In situ diffusion experiments: Effect of water sampling on tracer concentrations and parameters

Versión aceptada de https://doi.org/10.1016/j.pce.2010.04.009[Abstract:] In situ diffusion experiments are performed at underground research laboratories (URL) in clay formations to overcome the limitations of laboratory diffusion experiments and investigate scale effects. Tracers are monitored in the circulation system by aliquoting the solution at selected times. The extracted samples may be replaced with the same volume of synthetic unspiked water as it is done in the Bure URL (France). Sampling with replacement induces the tracer dilution. In the sampling method used in the Mont Terri URL (Switzerland), on the other hand, sampling volumes are not replaced. In this case, there is no tracer dilution, but the volume in circulation decreases progressively. Water sampling is commonly disregarded in the numerical interpretation of such experiments. However, water sampling may induce changes in the tracer concentrations and errors in the estimated diffusion and sorption parameters. Such errors have been analyzed here with a numerical model which accounts for sample extraction, sample replacement and the changes in the volume of the circulation system. These errors have been analyzed for HTO, 36Cl−, 22Na+ and 134Cs+ in the DIR2003 and EST 208 experiments at the Bure URL and for HTO, HDO, Br−, 22Na+, 85Sr2+, 133Ba2+ and 137Cs+ in the DR experiments at the Mont Terri URL. The effect of water sampling on the relative tracer concentrations depends on the sampling method and frequency, the volume of the sample and the volume of the circulation system. Water sampling causes minor differences in relative tracer concentrations in the DR experiments and its effect can be disregarded. In the DIR2003 experiment, however, the differences induced by aliquoting cannot not be neglected. 36Cl− is the tracer most affected by sampling and 134Cs+ is the least influenced. The identifiability analysis of the EST208 experiment reveals that failing to account for the effect of water sampling may lead to a significant overestimation of diffusion and sorption parameters, especially for 36Cl−. The results of our analysis indicate that the effects of the water sampling should not be neglected without a careful in-depth analysis.This work was supported by ANDRA and the Spanish Ministry of Science and Technology through the Research Project CGL2006-09080 and a FPI Research Scholarship awarded by the Spanish Ministry to the first author. We are grateful to the two anonymous reviewers for their comments and recommendations which improved the paper

Les rhyolites à topaze de la région de San Luis Potosi (Mexique) : caractéristiques des laves et conditions de croissance des topazes

Au Mexique, les rhyolites à topaze se rencontrent principalement dans la région de San Luis Potosi (Etats de San Luis Potosi et de Guanajuato). Ces rhyolites se présentent sous la forme de dômes mis en place lors d’une tectonique en extension d’âge tertiaire. Trois dômes du champv olcanique de San Luis Potosi ont été retenus pour cette étude en fonction de la couleur, de la forme et de la taille des topazes qu’ils contiennent afin de déterminer les caractéristiques des laves et les conditions de croissance des topazes. Ces rhyolites, riches en silice et alcalins, sont méta-alumineuses à légèrement hyper-alumineuses. Elles sont enrichies en fluor et en éléments incompatibles comme Rb, Cs, Ta, U et Th et appauvries en Ba, Sr, Ca, Mg, Ti et Ni. De ce fait elles apparaissent très comparables aux rhyolites à topaze de l’ouest des Etats-Unis. Le Cerro El Gato contient des topazes soit de couleur ambre, soit incolores qui ont cristallisé dans des géodes et des fractures. En prenant en compte le milieu de croissance, la morphologie, la composition chimique et les caractéristiques RPE de ces différents cristaux, il est possible de proposer un modèle de cristallisation et d’expliquer les différences de couleur observées. Ainsi, les topazes incolores du Cerro El Gato ont cristallisé à une température supérieure à 500oC (absence de centres colorés) et à partir de fluides enrichis en éléments lessivés de la lave, alors que les topazes de couleur ambre ont cristallisé à une température inférieure à 500oC (présence de centres colorés) et à partir d’un fluide plus riche en éléments volatils (As). Les topazes du Cerro el Lobo ont des caractéristiques intermédiaires entre celles des topazes incolores et des topazes ambres du Cerro El Gato

