Effect of reducing groundwater on the retardation of redox-sensitive radionuclides

Occurrence and Geochemistry of Lead-210 and Polonium-210 Radionuclides in Public-Drinking-Water Supplies from Principal Aquifers of the United States

On the basis of lifetime cancer risks, lead-210 (²¹⁰Pb) and polonium-210 (²¹⁰Po) ≥ 1.0 and 0.7 pCi/L (picocuries per liter), respectively, in drinking-water supplies may pose human-health concerns. ²¹⁰Pb and ²¹⁰Po were detected at concentrations greater than these thresholds at 3.7 and 1.5%, respectively, of filtered untreated groundwater samples from 1263 public-supply wells in 19 principal aquifers across the United States. Nationally, 72% of samples with radon-222 (²²²Rn) concentrations > 4000 pCi/L had ²¹⁰Pb ≥ 1.0 pCi/L. ²¹⁰Pb is mobilized by alpha recoil associated with the decay of ²²²Rn and short-lived progeny. ²¹⁰Pb concentrations ≥ 1.0 pCi/L occurred most frequently where acidic groundwaters inhibited ²¹⁰Pb readsorption (felsic-crystalline rocks) and where reducing alkaline conditions favored dissolution of iron-manganese- (Fe-Mn-) oxyhydroxides (which adsorb ²¹⁰Pb) and formation of lead-carbonate complexes (enhancing lead (Pb) mobility). ²¹⁰Po concentrations ≥ 0.7 pCi/L occurred almost exclusively in confined Coastal Plain aquifers where old (low percent-modern carbon-14) groundwaters were reducing, with high pH (>7.5) and high sodium/chloride (Na/Cl) ratios resulting from cation exchange. In high-pH environments, aqueous polonium (Po) is poorly sorbed, occurring as dihydrogen polonate (H₂PoO₃(aq)) or, under strongly reducing conditions, as a hydrogen-polonide anion (HPo^-). Fe-Mn- and sulfate-reduction and cation-exchange processes may mobilize polonium from mineral surfaces. Po²⁺ occurrence in low-to-neutral-pH waters is attenuated by adsorption.

Plutonium Desorption from Nuclear Melt Glass-Derived Colloids and Implications for Migration at the Nevada National Security Site, USA

The migration of low levels of plutonium has been observed at the Nevada National Security Site (NNSS) and attributed to colloids. To better understand the mechanism(s) of colloid-facilitated transport at this site, we performed flow cell desorption experiments with mineral colloid suspensions produced by hydrothermal alteration of NNSS nuclear melt glass, residual material left behind from nuclear testing. Three different colloid suspensions were used: (1) colloidal material from hydrothermal alteration of nuclear melt glass at 140 °C; (2) at 200 °C; and (3) plutonium sorbed to SWy-1 montmorillonite at room temperature. The 140 °C sample contained only montmorillonite, while zeolite and other phases were present in the 200 °C sample. Overall, more plutonium was desorbed from the 140 °C colloids (ca. 9-16%) than from the 200 °C colloids (ca. 4-8%). Furthermore, at the end of the 4.5 day flow cell experiments, the desorption rates for the 140 °C colloids and the Pu-montmorillonite colloids were similar while the desorption rates from the 200 °C colloids were up to an order of magnitude lower. We posit that the formation of zeolites and clays hydrothermally altered at 200 °C may lead to a more stable association of plutonium with colloids, resulting in lower desorption rates. This may give rise to more extensive colloid-facilitated transport and help explain why trace levels of plutonium are found downgradient from their original source decades after a nuclear detonation. Interestingly, in the case of cesium (a co-contaminant of plutonium), no difference was observed between the 140 and 200 °C colloids. This reflects intrinsic differences between cesium and plutonium sorption/desorption behavior (charge, cation size) and suggests that the Cs sorption mechanism (cation exchange) is not similarly affected by colloid formation temperature.

The speciation, transformation kinetics and fate of spiked Pu (IV) in highly saline groundwater

