Potassium and Calcium Isotopic Fractionation by Plants (Soybean [<i>Glycine max</i>], Rice [<i>Oryza sativa</i>], and Wheat [<i>Triticum aestivum</i>])

We conducted hydroponic experiments growing soybean (Glycine max), rice (Oryza sativa), and wheat (Triticum aestivum) under K and Ca replete conditions to establish the degree of K isotopic fractionation by plants, and compare the isotopic fractionation of Ca and K. Each of the test plants displays fractionation relative to the growth solution favoring the light isotopes of K and Ca. The average δ41K values of the roots from the three plant species were similar, and have an overall average of −0.55 ± 0.24‰ 2s, while the overall average δ44Ca for roots is −0.67 ± 0.44. For leaves, the overall average of δ41K is −0.97 ± 0.4‰, compared to an overall average leaf δ44Ca of −0.83 ± 0.09‰. In the case of the soybean plants, the lightest K and Ca occurs in the stems with average δ41K of −1.31 ± 0.40‰ 2s and average δ44Ca of −1.20 ± 0.19 ‰ 2s. We present a simple box model involving the relative fluxes of K and its isotopic fractionation that reproduces our K isotopic observations and suggests a fractionation of ∼0.8‰ with K uptake from solution by roots. Directly comparing the per amu fractionation of K and Ca reveals an average factor of 2.05 ± 0.50 2s greater fractionation of K isotopes which may reflect their different roles and behaviors in plants

Tracking Sources of Unsaturated Zone and Groundwater Nitrate Contamination Using Nitrogen and Oxygen Stable Isotopes at the Hanford Site, Washington

The nitrogen and oxygen isotopic compositions of nitrate in pore water extracts from unsaturated zone (UZ) core samples and groundwater samples indicate at least four potential sources of nitrate in groundwaters at the U.S. DOE Hanford Site in south-central Washington. Natural sources of nitrate identified include microbially produced nitrate from the soil column (δ15N of 4−8‰, δ18O of −9 to 2‰) and nitrate in buried caliche layers (δ15N of 0−8‰, δ18O of −6 to 42‰). Isotopically distinct industrial sources of nitrate include nitric acid in low-level disposal waters (δ15N ≈ 0‰, δ18O ≈ 23‰) and co-contaminant nitrate in high-level radioactive waste from plutonium processing (δ15N of 8−33‰, δ18O of −9 to 7‰). The isotopic compositions of nitrate from 97 groundwater wells with concentrations up to 1290 mg/L NO3- have been analyzed. Stable isotope analyses from this study site, which has natural and industrial nitrate sources, provide a tool to distinguish nitrate sources in an unconfined aquifer where concentrations alone do not. These data indicate that the most common sources of high nitrate concentrations in groundwater at Hanford are nitric acid and natural nitrate flushed out of the UZ during disposal of low-level wastewater. Nitrate associated with high-level radioactive UZ contamination does not appear to be a major source of groundwater nitrate at this time

Radiogenic ⁴⁰Ca in Seawater: Implications for Modern and Ancient Ca Cycles

Radiogenic 40Ca is preferentially concentrated in the continental crust through the decay of radioactive 40K and may have the potential to be used as a tracer for Ca fluxes to the ocean through time. Numerous published flux estimates suggest that rivers are the dominant source of Ca to the oceans. This conflicts, however, with conclusions drawn from previous radiogenic Ca data suggesting that seawater Ca has been dominated by weathering/hydrothermal alteration of oceanic crust throughout Earth history. We attempt to address this discrepancy by carrying out a larger number of radiogenic Ca measurements on materials that represent modern seawater, marine carbonates, and ocean floor basalt. We find that the 40Ca/44Ca composition of the oceanic crust and mantle appear to be different from modern seawater and marine carbonates, such that the latter are measurably enriched in radiogenic 40Ca (εCa = +1.1 ± 0.3, 2SE) relative to basalts and deep-sea hydrothermal fluid. This observation is consistent with most other data available in the literature. The results are also consistent with Sr isotope data and confirm that continental sources of Ca (mainly from rivers and groundwater) dominate the modern seawater budget. We find that off-axis Ca fluxes from the low temperature alteration of the oceanic crust are not large enough to change this balance. The Ca isotope data measured and compiled here also suggest that bulk-silicate earth 40Ca/44Ca is 1.2 ± 0.3 ε-units lower than reference material SRM915a and that variations in seawater εCa in the geologic past are likely too small to be resolved with current analytical techniques

