Applied Geochemistry (v.20, #2)
IAGC membership form (III).
Controls on the genesis of some high-fluoride groundwaters in India by Gunnar Jacks; Prosun Bhattacharya; Vikas Chaudhary; K.P. Singh (221-228).
India has an increasing incidence of fluorosis, dental and skeletal, with some 62 million people at risk. High fluoride groundwaters are present especially in the hard rock areas south of the Ganges valley and in the arid north-western part of the country. The phenomenon is related to groundwater with residual alkalinity ( Ca 2 + < HCO 3 - ) . Fluoride concentrations are governed by adsorption equilibria and by fluorite solubility. Evapotranspiration leads to a precipitation of calcite, a lowering of Ca activity and increase in Na/Ca ratios, and this allows an increase in F− levels. In southern India, Mg seems to be controlled by dolomite, while sepiolite and palygorskite are Mg sinks in Rajasthan but may then release F− under alkaline conditions. The latter two minerals are probably also important sources and sinks for F− in the hydroxy-positions. The increase in the extent of sodic soils as a result of irrigation is a contributing factor to the increasing incidence of fluorosis. Remedial measures including addition of gypsum and rainwater harvesting are needed even in areas where the sodicity does not cause structural problems in the soil.
Arsenic fractionation in a fine soil fraction and influence of various anions on its mobility in the subsurface environment by Kok-Hui Goh; Teik-Thye Lim (229-239).
The degree to which anions can affect As mobility in subsurface environments depends on As speciation and fractionation in the soil. In this study, the fine soil fraction of an acidic soil was deliberately contaminated with arsenate (As(V)) and adequately aged for more than 220 days. A selective sequential extraction (SSE) scheme was developed to fractionate As in the soil into 6 fractions, while the influence of various anions on As mobility was assessed by evaluating its extractability by various salts such as Na3PO4, Na2CO3, Na2SO4 and NaCl. The variables of the extraction experiment were type of salt, salt concentration and reaction time. Fractionations of As in soil samples after extraction with the salts were also examined and the results compared against the initial As fractionation. The SSE results showed that the majority of the As was strongly adsorbed via surface complexation in the fine soil. A considerable amount of As was also associated with the exchangeable fraction, amorphous Fe oxyhydroxides, and the residual fraction. Abilities of the anions in mobilizing As bound in the fine soil followed the order of PO 4 3 - ≫ CO 3 2 - > SO 4 2 - ≈ Cl - . Arsenic mobilization by PO 4 3 - was believed to be due primarily to a ligand exchange mechanism, through which the strongly bound As was replaced by the PO 4 3 - anion. Anion exchange might also contribute to the mobilization of As in this study. Increase in the concentration of the anions tested did not further mobilize As from the soil, with the exception of PO 4 3 - . The kinetics of As release in the PO 4 3 - solution could be best described by the Elovich model, which represents an exponential decrease in As desorption rate as more As was extracted from the soil.
Identification and regional quantification of hydrochemical processes at the contact zone between anoxic groundwater and surface water in poldered floodplains (Oderbruch polder, Germany) by C. Merz; P. Schuhmacher; A. Winkler; A. Pekdeger (241-254).
The hydrochemistry in the largest polder of the Oder River, named Oderbruch, is affected by long-term infiltration of water from the Oder into the aquifer below an alluvial loamy top layer of the polder. These exceptional hydraulic conditions are a result of dyke constructions which were built more than 250 a ago. The objective of this investigation is a better understanding and a characterisation of the contact zone between the anaerobic groundwater and the surface water of a vast drainage system. Induced by changing water levels, different hydraulic conditions occur, which strongly influence the hydrochemistry of the shallow aquifer and therefore the natural sink function of the polder area.Field investigations with a hydrochemical and hydraulic characterisation of selected drainage ditch locations show considerable chemical interactions between groundwater and surface water. Depending on the drainage ditch type, which is defined by the hydraulic situation, the redox processes create a chemical gradient combined with a distinct enrichment of Fe and Mn. The source of the high amounts of Fe and Mn in the groundwater are reduced Fe- and Mn-hydroxides from the aquifer sediments.Under exfiltrating conditions interrupted by dry phases, more than 50 g kg−1 Fe and 0.25 g kg−1 Mn have accumulated in the drainage ditch floor sediments since the construction of the drainage ditches 35 a ago. The results show a very effective fixation of trace metals in the drainage ditch sediments under these conditions. Under permanent exfiltration conditions, the enrichment of Fe and Mn is relatively low. The maximum Fe content was 4 g kg−1 sediment and the Mn content reached only 0.4 g kg−1. This is less than 10% of the mobile Fe2+ and less than 1% of the Mn2+ which migrates from the aquifer into the surface water.
