Controls on Ra during raffinate neutralization at the Key Lake uranium mill, Saskatchewan, Canada

This study was conducted to define the geochemical controls on ²²⁶Ra during raffinate (pH = 1.2) neutralization to pH 10 at the Key Lake U mill in northern Saskatchewan, Canada. High activities (120–150 Bq/L) of aqueous phase ²²⁶Ra are present in raffinate produced during milling of U ore. The solubility control of ²²⁶Ra in the SO₄-rich, hydrometallurgical raffinate solutions often involves the addition of BaCl₂ to form a radium-barite co-precipitate (Ba(Ra)SO₄). As such, neutralization experiments were conducted with samples of mill raffinate using Ca(OH)₂ or NaOH with and without the addition of BaCl₂. Radium-226 activity decreased from 150 to <4 Bq/L for all combinations of neutralizing agents with Ca(OH)₂ + BaCl₂ being the most effective combination (final activity ∼1.0 Bq/L; ∼99.3% removal). In the absence of BaCl₂, Ca(OH)₂ more efficiently removed ²²⁶Ra than NaOH between pH 4 and 8, due to the co-precipitation of ²²⁶Ra with gypsum. Overall, neutralization with the addition of BaCl₂ reduced ²²⁶Ra activities at lower pH values (by pH 4.5), due to co-precipitation of ²²⁶Ra with BaSO₄. At varying concentrations of BaCl₂, aqueous phase activities of ²²⁶Ra converged, but did not attain steady-state values during neutralization and would continue to decrease with time. Sequential extractions indicated that ²²⁶Ra in precipitates formed during neutralization of the mill raffinate is dominated by amorphous and crystalline Fe hydroxide phases, consistent with raffinate neutralization experiments that showed that adsorption onto ferrihydrite can remove most ²²⁶Ra in the raffinate. Data generated in this study are being used to define the long-term geochemical controls on ²²⁶Ra in U mill processes and tailings.     

The mobility of radium-226 and trace metals in pre-oxidized subaqueous uranium mill tailings

The exchange of ²²⁶Ra and trace metals across the tailings-water interface and the mechanisms governing their mobility were assessed via sub-centimetre resolution profiling of dissolved constituents across the tailings–water interface in Cell 14 of the Quirke Waste Management Area at Rio Algom's Quirke Mine, near Elliot Lake, Ontario, Canada. Shallow zones (<1.5 m water depth) are characterized by sparse filamentous vegetation, well-mixed water columns and fully oxygenated bottom waters. Profiles of dissolved O₂, Fe and Mn indicate that the tailings deposits in these areas are sub-oxic below tailings depths of ∼3 cm. These zones exhibit minor remobilization of Ra in the upper 5 cm of the tailings deposit; ²²⁶Ra fluxes at these sites are relatively small, and contribute negligibly to the water column activity of ²²⁶Ra. The shallow areas also exhibit minor remobilization of Ni, As, Mo and U. The release of these elements to the water cover is, however, limited by scavenging mechanisms in the interfacial oxic horizons. The presence of thick vegetation (Chara sp.) in the deeper areas (>2 m water depth) fosters stagnant bottom waters and permits the development of anoxia above the benthic boundary. These anoxic tailings are characterized by substantial remobilization of ²²⁶Ra, resulting in a relatively large flux of ²²⁶Ra from the tailings to the water column. The strong correlation between the porewater profiles of ²²⁶Ra and Ba (r²=0.99), as well as solubility calculations, indicate that the mobility of Ra is controlled by saturation with respect to a poorly ordered and/or impure barite phase [(Ra,Ba)SO₄]. In the anoxic zones, severe undersaturation with respect to barite is sustained by microbial SO₄ reduction. Flux calculations suggest that the increase in ²²⁶Ra activity in the water cover since 1995 (from <0.5 to 2.5 Bq l⁻¹) can be attributed to an increase in the spatial distribution of anoxic bottom waters caused by increased density of benthic flora. The anoxic, vegetated areas also exhibit minor remobilization with respect to dissolved As, Ni and Zn. The removal of trace metals in the anoxic bottom waters appears to be limited by the availability of free sulphide. Collectively, the data demonstrate that while the water cover over the U mill tailings minimizes sulphide oxidation and metal mobility, anoxic conditions which have developed in deeper areas have led to increased mobility of ²²⁶Ra.     

