Applied Geochemistry (v.26, #6)

Foreword by Jean-Claude Petit (929-930).

Biogeochemical processes in a clay formation in situ experiment: Part A – Overview, experimental design and water data of an experiment in the Opalinus Clay at the Mont Terri Underground Research Laboratory, Switzerland by P. Wersin; O.X. Leupin; S. Mettler; E.C. Gaucher; U. Mäder; P. De Cannière; A. Vinsot; H.E. Gäbler; T. Kunimaro; K. Kiho; L. Eichinger (931-953).
► The composition was affected by the complex interplay of diffusion, mineral and surface reactions. ► The 13C signals for carbon species showed significant variations which could only be partly explained. ► The main cations remained remarkably constant during the experiment. ► This underlines the strong buffering via cation exchange and carbonate dissolution/precipitation.An in situ test in the Opalinus Clay formation, termed porewater chemistry (PC) experiment, was carried out for a period of 5 years. It was based on the concept of diffusive equilibration whereby a traced water with a composition close to that expected in the formation was continuously circulated and monitored in a packed-off borehole. The main original focus was to obtain reliable data on the pH/pCO2 conditions of the porewater, but because of unexpected microbiologically-induced redox reactions, the objective was extended to elucidate the biogeochemical processes occurring in the borehole and to understand their impact on pH/pCO2 and porewater chemistry in the low permeability clay formation.The behaviour of the conservative tracers 2H and Br could be explained by diffusive dilution in the clay and moreover the results showed that diffusive equilibration between the borehole water and the formation occurred within about 3 year’s time. However, the composition and pH/pCO2 conditions differed considerably from those of the in situ porewater. Thus, pH was lower and pCO2 was higher than indicated by complementary laboratory investigations. The noted differences are explained by microbiologically-induced redox reactions occurring in the borehole and in the interfacial wall area which were caused by an organic source released from the equipment material. The degradation of this source was accompanied by sulfate reduction and – to a lesser extent – by methane generation, which induced a high rate of acetogenic reactions corresponding to very high acetate concentrations for the first 600 days. Concomitantly with the anaerobic degradation of an organic source, carbonate dissolution occurred and these processes resulted in high pCO2 and alkalinities as well as drop in pH. Afterwards, the microbial regime changed and, in parallel to ongoing sulfate reduction, acetate was consumed, leading to a strong decrease in TOC which reached background levels after about 1200 days. In spite of the depletion of this organic perturbation in the circuit water, sulfate reduction and methanogenesis continued to occur at a constant rate leading to near-to-constant concentrations of sulfate and bicarbonate as well as pH/pCO2 conditions until the end of the experiment. The main sink for sulphur was iron sulfide, which precipitated as FeS (am) and FeS2.The chemical and isotopic composition was affected by the complex interplay of diffusion, carbon degradation rates, mineral equilibria and dissolution rates, iron sulfide precipitation rates, and clay exchange reactions. The 13C signals measured for different carbon species showed significant variations which could only be partly explained. The main cations, such as Na, Ca and Mg remained remarkably constant during the experiment, thus indicating the strong buffering of the formation via cation and proton exchange as well as carbonate dissolution/precipitation reactions.

► A 5-year in situ porewater chemistry experiment in Opalinus Clay was overcored and examined. ► A microbial perturbation induced sulfate reduction, pH decrease and alkalinity / PCO2 increase. ► Changes to mineralogy, isotopic composition and bulk properties could not be detected. ► Precipitation of Fe-sulfides and carbonate occurred at the interface of the test interval. ► The chemical perturbation was effectively buffered by the claystone’s large capacity.An in situ Porewater Chemistry (PC) experiment in the Opalinus Clay formation was carried out at the Mont Terri underground rock laboratory (Jura Mountains, Switzerland) for a period of 5 a. A traced water with a composition close to that expected in the formation was continuously circulated and monitored in a packed-off borehole to achieve diffusive equilibration. An unwanted microbial perturbation changed the water composition, characterized by reduction of SO4 combined with increasing sulfide, increasing alkalinity, decreasing pH and increasing P(CO2). In contrast, the main cations (Na, Ca, Mg) remained remarkably constant during the experiment, thus indicating the strong buffering of the formation via cation and proton exchange as well as carbonate dissolution/precipitation reactions.After 5 a, the 4.5 m long vertical test interval was overcored and Opalinus Clay samples were analyzed along ca. 15 cm long radial profiles. The analytical investigations included mineralogy (XRD, SEM-EDX), bulk parameters (water content, density, C, S), cation exchange capacity and occupancy, aqueous leachates for Cl, Br, SO 4 2 - and water and carbonate stable isotopes. Emphasis was put on best sample preparation and conservation techniques. Results show that the distribution of non-reactive tracers (Br and 2H) follows the expected out/in-diffusion profiles compatible with the time-dependent boundary conditions in the test interval of the borehole. Although some experimental features remain unresolved (e.g. high content of leachable SO 4 2 - compared to the test interval), the distribution of reactive tracers (in porewater, on the clay exchanger and in the solid phase) demonstrate the very extensive buffer capacity of the Opalinus Clay formation towards chemical disturbances, such as those induced by microbial SO4 reduction and oxidation of an organic C source.

