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.     

Mode of occurrence and environmental mobility of oil-field radioactive material at US Geological Survey research site B,Osage-Skiatook Project,northeastern Oklahoma

Two samples of produced-water collected from a storage tank at US Geological Survey research site B, near Skiatook Lake in northeastern Oklahoma, have activity concentrations of dissolved ²²⁶Ra and ²²⁸Ra that are about 1500 disintegrations/min/L (dpm/L). Produced-water also contains minor amounts of small (5–50 μm) suspended grains of Ra-bearing BaSO₄ (barite). Precipitation of radioactive barite scale in the storage tank is probably hindered by low concentrations of dissolved SO₄ (2.5 mg/L) in the produced-water. Sediments in a storage pit used to temporarily collect releases of produced-water have marginally elevated concentrations of “excess” Ra (several dpm/g), that are 15–65% above natural background values. Tank and pit waters are chemically oversaturated with barite, and some small (2–20 μm) barite grains observed in the pit sediments could be transferred from the tank or formed in place. Measurements of the concentrations of Ba and excess Ra isotopes in the pit sediments show variations with depth that are consistent with relatively uniform deposition and progressive burial of an insoluble Ra-bearing host (barite?). The short-lived ²²⁸Ra isotope (half-life = 5.76 a) shows greater reductions with depth than ²²⁶Ra (half-life = 1600 a), that are likely explained by radioactive decay. The ²²⁸Ra/²²⁶Ra activity ratio of excess Ra in uppermost pit sediments (1.13–1.17) is close to the ratio measured in the samples of produced-water (0.97, 1.14). Declines in Ra activity ratio (excess) with sediment depth can be used to estimate an average rate of burial of 4 cm/a for the Ra-bearing contaminant. Local shallow ground waters contaminated with NaCl from produced-water have low dissolved Ra (<20 dpm/L) and also are oversaturated with barite. Barite is a highly insoluble Ra host that probably limits the environmental mobility of Ra at site B.     

Ra/Ra and Ra/Ba ratios to track barite formation and transport in the water column

We measured ²²⁸Ra_ex/²²⁶Ra_ex and ²²⁶Ra_ex/Ba_ex ratios in suspended and sinking particles collected at the Oceanic Flux Program (OFP) time-series site in the western Sargasso Sea and compared them to seawater ratios to provide information on the origin and transport of barite (BaSO₄) in the water column. The ²²⁸Ra_ex/²²⁶Ra_ex ratios of the suspended particles down to 2000 m are nearly identical to those of seawater at the same water depth. These ratios are much lower than expected if suspended barite was produced in surface waters and indicate that barite is produced throughout the mesopelagic layer. The ²²⁸Ra_ex/²²⁶Ra_ex activity ratios of sinking particles collected at 1500 and 3200 m varied mostly between 0.1 and 0.2, which is intermediate between the seawater ratio at these depths (<0.03) and the seawater ratios found in the upper 250 m (0.31-0.42). This suggests that excess Ba (i.e., Ba_ex = Ba_total − Ba_lithogenic), considered to be mainly barite, present in the sinking flux is a mixture of crystals formed recently in the upper water column, formed several years earlier in the upper water column, or formed recently in deeper waters. We observe a sizeable temporal variability in the ²²⁸Ra_ex/²²⁶Ra_ex ratios of sinking particles, which indicates temporal variability in the relative proportion of barite crystals originating from surface (with a high ²²⁸Ra_ex/²²⁶Ra_ex ratio) and mesopelagic (with a low ²²⁸Ra_ex/²²⁶Ra_ex ratio) sources. However, we could not discern a clear pattern that would elucidate the factors that control this variability. The ²²⁶Ra/Ba ratios measured in seawater are consistent with the value reported from the GEOSECS expeditions (2.3 dpm μmol⁻¹) below 500 m depth, but are significantly lower in the upper 500 m. High ²²⁶Ra_ex/Ba_ex ratios and elevated Sr concentrations in suspended particles from the upper water column suggest preferential uptake of ²²⁶Ra over Ba during formation of SrSO₄ skeletons by acantharians, which must contribute to barite formation in shallow waters. Deeper in the water column the ²²⁶Ra_ex/Ba_ex ratios of suspended particles are lower than those of seawater. Since ²²⁸Ra_ex/²²⁶Ra_ex ratios demonstrate that suspended barite at these depths has been produced recently and in situ, their low ²²⁶Ra_ex/Ba_ex ratios indicate preferential uptake of Ba over Ra in barite formed in mesopelagic water.     

