• Economic and Environmental Geology
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• v.42 no.5
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• pp.471-477
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• 2009

An experimental removal of dissolved uranium (U) exsiting as uranyl ion (${UO_2}^{2+}$) was carried out using Shewanella p., iron-reducing bacterium. By the microbial reductive reaction, initial U concentration ($50{\mu}M$) was constantly decreased, and most U were removed from solution after 2 weeks. Major mechanism that U was removed from the solution was adsorption, precipitation and mineralization on the microbe surface. Under the transmission electron microscopy, the U adsorbed on the microbe was observed as being crystallized and eventually enlarged to several ${\mu}m$ sizes of minerals by combining with individual microbes and organic exudates. It seems that such U growth and mineralization on the microbial surface could affect the U behavior in a radioactive waste disposal site. Thus, the biogechemical reaction of metal-reducing bacteria observed in this experiment could give an affirmative measure that the microbial activity may retard U movement in subsurface environment.

• Journal of Nuclear Fuel Cycle and Waste Technology(JNFCWT)
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• v.16 no.3
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• pp.281-290
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• 2018

In order to understand the long-term behavior of radionuclides in granite environments, geochemical behavior characteristics of uranium in granitic host rock of KURT (KAERI Underground Research Tunnel) were investigated by dissolution experiment with different reaction time and solutions. In the dissolution experiment, significantly increased dissolution levels of uranium from granite powder samples were identified during the reaction time of 0~10 days for reaction solutions ($UD-CO_3$ and UD-Bg) containing a large amount of $CO_3{^{2-}}$. On the other hand, significantly increased dissolution levels of uranium were also identified for reaction solutions containing Na and Ca after 60 days. Dissolution of uranium continuously increased in reaction solutions of $UD-CO_3$ ($44.61{\mu}g{\cdot}L^{-1}$), UD-Bg ($41.01{\mu}g{\cdot}L^{-1}$), UD-Na ($26.87{\mu}g{\cdot}L^{-1}$), UD-Ca ($20.26{\mu}g{\cdot}L^{-1}$), UD-CaSi ($17.03{\mu}g{\cdot}L^{-1}$), and UD-Si ($10.47{\mu}g{\cdot}L^{-1}$) in the experimental period of ~270 days. However, after day 270, dissolution of uranium showed a decreasing tendency. This is thought to have occurred because existing uranium in granite samples reached the limit of dissolution by interaction with reaction solutions. Concentrations of dissolved uranium and points of maximum concentration value were found to differ depending on the $CO_3{^{2-}}$ presence in the mixed reaction solution and on the geochemical type of the water. It is estimated that differences in the reaction rate between the granite sample and the reaction solution are due to the influence of dissolved ions in the reaction solution.

• The Journal of Engineering Geology
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• v.29 no.4
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• pp.495-508
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• 2019

We measured the concentrations of natural radionuclides (uranium and radon) and major elements in groundwater collected from forty wells located in Wonju area to investigate the hydrochemistry and the occurrence of these radionuclides. The range of electrical conductivity (EC) value in the study area was 73~400 μS/cm. In addition to the increase of EC value, the content of cations and anions also tends to increase. Uranium concentrations ranged from 0.06~50.5 ㎍/L (median value, 1.55 ㎍/L) and radon concentrations ranged from 67~8,410 pCi/L (median value, 1,915 pCi/L). Uranium concentrations in 3 well, 7.5% of the samples, exceeded 30 ㎍/L, the maximum contaminant level (MCL) proposed by the US Environmental Protection Agency (EPA), based on the chemical toxicity of uranium. Radon concentrations in 9 wells, 22.5% of the samples, and 1 well, 2.2% of the samples, exceeded 4,000 pCi/L (AMCL of the US EPA) and 8,100 pCi/L (Finland's guideline level), respectively. Concentrations of uranium and radon related to geology of the study area showd the highest values in groundwater of the biotite granite area. Uranium and radon contents in the groundwater are comparatively low compared to those in other countries with similar geological settings. It is likely that the measured value was lower than the actual content due to the inflow of shallow groundwater by the lack of casing and grouting.

• Economic and Environmental Geology
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• v.51 no.6
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• pp.553-566
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• 2018

The occurrence of natural radionuclides like uranium and radon in groundwater was hydrochemically examined based on 40 well groundwaters in Mungyeong area. The range of electrical conductivity (EC) value in the study area was $68{\sim}574{\mu}S/cm$. In addition to the increase of EC value, the content of cations and anions also tends to increase. Uranium concentrations ranged from $0.03{\sim}169{\mu}g/L$ (median value, $0.82{\mu}g/L$) and radon concentrations ranged from 70~30,700 pCi/L (median value, 955 pCi/L). Only 1 out of 40 wells (2.5%) showed uranium concentration exceeding the maximum contaminant level (MCL; $30{\mu}g/L$) proposed by the US Environmental Protection Agency (EPA). Radon concentrations of eight wells (20%) exceeded AMCL(Alternative maximum contaminant level) of the US EPA (4,000 pCi/L). Four out of those eight wells even exceeded Finland's guideline level (8,100 pCi/L). When concentrations of uranium and radon were investigated in terms of geology, the highest values are generally associated with granite. The uranium and radon levels observed in this study are low in comparison to those of other countries with similar geological settings. It is likely that the measured value was lower than the actual content due to the inflow of shallow groundwater by the lack of casing and grouting.