Effects of thermal cracking of hydrocarbons on the homogenization temperature of fluid inclusions from the Niigata oil and gas fields, Japan by Susumu Okubo (255-260).
The changes of the homogenization temperature (Th), when hydrocarbons in inclusions were cracked thermally, were experimentally evaluated. In the experiments, the hydrocarbon-inclusions in quartz were heated in an autoclave, and the Th between before and after heating were compared. In the experiments, the Th decreased after heating. The amount of decrease became greater as the heating temperature or the duration time or both increased. The experiments also revealed that the samples with higher Th showed a greater decrease after heating. Because the experiments have been performed in a closed system, mass transfer between inclusions and the enclosing medium was impossible. No volumetric changes in the hydrocarbon inclusions were observed after heating. Therefore, the temperature depression after heating should be caused by the pyrolysis of the hydrocarbons in the inclusions.In the Shin-Kumoide SK-1D well of the Niigata oil and gas field in Japan, the Th of hydrocarbon inclusions decreases from 73 to 30 °C between 4040 and 4810 m in depth successively. This temperature range is lower than the present formation temperature by from 60 to 140 °C, and shows a trend opposite to the normal geothermal gradient. Therefore, it should reflect the degree of the thermal cracking after trapping rather than the trapping temperature.
Influence of upwelling Zechstein sulphate on the concentration and isotope signature of sedimentary sulphides in a fluvioglacial sand aquifer by G. Mallén; R. Trettin; M. Gehre; R. Flynn; T. Grischek; W. Nestler (261-274).
Very low concentrations of total S, mainly sedimentary sulphides, were quantitatively extracted from Quaternary sands of the Elbe Basin, using HNO3, Br2 and HCl, to distinguish 3 aquifer zones:The latter originated from dissolved Zechstein SO4, which was reduced during upwelling through the organic-rich Tertiary aquiclude. H2S and HS− reacted and precipitated with Fe and other metal ions shortly after migration into the Corg-poor Quaternary aquifer. The sulphides yield valuable information concerning the ascent of confined saline solutions from isolated Zechstein evaporites inside the “Mühlberger Graben”, which is covered by Cenozoic sediments and whose extension and boundaries are therefore not well defined. Only a few locations, close to faults and geological windows, show deep-water admixture sufficiently strong to cause visible changes in hydrochemistry and isotopic ratios of SO4 and DIC directly above the base of the Quaternary. Sulphides showing different origins may possibly be used in other areas to provide information concerning underlying geology and hydrodynamics.
Coupled chemistry and transport modelling of sulphidic waste rock dumps at the Aitik mine site, Sweden by Claire M. Linklater; Daniel J. Sinclair; Paul L. Brown (275-293).
The recently developed geochemical modelling code, SULFIDOX, has been applied to simulate weathering of a waste rock dump at the Aitik mine site, Sweden. SULFIDOX models the key chemical and physical processes in the dump temporally and spatially (in two dimensions). The following processes are represented: gas and heat transport; water infiltration; aqueous speciation; mineral dissolution/oxidation and precipitation.Field observations at the site suggest that sulphide oxidation rates within the dump are variable. Although the major part of the dump is oxidising slowly, there are pockets of more highly oxidising material, particularly toward the dump edges. Using SULFIDOX, several models of the dump were investigated: (i) a dump wholly comprised of slowly oxidising material (representing a case where water flow paths are such that no rapidly oxidising regions are accessed); (ii) a dump wholly comprised of the more rapidly oxidising material (representing the opposite (and probably unlikely) extreme, where water flows only through rapidly oxidising regions in the dump); and (iii) a dump comprising a mixture of both slowly and more rapidly oxidising material, that more closely represents the mix of material in the dump.All the models studied gave O2 depth profiles consistent with those observed in probe holes at the site, and confirmed that only a minimal amount of heat production would be expected in the dump due to the role of exothermic sulphide oxidation reactions. The models suggested that a medium-term steady-state, with respect to effluent chemistry, would be achieved after 3–4 years. Based on sulphide consumption rates during this steady-state period, the time periods required to consume all the sulphide in the dump range from a few hundred to many thousands of years. Using the mixed model, and based on a mixture containing 86% slowly and 14% rapidly oxidising material, the calculated effluent chemistry was in good agreement with the observed effluent chemistry. Improvements with respect to the K concentrations were possible by including precipitation of a K-bearing secondary mineral such a K-jarosite in the model. Results from the more rapidly oxidising model suggested that gypsum precipitation might be expected in those regions of the dump containing this material.In summary, the SULFIDOX modelling code has been used successfully to reproduce observed data for the Aitik waste-rock dump. Using SULFIDOX, valuable insight was gained in relation to the temporal and spatial evolution of the dump.