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.     

The source of radium in anomalous accumulations near sandstone escarpments,Australia

The source of Ra has been determined in water samples from four areas in Australia where anomalously high surface concentrations of²²⁶Ra have accumulated from groundwaters. All four anomalies were located adjacent to sandstone formations, and the groundwaters, which were generally all acidic and low in dissolved salts, appeared to be meteoric water with short ground-residence times. Uranium,²²⁶Ra and²²⁸Ra concentrations of waters feeding the anomalous areas were comparable to those found in standing waters within the sandstones. The²²⁶Ra/²²⁸Ra isotopic ratios were distributed about a median of 1.1 which suggests that the waters are in contact with rocks with near-normal U/Th ratios and, hence, that the Ra in the anomalies was derived from within the sandstones.The presence of the short-lived Ra isotopes,²²³Ra and²²⁴Ra, in high concentrations in most spring waters feeding these anomalies suggests that Ra enters groundwaters by recoil following alpha decay of a precursor parent radionuclide within mineral grains. Thus, although three of the areas were considered prospective for U, the radioactive anomalies studied appear to be due to natural transfer of Ra from the sandstones to the surface environment. In no case were the anomalies related to nearby known or undiscovered U deposits. Accordingly, a geochemical procedure, which includes Ra isotopic measurements, is recommended for evaluating radioactive anomalies for U exploration. This procedure should enable selection of only those anomalies with the highest potential for further exploration by more expensive techniques.     

Relationships between radium and radon occurrence and hydrochemistry in fresh groundwater from fractured crystalline rocks, North Carolina (USA)

Naturally-occurring radionuclides (uranium, radium, and radon), major dissolved constituents, and trace elements were investigated in fresh groundwater in 117 wells in fractured crystalline rocks from the Piedmont region (North Carolina, USA). Chemical variations show a general transition between two water types: (1) slightly acidic (pH 5.0–6.0), oxic, low-total dissolved solids (TDS) waters, and (2) near neutral, oxic to anoxic, higher-TDS waters. The uranium, radium, and radon levels in groundwater associated with granite (Rolesville Granite) are systematically higher than other rock types (gneiss, metasedimentary, and metavolcanic rocks). Water chemistry plays a secondary role on radium and radon distributions as the ²²²Rn/²²⁶Ra activity ratio is correlated with redox-sensitive solutes such as dissolved oxygen and Mn concentrations, as well as overall dissolved solids content including major divalent cations and Ba. Since ²²⁴Ra/²²⁸Ra activity ratios in groundwater are close to 1, we suggest that mobilization of Ra and Rn is controlled by alpha recoil processes from parent nuclides on fracture surfaces, ruling out Ra sources from mineral dissolution or significant long-distance Ra transport. Alpha recoil is balanced by Ra adsorption that is influenced by redox conditions and/or ion concentrations, resulting in an approximately one order of magnitude decrease (~ 20,000 to ~ 2000) in the apparent Ra distribution coefficient between oxygen-saturated and anoxic conditions and also across the range of dissolved ion concentrations (up to ~ 7 mM). Thus, the U and Th content of rocks is the primary control on observed Ra and Rn activities in groundwater in fractured crystalline rocks, and in addition, linked dissolved solids concentrations and redox conditions impart a secondary control.     

Release ofRa from uranium mill tailings by microbial Fe(III) reduction

Uranium mill tailings were anaerobically incubated in the presence of H₂ with Alteromonas putrefaciens, a bacterium known to couple the oxidation of H₂ and organic compounds to the reduction of Fe(III) oxides. There was a direct correlation between the extent of Fe(III) reduction and the accumulation of dissolved²²⁶Ra. In sterile tailings in which Fe(III) was not reduced, there was negligible leaching of²²⁶Ra. The behavior of Ba was similar to that of Ra in inoculated and sterile systems. These results demonstrate that under anaerobic conditions, microbial reduction of Fe(III) may result in the release of dissolved²²⁶Ra from uranium mill tailings.     

