Influence of halogen processes on the hydrosphere,lithosphere, and biosphere

Great scales of halogen processes and aggressiveness of their liquid products, the brines, suggest significant and complex role of these processes in the geological evolution of the Earth. Evaporite process extracted huge masses of salts from oceans and transformed marginal seas into biotically unsuitable environments, many times increasing their salinity. Prolonged (10⁴–10⁶ years) formation of saliferous basins was accompanied by similarly protracted descending filtration of brines, which played an important role in the mineral transformation of the rocks of sedimentary shell and its water component. The study of behavior of halogen brines at the sedimentation and post-sedimentation stages makes it possible to decipher many aspects of genetic links of ore- and naphthidogenesis with halogenesis and to obtain the quantitative estimate of these links. Mechanisms and results of the influence of halogen processes on the hydrosphere, lithosphere, and biosphere were analyzed and a general model of halogen press on the aforementioned spheres was developed.     

Advances in geochemical research for the underground disposal of high-level, long-lived radioactive waste in a clay formation

One of the options for the long-term confinement of high-level, long-lived radioactive waste is the disposal in deep geological formations. In France, this option is particularly studied in a 155-Ma-old Jurassic clay formation located in the eastern part of the Paris Basin. Fifteen years of research and the construction of an underground research laboratory have provided a large set of data that allows the feasibility of the geological disposal to be evaluated from a scientific standpoint. Geochemical aspects of this research are of major importance because they provide essential information on both the characteristics of the geological medium and the long-term behaviour of the waste and the engineered system.     

Geochemical and environmental processes affecting radionuclide migration from a formerly used seepage trench

The disposal of liquid waste, containing about 0.3 million curies (10⁷ GBq) of fission nuclides, activation products, actinides and transuranics, in a shallow land seepage trench from 1962 to 1966, provided a field opportunity for investigating the chemical, geological, and hydrological processes which affect contaminant migration in soils and weathered bedrock. Gamma-log profiles of wells near the trench indicate that the waste liquids seeped along discrete layers parallel to bedding and along the strikes of faults and folds. Most of the radioactivity measured in the groundwaters consisted of ³H, ⁹⁹Tc, ⁶⁰Co, and ²³³U. The mobility of ⁹⁹Tc, ⁶⁰Co, and ²³³U has been attributed to low molecular weight anionic complexing. Concentrations of ⁹⁰Sr and ¹³⁷Cs in the groundwaters were extremely low because of the chemical treatments and precautions taken to establish and maintain an alkaline environment near the trench, which allows for ⁹⁰Sr sorption and precipitation, and because of the strong tendency for ¹³⁷Cs to be selectively sorbed by illite. Plutonium isotopic ratios indicate that much of the plutonium contamination near the trench results from the migration of ²⁴²Cm and ²⁴⁴Cm and their subsequent decay to ²³⁸Pu and ²⁴⁰Pu.Radionuclide concentrations in the groundwaters near the north end of the trench undergo seasonal variations, with the highest activities occurring in the spring and after prolonged rainfall. This suggests that contamination may be leached from the trench or from the relict waste migration layers when the groundwater level rises to saturate these zones or when precipitation infiltrates into the trench or along these relict migration layers during drainage. Suspected transport pathways from the trench to a nearby seep area appear to be associated with fault zones and limbs of a plunging limestone fold.     

Will the river Irtysh survive the year 2030? Impact of long-term unsuitable land use and water management of the upper stretch of the river catchment (North Kazakhstan)

The Irtysh river basin all the way from river spring in China across Kazakhstan as far as the Russian part of Siberia is among the most ecologically endangered and affected regions on our planet. The study provides a summary of the historical reasons for anthropological interventions in this area, which began with the construction of plants of the military—industrial complexes in the forties of the last century during World War II. These plants have a major share in extreme high concentrations of heavy metals in surface as well in groundwaters locally. The Semipalatinsk nuclear polygon plays a specific role as a source of contamination of local waters. The release of top secret data enabled us to gain knowledge about serious problems related to high radioactivity of groundwaters, which should spread uncontrollably through a system of secondary fissures activated by nuclear blasts. Another serious problem in this region is the quantitative aspect of contamination. Model simulations of water balance indicate that large industrial development in the spring area in China and continuously increasing water consumption in Kazakhstan may lead to desiccation of the lower stretch of this large river in Siberia during the summer months of 2030.     

