Uranium Series Radionuclides Research Articles

210Pb and 210Po in sediments and suspended matter in the Tagus estuary, Portugal. Local enhancement of natural levels by wastes from phosphate ore processing industry

Results of analyses of uranium series radionuclides in phosphate ore and in wastes released by the phosphate fertilizer industry confirm their potential for the enhancement of environmental radioactivity levels. Therefore, concentrations of 210Pb and 210Po were measured in bottom sediments and suspended matter in the Tagus estuary, Portugal, to assess the enhancement of radioactivity due to wastes from the phosphate industry. The concentration of 210Pb in surface sediments in the estuary increased inversely with sediment grain-size; conversely, increased percentage of sand has a dilution effect on the concentration of 210Pb measured in bulk sediment samples. By normalizing the data to the < 63-μm fraction, the naturally-occuring 210Pb in sediments was found to be 68 ± 19 Bq kg−1 (dry wt.) in background sediments. Higher 210Pb levels, up to 1580 Bq kg−1 (dry wt.), were measured in some bulk sediment samples. It was verified that this radionuclide has been introduced by the discharge of wastes from a phosphate fertilizer plant but enhanced concentrations are localized near the point of discharge. In other zones of the estuary, the concentrations of 210Pb in sediments and suspended matter were generally below those measured in the zone of phosphatic releases at the Barreiro Peninsula. Concentrations higher than the predicted average concentration of unsupported 210Pb from natural sources (atmospheric deposition, river input) were also measured in the upper estuary, both in bottom sediments and in suspended matter. It is suggested that these relatively elevated concentrations are due to the highly efficient scavenging of soluble naturally-occurring unsupported 210Pb onto suspended matter and to co-precipitation with iron-manganese hydroxides in the fresh water-salt water mixing zone.

Groundwater geochemistry in the Koongarra ore deposit, Australia (II): Activity ratios and migration mechanisms of uranium series radionuclides.

The concentrations of uranium series radionuclides in groundwater were determined to investigate the migration behavior of radionuclides in the Koongarra ore deposit. Particular attention was given to 238U and alpha-emitting radionuclides in its decay chain, including 234U, 230Th, 226Ra, and 222Rn, and beta-emitting 210Pb. Disequilibrium between various members of the 238U decay chain in the Koongarra system arises from a combination of factors, including differences in solubility, surface affinity, the degree of weathering, diffusion of gaseous 222Rn, alpha-recoil effects and redox processes. Measured groundwater 234U/238U activity ratios were below unity in the surficial weathered zone (shallower than about 20 m depth), and greater than unity in the deeper unweathered zone (>30 m depth). These were attributed to various mechanisms related to the alpha-recoil process. Groundwater concentrations of 230Th, and also 230Th/238U ratios were extremely low, indicating that thorium is immobile in this system. Radium-226 was relatively immobile in groundwaters of the weathered zone, with lower 226Ra/238U ratios than deeper groundwaters. This was attributed to co-precipitation of radium together with manganese and ferric hydroxides at the base of the weathered zone, and also to the greater abundance of radium-sorbing minerals in the weathered zone. Large excess concentrations of 222Rn were found in most Koongarra groundwaters, indicating substantial loss of 222Rn from the solid phase despite its short half-life. The 210Pb/222Rn ratios were relatively constant and it was possible to compute an average scavenging residence time for 210Pb in the groundwater of about 6 days using a simple box model. The patterns of dispersion of uranium series radionuclides in Koongarra groundwaters also suggest that present-day migration is toward the south of the orebody. This conclusion is in agreement with the outcome of the geochemistry study.

Sediment yield determination using uranium-series radionuclides: the case of The Wash and Fenland drainage basin, eastern England

Water and sediment samples were collected from the Fenland drainage basin to assess the fluvial contribution to the sediment budget of The Wash, on the east coast of England. Using the uranium-series disequilibrium found in the products of the weathering system (river sediment and water), the ratio of material carried in solid form to that dissolved ( U s/ U w) was assessed and the total flux of weathered material (sediment yield) was quantified. The results show that the U s/ U w ratio is controlled to a degree by catchment geology and hydrology, and that there is a marginal seasonal increase from between 3.28 ± 3.95 and 4.60 ± 0.44 (summer mean and weighted mean) to between 5.35 ± 3.05 and 6.07 ± 0.63 (winter mean and weighted mean). The best estimate of annual sediment flux from the Fenland to The Wash is between 2800 and 17,700 tonnes yr −1, equivalent to a rate of 0.22–1.42 tonnes km −2 yr −1. This is lower than previous estimates for the region, but reinforces the argument that sediment reworking within The Wash and erosion of the adjacent coasts and sea beds are important sources of sediment for the intertidal flats and salt marshes.