Application of226Ra,228Ra,223Ra, and224Ra in coastal waters to assessing coastal mixing rates and groundwater discharge to oceans

The fate of dissolved material delivered to the coastal ocean depends on its reactivity and the rate at which it is mixed offshore. To measure the rate of exchange of coastal waters, we employ two short-lived radium isotopes,²²³Ra and²²⁴Ra. Along the coast of South Carolina, shore-perpendicular profiles of²²³Ra and²²⁴Ra in surface waters show consistent gradients which may be modeled to yield eddy diffusion coefficients of 350–540 m²s⁻¹. Coupling the exchange rate with offshore concentration gradients yields estimates of offshore fluxes of dissolved materials. For systems in steady state, the offshore fluxes must be balanced by new inputs from rivers, groundwater, sewers or other sources. Two tracers that show promise in evaluating groundwater input are barium and²²⁶Ra. These tracers have high relative concentrations in the fluids and low-reactivity in the coastal ocean. Applying the eddy diffusion coefficients to the offshore gradient of²²⁶Ra concentration provides an estimate of the offshore flux of²²⁶Ra. Measuring the concentrations of²²⁶Ra in subsurface fluids provides an estimate of the fluid flux necessary to provide the²²⁶Ra. These estimates indicate that the volume of groundwater required to support these fluxes is of the order of 40% of the surface water flow.     

Ra-226 and Rn-222 in saline water compartments of the Aral Sea region

The Aral Sea has been shrinking since 1963 due to extensive irrigation and the corresponding decline in the river water inflow. Understanding of the current hydrological situation demands an improved understanding of the surface water/groundwater dynamics in the region. ²²²Rn and ²²⁶Ra measurements can be used to trace groundwater discharge into surface waters. Data of these radiometric parameters were not previously available for the study region. We determined ²²²Rn activities after liquid phase extraction using Liquid Scintillation Counting (LSC) with peak-length discrimination and analyzed ²²⁶Ra concentrations in different water compartments of the Amu Darya Delta (surface waters, unconfined groundwater, artesian water, and water profiles from the closed Large Aral Sea (western basin).The water samples comprise a salinity range between 1 and 263 g/l. The seasonal dynamics of solid/water interaction under an arid climate regime force the hydrochemical evolution of the unconfined groundwater in the Amu Darya Delta to high-salinity Na(Mg)Cl(SO₄) water types. The dissolved radium concentrations in the waters were mostly very low due to mineral over-saturation, extensive co-precipitation of radium and adsorption of radium on coexisting solid substrates.The analysis of very low ²²⁶Ra concentrations (<10 ppq) at remote study sites is a challenge. We used the water samples to test and improve different analytical methods. In particular, we modified a procedure developed for the α-spectrometric determination of ²²⁶Ra after solid phase extraction of radium using 3M Empore™ High Performance Extraction Disks (Purkl, 2002) for the analysis of the radionuclide using an ICP sector field mass spectrometer. The ²²⁶Ra concentration of 17 unconfined groundwater samples ranged between 0.2 and 5 ppq, and that of 28 artesian waters between <0.2 and 13 ppq. The ICP-MS results conformed satisfactorily to analytical results based on γ-measurements of the ²²²Rn ingrowth after purging and trapping on super-cooled charcoal. The ²²⁶Ra concentrations were positively correlated with the salinity and the dissolved NaCl concentrations. The occurrence of unusually high ²²⁶Ra activities is explained by radium release from adsorption sites with increasing salinity. The inferred spatial variability of ²²²Rn in the Aral Sea and of ²²²Rn and ²²⁶Ra in the groundwater of the Amu Darya Delta is discussed in the context of our own previous hydrochemical studies in the study sites. Relatively low ²²²Rn activities in the unconfined GW (1–9.5 Bq/l) indicate the alluvial sediments hosting the GW to be a low-²³⁸U(²²⁶Ra) substrate. Positive correlations between U and ²²⁶Ra, and U and ²²²Rn are likely related to locally deposited Fe(Mn)OOH precipitates. The ²²²Rn activity of the GW, however, distinctly exceeds the ²²²Rn concentration in the Aral Sea (10 mBq/l), in principle, making ²²²Rn a sensitive tracer for the inflow of GW. The high water volume of the Large Aral Sea and wind induced mixing of its water body, however, hamper the detection of local groundwater inflow.     