Hydrogeochemical evolution of confined groundwater in northeastern Osaka Basin, Japan: estimation of confined groundwater flux based on a cation exchange mass balance method by Masaru Yamanaka; Takanori Nakano; Norio Tase (295-316).
A confined aquifer system has developed in argillaceous marine and freshwater sediments of Pliocene–Holocene age in the northeastern Osaka Basin (NEOB) in central Japan. The shallow groundwater (<100 m) in the system is recharged in a northern hilly to mountainous area with dominantly Ca-HCO3 type water, which changes as it flows toward the SW to Mg-HCO3 type and then to Na-HCO3 type water. Comparison of the chemical and Sr isotopic compositions of the groundwater with those of the bulk and exchangeable components of the underground sediments indicates that elements leached from the sediments contribute negligibly to the NEOB aquifer system. Moreover, model calculations show that contributions of paleo-seawater in the deep horizon and of river water at the surface are not major factors of chemical change of the groundwater. Instead, the zonal pattern of the HCO3-dominant groundwater is caused by the loss of Ca2+ from the water as it is exchanged for Mg2+ in clays, followed by loss of Mg + Ca as they are exchanged for Na + K in clays between the Ca-HCO3 type recharge water and the exchangeable cations in the clay layers, which were initially enriched in Na+. Part of this process was reproduced in a chromatographic experiment in which Na type water with high 87Sr/86Sr was obtained from Mg type water with low 87Sr/86Sr by passing it through marine clay packed in a column. The flux of recharge water into the confined aquifer system according to this chromatographic model is estimated to be 0.99 mm/day, which is compatible with the average recharge flux to unconfined groundwater in Japan (1 mm/day).
Fluid geochemistry of the Sardinian Rift-Campidano Graben (Sardinia, Italy): fault segmentation, seismic quiescence of geochemically “active” faults, and new constraints for selection of CO2 storage sites by M. Angelone; C. Gasparini; M. Guerra; S. Lombardi; L. Pizzino; F. Quattrocchi; E. Sacchi; G.M. Zuppi (317-340).
Sardinia is typically seismically quiescent, displaying an almost complete lack of historical earthquakes and instrumentally recorded seismicity. This evidence may be in agreement with the presence of a ductile layer in the northern sector of the island, as suggested by the He isotopic signature in fluids rising to the surface through quiescent fault systems. The fault systems have been found to be “segmented” and therefore isolated in fluid circulation. The study of fluid behaviour along fault systems becomes strategically important when applied to solve some geological risk assessments such as Rn-indoor, or to define geological structures like potential CO2 storage sites. Both of these have been recently requested by the exploitation in Italy of the Euratom Directive and the evolution of the KyotoProtocol policy.Four water-dominated hydrothermal areas of Sardinia, located along regional fault systems, were considered: Campidano Graben, Tirso Valley, Logudoro and Casteldoria. A fluid geochemical survey was carried out taking into account physical–chemical and environmental parameters, major elements within gaseous and liquid phases, a few minor and trace elements, selected isotope ratios (2H, 18O, 13C, 3He/4He), 222Rn concentration, and some dissolved gases.Two different fluids have been recognised as regards both water chemistry and dissolved gases: (i) CO2-rich gases, poor in He and Rn, with a relatively high 3He/4He ratio (up to R/R a = 2.32), associated with Na–HCO3–(Cl) thermal and cold groundwater; (ii) gases rich in He and N2, poor in CO2 and Rn, with a low 3He/4He ratio, associated with alkaline thermal and cold waters. The distribution of these two groups of fluids characterises the Sardinian tectonic systems. In fact, gas fluxes are not homogeneous, being mainly related to the different fault segments and to the areas where Quaternary basalts crop out. The underground geochemical evolution of the Sardinian fluids, as a function of the geological and tectonic systems, provides some suggestions for solving one of the most important problems: CO2 geological sequestration. In order to reduce the CO2 excess produced by human activity, the best geological disposal sites are reservoirs with low hydraulic conductivity, sealed to fluid movement, or aquifers characterised by maximum pH buffering capacity of their mineralogical matrix. The knowledge of the role of faults, as permeability barriers or as deep fluid uprising pathways, is prerequisite.