Natural radionuclides in the Arabian Sea and Bay of Bengal: Distribution and evaluation of particle scavenging processes

Vertical and temporal variations in the activities of²³⁴Th,²¹⁰Po and²¹⁰Pb have been measured, in both dissolved and paniculate phases, at several stations in the eastern Arabian Sea and north-central Bay of Bengal. A comparative study allows us to make inferences about the particle associated scavenging processes in these two seas having distinct biogeochemical properties. A common feature of the²³⁴Th profiles, in the Arabian Sea and Bay of Bengal, is that the dissolved as well as total (dissolved + particulate) activity of²³⁴Th is deficient in the surface 200 m with respect to its parent,²³⁸U. This gross deficiency is attributed to the preferential removal of²³⁴Th by adsorption onto settling particles which account for its net loss from the surface waters. The scavenging rates of dissolved²³⁴Th are comparable in these two basins. The temporal variations in the²³⁴Th-²³⁸U disequilibrium are significantly pronounced both in the Arabian Sea and Bay of Bengal indicating that the scavenging rates are more influenced by the increased abundance of particles rather than their chemical make-up. In the mixed layer (0–50 m), the scavenging residence time of²³⁴Th ranges from 30 to 100 days. The surface and deep waters of both the seas show an enhanced deficiency of dissolved²¹⁰Po relative to²¹⁰Pb and that of²¹⁰Pb relative to²²⁶Ra. The deficiencies of both²¹⁰Po and²¹⁰Pb in the dissolved phases are not balanced by their abundance in the particulate form indicating a net loss of both these nuclides from the water column. The scavenging rates of²¹⁰Po and²¹⁰Pb are significantly enhanced in the Bay of Bengal compared to those in the Arabian Sea. The mean dissolved²¹⁰Po/²¹⁰Pb and²¹⁰Pb/²²⁶Ra activity ratios in deep waters of the Bay of Bengal are ∼ 0.7 and 0.1, respectively, representing some of the most pronounced disequilibria observed to date in the deep sea. The Bay of Bengal and the Arabian Sea appear to be the regions of most intense particle moderated scavenging processes in the world oceans. This is evidenced by the gross disequilibria exhibited by the three isotope pairs used in this study.     

Crustal rifting,and related thermo-mechanical processes in the lithosphere beneath Iceland

Recent observations of crustal rifting in the NE-Iceland axial zone are summarized and discussed. The thermal state of the crust in the axial zone is discussed on the basis of thermal gradient measurements, magnetotelluric soundings, and modelling of the accretion mechanism. The three independent methods are in good general agreement. Thermal modelling indicates that a zone of partial melting generally exists below approximately 6 km depth in the axial zone. It is suggested, that thermal stresses induced by the cooling of the lithosphere as it moves away from the axis are largely responsible for the off-axis volcanism, and that the thermal stresses also contribute to enhancing the vertical permeability for geothermal waters at intermediate crustal depths on the flanks of the axial zone.