Kinetic and thermodynamic factors controlling manganese concentrations in oceanic waters

Data are presented describing the changes in the distribution of dissolved and particulate Mn observed over a 16-month period in the periodically anoxic waters of Saanich Inlet, a fjord located on the coast of Vancouver Island, British Columbia. During the spring and summer when the bottom waters were anoxic, a dense cloud of particulate Mn was found at mid-depths where Mn²⁺ enriched anoxic bottom waters were mixing with oxygenated waters; then, during the autumn aand winter following an intrusion which reoxygenated the bottom water, an intense precipitation of Mn was observed throughout the entire water column. During this latter period, dissolved Mn concentrations in the bottom water, which exceeded 1000 nmol/l under anoxic conditions, decreased towards a lower limit of 1.6 nmole/l, a value comparable to that observed in Pacific Ocean waters of similar pH and dissolved oxygen content. Mn in the particulate matter collected just above the oxic-anoxic interface was found to have an average oxidation number of +3.05; and, on this basis, it is proposed that dissolved Mn concentrations in oceanic waters are controlled by the precipitation of the metastable oxide mineral manganite (γ-MnOOH), a hypothesis consistent with the fact that dissolved Mn values in subsurface Pacific Ocean waters closely approach the equilibrium solubility of this phase. Temporal and spatial gradients in the particulate Mn distribution were used to calculate the in situ rate of Mn precipitation, and the results of these calculations then were fitted to theoretical rate equations which suggest that the precipitation of Mn is controlled by 2 parallel processes: bacterial oxidation and an inorganic autocatalytic oxidation reaction.     

The geochemistry of Ca,Sr, Ba and Ra sulfates in some deep brines from the Palo Duro Basin,Texas

The geochemistry of Ca, Sr, Ba and Ra sulfates in some deep brines from the Palo Duro Basin of north Texas, was studied to define geochemical controls on radionuclides such as ⁹⁰Sr and ²²⁶Ra. Published solubility data for gypsum, anhydrite, celestite, barite and RaSO₄ were first reevaluated, in most cases using the ion interaction approach of Pitzer, to determine solubility products of the sulfates as a function of temperature and pressure. Ionic strengths of the brines were from 2.9 to 4.8 m, their temperatures and pressures up to 40°C and 130 bars. Saturation indices of the sulfates were computed with the ion-interaction approach in one brine from the arkosic granite wash fades and four from the carbonate Wolfcamp Formation. All five brines are saturated with respect to gypsum, anhydrite and celestite, and three of the five with respect to barite. All are undersaturated by from 5 to 6 orders of magnitude with respect to pure RaSO₄. ²²⁶Ra concentrations in the brines, which ranged from 10^?11.3 to 10^?12.7 m, are not controlled by RaSO₄ solubility or adsorption, but possibly by the solubility of trace Ra solid solutions in sulfates including celestite and barite.     

On the distribution of iron and manganese at the sediment/water interface: thermodynamic versus kinetic control

Iron and manganese solubility at the sediment/water interface has been studied at a water depth of 20 m in Kiel Bight, Western Baltic. By means of an in situ bell jar system enclosing 3.14 m² sediment surface and 2094 l water a complete redox turn-over in the bottom water was simulated in an experiment lasting 99 days. The concentration of dissolved Fe in the bell jar water never exceeded 0.041 μmol · dm^?3during the first 50 days of the experiment and then rose abruptly as the Eh fell from +600 to ?200 mV. The concentration of dissolved Fe under oxic and anoxic conditions seems to be limited by equilibria with solid Fe-phases (hydroxides and amorphous sulphide, respectively). In contrast to Fe, manganese was released continuously from the bottom during the first 50 days of the experiment leading to exponentially increasing manganese concentrations in the bell jar water. During this time dissolved O₂ had become ready depleted and pH had dropped from 8.3 to 7.5. Contrary to iron, manganese being solubilized in reduced sediment layers can penetrate oxic strata in metastable form due to slow oxidation kinetics; when the redoxcline moves upwards Mn²⁺ is enriched in bottom waters. The maximum concentration of dissolved Mn under anoxic conditions is controlled by a solid phase with solubility properties similar to MnCO₃ (rhodochrosite). Bottom water enrichment in dissolved Mn²⁺ could be traced to originate from excess solid manganese within the top 3 cm of the sediment.     

An evaluation of copper remobilization from mine tailings in sulfidic environments

A laboratory-based assessment of copper remobilization from Cu-rich mine tailings exposed to anoxic, sulfide rich waters was performed. The results from incubation experiments, conducted over a 20 day period, were compared to thermodynamic modelling calculations of copper speciation in sulfidic waters. The tailings materials were observed to react rapidly with added sulfide, consuming 159 μmol HS⁻ g⁻¹ (dry wt) within a 24 h period. The consumption of sulfide was attributed to a two stage process involving the reduction of Fe-hydroxy phases by sulfide followed by reaction with available Fe²⁺ and Cu²⁺ resulting in the formation Fe- and Cu-sulfide phases. During incubation experiments, the dissolved copper concentrations in the absence of sulfide were approximately 0.31 μmol l⁻¹, whereas in the presence of sulfide (0.5–5 mM) concentrations were typically 0.24 μmol l⁻¹. The experiments did not indicate enhanced solubility owing to the formation of soluble copper sulfide species. The predictions (based on the most recent thermodynamic data for aqueous Cu-sulfide and Cu-polysulfide species) did not accurately explain the laboratory observations. Model predictions were greatly influenced by the assumptions made about the oxidation state of copper under anoxic conditions and the solid sulfide phase controlling copper solubility. The study emphasizes the limitations of modelling copper speciation in sulfidic waters and the need for laboratory or field verification of predictions.     