Dealing with outliers and censored values in multi-element geochemical data – a visualization approach using XmdvTool by Katrin Grünfeld (341-352).
Dealing with geochemical data also means coping with their underlying limitations that are related to sampling, analytical techniques, and other characteristics of the data. This paper discusses the issue of data cleaning, using a regional geochemical dataset of 6 heavy metals in glacial till. Interactive data manipulation techniques provided in the freeware visualization system XmdvTool were used for exploring both metal concentrations reported as under the detection limit, and high or extreme values (outliers) in the dataset. The proposed integrated visual evaluation (IVE) approach for selective removal of outliers outperformed simple removal of the highest concentrations of metals, showing that existing spatial multi-element fingerprints in data could be recognized and preserved by IVE. The uniqueness of visualization is in simultaneous display of both multivariate and spatial information. Being simple and interactive, integrated visual evaluation can be recommended as a valuable complementary tool in cleaning and analysing multi-element geochemical data.
Lead isotopes in tap water: implications for Pb sources within a municipal water supply system by Zhongqi Cheng; Kenneth A. Foland (353-365).
Residential tap waters were investigated to examine the feasibility of using isotopic ratios to identify dominant sources of water Pb in the Columbus (Ohio, USA) municipal supply system. Overall, both the concentrations, which are generally low (0.1–28 μg/L), and isotopic compositions of tap water Pb show wide variations. This contrasts with the situation for a limited number of available service lines, which exhibit only a limited Pb-isotope variation but contain Pb of two very different types with one significantly more radiogenic than the other. Most tap water samples in contact with Pb service lines have Pb-isotope ratios that are different from the pipe Pb. Furthermore, the Pb isotope compositions of sequentially drawn samples in the same residence generally are similar, but those from separate residences are different, implying dominant Pb sources from domestic plumbing. A separate pilot study at two residences without Pb service lines shows isotopic similarity between water and solders in each house, further suggesting that the major Pb sources are domestic in these cases and dominated by Pb from solder joints. Although complicated by the broad range of overall Pb-isotope variations observed and limited by sample availability, the results suggest that Pb isotopes can be used effectively to constrain the sources of Pb in tap waters, especially for individual houses where multiple source candidates can be identified.
Saturated and aromatic diterpenoids and triterpenoids in Eocene coals and mudstones from China by Jincai Tuo; R. Paul Philp (367-381).
Several series of saturated, diaromatic, triaromatic C-ring cleaved and triaromatic diterpenoids and triterpenoids have been detected in 4 immature coal and mudstone samples. A number of these compounds appear to represent intermediates in a series of postulated pathways for progressive aromatization of biogenic diterpenoids and triterpenoids. Diagenetic pathways for the formation of tricyclic aromatic hydrocarbons from abietane and pimarane type diterpenoid precursors and for the formation of diaromatic, triaromatic C-cleaved and triaromatic hydrocarbons from β-amyrin and other triterpenoid precursors are proposed. Saturated and aromatized abietanes, pimaranes and phyllocladanes, which are the most abundant compounds in all 4 samples, indicate a predominant higher plant input which can be related to the Coniferales group but not to individual plant families. β-Amyrin and other triterpenoid-derived triaromatic and triaromatic C-ring cleaved hydrocarbons with triterpenoid structures are thought to be characteristic for angiosperms. The relative concentrations of the triaromatic and triaromatic C-ring cleaved hydrocarbons are higher in samples 9602 (mudstone) and 9603 (coal) than samples 9601 (coal) and 9604 (mudstone) indicating samples 9602 (mudstone) and 9603 (coal) contain relatively more angiosperm derived organic matter than samples 9601 (coal) and 9604 (mudstone). The distribution patterns and the relative concentrations of saturated and aromatic diterpenoids and triterpenoids thus are valuable markers for the determination of the relative contents of biological sources of organic material in geological samples.