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Zusammenfassung Neue Beobachtungen von Driftbewegungen der Erdkruste in dem zentralen Bereich der aktiven Riftzone in Nordost-Island werden zusammengefaßt und diskutiert. Der thermische Zustand der Zentralzone wird diskutiert an Hand von Temperaturgradientmessungen, magnetotellurischen Messungen und Modelrechnungen, die die Entstehung der Kontinentalplatten beschreiben. Die Ergebnisse der drei unabhängigen Methoden sind in guter Übereinstimmung miteinander. Thermische Modelrechnungen weisen darauf hin, daß in einer Tiefe von etwa 6 km unterhalb der zentralen Zone, eine Schicht von teilweise geschmolzenen Gesteinsmassen allgemein vorhanden ist. Es wird vorgeschlagen, daß thermische Spannungen, die durch die Abkühlung der Lithosphäre auf seiner Bewegung weg von der zentralen Riftzone erzeugt werden, hauptsächlich für die vulkanische Aktivität außerhalb der Zentralzone verantwortlich sind. Die thermischen Spannungen erhöhen vermutlich auch eine vertikale Permeabilität für geothermales Wasser in der mittleren Kruste an den Flanken der aktiven Zone.

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Résumé L'auteur résume et discute des observations récentes de formation de rift crustal dans la zone axiale du NE de l'Islande. Il discute l'état thermique de la croûte dans la zone axiale sur la base de mesures du gradient thermique, de sondages magnéto-telluriques et d'un modèle du mécanisme d'accroissement. Ces trois méthodes indépendantes sont en général en bon accord. Le modèle thermique indique qi'il existe généralement une zone de fusion partielle en-dessous d'une profondeur approximative de 6 km dans la zone axiale. Il est suggéré que des tensions thermales induites par le refroidissement de la lithosphere au fur et à mesure qu'il s'écarte de l'axe sont largement responsables du volcanisme à l'écart de l'axe, et que les tensions thermiques contribuent aussi à activer la perméabilité, suivant la verticale, pour les eaux chaudes aux profondeurs crustales moyennes sur les flancs de la zone axiale.

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The use of natural analogues to assess radionuclide transport

Predicting the long-term safety and performance of a repository for intermediate- to high-level radioactive wastes requires the evaluation of various scenarios which may influence the integrity of the repository system. One such scenario, radionuclide transport, is described, and some of the geochemical processes which serve to enhance or retard transport are illustrated with examples selected from natural analogue or natural system studies. These studies, as distinct from laboratory simulations, help considerably in understanding how near- and far-field radionuclide transport mechanisms work over geological timescales (thousands to hundreds of thousands of years), more in line with the anticipated lifespan of a repository.

Processes addressed include: stability of the spent fuel UO₂ matrix; bentonite backfill as a barrier to radionuclide diffusion; retention of radionuclides by absorption on fracture minerals; transport and diffusion of radionuclides controlled by interconnected bedrock porosity; geochemical influence and behaviour of repository construction materials on radionuclide transport.     

Characterization of sediments in an abandoned mining area; a case study of Mansfeld region,Germany

The distribution of selected heavy metals, including some radionuclides, metalloids and non-metals was determined in stream sediments in a region influenced by abandoned copper mining and ore processing activities. A considerable amount of the ore processing waste with a very complex composition and highly elevated concentrations of zinc, sulfur, lead, copper, arsenic, and a lot of other elements in the range between 100 and 1,000 mg/kg (Sb, Mn, Ni, Cr, Cd, Hg, and Ag) was piled up on mine dumps. The dispersion of the pollutants originating from this source and their environmental impact were investigated. Both, sediments and original waste material were studied to indicate the pathways and the mobilization behavior of different pollutants. For this purpose, the process of the elution of pollutants by application of different fractionation schemes was studied. The capabilities of different analytical techniques are shown for the analysis of solid samples (X-ray fluorescence spectrometry, Gamma-spectrometry) and liquid ones (ICP-atomic emission spectrometry, ICP-mass spectrometry and different techniques of atomic absorption). Additionally, the coupling of ion chromatography and ICP-MS detection was used to study the distribution of arsenic species in the sediment cores of a lake which acts as a natural sink for the region.     