Geological analogue for circumneutral pH mine tailings: implications for long-term storage,Macraes Mine,Otago, New Zealand

Tailings from the Macraes Au mine cyanidation process are stored in an impoundment about 0.6 km² and 80 m deep whose pH is maintained near 8 by the neutralizing capacity of the gangue minerals. The tailings are sandy (>50 μm particles), have a hydraulic conductivity of about 10⁻² m/day, and contain 0.1–1.0 wt.% S and 0.1–1.5 wt.% graphitic C from the primary deposit. Concentrations of As in the pore water of the mixed tailings, which are a combination of various tailings types, range from 0.1 to 20 ppm, HCO₃^- is 100 to 200 ppm, and dissolved SO₄ is 100–1700 ppm. The mixed tailings will be stored in this impoundment in perpetuity after mining ceases. Confidence in the long-term pH stability of these tailings can be gained from examination of mineralogically and chemically similar geological analogues in the immediate vicinity. A sequence, typically about 5 m thick, of sands and gravels derived from the Macraes mineralized zone 12–28 ka ago contains rounded detrital sulfide mineral grains which are unoxidized despite their close proximity to the surface and the occasional incursion of oxygenated waters. These sediments have a hydraulic conductivity of about 10⁻⁴ m/day. Saturating water pH is currently 7–8. Sands with 0.2–0.8 wt.% organic C host SO₄-reducing bacteria (SRB), and local cementation by authigenic framboidal pyrite has occurred. SRB were found in water-saturated sediments with decreased hydraulic conductivity and alkaline and anoxic conditions. These bacteria are involved in the formation of authigenic framboidal pyrite, reducing the cycling of dissolved Fe in the sediments. Carbon is not a limiting factor in this process as organic matter is present in the sandstone and ground water contains up to 180 ppm HCO₃^-. Comparison of the 28 ka old sediments with the modern tailings suggests that the chemical behaviour of the two will be similar, possibly with the crystallization of authigenic pyrite in the tailings over the long term. As long as the present slightly anoxic and circumneutral pH environmental conditions are maintained in the mixed tailings impoundment, sulfide decomposition and acidification are unlikely.     

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.     

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.     

Radium isotopic measurements in the search for uranium in palaeodrainage channels

Analyses have been made of many groundwater samples, some of which were collected from the vicinity of uranium deposits and others from unmineralized areas, for dissolved uranium and for the four naturally occurring isotopes of radium: ²²⁶Ra (²³⁸U decay series, y), ²²⁸Ra and ²²⁴Ra (²³²Th decay series, y and 3.8 d) and ²²³Ra (²³⁵U decay series, d). The radium isotopes ²²⁶Ra, ²²⁴Ra and ²²³Ra, are measured by alpha-spectrometry after extraction from a water sample soon after collection and ²²⁸Ra at a later time by determining the amount of ingrown ²¹²Po.     

扬子板块北缘下志留统龙马溪组重晶石结核特征及其成因机制分析

在扬子板块北缘城口明中剖面及巫溪徐家坝剖面下志留统龙马溪组底部的硅质岩、泥岩中发现椭球状重晶石结核。其矿物组成主要为重晶石颗粒与作为"基质"的黄铁矿、粘土矿物和石英。通过重晶石岩石学、矿物学及锶同位素分析表明,重晶石结核形成于早期成岩阶段松软沉积物的孔隙水中。上升洋流带来丰富的营养及富钡物质,表层海水的高初始生产力促使生物繁盛,海水中的钡通过生物作用富集形成生物钡,生物钡(bio-barite)在埋藏过程中的硫酸盐耗竭区(sulfate depleted zones)通过硫酸盐细菌作用(BSR)溶解被激活提供了钡的来源。围岩岩性(黑色泥岩和硅质泥岩)表明重晶石结核形成于缺氧的环境中。上述研究对深入理解早志留世时期扬子板块北缘古海洋环境有一定的启示作用。     

