Richard T. Lowson

234U238U and 230Th234U activity ratios in the colloidal phases of aquifers in lateritic weathered zones

A procedure was developed for comparing solute and colloid phases of groundwaters in contact with uranium ore bodies at Nabarlek and Koongarra in the Alligator Rivers region, Northern Territory, Australia. Single-pass ultrafiltration of large volumes with cut-offs of 18 nm and 1 {mu}m was used. Colloids were composed of Fe and Si species with sorbed U and U daughters. Uranium isotopes were mostly present as soluble species. Thorium was significantly associated with the colloids. The {sup 234}U/{sup 238}U activity ratios (ARs) were similar in solute and colloid phases close to the ore bodies but further down-gradient colloids were generally more depleted of {sup 234}U than the solute. The {sup 230}Th/{sup 234}U ARs rose from very low values for both solute and colloid phases close to the ore bodies through several orders of magnitude to much higher values further down-gradient. Colloid {sup 230}Th/{sup 234}U ARs were always significantly greater than solute ARs. Results were consistent with a systematic leaching of U from colloids going down-gradient and very little mobilization of ore body. {sup 230}Th relative to U. Ubiquitous complexes {sup 232}Th appeared to suppress the solubility of {sup 230}Th.

A vibrational spectroscopic 18O tracer study of pyrite oxidation

Pyrite was oxidised under 18O2 gas in H216O solutions, with and without added ferric ion, and the sulfate produced was analysed by vibrational spectroscopy to determine the relative amounts of sulfate isotopomers (S16On18O4−n2−) formed. At 70°C and pH 1, with no added Fe3+, the majority of the sulfate formed was that which derived all four oxygen atoms from water (i.e., S16O42−), but significant amounts of two other isotopomers, S16O318O2− and S16O218O22−, which derive one or two oxygen atoms from molecular oxygen were observed. When Fe3+ was added at the start under identical conditions, no S16O218O22− was observed. The major isotopomer formed was still S16O42−, with S16O318O2− present as a minor product. Experiments which were performed at initial pH 7 yielded similar results, as did others performed at 20°C, although the amounts of the minor isotopomers formed vary with temperature. All of the results were confirmed by performing identical experiments with the source of the oxygen isotopes reversed, that is, by oxidising pyrite under air in H218O solutions and obtaining the same products in isotopic reverse.

sup 234 U/ sup 238 U and sup 230 Th/ sup 234 U activity ratios in the collidal phases of aquifers in lateritic weathered zones

A procedure was developed for comparing solute and colloid phases of groundwaters in contact with uranium ore bodies at Nabarlek and Koongarra in the Alligator Rivers region, Northern Territory, Australia. Single-pass ultrafiltration of large volumes with cut-offs of 18 nm and 1 {mu}m was used. Colloids were composed of Fe and Si species with sorbed U and U daughters. Uranium isotopes were mostly present as soluble species. Thorium was significantly associated with the colloids. The {sup 234}U/{sup 238}U activity ratios (ARs) were similar in solute and colloid phases close to the ore bodies but further down-gradient colloids were generally more depleted of {sup 234}U than the solute. The {sup 230}Th/{sup 234}U ARs rose from very low values for both solute and colloid phases close to the ore bodies through several orders of magnitude to much higher values further down-gradient. Colloid {sup 230}Th/{sup 234}U ARs were always significantly greater than solute ARs. Results were consistent with a systematic leaching of U from colloids going down-gradient and very little mobilization of ore body. {sup 230}Th relative to U. Ubiquitous complexes {sup 232}Th appeared to suppress the solubility of {sup 230}Th.

Thorium-uranium disequilibrium dating of Late Quaternary ferruginous concretions and rinds

Radioactive ferruginous nodules from the Alligator Rivers Uranium Province, Northern Territory, Australia, were systematically examined by optical microscopy, XRD, alpha- and fission-track autoradiography, and analysed for U, Th, Ra and major stable elements. Correlation of autoradiographs with microscopic structure and analyses of selective extractions confirmed that U and Th were strongly associated with Fe oxides. Fission-track autoradiography showed no concentration gradient evidence for postdepositional leaching of uranium. Strong acid leaching studies showed that irreversibly adsorbed U, and authigenic 234U and 230Th, but not 226Ra, are quantitatively retained by accumulated oxide/oxyhydroxide. Correlation of the groundwater activity ratio with oxidic 234U/238U activity ratios indicated the latter was radiogenically consistent with oxidic 230Th/234U activity ratios < 1. Dense, accumulated Fe/Mn oxide matrices are apparently capable of forming radiogenic closed systems in respect of the 230U → 234U → 230Th decay set. 234U/238U and 230Th/234U activity ratios were used to derive similar mean ages of rinds for four nodules in the same horizon. Th/U ages were also determined, using a Th-index detrital correction method previously used for impure carbonates, of pedogenic Fe/Mn accumulations having background concentrations of U and Th from other locations in Australia. These ages were compared with TL and 14C ages of the-host sediments. The comparisions suggest that Th/U dating could be used to age indurated layers of Fe/Mn oxides in soil horizons < 350 ka old.