US Geological Survey research on the environmental fate of uranium mining and milling wastes

Studies by the US Geological Survey (USGS) of uranium mill tailings (UMT) have focused on characterizing the forms in which radionuclides are retained and identifying factors influencing the release of radionuclides to air and water. Selective extraction studies and studies of radionuclide sorption by and leaching from components of UMT showed alkaline earth sulfate and hydrous ferric oxides to be important hosts of radium-226 (²²⁶Ra) in UMT. Extrapolating from studies of barite dissolution in anerobic lake sediments, the leaching of²²⁶Ra from UMT by sulfate-reducing bacteria was investigated; a marked increase in²²⁶Ra release to aqueous solution as compared to sterile controls was demonstrated. A similar action of iron(III)-reducing bacteria was later shown. Ion exchangers such as clay minerals can also promote the dissolution of host-phase minerals and thereby influence the fate of radionuclides such as²²⁶Ra. Radon release studies examined particle size and ore composition as variables. Aggregation of UMT particles was shown to mask the higher emanating fraction of finer particles. Studies of various ores and ore components showed that UMT cannot be assumed to have the same radon-release characteristics as their precursor ores, nor can²²⁶Ra retained by various substrates be assumed to emanate the same fraction of radon. Over the last decade, USGS research directed at offsite mobility of radionuclides from uranium mining and milling processes has focused on six areas: the Midnite Mine in Washington; Ralston Creek and Reservoir, Colorado; sites near Canon City, Colorado; the Monument Valley District of Arizona and Utah; the Cameron District of Arizona; and the Puerco River basin of Arizona and New Mexico.     

Development of a reactive zone technology for simultaneous in situ immobilisation of radium and uranium

Simultaneous in situ immobilisation of uranium (U) and radium (²²⁶Ra) by injectible amounts of grey cast iron (gcFe), nano-scale iron (naFe) and a gcFe/MnO₂ mixture (1:1) was studied in batch and column tests. Both 0.5 g/L naFe and gcFe are effective in ²²⁶Ra and U removal from mine water, whereas MnO₂ addition clearly increased the efficiency of gcFe for ²²⁶Ra and U immobilisation. In a column test with 0.6 wt% gcFe/MnO₂ mixture (1:1), neither ²²⁶Ra nor U was detected in the effluent after replacement of 45 pore volumes. A sequential extraction under flow condition revealed ²²⁶Ra to be mostly occluded in manganese oxides. Uranium was mostly sorbed onto poorly crystalline iron hydroxides, but a significant part was found to be occluded in manganese oxides also. The results of this study suggest that MnO₂ promotes iron hydroxide formation under slightly reducing environmental conditions resulting in an increased pollutant retention capacity.     

An improved method for Ra isotope (226Ra, 228Ra, 224Ra) measurements by gamma spectrometry in natural waters: application to CO2-rich thermal waters from the French Massif Central

An improved method was developed to measure ²²⁶Ra, ²²⁸Ra and ²²⁴Ra in freshwaters by gamma spectrometry. Radium was selectively extracted from acidified samples using specific filters (3M EMPORE™ Radium Rad disks). The latter was subsequently analysed by gamma spectrometry. Simultaneous and direct determination of the activities of the three isotopes was performed by comparison of gamma rays of the Radium Rad disks with those of a calibrated standard disk. This efficient and reliable method allowed a reduction of sample processing to a few hours.This technique was applied to analyse the Ra isotope compositions of several CO₂-rich hydrothermal springs of the western border of the Limagne graben (French Massif Central). The studied springs emerge from a succession of granitic outcrops lined up along a major fault. Their chemical compositions evolve from calcic and magnesian chloro-bicarbonated to sodic bicarbonated. All the springs display high Ra activities, probably linked to high CO₂ content and/or high cation content of these waters, with various Ra isotope ratios. ²²⁶Ra activity ranges from 588 to 2287 mBq/l and ²²⁸Ra activity from 260 to 1590 mBq/l, whereas ²²⁴Ra displays an activity between 245 and 1808 mBq/l. Four of the six analysed springs have (²²⁸Ra/²²⁶Ra) activity ratios lower than 0.7, thus, significantly lower than the ratio expected from an interaction with a calc-alkaline granitoid (typically having (²³²Th/²³⁸U) activity ratio between 1 and 2). Low (²²⁸Ra/²²⁶Ra) ratio (0.27) of the northern water (Montpensier) suggests the existence in this area of a zone of U concentration, possibly resulting from U mobilization and accumulation induced by previous hydrothermal events. The (²²⁴Ra/²²⁸Ra) ratios display smaller variations. They suggest short transit times from the zone of Ra leaching to the surface (a few days) or a very shallow addition of ²²⁴Ra (e.g., from a localised zone where ²²⁸Th could be preferentially adsorbed on the mineral surfaces). In some cases, these ratios might be used to infer differences in transit times of waters between neighboring springs.     