Naturally occurring radionuclides and their geochemical interactions at a geothermal site in the North German Basin

The activities of the most common, naturally occurring radionuclides ²³⁸U, ²²⁶Ra, ²¹⁰Pb, ²²⁸Ra, ²²⁸Th, and ⁴⁰K were measured by gamma-ray spectrometry in samples from reservoir rocks, geothermal fluids, and mineral precipitates at the geothermal research site Groß Schönebeck (North German Basin). Results demonstrated that the specific activity of the reservoir rock is within the range of the mean concentration in the upper earth crust of <800 Bq/kg for ⁴⁰K and <60 Bq/kg for radionuclides of the ²³⁸U and ²³²Th series, respectively. The geothermal fluid showed elevated activity concentrations (up to 100 Bq/l) for ²²⁶Ra, ²¹⁰Pb, and ²²⁸Ra, as compared to concentrations found in natural groundwater. Their concentration in filter residues even increased up to 100 Bq/g. These residues contain predominantly two different mineral phases: a Sr-rich barite (Sr, BaSO₄) and laurionite (PbOHCl), which both precipitate upon cooling from the geothermal fluid. Thereby they presumably enrich the radionuclides of Ra (by substitution of Ba) and Pb. Analysis of these precipitates further showed an increased ²²⁶Ra/²²⁸Ra ratio from around 1–1.7 during the initial months of fluid production indicating a change in fluid composition over time which can be explained by different contributions of stimulated reservoir rock areas to the overall produced fluid.     

Radium uptake during barite recrystallization at 23 ± 2 °C as a function of solution composition: An experimental Ba and Ra tracer study

High-purity synthetic barite powder was added to pure water or aqueous solutions of soluble salts (BaCl₂, Na₂SO₄, NaCl and NaHCO₃) at 23 ± 2 °C and atmospheric pressure. After a short pre-equilibration time (4 h) the suspensions were spiked either with ¹³³Ba or ²²⁶Ra and reacted under constant agitation during 120-406 days. The pH values ranged from 4 to 8 and solid to liquid (S/L) ratios varied from 0.01 to 5 g/l. The uptake of the radiotracers by barite was monitored through repeated sampling of the aqueous solutions and radiometric analysis. For both ¹³³Ba and ²²⁶Ra, our data consistently showed a continuous, slow decrease of radioactivity in the aqueous phase.Mass balance calculations indicated that the removal of ¹³³Ba activity from aqueous solution cannot be explained by surface adsorption only, as it largely exceeded the 100% monolayer coverage limit. This result was a strong argument in favor of recrystallization (driven by a dissolution-precipitation mechanism) as the main uptake mechanism. Because complete isotopic equilibration between aqueous solution and barite was approached or even reached in some experiments, we concluded that during the reaction all or substantial fractions of the initial solid had been replaced by newly formed barite.The ¹³³Ba data could be successfully fitted assuming constant recrystallization rates and homogeneous distribution of the tracer into the newly formed barite. An alternative model based on partial equilibrium of ¹³³Ba with the mineral surface (without internal isotopic equilibration of the solid) could not reproduce the measured activity data, unless multistage recrystallization kinetics was assumed. Calculated recrystallization rates in the salt solutions ranged from 2.8 × 10⁻¹¹ to 1.9 × 10⁻¹⁰ mol m⁻² s⁻¹ (2.4-16 μmol m⁻² d⁻¹), with no specific trend related to solution composition. For the suspensions prepared in pure water, significantly higher rates (∼5.7 × 10⁻¹⁰ mol m⁻² s⁻¹ or ∼49 μmol m⁻² d⁻¹) were determined.Radium uptake by barite was determined by monitoring the decrease of ²²⁶Ra activity in the aqueous solution with alpha spectrometry, after filtration of the suspensions and sintering. The evaluation of the Ra uptake experiments, in conjunction with the recrystallization data, consistently indicated formation of non-ideal solid solutions, with moderately high Margules parameters (W_AB = 3720-6200 J/mol, a₀ = 1.5-2.5). These parameters are significantly larger than an estimated value from the literature (W_AB = 1240 J/mol, a₀ = 0.5).In conclusion, our results confirm that radium forms solid solutions with barite at fast kinetic rates and in complete thermodynamic equilibrium with the aqueous solutions. Moreover, this study provides quantitative thermodynamic data that can be used for the calculation of radium concentration limits in environmentally relevant systems, such as radioactive waste repositories and uranium mill tailings.