The mobility of plutonium (Pu) in groundwater is dependent of its speciation distribution and transformation. The speciation and transformation kinetics of Pu(IV) and its colloids in highly saline groundwater have, however, been rarely studied. In the present study, groundwater (Ionic strength 1 M) from Dunhuang region, NW China, was collected for investigating the speciation, transformation kinetics and fate of spiked Pu (IV) with aging time. The results showed that ~99% of the spiked Pu (IV) (over initial concentration c₀ range 2.5 × 10^-10-7.8 × 10^-7 mol·L^-1) was easily associated with the natural colloids and transformed into relatively unstable Pu pseudo-colloids in 1 day, which then gradually deposited and/or adsorbed on the container walls with aging. The suspended Pu pseudo-colloids decreased in similar exponential models, with rate equations r(t) = -3.1 × 10^-10e^{- t/4} and -1.3 × 10^-8e^-/3 for c₀ = 1.25 × 10^-9 mol·L^-1and 4.17 × 10^-8 mol·L^-1, respectively. The chemical speciation of the suspended colloidal Pu was dominated by "Fe/Mn Oxides" at the early time, while "Carbonates" with slower depositing rate (r(t) = -6.9 × 10^-12e^{- 0.149t}) dominated it (~82%) at equilibrium state. Whatever the c₀ was, the concentration of dissolved Pu (i.e., the apparent solubility of Pu) kept at 0.7 × 10^-11 mol·L^-1 over aging. The valence of dissolved Pu was dominated by Pu(IV) at early time, while Pu(V + VI) would become dominant (~95%) at equilibrium state with transformation rate of r(t) = -92.9e^{- t/16.6} + 96.9. The equilibrium times of Pu deposition (and/or adsorption), speciation transformation of the suspended colloidal Pu, and valence change of the dissolved Pu were 30 d, 80 d and 120 d, respectively. The kinetic process for each Pu species could be well fitted with exponential model. These results suggest that the majority of released Pu(IV) into highly saline groundwater will be easily associated with natural aquatic colloids and then become immobile in short time due to deposition (and/or adsorption) onto the environmental medium, but potential migration risk caused by stable suspended Pu colloids cannot be ignored.

Hydrothermal Alteration of Nuclear Melt Glass, Colloid Formation, and Plutonium Mobilization at the Nevada National Security Site, U.S.A

Approximately 2.8 t of plutonium (Pu) has been deposited in the Nevada National Security Site (NNSS) subsurface as a result of underground nuclear testing. Most of this Pu is sequestered in nuclear melt glass. However, Pu migration has been observed and attributed to colloid facilitated transport. To identify the mechanisms controlling Pu mobilization, long-term (∼3 year) laboratory nuclear melt glass alteration experiments were performed at 25 to 200 °C to mimic hydrothermal conditions in the vicinity of underground nuclear tests. The clay and zeolite colloids produced in these experiments are similar to those identified in NNSS groundwater. At 200 °C, maximum Pu and colloid concentrations of 30 Bq/L and 150 mg/L, respectively, were observed. However, much lower Pu and colloid concentrations were observed at 25 and 80 °C. These data suggest that Pu concentrations above the drinking water Maximum Contaminant Levels (0.56 Bq/L) may exist during early hydrothermal conditions in the vicinity of underground nuclear tests. However, formation of colloid-associated Pu will tend to decrease with time as nuclear test cavity temperatures decrease. Furthermore, median colloid concentrations in NNSS groundwater (1.8 mg/L) suggest that the high colloid and Pu concentrations observed in our 140 and 200 °C experiments are unlikely to persist in downgradient NNSS groundwater. While our experiments did not span all groundwater and nuclear melt glass conditions that may be present at the NNSS, our results are consistent with the documented low Pu concentrations in NNSS groundwater.

Incorporation of Metals into Calcite in a Deep Anoxic Granite Aquifer

Understanding metal scavenging by calcite in deep aquifers in granite is of importance for deciphering and modeling hydrochemical fluctuations and water-rock interaction in the upper crust and for retention mechanisms associated with underground repositories for toxic wastes. Metal scavenging into calcite has generally been established in the laboratory or in natural environments that cannot be unreservedly applied to conditions in deep crystalline rocks, an environment of broad interest for nuclear waste repositories. Here, we report a microanalytical study of calcite precipitated over a period of 17 years from anoxic, low-temperature (14 °C), neutral (pH: 7.4-7.7), and brackish (Cl: 1700-7100 mg/L) groundwater flowing in fractures at >400 m depth in granite rock. This enabled assessment of the trace metal uptake by calcite under these deep-seated conditions. Aquatic speciation modeling was carried out to assess influence of metal complexation on the partitioning into calcite. The resulting environment-specific partition coefficients were for several divalent ions in line with values obtained in controlled laboratory experiments, whereas for several other ions they differed substantially. High absolute uptake of rare earth elements and U(IV) suggests that coprecipitation into calcite can be an important sink for these metals and analogousactinides in the vicinity of geological repositories.