Radiogenic ⁴⁰Ca in Seawater: Implications for Modern and Ancient Ca Cycles

Radiogenic 40Ca is preferentially concentrated in the continental crust through the decay of radioactive 40K and may have the potential to be used as a tracer for Ca fluxes to the ocean through time. Numerous published flux estimates suggest that rivers are the dominant source of Ca to the oceans. This conflicts, however, with conclusions drawn from previous radiogenic Ca data suggesting that seawater Ca has been dominated by weathering/hydrothermal alteration of oceanic crust throughout Earth history. We attempt to address this discrepancy by carrying out a larger number of radiogenic Ca measurements on materials that represent modern seawater, marine carbonates, and ocean floor basalt. We find that the 40Ca/44Ca composition of the oceanic crust and mantle appear to be different from modern seawater and marine carbonates, such that the latter are measurably enriched in radiogenic 40Ca (εCa = +1.1 ± 0.3, 2SE) relative to basalts and deep-sea hydrothermal fluid. This observation is consistent with most other data available in the literature. The results are also consistent with Sr isotope data and confirm that continental sources of Ca (mainly from rivers and groundwater) dominate the modern seawater budget. We find that off-axis Ca fluxes from the low temperature alteration of the oceanic crust are not large enough to change this balance. The Ca isotope data measured and compiled here also suggest that bulk-silicate earth 40Ca/44Ca is 1.2 ± 0.3 ε-units lower than reference material SRM915a and that variations in seawater εCa in the geologic past are likely too small to be resolved with current analytical techniques

Isotopic Tracking of Hanford 300 Area Derived Uranium in the Columbia River

Our objectives in this study are to quantify the discharge rate of uranium (U) to the Columbia River from the Hanford Site’s 300 Area and to follow that U downriver to constrain its fate. Uranium from the Hanford Site has variable isotopic composition due to nuclear industrial processes carried out at the site. This characteristic makes it possible to use high-precision isotopic measurements of U in environmental samples to identify even trace levels of contaminant U, determine its sources, and estimate discharge rates. Our data on river water samples indicate that as much as 3.2 kg/day can enter the Columbia River from the 300 Area, which is only a small fraction of the total load of dissolved natural background U carried by the Columbia River. This very low level of Hanford-derived U can be discerned, despite dilution to <1% of natural background U, 400 km downstream from the Hanford Site. These results indicate that isotopic methods can allow the amounts of U from the 300 Area of the Hanford Site entering the Columbia River to be measured accurately to ascertain whether they are an environmental concern or insignificant relative to natural uranium background in the Columbia River

Radiogenic ⁴⁰Ca in Seawater: Implications for Modern and Ancient Ca Cycles

Radiogenic 40Ca is preferentially concentrated in the continental crust through the decay of radioactive 40K and may have the potential to be used as a tracer for Ca fluxes to the ocean through time. Numerous published flux estimates suggest that rivers are the dominant source of Ca to the oceans. This conflicts, however, with conclusions drawn from previous radiogenic Ca data suggesting that seawater Ca has been dominated by weathering/hydrothermal alteration of oceanic crust throughout Earth history. We attempt to address this discrepancy by carrying out a larger number of radiogenic Ca measurements on materials that represent modern seawater, marine carbonates, and ocean floor basalt. We find that the 40Ca/44Ca composition of the oceanic crust and mantle appear to be different from modern seawater and marine carbonates, such that the latter are measurably enriched in radiogenic 40Ca (εCa = +1.1 ± 0.3, 2SE) relative to basalts and deep-sea hydrothermal fluid. This observation is consistent with most other data available in the literature. The results are also consistent with Sr isotope data and confirm that continental sources of Ca (mainly from rivers and groundwater) dominate the modern seawater budget. We find that off-axis Ca fluxes from the low temperature alteration of the oceanic crust are not large enough to change this balance. The Ca isotope data measured and compiled here also suggest that bulk-silicate earth 40Ca/44Ca is 1.2 ± 0.3 ε-units lower than reference material SRM915a and that variations in seawater εCa in the geologic past are likely too small to be resolved with current analytical techniques

Radiogenic ⁴⁰Ca in Seawater: Implications for Modern and Ancient Ca Cycles

Radiogenic 40Ca is preferentially concentrated in the continental crust through the decay of radioactive 40K and may have the potential to be used as a tracer for Ca fluxes to the ocean through time. Numerous published flux estimates suggest that rivers are the dominant source of Ca to the oceans. This conflicts, however, with conclusions drawn from previous radiogenic Ca data suggesting that seawater Ca has been dominated by weathering/hydrothermal alteration of oceanic crust throughout Earth history. We attempt to address this discrepancy by carrying out a larger number of radiogenic Ca measurements on materials that represent modern seawater, marine carbonates, and ocean floor basalt. We find that the 40Ca/44Ca composition of the oceanic crust and mantle appear to be different from modern seawater and marine carbonates, such that the latter are measurably enriched in radiogenic 40Ca (εCa = +1.1 ± 0.3, 2SE) relative to basalts and deep-sea hydrothermal fluid. This observation is consistent with most other data available in the literature. The results are also consistent with Sr isotope data and confirm that continental sources of Ca (mainly from rivers and groundwater) dominate the modern seawater budget. We find that off-axis Ca fluxes from the low temperature alteration of the oceanic crust are not large enough to change this balance. The Ca isotope data measured and compiled here also suggest that bulk-silicate earth 40Ca/44Ca is 1.2 ± 0.3 ε-units lower than reference material SRM915a and that variations in seawater εCa in the geologic past are likely too small to be resolved with current analytical techniques