In-situ diffusion of HTO, 22Na+, Cs+ and I- in Opalinus Clay at the Mont Terri underground rock laboratory

The diffusion properties of the Opalinus Clay were studied in the underground research laboratory at Mont Terri (Canton Jura, Switzerland) and the results were compared with diffusion data measured in the laboratory on small-scale samples. The diffusion of HTO, Na-22(+), Cs+ and I- were investigated for a period of 10 months. The diffusion equipment used in the field experiment was designed in such a way that a solution of tracers was circulated through a sintered metal screen placed at the end of a borehole drilled in the formation. The concentration decrease caused by the diffusion of tracers into the rock could be followed with time and allowed first estimations of the effective diffusion coefficient. After 10 months, the diffusion zone was over-cored and the tracer profiles measured. From these profiles, effective diffusion coefficients and rock capacity factors Could be extracted by applying a two-dimensional transport model including diffusion and sorption. The simulations were done with the reactive transport code CRUNCH. In addition, results obtained from through-diffusion experiments oil small-sized samples with HTO, Cl-36(-) and Na-22(+) are presented and compared with the in situ data. In all cases. excellent agreement between the two data sets exists. Results for Cs+ indicated five times higher diffusion rates relative to HTO. Corresponding laboratory diffusion measurements are still lacking. However. our Cs+ data are in qualitative agreement wish through-diffusion data for Callovo-Oxfordian argillite rock samples. which also indicate significantly higher effective diffusivities for Cs+ relative to HTO

Géochimie isotopique des gaz rares des eaux souterraines et des sédiments de l'Est du Bassin parisien

Dewonck S., Marty B., Tolstikhin I., Aranyossy Jean-François. Géochimie isotopique des gaz rares des eaux souterraines et des sédiments de l'Est du Bassin parisien. In: Transferts dans les systèmes sédimentaires : de l'échelle du pore à celle du bassin. Réunion spécialisée SGF-TRABAS/CNRS, Paris 27-28 septembre 1999. Résumés. Strasbourg : Institut de Géologie – Université Louis-Pasteur, 1999. pp. 63-66. (Sciences Géologiques. Mémoire, 99

Géochimie isotopique des gaz rares des eaux souterraines et des sédiments de l'Est du Bassin parisien

Dewonck S., Marty B., Tolstikhin I., Aranyossy Jean-François. Géochimie isotopique des gaz rares des eaux souterraines et des sédiments de l'Est du Bassin parisien. In: Transferts dans les systèmes sédimentaires : de l'échelle du pore à celle du bassin. Réunion spécialisée SGF-TRABAS/CNRS, Paris 27-28 septembre 1999. Résumés. Strasbourg : Institut de Géologie – Université Louis-Pasteur, 1999. pp. 63-66. (Sciences Géologiques. Mémoire, 99

Normalized sensitivities and parameter identifiability of in situ diffusion experiments on Callovo-Oxfordian clay at Bure site