Reductive immobilization of pertechnetate in soil and groundwater using synthetic pyrite nanoparticles

Radioactive technetium (⁹⁹Tc) is of intense concern because of its toxicity and high mobility in the environment. Reduction of Tc(VII) to Tc(IV) decreases the mobility and availability of technetium in soil and groundwater. In this study, pyrite nanoparticles (FeS₂) were synthesized, characterized and tested for immobilizing/removing ⁹⁹Tc(VII) in soil and groundwater through batch and column experiments. Influences of particle dosage, dissolved organic matter (DOM), and pH on the reductive immobilization kinetics were examined. At a dosage of 0.28 g/L as Fe, the pyrite nanoparticles were able to rapidly and completely remove 4.88 × 10^-7 M of Tc(VII) by converting it to insoluble Tc(IV), with a retarded first-order rate constant of 0.30 h^-1. The presence of high concentrations of DOM only moderately inhibited the reduction effectiveness, and acidic pH was more favorable for Tc(VII) reduction. Column experiments showed that embedding a 0.8 cm pyrite layer of the material in a soil bed, simulating a permeable reactive barrier, was able to retard technetium transport 710 times more than a model sandy soil. The results demonstrated that the pyrite particles may serve as a long-lasting reactive material to remediate Tc-contaminated soil, groundwater and solid wastes.

Plutonium(IV) and (V) Sorption to Goethite at Sub-Femtomolar to Micromolar Concentrations: Redox Transformations and Surface Precipitation

Pu(IV) and Pu(V) sorption to goethite was investigated over a concentration range of 10(-15)-10(-5) M at pH 8. Experiments with initial Pu concentrations of 10(-15) - 10(-8) M produced linear Pu sorption isotherms, demonstrating that Pu sorption to goethite is not concentration-dependent across this concentration range. Equivalent Pu(IV) and Pu(V) sorption Kd values obtained at 1 and 2-week sampling time points indicated that Pu(V) is rapidly reduced to Pu(IV) on the goethite surface. Further, it suggested that Pu surface redox transformations are sufficiently rapid to achieve an equilibrium state within 1 week, regardless of the initial Pu oxidation state. At initial concentrations >10(-8) M, both Pu oxidation states exhibited deviations from linear sorption behavior and less Pu was adsorbed than at lower concentrations. NanoSIMS and HRTEM analysis of samples with initial Pu concentrations of 10(-8) - 10(-6) M indicated that Pu surface and/or bulk precipitation was likely responsible for this deviation. In 10(-6) M Pu(IV) and Pu(V) samples, HRTEM analysis showed the formation of a body centered cubic (bcc) Pu4O7 structure on the goethite surface, confirming that reduction of Pu(V) had occurred on the mineral surface and that epitaxial distortion previously observed for Pu(IV) sorption occurs with Pu(V) as well.

Colloid-borne forms of tetravalent actinides: a brief review

Tetravalent actinides, An(IV), are usually assumed to be little mobile in near-neutral environmental waters because of their low solubility. However, there are certain geochemical scenarios during which mobilization of An(IV) in a colloid-borne (waterborne) form cannot be ruled out. A compilation of colloid-borne forms of tetravalent actinides described so far for laboratory experiments together with several examples of An(IV) colloids observed in field experiments and real-world scenarios are given. They are intended to be a knowledge base and a tool for those who have to interpret actinide behavior under environmental conditions. Synthetic colloids containing structural An(IV) and synthetic colloids carrying adsorbed An(IV) are considered. Their behavior is compared with the behavior of An(IV) colloids observed after the intentional or unintentional release of actinides into the environment. A list of knowledge gaps as to the behavior of An(IV) colloids is provided and items which need further research are highlighted.

Modeling of the fate of radionuclides in urban sewer systems after contamination due to nuclear or radiological incidents

After an accidental radioactive contamination by aerosols in inhabited areas, the radiation exposure to man is determined by complex interactions between different factors such as dry or wet deposition, different types of ground surfaces, chemical properties of the radionuclides involved and building development as well as dependence on bomb construction e.g. design and geometry. At short-term, the first rainfall is an important way of natural decontamination: deposited radionuclides are washed off from surfaces and in urban areas the resulting contaminated runoff enters the sewer system and is collected in a sewage plant. Up to now the potential exposure caused by this process has received little attention and is estimated here with simulation models. The commercial rainfall-runoff model for urban sewer systems KANAL++ has been extended to include transport of radionuclides from surfaces through the drainage to various discharge facilities. The flow from surfaces is modeled by unit hydrographs, which produce boundary conditions for a system of 1d coupled flow and transport equations in a tube system. Initial conditions are provided by a map of surface contamination which is produced by geo-statistical interpolation of γ-dose rate measurements taking into account the detector environment. The corresponding methodology is implemented in the Inhabited Area Monitoring Module (IAMM) software module as part of the European decision system JRODOS. A hypothetical scenario is considered where a Radiation Dispersal Device (RDD) with Cs-137 is detonated in a small inhabited area whose drainage system is realistically modeled. The transition of deposited radionuclides due to rainfall into the surface runoff is accounted for by different nuclide-specific entrainment coefficients for paved and unpaved surfaces. The concentration of Cs-137 in water is calculated at the nodes of the drainage system and at the sewage treatment plant. The external exposure to staff of the treatment plant is estimated. For Cs-137 radiation levels in the plant are low since wash-off of cesium from surfaces is an ineffective process.