Studying the migration behaviour of selenate in Boom Clay by electromigration

For the disposal of high-level waste (HLW) in a deep geological formation as Boom Clay, safety assessment studies have shown that long lived ⁷⁹Se is one of the more critical fission products. Therefore, the knowledge of its migration properties (diffusion, retention) through the geological barrier (Boom Clay) is of paramount importance. The migration behaviour of selenium strongly depends on its speciation. Under reducing conditions, selenide would be the dominant species and selenium migration would mainly be controlled by the low solubility of Se(−II)-bearing minerals. However Se species are often found in redox disequilibrium and more oxidized species might also coexist. Therefore, the study of selenate migration requires attention, as it might be the most mobile selenium species in the host rock. Electromigration experiments performed with a ⁷⁵Se-labeled selenate in Boom Clay indicate a high mobility for this species. The apparent diffusion coefficient (D_app) of selenate in Boom Clay is estimated from electromigration experiments performed under different electric fields. Using two independent approaches, the value of D_app for selenate is shown to fall in the range from 1.7×10⁻¹¹ to 6.2×10⁻¹¹ m² s⁻¹. Moreover, no reduction of selenate in Boom Clay was observed.     

Natural radionuclides as tracers of coastal biogeochemical processes

This paper presents an overview of the ways that natural radionuclides have been used as tracers of biogeochemical processes occurring in the coastal ocean. The radionuclides involved include those in the uranium and thorium decay series and those produced in the atmosphere by the interaction of cosmic rays with atmospheric gases (i.e., cosmogenic radionuclides). The property of radioactivity provides a chronometer with which to measure the rates of oceanic processes, and the fundamental biogeochemical behavior of the radionuclide determines which process(es) it may be used to trace. Examples from recent research will be presented in the talk.     

The significance of microbial processes in hydrogeology and geochemistry

2 (methanogenesis). In aquifers contaminated by anthropogenic contaminants, an excess of available organic carbon often exists, and microbial metabolism is limited by the availability of electron acceptors. In addition to changes in groundwater chemistry, the solid matrix of the aquifer is affected by microbial processes. The production of carbon dioxide and organic acids can lead to increased mineral solubility, which can lead to the development of secondary porosity and permeability. Conversely, microbial production of carbonate, ferrous iron, and sulfide can result in the precipitation of secondary calcite or pyrite cements that reduce primary porosity and permeability in groundwater systems. Received, January 1999/Revised, July 1999, August 1999/Accepted, October 1999     

A temporary sampling technique for investigating groundwater quality near the ground surface

Improved knowledge of processes determining groundwater quality is an important precondition for the solution of various ecological and water management problems. In areas with highly fluctuating groundwater levels, time-limited access, local pollution sources or temporary interactions between surface water and groundwater, a temporary groundwater sampling technique could be of advantage. Furthermore, depth-specific sampling is of high value for investigating groundwater pollution related to seepage or surface water infiltration. A stainless steel core probe has been developed to obtain groundwater samples and to measure the hydraulic head distribution at various defined depths. The sampling technique is applicable only for non-volatile water constituents. An advantage of the core probe is that it can be driven into soil or sediments using ordinary low cost percussion equipment. The probe enables hydraulic head measurements and water sampling over vertical intervals of 0.3 m. Results from field experiments using the stainless steel core probe were in good correspondence with results from groundwater sampling at nearby observation wells. In the upper layer of the aquifer, the intrinsic spatial change in concentrations of sulphate, chloride and other water constituents is a function of distance between observation points and groundwater surface. Results indicate strong effects of a fluctuating groundwater level on groundwater quality at certain depths.     