Solubility and dissimilatory reduction kinetics of iron(III) oxyhydroxides: A linear free energy relationship

Rates of reduction of Fe(III) oxyhydroxides by the bacterium Shewanella putrefaciens were measured as a function of the bacterial density and the Fe(III) substrate concentration. The results show that an earlier reported positive correlation between the solubility products (^*K_so) and the maximum cell-specific reduction rates (v_max) of predominantly poorly crystalline Fe(III) oxyhydroxides also applies to insoluble and crystalline Fe(III) oxyhydroxides. The mineral solubilities were measured by a dialysis bag technique under acidic conditions (pH 1 up to 2.5) at 25 °C. Initial iron reduction rates by S. putrefaciens were determined in the presence of excess lactate as electron donor. In all cases, the microbial reduction rate exhibited saturation behavior with respect to the Fe(III) oxyhydroxide concentration. On a double logarithmic scale, the maximum rates v_max and the solubility products defined a single linear free energy relationship (LFER) for all the Fe(III) oxyhydroxides considered. The solubility provided a better predictor of v_max than the specific surface area of the mineral phase. A rate limitation by the electron transfer between an iron reductase and a Fe(III) center, or by the subsequent desorption of Fe²⁺ from the iron oxide mineral surface, are both consistent with the observed LFER.     

Speciation and characterization of arsenic in gold ores and cyanidation tailings using X-ray absorption spectroscopy

The knowledge of mineralogy and molecular structure of As is needed to better understand the stability of As in wastes resulting from processing of gold ores. In this study, optical microscopy, scanning electron microscopy, electron microprobe, X-ray diffraction and X-ray absorption fine structure (XAFS) spectroscopy (including both XANES and EXAFS regimes) were employed to determine the mineralogical composition and local coordination environment of As in gold ores and process tailings from bench-scale tests designed to mimic a common plant practice. Arsenic-bearing minerals identified in the ores and tailings include iron (III) oxyhydroxides, scorodite (FeAsO₄·2H₂O), ferric arsenates, arseniosiderite (Ca₂Fe₃(AsO₄)₃O₂·3H₂O), Ca-Fe arsenates, pharmacosiderite (KFe₄(AsO₄)₃(OH)₄·6-7H₂O), jarosite (K₂Fe₆(SO₄)₄(OH)₁₂) and arsenopyrite (FeAsS). Iron (III) oxyhydroxides contain variable levels of As from trace to about 22 wt% and Ca up to approximately 9 wt%.Finely ground ore and tailings samples were examined by bulk XAFS and selected mineral grains were analyzed by microfocused XAFS (micro-EXAFS) spectroscopy to reconcile the ambiguities of multiple As sources in the complex bulk EXAFS spectra. XANES spectra indicated that As occurs as As⁵⁺in all the samples. Micro-EXAFS spectra of individual iron (III) oxyhydroxide grains with varying As concentrations point to inner-sphere bidentate-binuclear arsenate complexes as the predominant form of As. There are indications for the presence of a second Fe shell corresponding to bidentate-mononuclear arrangement. Iron (III) oxyhydroxides with high As concentrations corresponding to maximum adsorption densities probably occur as nanoparticles. The discovery of Ca atoms around As in iron (III) oxyhydroxides at interatomic distances of 4.14-4.17 Å and the coordination numbers suggest the formation of arseniosiderite-like nanoclusters by coprecipitation rather than simple adsorption of Ca onto iron (III) oxyhydroxides. Correlation of Ca with As in iron (III) oxyhydroxides as determined by electron microprobe analysis supports the coprecipitate origin for the presence of Ca in iron (III) oxyhydroxides.The samples containing higher abundances of ferric arsenates released higher As concentrations during the cyanidation tests. The presence of highly soluble ferric arsenates and Ca-Fe arsenates, and relatively unstable iron (III) oxyhydroxides with Fe/As molar ratios of less than 4 in the ore and process tailings suggests that not only the tailings in the impoundment will continue to release As, but also there is the potential for mobilization of As from the natural sources such as the unmined ore.     