Timing the Onset of Sulfate Reduction over Multiple Subsurface Acetate Amendments by Measurement and Modeling of Sulfur Isotope Fractionation

Stable isotope fractionations of sulfur are reported for three consecutive years of acetate-enabled uranium bioremediation at the US Department of Energy’s Rifle Integrated Field Research Challenge (IFRC) site. The data show a previously undocumented decrease in the time between acetate addition and the onset of sulfate reducing conditions over subsequent amendments, from 20 days in the 2007 experiment to 4 days in the 2009 experiment. Increased sulfide concentrations were observed at the same time as δ³⁴S of sulfate enrichment in the first year, but in subsequent years elevated sulfide was detected up to 15 days after increased δ³⁴S of sulfate. A biogeochemical reactive transport model is developed which explicitly incorporates the stable isotopes of sulfur to simulate fractionation during the 2007 and 2008 amendments. A model based on an initially low, uniformly distributed population of sulfate reducing bacteria that grow and become spatially variable with time reproduces measured trends in solute concentration and δ³⁴S, capturing the change in onset of sulfate reduction in subsequent years. Our results demonstrate a previously unrecognized hysteretic effect in the spatial distribution of biomass growth during stimulated subsurface bioremediation

Isotopic Evidence for Reductive Immobilization of Uranium Across a Roll-Front Mineral Deposit

We use uranium (U) isotope ratios to detect and quantify the extent of natural U reduction in groundwater across a roll front redox gradient. Our study was conducted at the Smith Ranch-Highland in situ recovery (ISR) U mine in eastern Wyoming, USA, where economic U deposits occur in the Paleocene Fort Union formation. To evaluate the fate of aqueous U in and adjacent to the ore body, we investigated the chemical composition and isotope ratios of groundwater samples from the roll-front type ore body and surrounding monitoring wells of a previously mined area. The ²³⁸U/²³⁵U of groundwater varies by approximately 3‰ and is correlated with U concentrations. Fluid samples down-gradient of the ore zone are the most depleted in ²³⁸U and have the lowest U concentrations. Activity ratios of ²³⁴U/²³⁸U are ∼5.5 up-gradient of the ore zone, ∼1.0 in the ore zone, and between 2.3 and 3.7 in the down-gradient monitoring wells. High-precision measurements of ²³⁴U/²³⁸U and ²³⁸U/²³⁵U allow for development of a conceptual model that evaluates both the migration of U from the ore body and the extent of natural attenuation due to reduction. We find that the premining migration of U down-gradient of the delineated ore body is minimal along eight transects due to reduction in or adjacent to the ore body, whereas two other transects show little or no sign of reduction in the down-gradient region. These results suggest that characterization of U isotopic ratios at the mine planning stage, in conjunction with routine geochemical analyses, can be used to identify where more or less postmining remediation will be necessary

Isotopic and Geochemical Tracers for U(VI) Reduction and U Mobility at an in Situ Recovery U Mine

In situ recovery (ISR) uranium (U) mining mobilizes U in its oxidized hexavalent form (U­(VI)) by oxidative dissolution of U from the roll-front U deposits. Postmining natural attenuation of residual U­(VI) at ISR mines is a potential remediation strategy. Detection and monitoring of naturally occurring reducing subsurface environments are important for successful implementation of this remediation scheme. We used the isotopic tracers ²³⁸U/²³⁵U (δ²³⁸U), ²³⁴U/²³⁸U activity ratio, and ³⁴S/³²S (δ³⁴S), and geochemical measurements of U ore and groundwater collected from 32 wells located within, upgradient, and downgradient of a roll-front U deposit to detect U­(VI) reduction and U mobility at an ISR mining site at Rosita, TX, USA. The δ²³⁸U in Rosita groundwater varies from +0.61‰ to −2.49‰, with a trend toward lower δ²³⁸U in downgradient wells. The concurrent decrease in U­(VI) concentration and δ²³⁸U with an ε of 0.48‰ ± 0.08‰ is indicative of naturally occurring reducing environments conducive to U­(VI) reduction. Additionally, characteristic ²³⁴U/²³⁸U activity ratio and δ³⁴S values may also be used to trace the mobility of the ore zone groundwater after mining has ended. These results support the use of U isotope-based detection of natural attenuation of U­(VI) at Rosita and other similar ISR mining sites