Biogeochemical processes in a clay formation in situ experiment: Part C – Organic contamination and leaching data by P. De Cannière; J. Schwarzbauer; P. Höhener; G. Lorenz; S. Salah; O.X. Leupin; P. Wersin (967-979).
► The cause of a microbial perturbation of an in situ geochemistry experiment in clay is identified. ► Bacterial activity plays a key role in restoring highly reducing conditions in porewater. ► Bacterial activity was fuelled by contamination of porewater by dissolved organic carbon (DOC). ► Glycerol released from pH–Eh electrode polymer gel was the main source of organic carbon. ► Constant water recirculation in close loop aggravated the contamination of porewater by DOC.Data interpretation of the Porewater Chemistry (PC) experiment at the Mont Terri Rock Laboratory has led to unexpected observations of anaerobic microbial processes which caused important geochemical perturbations of the Opalinus Clay water in the borehole. The increases of acetate to 146 mg C/L, of DIC to 109 mg C/L and of CH4 to 0.5 mg C/L were unexpected and could not be explained without the presence of a C source in the system. The organic C fuelling the observed microbial activity was until then unknown. Leaching tests were performed on several polymers used for the fabrication of the PC equipment to identify the source of organic matter (OM). Polyethylene (PE) appears to be very inert and does not release detectable concentrations of dissolved organic C (DOC) (<1 ppb) into the water. Polyurethane (PU) leaches out a dozen different organic compounds accounting for only 13 μg DOC/g PU. Under the conditions of the leaching tests, 1 g of polyamide (PA, Nylon) also releases ∼512 μg of the plasticizer N-Butyl-Benzene-Sulfonamide (NBBS). Soaking tests with polyethylene samples immersed in acetone under conditions similar to those used to remove grease spots on the porous PE filter prior to installation showed that acetone could have been trapped in the PE filter, corresponding to an initial concentration of 1.5 g acetone/L of water. However, the accumulated amount of organic C taken into account from all these components was insufficient to satisfactorily explain the observed microbially mediated reducing perturbation. Finally, large amounts of dissolved organic C were found to be released in the system by the jelly polymer filling the reference compartment of the pH and Eh electrodes permanently installed over 5 years in flow-through cells on the water circulation loop of the PC experiment. Glycerol was further identified by chromatographic analysis as the main organic compound released by the electrodes. From the analysis results, as well as from the geochemical calculations, the most likely primary organic C source fuelling the microbial perturbation was glycerol released from the polymeric gel filling the reference electrodes (1.6 g glycerol/electrode). Other sources, such as acetone, may also have contributed to microbial processes, but only to a minor extent.