Melt migration and melt-rock reaction in the Alpine-Apennine peridotites:Insights on mantle dynamics in extending lithosphere

The compositional variability of the lithospheric mantle at extensional settings is largely caused by the reactive percolation of uprising melts in the thermal boundary layer and in lithospheric environments.The Alpine-Apennine(A-A)ophiolites are predominantly constituted by mantle peridotites and are widely thought to represent analogs of the oceanic lithosphere formed at ocean/continent transition and slow-to ultraslow-spreading settings.Structural and geochemical studies on the A-A mantle peridotites have revealed that they preserve significant compositional and isotopic heterogeneity at variable scale,reflecting a long-lived multi-stage melt migration,intrusion and melt-rock interaction history,occurred at different lithospheric depths during progressive uplift.The A-A mantle peridotites thus constitute a unique window on mantle dynamics and lithosphere-asthenosphere interactions in very slow spreading environments.In this work,we review field,microstructural and chemical-isotopic evidence on the major stages of melt percolation and melt-rock interaction recorded by the A-A peridotites and discuss their consequences in creating chemical-isotopic heterogeneities at variable scales and enhancing weakening and deformation of the extending mantle.Focus will be on three most important stages:(i)old(pre-Jurassic)pyroxenite emplacement,and the significant isotopic modification induced in the host mantle by pyroxenite-derived melts,(ii)melt-peridotite interactions during Jurassic mantle exhumation,i.e.the open-system reactive porous flow at spinel facies depths causing bulk depletion(origin of reactive harzburgites and dunites),and the shallower melt impregnation which originated plagioclase-rich peridotites and an overall mantle refertilization.We infer that migrating melts largely originated as shallow,variably depleted,melt fractions,and acquired Si-rich composition by reactive dissolution of mantle pyroxenes during upward migration.Such melt-rock reaction processes share significant similarities with those documented in modern oceanic peridotites from slow-to ultraslow-spreading environments and track the progressive exhumation of large mantle sectors at shallow depths in oceanic settings where a thicker thermal boundary layer exists,as a consequence of slow-spreading rate.     

Sedimentological study of the Chernobyl NPP site to schematise radionuclide migration conditions

The approach, methods and results of a sedimentological study of a near-surface stratum of Late Pleistocene-Holocene deposits in the near-zone (5–10 km radius) of the Chernobyl Nuclear Power Plant are presented. Sedimentological analyses are carried out at three levels of detail: regional-, local- and object-scale. The unsaturated zone and unconfined aquifer at the site are composed of two main genetic types of deposits, aeolian and alluvial, including several dynamic facies. Consideration of lithological properties leads to following ranking of the main genetic sediments facies with respect to radionuclide migration retardation potential: aeolian <alluvial channel <alluvial overbank <alluvial abandoned channel. Based on sedimentological interpretations, the geological environment is schematised into a set of typical geological sections possessing different radionuclide retardation potential.     

Transport of colloidal contaminants in groundwater: radionuclide migration at the Nevada test site

Large-volume groundwater samples were collected at the Nevada Test Site from within a nuclear detonation cavity and from approximately 300 m outside the cavity. The samples were filtered and ultrafiltered, and the filtrates and various particle size fractions were analyzed for chemical composition and radionuclide activity. In samples from both locations, approximately 100% of the transition element (Mn, Co) and lanthanide (Ce, Eu) radionuclides were associated with colloids. Their presence outside the cavity indicates transport in the colloidal form. Distribution coefficients calculated for Ru, Sb, and Cs nuclides from both field sample locations indicate equilibrium partitioning on the 0.05-0.003 μm colloids. Calculation of transport efficiencies relative to colloid mass concentrations and dissolved neutral or anionic nuclides indicates that both the cations and the radiolabelled colloids appear to experience capture by or exchange with immobile aquifer surfaces.     

Column experiment for assessing microbial behavior around radioactive waste repositories,including migration of potentially radionuclide-accumulating bacteria