Geochemistry of radium in soils of the Eastern United States

The abundance and chemical/mineralogical form of ²²⁶Ra, ²³⁸U and ²³²Th were determined on samples of soil and associated vegetation at 12 sites in the eastern United States. Progressive, selective chemical extraction plus size fractionation determined the abundance and radiometric equilibrium condition of these nuclides in 6 operationally defined soil fractions: exchangeable cations, organic matter, “free” Fe-oxides, sand, silt, and clay.In soils, profile-averaged ²²⁶Ra/²³⁸U activity ratios (AR) are within 10% of unity for most sites, implying little fractionation of U and Ra when the entire soil profile is considered. However, ²²⁶Ra greatly exceeds ²³⁸U activity in most surface soil (AR up to 1.8, av 1.22), in vegetation (AR up to 65, av. 2.8), in the exchangeable+organic fraction (AR up to 30, av. 13), in some soil Fe oxides (AR up to 3.5, av. 0.83) and in the C horizons of deeply weathered soils (AR up to 1.5).A major factor in Ra behavior is uptake by vegetation, which concentrates Ra>U and moves Ra from deeper soil to surface soil. Vegetation is capable of creating the observed Ra excess in typical surface soil horizons (AR up to 1.8, av. 1.22) in about 1000 a. Of the total Ra in an average A horizon, 42% occurs as exchangeable ions and in organic matter, but only 6–8% of the parent U and Th occur in these soil forms. In contrast, U is slightly enriched relative to Ra in Fe-oxides of A horizons, implying rapid chemical partition of vegetation-cycled U and Ra.In deeper horizons, transfer by vegetation and/or direct chemical partitioning of Ra into organic and exchangeable forms provides a source for unsupported ²²⁶Ra in Ra-rich organic matter, and leaves all soil minerals Ra-poor (AR=0.73). Organic matter evidently has a strong affinity for Ra.The phenomena discussed above are relevant to evaluation of indoor Rn hazard, and behavior of Ra at sites affected by radioactive waste disposal, phosphate tailings, Ra-rich brine, and uraniferous fertilizer.     

Arsenic behaviour in gold-ore mill tailings,Massif Central,France: hydrogeochemical study and investigation of in situ redox signatures

Historical Au-ore exploitation at the Chéni mine in the Massif Central, France, generated 525,000 tonnes of finely ground mill tailings deposited in a heap that has spread with time into three settling basins. The tailings, which are rich in quartz (80%), mica and clay minerals (10% of illite, smectite, kaolinite and chlorite), feldspars (5%) but poor in carbonates (<1%), also contain sulphides (around 5%, mainly pyrite and arsenopyrite). Arsenic content of the tailings is around 6 g kg. This paper describes the geochemistry of drainage waters, with special attention paid to in situ values of the three major redox couples, namely Fe(II)/Fe(III), As(III)/As(V) and S(IV)/S(VI). The water samples range from acidic and oxidized (pH 2.9, E_h +700 mV) to moderate pH and weakly reducing (pH 7.6, E_h 15 mV). The waters are rich in SO₄ and Ca and have variable As (0.05–95 mg L⁻¹) and Fe concentrations (0.07–141 mg L⁻¹). Reduced As(III) species predominate over As(V) species (As(III)/As(V) up to 21), whereas oxidized forms of Fe and S are favoured (Fe(II)/Fe(III) up to 0.5, and S(IV)/S(VI) up to 1).Thermodynamic calculations were performed with the PHREEQC and EQ3NR codes based on a revised As database to evaluate saturation indices (SI) of the waters in relation to the main minerals and define which redox couples control the redox state of the system. The important role of carbonates, though only present in small amounts, explains the acid buffering generated by the oxidation of sulphides for waters in the pH 7–7.5 range. Measured E_h appears to fall between the calculated E_h of the Fe(II)/Fe(III) couple and that of the As(III)/As(V) couple, illustrating redox disequilibrium.     