Spatial variability of arsenic in some iron-rich deposits generated by acid mine drainage by Alexandra Courtin-Nomade; Cécile Grosbois; Hubert Bril; Christophe Roussel (383-396).
Potential contamination of rivers by trace elements can be controlled, among others, by the precipitation of oxyhydroxides. The streambed of the studied area, located in “La Châtaigneraie” district (Lot River Basin, France), is characterised by iron-rich ochreous deposits, acidic pH (2.7–4.8) and SO4–Mg waters. Beyond the acid mine drainage, the presence of As both in the dissolved fraction and in the deposits is also a problem. Upstream, at the gallery outlet, As concentrations are high (Asmax = 2.6 μmol/l and up to 5 wt% locally, respectively, in the dissolved and in the solid fractions). Downstream, As concentrations decrease below 0.1 μmol/l in the dissolved fraction and to 1327 mg/kg in the solid fraction. This natural attenuation is related to the As retention within ochreous precipitates (amorphous to poorly crystalline Fe oxyhydroxides, schwertmannite and goethite), which have great affinities for this metalloid. Upstream, schwertmannite is dominant while downstream, goethite becomes the main mineral. The transformation of schwertmannite into goethite is observed in the upstream deposits as schwertmannite is unstable relative to goethite. Furthermore, thermodynamic calculations indicate that the downstream goethite is not able to precipitate in situ according to the water chemistry. Goethite mainly results from the transformation of schwertmannite and its solid transport downstream.Moreover, as highlighted by leaching experiments carried out on the ochreous precipitates, this transformation does not seem to affect the As-retention in solids as no release of As was observed in the solution. Arsenic may either be strongly trapped by co-precipitation in the present minerals or it may be quickly released and re-adsorbed on the precipitate surface.
Effect of the addition of gypsum- and lime-rich industrial by-products on Cd, Cu and Pb availability and leachability in metal-spiked acid soils by F. Garrido; V. Illera; M.T. García-González (397-408).
In situ stabilization of metals in contaminated soils by addition of industrial by-products is an attractive remediation technique. In this work, mixtures of metal-spiked (Cd, Cu and Pb) samples from the Ap horizons of two acid soils and solid samples of 4 industrial by-products (red gypsum, phosphogypsum, sugar foam and dolomitic residue) were incubated in order to assess the efficiency of the by-products for the in situ remediation of contaminated soils. Single (DTPA extraction and TCLP test) and sequential chemical extraction procedures were used to examine the availability and potential mobility of the metals retained as a result of the treatments. Both the sugar foam and dolomitic residue reduced the availability and mobility of Cd, Cu and to a lesser extent, Pb. On the other hand, red gypsum and phosphogypsum proved effective in immobilizing Pb and, to a lesser extent, Cu. However, both gypsum-like by-products were not so effective in reducing Cd availability and mobility.
A simplified method to evaluate the swelling capacity evolution of a bentonite barrier related to geochemical transformations by G. Montes-H; B. Fritz; A. Clement; N. Michau (409-422).
Bentonites have been proposed as buffer material for barriers in geological disposal facilities for radioactive waste. This material is expected to fill up by swelling the void between the canisters containing the waste and the surrounding ground. However, the bentonite barriers may be submitted to changes of humidity, temperature variation, fluid interaction, mass transport, etc. This could modify the physico-chemical performance of the barrier, mainly on the interface with the steel container and with the geological barrier. The engineered barrier development necessitates thus the study of the physico-chemical stability of its mineral component as a function of time under the conditions of the repository in the long-term. The aim of this paper is to apply a simplified method (volume balance in a saturated medium) to evaluate the swelling capacity evolution of a bentonite barrier because of their geochemical transformations by using a thermo-kinetic hydrochemical code (KIRMAT: Kinetic Reactions and Mass Transport “1D”).The system modelled here consists of 1 m thick zone of water-saturated engineered barrier. This non-equilibrated system is placed in contact with a geological fluid on one side, which is then allowed to diffuse into the barrier, while the other side is kept in contact with iron-charged solution (0.001 mol/kg H2O). The initial reducing conditions ( P O 2 ≅ 0 ; Eh = −200 mV) and a constant reaction temperature (100 °C) were considered.In the current study the decay of swelling capacity was considered directly proportional on the volume of transformed montmorillonite (cation exchange + geochemical transformation), taking into account that it may be partially compensated by the volume of neo-formed swelling-clays.The results showed that the swelling capacity of the engineered barrier is not drastically affected after about 3000 years of reaction and transport because the volume of neo-formed swelling clays is almost directly proportional on the volume of transformed montmorillonite. In fact, a graphical method predicted that the decay of swelling capacity of the engineered barrier lies between 11% and 14% when the montmorillonite is completely transformed (cation exchange + minimal geochemical transformation) in the system.