Measurement of Uranium Series Radionuclides in Rock and Groundwater at the Koongarra ore Deposit, Australia, by Gamma Spectrometry

AbstractGamma spectrometry without any self-absorption correction was developed to measure low energy gamma rays emitted by uranium and actinium series radionuclides in rock samples and groundwater residues collected at the Koongarra ore deposit, Australia. Thin samples were prepared to minimize the self-absorption by uranium in the samples. The present method gave standard deviations of 0.9 to 18% for the measurements of concentrations of uranium and actinium series radionuclides. The concentrations of238U,230Th and235U measured by gamma spectrometry were compared with those by alpha spectrometry that requires a complicated chemical separation procedure. The results obtained by both methods were in fairly good agreement, and it was found that the gamma spectrometry is applicable to rock and groundwater samples having uranium contents up to 8.1% (103Bq/g) and 3 Bq/1 of238U, respectively. The detection limits were calculated to be of the order of 10−2Bq/g for rock samples and 10−2Bq/1 for groundwater samples. The concentrations of uranium and actinium series radionuclides can be determined precisely in these samples using gamma spectrometry without any self-absorption correction.

Uranium-series radionuclides in fluids and solids, Milk River aquifer, Alberta, Canada

Samples from 21 wells completed in the Milk River aquifer were sampled and analysed for U isotopes. In general, U concentrations decrease with distance from the recharge area. Groundwater samples can be delineated into a small oxic group (U content ∼ 10 1t-5 mmol/1) and a more predominant anoxic group (U content of ∼ 10 −7 mmol/1). All groundwater samples were undersaturated with respect to the common U minerals such as uraninite, coffinite and rutherfordine. The dominant U complexes were uranyl carbonates. The 234U/ 238U activity ratios increased from 2 to 11 with distance downflow of the outcrop. The highest activity ratio of 11.8 was measured at a well-defined redox front, whereupon a steady decrease downdip to 4 was observed. This decrease was interpreted to be due to the decay of 234U excess with age of the groundwater and due to U loss by sorption/precipitation processes from solution to solid surfaces. Two U isotope dating models were used to estimate the age of the groundwater in the Milk River aquifer. Model (1) is a simple model of downdip decay of 234U excess in solution under steady-state conditions. Model (2) is a phenomenological model for the evolution of U isotope groundwater composition downdip based on a transport (dispersion) equation incorporating radioactive decay and assuming that sorption processes are of first-order kinetics. Both models were applied to the U isotope data collected along the same flow path in the aquifer. They yielded flowrates of 0.1–0.2 m/a and 0.2–0.6 m/a, respectively. The latter range is in good agreement with flowrates of 0.3 m/a obtained from hydraulic considerations. The lower value derived from Model (1) is interpreted in terms of U migration velocity rather than as groundwater flowrate. Model (2) was also applied to an additional flowpath. The calculated flowrate range of 0.1–0.4 m/a is in excellent agreement with the hydraulic model flowrate of 0.2 m/a. The main conclusion of this work is that U isotopes can be used for dating very old groundwaters provided the local hydrogeology and U geochemistry are understood well.

Ra-226 concentrations in otter, Lutra canadensis, trapped near uranium tailings at Elliot Lake, Ontario.

The Elliot Lake area of Ontario, is currently the major uranium producing region of Canada. It is estimated there are 120 million tons of uranium tailings spread over 600 ha in the vicinity of Elliot Lake. The transfer and fate of uranium-series radionuclides from tailing sites remain primary ecological concerns in these areas. It has been demonstrated that the levels of radionuclides, including Ra-226, are elevated in vegetation, small mammals and fish living on or near tailing disposal sites. However, the transfer potential of Ra-226 to predatory species has not been examined in detail. The objective of this study was to measure Ra-226 levels in otters (Lutra canadensis), captured near tailing sites, to provide further information on the fate of radionuclides in the environment.

Distribution of uranium series radionuclides in upland vegetation of northern Saskatchewan. I. Plant and soil concentrations

Levels of lead-210, polonium-210, radium-226, and uranium are reported for 10 plant species (2 conifer, 4 shrub, 3 lichen, and 1 moss species) and soils in northern Saskatchewan. Two localities were studied in each of two regions, one uraniferous, the other not. Nonvascular species showed the highest levels of lead-210, polonium-210, and uranium, and the shrubs, the highest levels of radium-226. The lichen and moss species show no significant difference in accumulation of lead-210 and polonium-210 among regions, reflecting the assumed atmospheric distribution and particulate accumulation of these radionuclides. Seven of eight species for which comparisons could be made showed significant differences in uranium accumulation among regions. High levels in the lichen and moss species suggest that the primary source of uranium in these species is not directly from the soil. In contrast to the other radionuclides, radium-226 only showed significant differences among localities within regions, this being due to high accumulations occurring at one locality in the nonuraniferous region.