Geochemistry of arsenic in uranium mine mill tailings,Saskatchewan, Canada

The Rabbit Lake U mine in-pit tailings management facility (TMF) (425 m long×300 m wide×91 m deep) is located in northern Saskatchewan, Canada. The objectives of this study were to quantify the distribution of As phases in the tailings and evaluate the present-day geochemical controls on dissolved As. These objectives were met by analyzing pore fluid samples collected from the tailings body for dissolved constituents, measuring Eh, pH, and temperature of tailings core and pore fluid samples, conducting sequential extractions on solid samples, conducting geochemical modeling of pore fluid chemistry using available thermodynamic data, and by reviewing historical chemical mill process records. Dissolved As concentrations in 5 monitoring wells installed within the tailings body ranged from 9.6 to 71 mg/l. Pore fluid in the wells had a pH between 9.3 and 10.3 and Eh between +58 and +213 mV. Sequential extraction analyses of tailings samples showed that the composition of the solid phase As changed at a depth of 34 m. The As above 34 m was primarily associated with amorphous Fe and metal hydroxides while the As below 34 m was associated with Ca, likely as amorphous poorly ordered calcium arsenate precipitates. The change in the dominant As solid phases at this depth was attributed to the differences in the molar ratio of Fe to As in the mill tailings. Below 34 m it was <2 whereas above 34 m it was >4. The high Ca/As ratio during tailings neutralization would likely precipitate Ca₄(OH)₂(AsO₄)₂:4H₂O type Ca arsenate minerals. Geochemical modeling suggested that if the pore fluids were brought to equilibrium with this Ca-arsenate, the long-term dissolved As concentrations would range between 13 and 126 mg/l.     

Reductive transformation of iron and sulfur in schwertmannite-rich accumulations associated with acidified coastal lowlands

We examined the transformations of Fe and S associated with schwertmannite (Fe₈O₈(OH)₆SO₄) reduction in acidified coastal lowlands. This was achieved by conducting a 91 day diffusive-flux column experiment, which involved waterlogging of natural schwertmannite- and organic-rich soil material. This experiment was complemented by short-term batch experiments utilizing synthetic schwertmannite. Waterlogging readily induced bacterial reduction of schwertmannite-derived Fe(III), producing abundant pore-water Fe^II, SO₄ and alkalinity. Production of alkalinity increased pH from pH 3.4 to pH ∼6.5 within the initial 14 days, facilitating the precipitation of siderite (FeCO₃). Interactions between schwertmannite and Fe^II at pH ∼6.5 were found, for the first time, to catalyse the transformation of schwertmannite to goethite (αFeOOH). Thermodynamic calculations indicate that this Fe^II-catalysed transformation shifted the biogeochemical regime from an initial dominance of Fe(III)-reduction to a subsequent co-occurrence of both Fe(III)- and SO₄-reduction. This lead firstly to the formation of elemental S via H₂S oxidation by goethite, and later also to formation of nanoparticulate mackinawite (FeS) via H₂S precipitation with Fe^II. Pyrite (FeS₂) was a quantitatively insignificant product of reductive Fe and S mineralization. This study provides important new insights into Fe and S geochemistry in settings where schwertmannite is subjected to reducing conditions.     

Ra/Ra and Ra/Ba ratios in the Western Mediterranean Sea: Barite formation and transport in the water column

²²⁶Ra, ²²⁸Ra and Ba distributions as well as ²²⁸Ra/²²⁶Ra and ²²⁶Ra/Ba ratios were measured in seawater, suspended and sinking particles at the DYFAMED station in the Western Mediterranean Sea at different seasons of year 2003 in order to track the build-up and fate of barite through time. The study of the ²²⁸Ra_ex/²²⁶Ra_ex ratios (Ra_ex = Ra activities corrected for the lithogenic Ra) of suspended particles suggests that Ba_ex (Ba_ex = Ba concentrations corrected for the lithogenic Ba, mostly barite) formation takes place not only in the upper 500 m of the water column but also deeper (i.e. throughout the mesopelagic layer). Temporal changes in the ²²⁸Ra_ex/²²⁶Ra_ex ratios of sinking particles collected at 1000 m depth likely reflect changes in the relative proportion of barite originating from the upper water column (with a high ²²⁸Ra/²²⁶Ra ratio) and formed in the mesopelagic layer (with a low ²²⁸Ra/²²⁶Ra ratio). ²²⁸Ra_ex/²²⁶Ra_ex ratios measured in sinking particles collected in the 1000 m-trap in April and May suggest that barite predominantly formed in the upper water column during that period, while barite found outside the phytoplankton bloom period (February and June) appears to form deeper in the water column. Combining ratios of both the suspended and sinking particles provides information on aggregation/disaggregation processes. High ²²⁶Ra_ex/Ba_ex ratios were also found in suspended particles collected in the upper 500 m of the water column. Because celestite is expected to be enriched in Ra [Bernstein R. E., Byrne R. H. and Schijf J. (1998) Acantharians: a missing link in the oceanic biogeochemistry of barium. Deep-Sea Res. II45, 491-505], acantharian skeletons may contribute to these high ratios in shallow waters. The formation of both acantharian skeletons and barite enriched in ²²⁶Ra may thus contribute to the decrease in the dissolved ²²⁶Ra activity and ²²⁶Ra/Ba ratios of surface waters observed between February and June 2003 at the DYFAMED station.     