Biogeochemical processes in a clay formation in situ experiment: Part D – Microbial analyses – Synthesis of results by S. Stroes-Gascoyne; C. Sergeant; A. Schippers; C.J. Hamon; S. Nèble; M.-H. Vesvres; V. Barsotti; S. Poulain; C. Le Marrec (980-989).
► Geochemical parameters were measured in Opalinus Clay porewater. ► Unexpected microbial activity occurred in porewater. ► Microorganisms were sustained by organic contamination. ► Microbial activity affected geochemical conditions. ► Fe(III)- reduction, sulphate-reduction and methanogenesis occurred.The purpose of the Porewater Chemistry (PC) experiment at the Mont Terri (MT) Underground Rock Laboratory (URL) was to measure geochemical parameters, such as pH, Eh and pCO2, in the porewater of the Opalinus Clay formation. Although the PC experiment was designed and implemented carefully from a geochemical perspective, conditions were not sterile and some microbial and nutrient contamination likely occurred. Microbial activity in the added synthetic porewater in the borehole was apparent shortly after initiation of the experiment and affected the geochemical parameters observed in the porewater. This paper summarizes the results from microbial analyses of post-termination PC water and overcore clay samples, conducted to attempt to elucidate the role of microbial activity in the evolution of the geochemical conditions in the PC experiment. Microbial analyses of the PC borehole water, and of clay overcore samples from around the borehole, were carried out at three laboratories and included both molecular biology and culturing methods.Results indicated the presence of heterotrophic aerobic and anaerobic organisms that resulted likely from the initial, non-sterile conditions, sustained by suspected contamination with organic matter (glycerol, acetone). The results also indicated the presence of NO3-reducers, Fe-reducers, SO4-reducers and methanogens (i.e., Bacteria as well as Archaea), suggesting a reducing environment with Fe(III)- and SO4 reduction, and methanogenesis occurring in the PC water and adjacent clay. A black precipitate containing pyrite (identified by XRD and SEM) and a strong H2S smell in the porewater confirmed the occurrence of SO4 reduction. Microorganisms identified in the porewater included Pseudomonas stutzeri, Bacillus licheniformis, Desulfosporosinus spp. and Hyphomonas spp. Species identified in enrichment cultures from the overcore samples included Pseudomonas stutzeri, three species of Trichococcus spp., Caldanaerocella colombiensis, Geosporobacter subterrenus and Desulfosporosinus lacus. Overall the results indicated a thriving microbial community in the PC water and adjacent clay in contrast to “undisturbed” Opalinus Clay for which limited evidence for a small viable microbial community has been given in a previous study.

► Equilibrium models of water-rock reactions in clay rocks are reviewed. ► Analyses of pore waters of the Opalinus Clay from boreholes in the Mont Terri URL, Switzerland, are tabulated. ► Results of modelling with various mineral controls are compared with the analyses. ► Best agreement results with calcite, dolomite and siderite or daphnite saturation, Na–K–Ca–Mg exchange and/or kaolinite, illite, quartz and celestite saturation. ► This approach allows calculation of the chemistry of pore water in clays too impermeable to yield water samples.The chemistry of pore water (particularly pH and ionic strength) is an important property of clay rocks being considered as host rocks for long-term storage of radioactive waste. Pore waters in clay-rich rocks generally cannot be sampled directly. Instead, their chemistry must be found using laboratory-measured properties of core samples and geochemical modelling. Many such measurements have been made on samples from the Opalinus Clay from the Mont Terri Underground Research Laboratory (URL). Several boreholes in that URL yielded water samples against which pore water models have been calibrated. Following a first synthesis report published in 2003, this paper presents the evolution of the modelling approaches developed within Mont Terri URL scientific programs through the last decade (1997–2009). Models are compared to the composition of waters sampled during dedicated borehole experiments. Reanalysis of the models, parameters and database enabled the principal shortcomings of the previous modelling efforts to be overcome. The inability to model the K concentrations correctly with the measured cation exchange properties was found to be due to the use of an inappropriate selectivity coefficient for Na–K exchange; the inability to reproduce the measured carbonate chemistry and pH of the pore waters using mineral–water reactions alone was corrected by considering clay mineral equilibria. Re-examination of the measured Ca/Mg activity ratios and consideration of the mineralogical composition of the Opalinus Clay suggested that Ca/Mg cation exchange rather than dolomite saturation may control the ratio of these ions in solution. This re-examination also suggests that the Ca/Mg ratio decreases with increasing pore-water salinity. Several possible reasons for this are proposed. Moreover, it is demonstrated that feldspar equilibria must not be included in Opalinus Clay modelling because feldspars are present only in very small quantities in the formation and because Na/K ratios measured in pore water samples are inconsistent with feldspar saturation. The principal need to improve future modelling is additional or better data on rock properties, in particular: (i) a more detailed identification of phases in the Opalinus Clay that include redox-sensitive elements together with evaluation of their thermodynamic properties; (ii) an improved understanding of the distribution of celestite throughout the Opalinus Clay for Sr/SO4 concentrations control; (iii) improvements in analytic and thermodynamic data for Ca–Mg rock cation exchange and mineral chemical properties and (iv) the measurement of composition and stability constants of clay minerals actually present in the formation.