DISTRIBUTION OF URANIUM- AND THORIUM SERIES RADIONUCLIDES IN MINERAL PHASES OF A WEATHERED LATERITIC TRANSECT OF A URANIUM ORE BODY

Abstract A surficial, weathered 400 m long profile of the Ranger One ore body #3 (Northern Territory, Australia) has been characterised to a depth of 10 m in terms of the total uranium, 238 U , 234 U , 232 Th , 230 Th and 228 Th distributions, and measurements of other geochemically important elements. The characterisation was limited to the 234 U , 230 Th and 228 Th , precipitation due to evaporation of ground- and interstitial water, and the adsorption of uranium and thorium within a time frame less than that required for 234 U / 238 U and 230 Th / 234 U to achieve secular radioactive equilibrium. The mineralogical composition of the soil is to some extent responsible for local variations in the measured properties. The differences in chemical speciation and behaviour of uranium and thorium enhance the variability in the 230 Th / 234 U activity ratios. The monsoonal climatic and hydrological variations intensify some of the observed effect.

Uranium and other contaminant migration in groundwater at a tropical Australian Uranium Mine

Hydrogeochemical modelling (utilising the modelling tools MODFLOW, MT3D and HARPHRQ) has been used in conjunction with laboratory-based experiments and a field monitoring program to investigate the fate of uranium and other contaminants in excess water sprayed on a 33 ha region of the Ranger Uranium Mine (RUM), northern Australia. The results indicate that uranium is retained in the surficial layer of the lateritic soils of the area. Conservative contaminants are not retained by the soils and are transported into the groundwater. Subsequently, they migrate relative to the groundwater flow rate towards the river system down hydraulic gradient of the irrigation area.

The use of kinetic modelling as a tool in the assessment of contaminant release during rehabilitation of a uranium mine

Abstract Geochemical kinetic modelling codes have been utilised in the assessment of the potential for contaminant release during the rehabilitation of mine sites. Two uranium mines in eastern Germany have been used as study sites for the application of the kinetic models. Results from the study will be used in the formulation of options for the rehabilitation of the two sites. The predicted results are in good agreement with results from other studies using different techniques and can also predict the water quality of pilot-scale flooding experiments.

Determination of sulphate isotopomers by vibrational spectroscopy

Abstract Vibrational spectra of the 16 O/ 18 O isotopomers of the sulphate anion are examined with a view to developing a spectrometric technique for the determination of these species. Such a technique could provide more information than mass spectroscopy alone in the analysis of sulphates formed in isotopic tracer experiments. In Raman solution spectra, each of the five isotopomers produces a single ν 1 , band with an intensity that varies linearly with the isotopomer concentration within a small range of experimental error. However, in the solid state Raman and FTIR spectra of 18 O-enriched BaSO 4 , the isotopomer ν 1 bands exhibit complicated splittings due to symmetry lowering, making quantitative analysis difficult. FTIR remains the most convenient qualitative technique, however, for both the speed and the ease of spectrum collection.

Oxygen-18 Sulfate Isotopomers Monitored by Infrared Spectroscopy

Infrared spectroscopy has been used with oxygen-18 isotopic tracer experiments to provide insight into the mechanism of formation of sulfate by the air oxidation of sulfite, thiosulfate, or tetrathionate ions. The relative amounts of each of the different isotope isomers, or isotopomers, can be observed qualitatively by their characteristic totally symmetric vibrational mode frequencies. The source of the 18O can be either dioxygen or the solvent water. By reversing the source of the isotope, one can check the results for isotopic exchange and/or impurities.

Application of the Uranium Decay Series to a Study of Ground Water Colloids

In recent years, the study of surface and groundwater colloids has gained considerable momentum for a number of reasons. These include the failure of computer models to account correctly for the observed mass transport, the concern of environmental regulating organisations that not all sources of these colloids had been accounted for, and the development of new techniques for the collection and study of very dilute colloid systems.