Raexcess/Ba growth rates and U-Th-Ra-Ba systematic of Baltic Mn/Fe crusts

We analyzed ²³⁸U, ²³⁴U, ²³²Th, ²³⁰Th, and ²²⁶Ra by thermal ionization mass spectrometry (TIMS) and Ba by inductively coupled plasma optical emission spectrometry (ICP-OES) on eight Mn/Fe crusts from the Mecklenburg Bay (SW Baltic) and on one from the Bothnian Bay (N Baltic) to test the ²²⁶Ra_ex/Ba ratio as potential geochronometer. ²²⁶Ra_ex/Ba ratios decrease as a function of depth within the concretions in all analyzed profiles. Calculated diffusion coefficients are relatively low (∼9 · 10⁻⁷ cm²/yr for Ra and 5 · 10⁻⁷ cm²/yr for Ba) and suggest that diffusion is negligible for the Ra and Ba record. In addition, ²²⁶Ra_ex/Ba ages are consistent and independent from the growth rate and growth direction within a crust. Thus, the decline in ²²⁶Ra_ex/Ba ratio is most likely due to radioactive decay of ²²⁶Ra_ex, although the influence of varying oxic conditions has still to be evaluated. ²²⁶Ra_ex/Ba growth rates range from 0.021 to 0.0017 mm/yr and tend to be lower than those calculated and based on stratigraphic methods (1 to 0.013 mm/yr). ²²⁶Ra_ex/Ba ages of concretions from shallow water environment (20 m depth, Mecklenburg Bay/SW Baltic) cover a time interval from 990 ± 140 yr to 4310 ± 310 yr BP corresponding to the stabilization of the sea level close to the present position about 5500 to 4500 yr ago. One sample from greater depth (70 m, Bothnian Bay-/N Baltic) showed a higher ²²⁶Ra_ex/Ba age of 6460 ± 520 yr BP.     

Seasonal and topographic variations in porewaters of a southeastern USA salt marsh as revealed by voltammetric profiling†

We report electrochemical profiles from unvegetated surficial sediments of a Georgia salt marsh. In creek bank sediments, the absence of ΣH₂S or FeS_aq and the presence of Fe(III)–organic complexes suggest that Mn and Fe reduction dominates over at least the top ca. 5 cm of the sediment column, consistent with other recent results. In unvegetated flats, accumulation of ΣH₂S indicates that SO₄ ^2- reduction dominates over the same depth. A summer release of dissolved organic species from the dominant tall form Spartina alterniflora, together with elevated temperatures, appears to result in increased SO₄ ^2- reduction intensity and hence high summer concentrations of ΣH₂S in flat sediments. However, increased bioturbation and/or bioirrigation seem to prevent this from happening in bank sediments. Studies of biogeochemical processes in salt marshes need to take such spatial and temporal variations into account if we are to develop a good understanding of these highly productive ecosystems. Furthermore, multidimensional analyses are necessary to obtain adequate quantitative pictures of such heterogeneous sediments.     

Combined hydrochemical,isotopic, and multivariate statistics techniques to assess the effects of discharges from a uranium mine on water quality in neighboring streams

The Caldas Uranium Mine (CUM), located on the Poços de Caldas Plateau in the southeastern region of Brazil, is presently undergoing a decommissioning process. The aim of the present investigation is to identify and characterize the effects of acid mine drainage (AMD) originating from the CUM on surface water quality. To achieve these aims, sampling stations were located at two AMD sources: the retention pond at the foot of waste rock pile#4 (WRP#4) and the settling pond that receives effluents from the tailings dam (TD). Ten additional sampling stations were located along watercourses in the vicinity, both downstream and upstream of the mine. Sampling was performed during the rainy and dry seasons in 2010 and 2011. The water analysis detected significant changes in pH, electrical conductivity, F^?, Cd, U, Zn, Al, Mn, As, Ca, SO₄ ^2?, Pb, ²³⁸U, ²²⁶Ra, ²¹⁰Pb, ²³²Th, ²²⁸Ra, and Mo in waters downstream of both pond discharge sites. It was demonstrated that the disequilibrium between ²²⁶Ra and ²³⁸U can be used to trace the extent of AMD impacts in nearby streams. Variations in ¹⁸O and ²H enabled the flow of mining-impacted water to be traced from the ponds to nearby streams. Multivariate analysis yielded a three-factor model: Factor 1 was interpreted as being associated with AMD (from WRP#4) and Factor 2 with a Ca–Mo relationship associated with the chemical constitution of the ore and with the treatment of tailings wastes in the area (from TD); Factor 3 was interpreted as being associated with the natural influence of geogenic processes on water quality in the area. The results of this study provide a scientific basis for recommending appropriate remedial actions during mine decommissioning.     