To assess microbial behavior at anticipated repositories of nitrate-containing radioactive waste such as TRU waste, we set up an anoxic single horizontal column filled with Pleistocene sand with indigenous microorganisms as model samples. The column was supplied with artificial groundwater containing nitrate and acetate for 9 weeks (Run 1) or nitrate-amended groundwater from the same Pleistocene stratum for 6 weeks (Run 2). Bacterial communities, including culturable denitrifiers, were established in the sand bed, resulting in acridine orange direct counts per pore water of 3 × 10⁸ cell mL⁻¹ in Run 1 and 5 × 10⁷ cell mL⁻¹ in Run 2 and nitrate-reducing activity per pore water of roughly 13 mg L⁻¹ d⁻¹ in Run 1 and 1–4 mg L⁻¹ d⁻¹ in Run 2. Eh and hydraulic conductivity declined in Run 1, indicating microbial activity capable of retarding radionuclide transport. However, the ratio of bacterial cell concentration found in the effluent water (free-living bacteria) to the total bacterial concentration in sand (R_mobile) exceeded 2%. This finding is relevant to the increase in radionuclide transport associated with free-living cells. As a tool for quantifying this influence, we introduced an index, K^d,att (distribution coefficient for microbes on sand particles), and calculated this value from the R_mobile value. By sensitivity analysis using a numerical simulation model (MINT), we then demonstrated that higher K^d,att values would suppress the detrimental effects of the free-living bacteria. Quantification of microbial influences can be made more realistic by obtaining K^d,att values in a column experiment and incorporating this index into radionuclide transport models.     

Contaminant migration at two low-level radioactive waste sites in arid western United States – A review

 Contamination of the unsaturated zone and ground water at the Beatty, Nevada and Richland, Washington low-level radioactive waste sites shows that pathways exist for rapid lateral and vertical migration of contaminants through unconsolidated clastic sediments that comprise the 100 m-thick unsaturated zones of those arid disposal sites. Disposal of liquid wastes at the Beatty site until 1975 may have contributed to rapid migration of contaminants, but negligible amounts of liquid wastes reportedly were disposed at the Richland LLRW site and similar problems of contaminant migration exist. Pathways for vertical migration in the unsaturated zone include fractures and, at Richland, clastic dikes; lateral migration pathways likely are facies-controlled. Disturbance of the disposal sites contributed to increased infiltration of the unlined waste trenches after closure; simulations that used Beatty sample data show dramatic increases in recharge with disturbances necessary to develop the site. Because neither an arid climate nor presence of a thick unsaturated zone offer effective barriers to ground-water contamination, reliance on those factors at proposed sites such as Ward Valley, California and elsewhere is unwarranted. Received: 4 February 1998 · Accepted: 4 May 1998     

Kaolinite as an in situ dosimeter for past radionuclide migration at the Earth's surface

The origin of 3 types of point defects (A-, A′- and B-centers) in kaolinite, due to natural irradiation and detected by electron paramagnetic resonance spectroscopy (EPR), has been demonstrated by artificial irradiation. The potential use of tracing the dynamics of the transfer of radionuclides through A-centers (i.e. the most stable centers) was qualitatively tested on different low-temperature alteration systems, some associated with U-concentrations. This paper proposes a quantitative approach to the reconstruction of the past migration of radionuclides by dosimetry of A-centers. With this aim in mind, the efficiency of α- and γ-radiations to produce A-centers was determined by experimental irradiation. Parameters extracted from A-center growth curves, together with their relationship with a parameter describing the degree of order of kaolinite, permitted (i) a definition to be made of the dose range in which a given kaolinite could be used as a dosimeter and (ii) the quantitative derivation of U-concentration from the cumulative dose (paleodose) of kaolinites. This was achieved by a formalism that accounted for the contribution of natural radiosources to the production of A-centers. The formalism was applied to the Nopal I U-deposit (Chihuhua, Mexico), considered as a natural analogue of a high level nuclear waste repository. Irrespective of the scenario considered, in terms of kaolinite age and of degree of isotopic disequilibrium in the system, A-center dosimetry permitted the determination of past occurrences of U which were several orders of magnitude higher than the present-day measured U-concentrations. Furthermore, this approach also provided evidence for several previous episodes of U-migration. EPR spectroscopy is thus a unique tool for the quantitative, indirect assessment of past radionuclide migration in the geosphere and kaolinite is a reliable in-situ dosimeter.