.8 (μmol cm‐2yr‐1), respectively. Measurements of solid phase iron species indicate that surficial sediments are enriched in easily‐reducible ferric oxyhydroxides. The enrichment is maintained by a combination of oxidation of Fe2+ diffusing upward from underlying anoxic sediments, as well as input of fresh sediment enriched in ferric oxyhydroxides. The three primary sources for iron converted to pyrite and the sequence in which they are utilized are: ferric oxyhydroxides, magnetite and other crystalline oxides, and “exchangeable” iron in phyllosilicates. The majority (50–80%) of the iron converted to pyrite is from the silicates, and budgetary calculations indicate the amount of iron released from San Pedro basin silicates agrees within 35% with the amount of magnesium removed from pore water to solid phases. Fe2+ is enriched in near‐surface pore waters because rates of dissolved iron production by oxyhydroxide reduction exceed rates of sulfate reduction and pyrite formation. At depth, pore waters are sulfidic because rates of sulfate reduction exceed rates of iron release from silicates. Sulfide produced at depth diffuses upward until it reaches sediments with available iron, causing a step‐like increase in solid phase sulfur concentration. Over 90% of the magnetite present in surficial sediments is dissolved at depth due to reaction with H2S. A model is developed to predict the depth at which magnetite dissolution should occur, based on sulfate reduction rates and the flux of ferric oxyhydroxides. The results of this model predict the onset of dissolution at depths of 5–40 cm in different basins and agree well with the observed depths of magnetite dissolution.
The rock magnetic properties of marine sediments from the California continental borderland (San ... more The rock magnetic properties of marine sediments from the California continental borderland (San Pedro, Santa Catalina, and San Nicolas basins) have been studied in order to quantitatively assess the effects of sediment diagenesis on magnetic minerals. Previous studies have noted that the natural remanent magnetization in these sediments, primarily carried by detrital magnetite, decays to 10% or less of its surface value soon after deposition. This decrease is caused by magnetite dissolution related to sediment diagenesis and is unrelated to paleoclimatic variations or changes in the regional influx of detrital magnetic material. Detailed rock magnetic measurements show that shifts to softer remanent coercivity and differences in the rate and degree of magnetic intensity loss with depth can be related to the dissolution process. The shift to softer remanent coercivity is related to a coarsening of the magnetic mineral grain sizes with depth due to preferential dissolution of the finest‐grained magnetic material. The intensity decreases, which are linearly proportional to magnetite concentration decreases, indicate that dissolution occurs with rate constants ranging from 0.3 to 1.6 kyr−1. The rate constants, sulfide concentrations, and magnetite grain size estimates from the borderland are consistant with previous studies of magnetite dissolution. Our results demonstrate the importance of both sulfide and magnetite surface area in the dissolution process. Anomalous peaks in viscous remanence within the sediments suggest the authigenic growth of greigite and its subsequent transformation to pyrite.
This is an annual status report on the results of research conducted on behalf of the US NRC by t... more This is an annual status report on the results of research conducted on behalf of the US NRC by the Center for Nuclear Waste Regulatory Analyses in support of activities under the Nuclear Waste Policy Act, as Amended. Nine specific projects are underway; eight of which are reported here. The Geochemistry project is using laboratory methods and computer calculations to assess key geochemical constraints and to evaluate sorptive properties of zeolites present at the proposed repository site. The Thermohydrology project has as its focus improved understanding of heat and fluid flow in unsaturated media. Laboratory, field, and calculational studies are combined in the Seismic Rock Mechanics project to examine the effects of repeated seismic loadings on the rock-mechanical and hydrological responses of rock masses. The Integrated Waste Package Experiments have been initiated to evaluate degradation modes of candidate waste container alloys. Three-dimensional computer analysis techniques are being used to investigate spatial variability of flow and transport in variably saturated fractured porous media in the Stochastic Flow and Transport project. The recently initiated Geochemical Analogs project seeks to investigate the role of such analogs in the licensing process, and is currently focused on characterizing and evaluating a potential site for investigation. The Sorption Modeling Project has as its objective the evaluation and eventual selection of model(s) of sorption processes which are deemed technically acceptable in the context of repository licensing. Finally, the Performance Assessment project is directed toward developing and evaluating methodologies for evaluation of the long-term performance of the proposed repository.
ABSTRACTSeveral radionuclides of the naturally occurring 238U, 235U and 232Th decay series have b... more ABSTRACTSeveral radionuclides of the naturally occurring 238U, 235U and 232Th decay series have been measured in high-temperature (300–350 °C) brines from several flow tests of the Sal ton Sea Scientific Drilling Project (SSSDP) well. Activities (dpm/kg) at the initial flow test of the well were 222Rn (2130), 226Ra (2200), 228Ra (1120), 224Ra (1010), 223Ra (6), 210Pb (3260), and 212Pb (2250). In contrast, activities of U and Th isotopes were much lower (< 1 dpm/kg). Following the first flow test (12/85), the well was deepened and sampled again during a short flow test (3/86). Radium and uranium concentrations were about two times lower during the second flow test, while other elements had similar concentrations. During a subsequent 20 day flow test of the well (6/88), flow rates were regulated at the wellhead to investigate reservoir characteristics, and 226Ra and 210Pb activities were found to be negatively correlated with the flow rate and positively correlated with the wellhead pressure. The 226Ra activity varied by a factor of five, roughly the variation in total flow rate, while well-head pressure varied only by 25% and 210Pb activity varied by a factor of two. Ratios of short-lived daughters to longer-lived parents increased in these flow tests. Results from mass balance calculations incorporating reaction kinetics suggest that differences in the degree of radium adsorption, rates of precipitation and dissolution of reservoir minerals exist in different flow zones.
The large uncertainties in many of the published dates for volcanic rocks in the Yucca Mountain r... more The large uncertainties in many of the published dates for volcanic rocks in the Yucca Mountain region (YMR) strongly affect probability and consequence models of volcanism, especially models focusing on Quaternary volcanism. Dating techniques for post-10 Ma basaltic rocks in the YMR have inherent limitations and uncertainties, which are rarely discussed in detail. Dates produced from the most widely available techniques have uncertainties that generally increase with decreasing age of the rock. Independent evaluation of most published dates is difficult due to a lack of information on analytical techniques, sample characteristics, and sources of error. Neogene basaltic volcanoes in the YMR have reported dates that generally reflect the precision and accuracy of the analytical technique, although the number of samples dated for each volcanic center only ranges from one to three. Dates for Quaternary volcanoes in the YMR have relatively large reported uncertainties and yield averages with large errors when reported uncertainty is propagated through statistical calculations. Using available data, estimates of the average ages of the Quaternary YMR volcanoes are 1.2±0.4 Ma for Crater Flat, 0.3±0.2 Ma for Sleeping Butte, and 0.1+0.05 Ma for Lathrop Wells. These dates generally do not represent the best dates possible with currently available geochronological techniques. Uncertainty in the age of Lathrop Wells is relatively small and does not affect current probability models significantly. However, the relatively large uncertainties in the ages of Crater Flat volcanoes strongly affect probability models.
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