Distribution of uranium series radionuclides in upland vegetation of northern Saskatchewan. II. Patterns of accumulation among species and localities

Multivariate methods are used to explore patterns of accumulation of radionuclides and stable elements among 10 plant species at four localities in northern Saskatchewan. Principal components analysis and canonical correlation analysis of the radionuclides and stable elements showed that lead-210 and polonium-210 are correlated with crustal elements distributed in the atmosphere and therefore are accumulated more abundantly by lichen and moss species than by vascular plants. Uranium showed a similar trend. Neither these radionuclides nor radium-226 showed strong correlations with other stable elements. The highest accumulation of uranium was in the Wollaston Lake region, especially for the nonvascular plants, where the soil levels are lowest. This tends to substantiate previous suggestions that uranium available for uptake by vascular plants is associated with groundwater rather than soil particles. Canonical variates analysis on radionuclide levels in vegetation by species group shows that trees, shrubs, lichens, and moss all have significantly different patterns of accumulation. The vascular and nonvascular groups are separated primarily by lead-210 and polonium-210 accumulation. The trees and shrubs are separated by radium-226 levels. A similar analysis by locality showed significant differences in radionuclide accumulation by vegetation in all possible pairs of localities, except between the two Wollaston Lake localities. The largest distances were between regions and were based primarily on uranium accumulation in the nonvascular plants. Differences in radium-226 levels among the shrub species are responsible for the significant difference between the two localities in the Churchill River region.

Uranium-series growth history of a quaternary phosphatic crust from the peruvian continental margin

A 20-mm-thick oriented phosphatic crust recovered together with its overlying (14 cm) and underlying (4 cm) associated sediment from the Peruvian sea floor has been analyzed in detail for uranium-series radionuclides in an attempt to determine its rate and direction of growth. Growth curves based upon 226Ra and 230Th ages show that this crust grew upward toward the sediment-water interface. Calculated growth rates in the range of 12–13 mm ka −1 are slightly higher but comparable to values previously reported. Crystallographical analyses of this phosphatic crust show a trend of decreasing unit-cell dimension a with sample age. The upward growth of a buried crust toward the sediment-water interface is consistent with results from recent pore-water studies of fluoride and phosphate in Peru shelf sediments.

226Ra in phosphate nodules from the Peru/Chile seafloor

226Ra and other uranium-series radionuclides have been measured in a suite of marine phosphorite samples from the upwelling area off Peru/Chile by gamma-ray spectrometry and radiochemical techniques. Our results lead to the following conclusions: (1) phosphorite nodules typically show unidirectional growth at rates of 1 to 10 mm/Kyr; (2) very young samples (less than a few thousand years) contain slight excess amounts of 226Ra probably derived from pore fluids; and (3) slow but persistent leakage of 226Ra out of phosphate nodules occurs resulting in systematically lower 226Ra ages compared to 230Th ages for samples older than about twenty thousand years. Radium fluxes from these phosphate nodules appear to be 1 to 3 orders of magnitude less than those calculated for deep-sea sediments and ferromanganese nodules.

An observation of rapid mobilisation of uranium and its decay products

Uranium series radionuclides were determined in a sample collected from a rainwater puddle at the Ranger Uranium Mine (Northern Territory, Australia). The 1 l sample contained 0.25 g of uranium with a 234U/ 238U activity ratio of 1.07. The activities of the 238U daughter products mobilised by the rainwater were 3.8 Bq for 230Th and 66 Bq for 226Ra. These values are 0.1% and 2.2%, respectively, of the activities required for secular equilibrium with 0.25 g uranium. The activities of two daughters of 235U were 12.5 Bq for 227Ac and between 18.7 and 32.8 Bq for 227Th, values that represent 9.0% and between 13 and 24%, respectively, of the activity required for secular equilibrium with 0.25 g uranium. The results indicate that 234U, 227Th and 227Ac were the most readily mobilised daughter radionuclides of the uranium and actinium series. If 227Th and 227Ac are generally mobile, this could account for the frequent observations of 223Ra in Australian ground waters in concentrations higher than would be expected for an isotope with a half-life of 11.3 days.