Biogeochemical prospecting for buried uranium mineralization and the investigation of black spruce trees as a tool for biogeochemical sampling and for environmental monitoring of radionuclide contamination

Nyade, Praise Kwasi (2014) Biogeochemical prospecting for buried uranium mineralization and the investigation of black spruce trees as a tool for biogeochemical sampling and for environmental monitoring of radionuclide contamination. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Surface geochemical methods were applied to delineate uranium (U) mineralization buried under till cover in the Jacque’s Lake area, Labrador. Samples analyzed included black spruce twigs, bark, and needles; Labrador tea leaves and stems, and B-horizon soil. These results were compared to defined radiometric U/Th anomalies in the study area. Relative U enrichment followed the order: spruce twigs>bark >Labrador tea stem>spruce needles>Labrador tea leaves. Spruce twigs displayed the greatest contrasts in U contents and exhibited a significant association with the U/Th anomalies. Twig U concentrations ranged between <0.01 to 61 ppm and were highly correlated with Pb, Ag, Sb, Ca, and Be. Areas with anomalous U concentration in <250 μm sized-fraction of B-horizon soil appeared displaced east of the defined U/Th responses and the near surface projection of the buried U mineralization. There is a lack of correlation between U concentration in B-horizon soil and the defined radiometric anomalies. The U hyperaccumulation by black spruce trees was investigated and the possible use of ²³⁸U/²³⁵U ratio as a tracer for fingerprinting the source and transport pathways of U taken up by vegetation was examined. The U accumulation property of the black spruce seedlings was evaluated by measuring the metal uptake in seedlings grown in peat and in soil treated with varying concentrations of uranyl nitrate. The bioavailability indexes of U in soil treated with 100, 250, and 400 mg U/Kg soil were estimated to be 0.21, 0.15 and 0.10 respectively. Uranium concentrations in seedling stems, the Biological Accumulation Coefficient (BAC), and Transfer Factors (TF) increased linearly with increasing bioavailable concentration of U in the substrates. The metal accumulation in the plant tissues followed the order stem > roots > needles. Although the trend of U accumulation was comparable for plants grown in U-dosed soil versus U-dosed peat, the BAC values for plants grown in U-spiked soils were higher suggesting that metal availability was substrate specific. The source of U taken up by the spruce seedlings was evaluated by Inductively Coupled Plasma-Quadrupole Mass Spectrometry (ICP-QMS) and Neutron Activation Analysis (NAA) analyses of ²³⁸U/²³⁵U ratios in stems of seedlings grown on substrates mounted in solutions spiked with different concentrations of DU. The ²³⁸U/²³⁵U ratio in the stems exhibited measureable deviations from the natural ratio (137.88) and ranged from 174-210 and is a consequence of the bulk uptake of U derived from the DU solutions. Elevated ²³⁸U/²³⁵U ratio in the substrates suggests upward migration of DUenriched solution along the substrate column. The precision (1σ) for ICP-QMS determination of ²³⁸U/²³⁵U in DU-spiked substrates, and the stems of seedlings, are 0.02%-2.7% and 0.05%-1.9%, respectively.

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