Tailings dam seepage at the rehabilitated Mary Kathleen uranium mine,Australia

This study reports on the seepage of metals, metalloids and radionuclides from the Mary Kathleen uranium mill tailings repository. Since rehabilitation in the 1980s, the capped tailings have developed a stratified hydrochemistry, with acid (pH 3.7), saline, metal-rich (Fe, Mn, Ni, U ± As, Pb, Zn), oxygenated (1.05 mg L⁻¹ DO), radioactive waters in the upper tailings pile and near-neutral pH (pH 7.57), metal-poor, reduced (0.08 mg L⁻¹ DO) waters at depth. Seepage (∼0.5 L s⁻¹) of acid (pH 5.5), metal-rich (Fe, Mn ± Ni, U, Zn), radioactive (U-235, U-238, Ra-226, Ra-228, Ac-227) waters occurs from the base of the tailings dam retaining wall into the former evaporation pond and local drainage system. Oxygenation of the seepage waters causes the precipitation of Fe and coprecipitation and adsorption of other metals (U, Y), metalloids (As), rare earth elements (Ce, La) and radionuclides (U-235, U-238). By contrast, alkalis and alkaline–earth elements (Ca, K, Mg, Na, Sr), Mn, sulfate and to some degree metals (U, Zn, Ni), rare earth elements (Ce, La) and radionuclides (U-235, U-238, Ra-226, Ra-228) remain in solution until pH neutralisation and evaporation lead to their precipitation in efflorescences and sulfate-rich evaporative sediments. While the release of contaminant loads from the waste repository through seepage is insignificant (e.g. ∼5 kg of U per year), surface waters downstream of the tailings impoundment possess TDS, U and SO₄ concentrations that exceed Australian water quality guideline values in livestock drinking water. Thus, in areas with a semi-arid climate, even insignificant load releases of contaminants from capped tailings repositories can still cause the deterioration of water quality in ephemeral creek systems.     

Radiological hazards of coal and ash samples collected from Xi’an coal-fired power plants of China

The natural radiological characteristics and their respective annual effective dose (AED) rates, produced by ²²⁶Ra, ²³²Th and ⁴⁰K in coal, fly ash and bottom ash from two large coal-fired power plants (CFPPs) of Xi’an were determined by means of γ-ray spectrometry. The average activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K in all ash samples (fly ash and bottom ash samples) from the two CFPPs were 67.6, 74.3 and 225.3 Bq kg⁻¹, respectively. The results are compared with data from other locations. To evaluate the radiological hazards of the natural radioactivity, the radium equivalent activity (Ra_eq), air absorbed dose rate (D), AED and external hazard index (H _ex) are compared with internationally accepted values. Ra_eq and H _ex of all samples except three fly ash samples were less than the limits of 370 Bq kg⁻¹ and unity, respectively. The average D and AED for ash samples were 86.8 nGy h⁻¹ and 0.11 mSv y⁻¹, respectively, which exceed the world average and Xi’an average values.     

Sorption of radium-226 from oil-production brine by sediments and soils

The sorption of²²⁶Ra from oil-production brine by soils and sediments was investigated. Sorption was rapid, and the percentage sorbed increased with brine dilution. Greatest removals of²²⁶Ra from sediments in the laboratory occurred with alkaline DTPA, HCl, and BaCl₂, with lesser removals using CaCl₂ and NaCl solutions. Digestion of sediments with NaOCl indicates that most of the native and sorbed²²⁶Ra is associated with the mineral rather than organic fraction of the sediments. Correlation analysis based on 14 soils indicates that the retention of²²⁶Ra may involve precipitation reactions associated with sulfate-bearing minerals, as well as ion-exchange reactions with the clay mineral fractions of surficial earth materials. Oceanographer, Naval Ocean Research and Development Activity, Code 334; NSTL Station, Mississippi 39529 NORDA Contribution #83:334:01.     