• The Journal of Engineering Geology
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• v.23 no.2
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• pp.171-186
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• 2013

A drilling project was undertaken to characterize the geochemical relationship and the occurrence of radioactive materials at a test site among public-use groundwaters previously known to have high occurrence of uranium and radon-222 in the Daejeon area. A borehole (121 m deep) was drilled and core rocks mainly consist of two-mica granite, and associated with pegmatite and dykes of intermediate composition. The groundwater samples collected at six different depths in the borehole by a double-packed system showed the pH values ranging from neutral to alkaline (7.10-9.3), and electrical conductivity ranging from 263 to 443 ${\mu}S/cm$. The chemical composition of the borehole groundwaters was of the $Ca-HCO_3(SO_4+Cl)$ type. The uranium and Rn-222 contents in the groundwater were 109-1,020 ppb and 9,190-32,800 pCi/L, respectively. These levels exceed the regulation guidelines of US EPA. The zone of the highest groundwater uranium content occurred at depths of 45 to 55m. The groundwater chemistry in this zone (alkaline, oxidated, and high in bicarbonate) is favorable for the dissolution of uranium into groundwater. The dominant uranium complex in groundwater is likely to be $(UO_2CO_3)^0$ or $(UO_2HCO_3)^+$. Radon-222 content in groundwater shows an increasing trend with depth. The uranium and thorium contents in the core were 0.372-47.42 ppm and 0.388-11.22 ppm, respectively. These levels are higher values than those previously been reported in Korea. Microscopic observations and electron microprobe analysis(EPMA) revealed that the minerals containing U and Th are monazite, apatite, epidote, and feldspar. U and Th in these minerals are likely to substitute for major elements in crystal lattice.

• The Journal of Engineering Geology
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• v.21 no.2
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• pp.163-178
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• 2011

A test borehole was drilled in the Cheongwon area to investigate the relationship between geochemical environment and the natural occurrence of radioactive materials (uranium and Rn-222) in borehole groundwater. The borehole encountered mainly biotite schist and biotite granite, with minor porphyritic granite and basic dykes. Six groundwater samples were collected at different depths in the borehole using the double-packed system. The groundwater pH ranges from 5.66 to 8.34, and the chemical type of the groundwater is Ca-$HCO_3$. The contents of uranium and Rn-222 in the groundwater are 0.03-683 ppb and 1,290-7,600 pCi/L, respectively. The contents of uranium and thorium in the rocks within the borehole are 0.51-23.4 ppm and 0.89-62.6 ppm, respectively. Microscope observations of the rock core and analyses by electron probe microanalyzer (EPMA) show that most of the radioactive elements occur in the biotite schist, within accessory minerals such as monazite and limenite in biotite, and in feldspar and quartz. The high uranium content of groundwater at depths of -50 to -70 m is due to groundwater chemistry (weakly alkaline pH, an oxidizing environment, and high concentrations of bicarbonate). The origin of Rn-222 could be determined by analyzing noble gas isotopes (e.g., $^3He/^4He$ and $^4He/^{20}Ne$).

• Journal of the Mineralogical Society of Korea
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• v.25 no.3
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• pp.155-162
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• 2012

We observed characteristic oxidation and dissolution phenomena induced by dissolved oxygen for mackinawite that is produced via sulfate-reducing bacteria (SRB) living in anaerobic environments such as soils and groundwater. We tried to recognize the role of the sulfide minerals that usually coexist with some stabilized radionuclides (e.g., reduced uranium), which can be reoxidized and redissolved by an oxygen-rich groundwater invaded into a contaminated area. The mackinawite produced by 'Desulfovibrio desulfuricans', a sulfate-reducing bacterium, was conducted to be dissolved for 2 weeks by some oxidants such as 'hydrogen peroxide' and 'sodium nitrite'. Although mineralogical oxidation and dissolution characteristics were different from each other according to the oxidants, the initially oxidized solution was early stabilized through the oxygen consumption by ${\mu}m$-sized sulfide particles and the resultant increase of sulfate in solution. From these results, we can anticipate that the large amount of sulfide minerals generated by SRB can not only repress the anoxic environment to be disturbed by the consumption of oxygen in groundwater, but also contribute to stabilize the reduced/precipitated radionuclides as a buffer material for a long time.