Biogeochemical processes in a clay formation in situ experiment: Part F – Reactive transport modelling by Christophe Tournassat; Peter Alt-Epping; Eric C. Gaucher; Thomas Gimmi; Olivier X. Leupin; Paul Wersin (1009-1022).
► Reactive transport modelling was used to simulate simultaneously solute transport, thermodynamic reactions, ion exchange and biodegradation during an in-situ experiment in a clay-rock formation. ► Opalinus clay formation has a high buffering capacity in terms of chemical perturbations caused by bacterial activity. ► Buffering capacity is mainly attributed to the carbonate system and to the reactivity of clay surfaces (cation exchange, pH buffering).Reactive transport modelling was used to simulate solute transport, thermodynamic reactions, ion exchange and biodegradation in the Porewater Chemistry (PC) experiment at the Mont Terri Rock Laboratory. Simulations show that the most important chemical processes controlling the fluid composition within the borehole and the surrounding formation during the experiment are ion exchange, biodegradation and dissolution/precipitation reactions involving pyrite and carbonate minerals. In contrast, thermodynamic mineral dissolution/precipitation reactions involving alumo-silicate minerals have little impact on the fluid composition on the time-scale of the experiment. With the accurate description of the initial chemical condition in the formation in combination with kinetic formulations describing the different stages of bacterial activities, it has been possible to reproduce the evolution of important system parameters, such as the pH, redox potential, total organic C, dissolved inorganic C and SO4 concentration. Leaching of glycerol from the pH-electrode may be the primary source of organic material that initiated bacterial growth, which caused the chemical perturbation in the borehole. Results from these simulations are consistent with data from the over-coring and demonstrate that the Opalinus Clay has a high buffering capacity in terms of chemical perturbations caused by bacterial activity. This buffering capacity can be attributed to the carbonate system as well as to the reactivity of clay surfaces.

Biogeochemical processes in a clay formation in situ experiment: Part G – Key interpretations and conclusions. Implications for repository safety by P. Wersin; S. Stroes-Gascoyne; F.J. Pearson; C. Tournassat; O.X. Leupin; B. Schwyn (1023-1034).
► From the results of the PC experiment it can be inferred that degradation of organic compounds may induce. ► Changes in pH and Eh which may affect the mobility of radionuclides eventually released from the waste. ► Such changes will be limited in space and time because of large buffering capacity and low permeability of clay. ► Nevertheless, amount of organic material in high level waste repositories should be kept small. ► This will ensure achievement of background concentrations within short time period after repository closure.The in situ porewater chemistry (PC) experiment carried out in the Opalinus Clay formation at the Mont Terri Rock Laboratory, Switzerland for a period of 5 a allowed the identification and quantification of the biogeochemical processes resulting from and affected by an anaerobic microbial disturbance. The unintentional release of degradable organic compounds (mainly glycerol) induced microbially-mediated SO4 reduction in the borehole with concomitant significant geochemical changes in the circulating water and the adjacent porewater. These changes included a decrease in SO 4 2 - concentration and pH and an increase in pCO2 and alkalinity relative to the non-affected formation water. However, the cation composition of the water and the mineralogy of the clay close to the borehole wall showed very little change. This is explained by (1) the strong chemical buffering processes in the clay and (2) by the diffusion-limited flux of solutes.With the aid of a reactive transport model with a minimum set of kinetic parameters for the hypothesised degradation reactions, the evolution of solutes in the borehole could be modelled adequately. The model was also applied to the prediction of restoration times upon depletion of the C source and results indicated restoration times to undisturbed conditions of about 15 a, but also highlighted the rather large uncertainties inherent in the geochemical model. Nevertheless, the simulations provided additional evidence of the high pH buffer capacity of the Opalinus Clay.The results from the microbiological investigations do not allow unambiguous identification of the origin of the microbial population in the borehole. Possible sources were the drilling procedure, the artificial porewater, and perhaps some revival of indigenous dormant strains. Regardless of the origin of the microbes, the results from the PC experiment underlined the importance of anaerobic microbial activity in the “disturbed” Opalinus Clay, facilitated by the introduction of space, water and organic material, in rapidly establishing very reducing conditions.The PC experiment also yielded valuable insight with regard to the safety of a high-level radioactive waste repository emplaced in Opalinus Clay. Anaerobic microbial perturbations in the clay host rock may occur from the construction and excavation procedures and emplaced organic by-products. The resulting effects on porewater chemistry, i.e., especially on pH and Eh, may affect the mobility of radionuclides eventually released from the waste. However, the overall results of the PC experiment suggest that such effects are temporary and spatially limited because of the large buffering capacity and diffusive properties of the clay formation. Nevertheless, the results also indicate that the amounts of organic materials in a high-level waste repository should be kept small in order to achieve background conditions within a short time period after repository closure. A further conclusion from the PC experiment is that commonly used equipment materials may not display commonly assumed inert behaviour. This particularly holds for the gel-type “robust” reference electrodes, which may release substantial amounts of glycerol.