Factors controlling tungsten concentrations in ground water, Carson Desert, Nevada by Ralph L. Seiler; Kenneth G. Stollenwerk; John R. Garbarino (423-441).
An investigation of a childhood leukemia cluster by US Centers for Disease Control and Prevention revealed that residents of the Carson Desert, Nevada, are exposed to high levels of W and this prompted an investigation of W in aquifers used as drinking water sources. Tungsten concentrations in 100 ground water samples from all aquifers used as drinking water sources in the area ranged from 0.27 to 742 μg/l. Ground water in which W concentrations exceed 50 μg/l principally occurs SE of Fallon in a geothermal area. The principal sources of W in ground water are natural and include erosion of W-bearing mineral deposits in the Carson River watershed upstream of Fallon, and, possibly, upwelling geothermal waters. Ground water in the Fallon area is strongly reducing and reductive dissolution of Fe and Mn oxyhydroxides may be releasing W; however, direct evidence that the metal oxides contain W is not available.Although W and Cl concentrations in the Carson River, a lake, and water from many wells, appear to be controlled by evaporative concentration, evaporation alone cannot explain the elevated W concentrations found in water from some of the wells. Concentrations of W exceeding 50 μg/l are exclusively associated with Na – HCO 3 and Na–Cl water types and pH > 8.0; in these waters, geochemical modeling indicates that W exhibits <10% adsorption. Tungsten concentrations are strongly and positively correlated with As, B, F, and P, indicating either common sources or common processes controlling their concentrations. Geochemical modeling indicates W concentrations are consistent with pH-controlled adsorption of W.The geochemical model PHREEQC was used to calculate IAP values, which were compared with published Ksp values for primary W minerals. FeWO4, MnWO4, Na2WO4, and MgWO4 were undersaturated and CaWO4 and SrWO4 were approaching saturation. These conclusions are tentative because of uncertainty in the thermodynamic data.The similar behavior of As and W observed in this study suggests ground water in areas where elevated As concentrations are present also may contain elevated W concentrations, particularly if there is a mineral or geothermal source of W and reducing conditions develop in the aquifer.
Influence of water-extractable organic matter from Opalinus Clay on the sorption and speciation of Ni(II), Eu(III) and Th(IV) by Martin A. Glaus; Bart Baeyens; Matthias Lauber; Thomas Rabung; Luc R. Van Loon (443-451).
The influence of water-extractable organic matter from 6 Opalinus Clay (OPA) samples from Mont Terri and Benken (Switzerland) on the sorption of Ni(II), Eu(III) and Th(IV) has been measured using an ion exchange technique. OPA is considered to be one of the potential host rocks for the deep geological disposal of high-level and long-lived intermediate-level radioactive waste in Switzerland. Within the range of estimated uncertainties, no significant differences in sorption were observed in most cases as compared with suitable synthetic waters devoid of organic C. Only in certain individual cases were slight reductions in sorption (less than a factor of 5) for Eu(III) and Ni(II) found. The results of accompanying laser fluorescence spectroscopy experiments did not show any influence of the extracts on Cm(III) speciation. This would suggest that the reduction of sorption occasionally observed in the ion exchange experiments is probably not caused by the formation of complexes between the radionuclides and the organic matter in the extracts, but is rather due to an underestimation of systematic uncertainties. From these findings, and from UV–VIS spectroscopic characterisation of the organic matter in the extracts, it can be concluded that only a negligible fraction of the organic matter present may be in the form of humic or fulvic acids. It is consequently justified to put aside overly conservative assumptions with respect to the complexing behaviour of the organic matter used towards the metal ions investigated and their chemical analogues. In view of the site-specific character of the present study, these conclusions may not be arbitrarily applied to other geological formations considered as possible host rocks for the disposal of radioactive waste.
by Nick Pearce (453-454).