• Economic and Environmental Geology
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• v.56 no.6
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• pp.847-870
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• 2023

The safe disposal of high-level radioactive waste requires accurate predictions of the long-term geochemical behavior of radionuclides. To achieve this, the present study was conducted to model geochemical behaviors of uranium (U), plutonium (Pu), and palladium (Pd) under different hydrogeochemical conditions that represent deep groundwater in Korea. Geochemical modeling was performed for five types of South Korean deep groundwater environment: high-TDS saline groundwater (G1), low-pH CO₂-rich groundwater (G2), high-pH alkaline groundwater (G3), sulfate-rich groundwater (G4), and dilute (fresh) groundwater (G5). Under the pH and Eh (redox potential) ranges of 3 to 12 and ±0.2 V, respectively, the solubility and speciation of U, Pu, and Pd in deep groundwater were predicted. The result reveals that U(IV) exhibits high solubility within the neutral to alkaline pH range, even in reducing environment with Eh down to -0.2 V. Such high solubility of U is primarily attributed to the formation of Ca-U-CO₃ complexes, which is important in both G2 located along fault zones and G3 occurring in granitic bedrocks. On the other hand, the solubility of Pu is found to be highly dependent on pH, with the lowest solubility in neutral to alkaline conditions. The predominant species are Pu(IV) and Pu(III) and their removal is predicted to occur by sorption. Considering the migration by colloids, however, the role of colloid formation and migration are expected to promote the Pu mobility, especially in deep groundwater of G3 and G5 which have low ionic strengths. Palladium (Pd) exhibits the low solubility due to the precipitation as sulfides in reducing conditions. In oxidizing condition, anionic complexes such as Pd(OH)₃^-, PdCl₃(OH)₂^-, PdCl₄^2-, and Pd(CO₃)₂^2- would be removed by sorption onto metal (hydro)oxides. This study will improve the understanding of the fate and transport of radionuclides in deep groundwater conditions of South Korea and therefore contributes to develop strategies for safe high-level radioactive waste disposal.

• Journal of the Mineralogical Society of Korea
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• v.20 no.3
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• pp.165-173
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• 2007

The KAERI Underground Research Tunnel (KURT) located in KAERI (Korea Atomic Energy Research Institute) was recently constructed following the site investigation in 2003. Its dimension is 180 m in length, 6 m in width, and 6 m in height, and it has a horseshoe-like cross-sec-lion and is located in the ground to the depth of 90 m. When the tunnel was dug into the ground with 100 m in length, fresh rocks, weathered rocks and fracture-filling materials were taken and examined by mineralogical and chemical analyses. There are phyllosilicate minerals such as illite, smectite and chlorite including calcite, which are filling some faults and cracks of the KURT rock. The illite and smectite usually coexist in the fracture, where their content ratio is different according to which mineral is predominant. There are high concentrations of U and Th in the rocks coated with iron-oxides and filled with secondary materials as compared with those in the fresh rocks. It seems that the radionuclides, which are slowly leached from the parent rocks or exist as a dissolved form in the groundwater and hydrothermal solution, may have been migrated along the fractures and thereafter selectively sorbed and coprecipitated on the iron-oxides and the fracture-filling materials. These results will be very useful far the evaluation of environmental factors affecting the nuclides migration and retardation when long-term safety is considered to the geological disposal of high-level radioactive wastes in the future.

• Korean Journal of Mineralogy and Petrology
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• v.35 no.3
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• pp.313-331
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• 2022

Natural or native abiotic molecular hydrogen (H₂) is a major component in natural gas, however yet its importance in the global energy sector's usage as clean and renewable energy is underestimated. Here we review the occurrence and geological settings of native hydrogen to demonstrate the much widesprease H₂ occurrence in nature by comparison with previous estimations. Three main types of source rocks have been identified: (1) ultramafic rocks; (2) cratons comprising iron (Fe²⁺)-rich rocks; and (3) uranium-rich rocks. The rocks are closely associated with Precambrian crystalline basement and serpentinized ultramafic rocks from ophiolite and peridotite either at mid-ocean ridges or within continental margin(Zgonnik, 2020). Inorganic geological processes producing H₂ in the source rocks include (a) the reduction of water during the oxidation of Fe²⁺ in minerals (e.g., olivine), (b) water splitting due to radioactive decay, (c) degassing of magma at low pressure, and (d) the reaction of water with surface radicals during mechanical breaking (e.g., fault) of silicate rocks. Native hydrogen are found as a free gas (51%), fluid inclusions in various rock types (29%), and dissolved gas in underground water (20%) (Zgonnik, 2020). Although research on H₂ has not yet been carried out in Korea, the potential H₂ reservoirs in the Gyeongsang Basin are highly probable based on geological and geochemical characteristics including occurrence of ultramafic rocks, inter-bedded basaltic layers and iron-copper deposits within thick sedimentary basin and igneous activities at an active continental margin during the Permian-Paleogene. The native hydrogen is expected to be clean and renewable energy source in the near future. Therefore it is clear that the origin and exploration of the native hydrogen, not yet been revealed by an integrated studies of rock-fluid interaction studies, are a field of special interest, regardless of the presence of economic native hydrogen reservoirs in Korea.