Estimation of radiation exposure in soils and organic (animal) and inorganic (chemical) fertilizers using active technique

In this study, activity concentrations of ⁴⁰K, ²²⁶Ra and ²³²Th in fertilized soil samples and different organic and inorganic fertilizers used in agricultural soil were analysed using gamma-ray spectrometry NaI (Tl) detector in order to access the implications of extended use of fertilizers in 2–3 years. The concentrations of radionuclides in some granular fertilizer brands were discovered to be higher for ⁴⁰K, ²²⁶Ra and ²³²Th than those obtained in leafy fertilizer, animal fertilizer and fertilized soil samples. From the results, the highest overall mean concentrations of the specific activities of ⁴⁰K, ²²⁶Ra and ²³²Th were 2301.8 (granular fertilizer), 42.5 (leafy fertilizer) and 327.1 (animal fertilizer) in Bq kg^?1, while the lowest values observed in the specific activities of the same radionuclides were 357.7 (leafy fertilizer), 28.1 (animal fertilizer) and 36.5 (animal fertilizer). The radiological hazards of the radium equivalent (Ra_eq), normative value (NRN), outdoor radium equivalent (Ra_eq-out), external hazard index (H _ext), internal hazard index (H _in), dose rate, annual effective dose rate, activity utilization index and concentration accumulation index (CAI) and Ra_FZ due to the presence of these radionuclides in the investigated samples were calculated. Nevertheless, some of the fertilizer brands have higher concentration values than the recommended limit, and the values of hazard indices of fertilizer brands used in the selected teaching and research farms were within acceptable limit. Therefore, the fertilized soil samples in the studied farms are safe.     

Precise two chronometer dating of Pleistocene travertine: The Th/U and Raex/Ra(0) approach

In order to determine the geochemical evolution of a freshwater limestone cave system located in central Switzerland (Hell Grottoes at Baar/Zug,) young postglacial tufaceous limestone and travertine precipitates were investigated using the ²³⁰Th/²³⁴U ingrowth system. Additional analyses of further radionuclides within the ²³⁸U decay chain, i.e. ²²⁶Ra and ²¹⁰Pb, showed that the Th/U chronometer started with insignificant inherited ²³⁰Th over the entire formation period of the travertine setting (i.e. ²³⁰Th(0)=0). A contribution from detrital impurities with ²³⁰Th/²³⁴U in secular equilibrium could be precisely subtracted by applying isochron dating of cogenetic phases and recently formed travertine. The resulting precise ²³⁰Th/²³⁴U formation ages were found to be consistent with the geological stratigraphy and were furthermore used to demonstrate the applicability of the next geologically important chronometer in the ²³⁸U-decay series, based on decay of excess ²²⁶Ra normalized to the initial, i.e.²²⁶Ra_ex/²²⁶Ra(0). This system is suitable for dating phases younger than 7000 yr when the correction of a detritus component increasingly limits the precision of the ²³⁰Th/²³⁴U chronometer. Analytical solutions of the coupled ²³⁴U/²³⁰Th/²²⁶Ra radionuclide system predicted that the ²²⁶Ra_ex/²²⁶Ra(0) chronometer is independent of the actual ²³⁰Th activity build up from decay of ²³⁴U, if the systems starts with zero inherited ²³⁰Th(0). The data set confirmed this hypothesis and showed furthermore that the initially incorporated ²²⁶Ra excess must have remained almost uniform in all limestone over a period of at least 7000 yr, i.e. 4–5 half-lives of ²²⁶Ra. This is concluded because (i) the ²²⁶Ra_ex/²²⁶Ra(0) ages agreed well with those derived from ²³⁰Th/²³⁴U, (ii) all data plot within uncertainty on the ²²⁶Ra_ex/²²⁶Ra(0) decay curve and (iii) the atomic Ba/Ca ratio was found to be constant in the travertine material independent of the sample ages. Provided that such boundary conditions hold, ²²⁶Ra_ex/²²⁶Ra(0) should be applicable to materials which are suitable for ²³⁰Th/²³⁴U dating in sedimentology and oceanography, i.e. travertine, corals, phosphorites, etc., and should strongly support ²³⁰Th/²³⁴U for samples that have been formed a few thousand years ago.