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).
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.
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.
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).
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.
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.
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Papers by Bret Leslie