DIR (Diffusion of Inert and Reactive tracers) is an experimental program performed by ANDRA at Bure underground research laboratory in Meuse/Haute Marne (France) to characterize diffusion and retention of radionuclides in Callovo-Oxfordian (C-Ox) argillite. In situ diffusion experiments were performed in vertical boreholes to determine diffusion and retention parameters of selected radionuclides. C-Ox clay exhibits a mild diffusion anisotropy due to stratification. Interpretation of in situ diffusion experiments is complicated by several non-ideal effects caused by the presence of a sintered filter, a gap between the filter and borehole wall and an excavation disturbed zone (EdZ). The relevance of such non-ideal effects and their impact on estimated clay parameters have been evaluated with numerical sensitivity analyses and synthetic experiments having similar parameters and geometric characteristics as real DIR experiments. Normalized dimensionless sensitivities of tracer concentrations at the test interval have been computed numerically. Tracer concentrations are found to be sensitive to all key parameters. Sensitivities are tracer dependent and vary with time. These sensitivities are useful to identify which are the parameters that can be estimated with less uncertainty and find the times at which tracer concentrations begin to be sensitive to each parameter. Synthetic experiments generated with prescribed known parameters have been interpreted automatically with INVERSE-CORE{sup 2D} and used to evaluate the relevance of non-ideal effects and ascertain parameter identifiability in the presence of random measurement errors. Identifiability analysis of synthetic experiments reveals that data noise makes difficult the estimation of clay parameters. Parameters of clay and EdZ cannot be estimated simultaneously from noisy data. Models without an EdZ fail to reproduce synthetic data. Proper interpretation of in situ diffusion experiments requires accounting for filter, gap and EdZ. Estimates of the effective diffusion coefficient and the porosity of clay are highly correlated, indicating that these parameters cannot be estimated simultaneously. Accurate estimation of D{sub e} and porosities of clay and EdZ is only possible when the standard deviation of random noise is less than 0.01. Small errors in the volume of the circulation system do not affect clay parameter estimates. Normalized sensitivities as well as the identifiability analysis of synthetic experiments have provided additional insight on inverse estimation of in situ diffusion experiments and will be of great benefit for the interpretation of real DIR in situ diffusion experiments

In situ diffusion in Callovo-Oxfordian mudstone

Communications Orale

Design and dimensionning of in situ diffusion experiments at Bure

Communication par affich

Diffusion of HTO, Br-, I-, Cs+, Sr-85(2+) and Co-60(2+) in a clay formation: Results and modelling from an in situ experiment in Opalinus Clay

The migration of radioactive and chemical contaminants in clay materials and argillaceous host rocks is characterised by diffusion and retention processes. Valuable information on such processes can be gained by combining diffusion studies at laboratory scale with field migration tests. In this work, the outcome of a multi-tracer in situ migration test performed in the Opalinus Clay formation in the Mont Terri underground rock laboratory (Switzerland) is presented. Thus, 1.16 x 10(5) Bq/L of HTO, 3.96 x 10(3) Bq/L of Sr-85, 6.29 x 10(2) Bq/L of Co-60, 2.01 x 10(-3) mol/L Cs, 9.10 x 10(-4) mol/L I and 1.04 x 10(-3) mol/L Br were injected into the borehole. The decrease of the radioisotope concentrations in the borehole was monitored using in situ gamma-spectrometry. The other tracers were analyzed with state-of-the-art laboratory procedures after sampling of small water aliquots from the reservoir. The diffusion experiment was carried out over a period of one year after which the interval section was overcored and analyzed. Based on the experimental data from the tracer evolution in the borehole and the tracer profiles in the rock, the diffusion of tracers was modelled with the numerical code CRUNCH. The results obtained for HTO (H-3), I- and Br- confirm previous lab and in situ diffusion data. Anionic fluxes into the formation were smaller compared to HTO because of anion exclusion effects. The migration of the cations Sr-85(2+), Cs+ and Co-60(2+) was found to be governed by both diffusion and sorption processes. For Sr-85(2+), the slightly higher diffusivity relative to HTO and the low sorption value are consistent with laboratory diffusion measurements on small-scale samples. In the case of Cs+, the numerically deduced high diffusivity and the Freundlich-type sorption behaviour is also supported by ongoing laboratory data. For Co, no laboratory diffusion data were yet available for comparison; however, the modelled data suggests that Co-60(2+) sorption was weaker than would be expected from available batch sorption data. Overall, the results demonstrate the feasibility of the experimental setup for obtaining high-quality diffusion data for conservative and sorbing tracers. (C) 2007 Elsevier Ltd. All rights reserved

In situ diffusion experiments in Callovo-Oxfordian mudstone

Communications OralesInternational audienc