Elsevier

Applied Geochemistry

Volume 26, Issue 6, June 2011, Pages 967-979
Applied Geochemistry

Biogeochemical processes in a clay formation in situ experiment: Part C – Organic contamination and leaching data

https://doi.org/10.1016/j.apgeochem.2011.03.006Get rights and content

Abstract

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.

Highlights

► 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.

Introduction

In order to gain deeper insight into geochemical processes regulating pH and Eh, the Porewater Chemistry (PC) experiment was started at the Mont Terri Rock Laboratory in 2002. This in situ experiment was based on the diffusive equilibration method. Synthetic porewater was recirculated in a packed-off borehole and allowed to equilibrate with the formation water via diffusion (Wersin et al., 2011a). The diffusive exchange process was monitored by conservative tracers (2H, Br) (Waber, 2002, Eichinger, 2003, Eichinger, 2004). Throughout the experiment disturbance by atmospheric O2 was minimised. In particular, the borehole was drilled with N2 and filled with Ar prior to equipment installation (Drouiller and Vinsot, 2002, De Cannière and Fierz, 2002, Wersin et al., 2011a). All parts contacting the circulating water were non-metallic to avoid any influence on redox potential measurements (Eh). The non-preservable parameters, such as pH, Eh, temperature, and hydraulic pressure, were continuously monitored in situ. Water samples were taken periodically and analysed for their chemical and isotopic composition. In addition, microbial analyses were also performed (Battaglia and Gaucher, 2003, Daumas, 2004, Ishii, 2004, Mauclaire, 2005, Stroes-Gascoyne et al., 2007, Stroes-Gascoyne et al., 2011) along with isotope analysis of the CO2 gas and the carbonate minerals in the samples (Waber, 2002, Eichinger, 2003, Eichinger, 2004, Wersin et al., 2011a, Tournassat et al., 2011).

The chemical composition of the water sampled from the PC experiment showed significant changes with time, especially for the major anions, pH, Eh, pCO2, dissolved inorganic and organic C (DIC, DOC), and the stable isotope composition (δ13C). In particular, the concentrations of SO42- and HCO3- were strongly affected. After a period of 2.3 a, HCO3- concentrations showed a 7-fold increase and SO42- concentrations decreased by a factor of 3 (Mettler, 2004, Wersin et al., 2011a). Concomitantly, pH values decreased from 7.8 to 6.7 and sulphide concentrations showed a strong rise. Eh values showed an initial rapid decrease, which was followed by a slow steady decrease down to −220 mV (SHE). The concentrations of DOC rose from 2–5 mg C/L to values >150 mg C/L of which most was acetic acid.

13C-DIC and 13C-DOC values showed some variations with time, but both parameters appear to have reached stable values at about −15‰ (Waber, 2002, Eichinger, 2003, Eichinger, 2004, Wersin et al., 2011a). 13C–CH4 showed strongly negative values of about −43‰ after 2.3 a. Microbial analysis indicated the presence of SO4-reducing and of some methanogenic bacteria in the circulating water (Battaglia and Gaucher, 2003, Ishii, 2004, Mauclaire, 2005, Stroes-Gascoyne et al., 2011).

Surprisingly also, the in situ data indicated pCO2 values as high as 10−1.3 bar in the latter stages of the PC experiment. These values were not consistent with CO2 measurements from core samples (pCO2 of about 10−2.7 bar) obtained by Gaucher (2002) and porewater data from a column infiltration experiment (Mäder, 2002, Mäder, 2005, Mäder and Gimmi, 2007), nor with other Mont Terri experiments suggesting lower in situ pCO2 values (10−3.5 bar) (Pearson et al., 2003, Pearson et al., 2011, Gaucher, 2004a, Gaucher, 2004b, Vinsot et al., 2005, De Cannière et al., 2008).

The chemical and isotopic data (Eichinger, 2003, Eichinger, 2004b; Wersin et al., 2011a) as well as geochemical modelling (Arcos, 2003, Gaucher, 2003, Gaucher, 2004a, Gaucher, 2004b; Pearson, 2003, Pearson, 2005, Pearson et al., 2011; Tournassat et al., 2011) confirmed the significance of microbially-mediated SO4-reduction occurring in the borehole. The reducing agent appears to be principally organic C. Although CH4 oxidation is commonly associated with SO4-reduction, the role of CH4 within this redox process is not conclusive, but both methanogenic fermentation and CH4 oxidation reactions appear to occur. The measured δ13C data in the circulating water are interpreted to be a result of concomitant organic C degradation and carbonate dissolution (Höhener, 2004; Tournassat et al., 2011, Pearson et al., 2011). Data indicate that glycerol (propane-1,2,3-triol) released from the polymer gel of pH–Eh electrodes emplaced in the water circulation loop served as a degrading C source fuelling the microbial processes. Acetone and organic matter released by the PE filter screen and other parts of the equipment made exclusively of polymeric materials could have contributed to the microbial degradation only to a minor extent.

The objective of the present work was to identify, or to exclude, possible sources of organic C released by different plastic materials, pH electrodes, or solvents, used in the equipment, or component cleaning of the PC experiment. This characterisation study was launched after the interpretation of test data obtained from the PC experiment and a literature survey on the biodegradation of synthetic materials done by Höhener, 2004, Höhener, 2005. Of particular interest were first the plasticizers that might have leached from soft polyurethane (PU) and polyamide (PA, Nylon).

Another initially suspected source of dissolved organic C in the water of the PC experiment was acetone. Indeed, the polyethylene (PE) filter for the PC experiment was washed with acetone before installation in order to remove traces of grease that were left from the manufacturing process. So, it might have been possible that the polyethylene was still soaked with acetone. Acetone is known to be biodegraded by fermentative (Platen et al., 1994) and SO4-reducing bacteria (Janssen and Schink, 1995a, Janssen and Schink, 1995b), or other complex degradation pathways (Platen and Schink, 1987).

Finally, the continuous increase of DOC observed by Leupin (unpublished data, pers. comm.) in the water circuit of another in situ Diffusion Retention (DR) experiment at Mont Terri gave a decisive clue to solve the long puzzling enigma related to the unidentified C source in the system. DOC accumulation in water of the DR experiment showed that the organic polymer gel used in the reference compartment of on-line pH and Eh electrodes was also likely the main contributor to the organic C inventory in the PC experiment. Hence, an essential and ultimate objective was to determine the nature and the concentration of organic C released by the electrodes and to establish a C mass balance with a consistent geochemical hypothesis for the C turnover in the system (see also Tournassat et al., 2011).

Section snippets

Nature and properties of equipment materials

The experimental setup and the installation procedure of the Mont Terri Porewater Chemistry (PC) experiment are described in detail by De Cannière and Fierz (2002) and by Wersin et al. (2011a). The borehole equipment consisted of a single packer system with a 4.5 m long filter section between the bottom of the hole and the packer, and an upper section sealed with epoxy resin (Sikadur 52) above the packer (Fig. 1) to isolate the test interval from the gallery and to prevent gas exchanges (O2, CO2

Leaching of selected polymer materials

The analyses reveal that polyethylene (PE) from the porous screen does not release any detectable quantity of contaminants into the bidistilled water. As observed on Fig. 2d, polyurethane (PU) from the packer sleeve releases many constituents (about a dozen have been identified and quantified) in water after leaching, but it accounts for only about 12.8 μg/g of dissolved organic matter. Finally, the polymer most prone to leaching is polyamide (PA). N-Butyl-Benzene-Sulfonamide (NBBS, CID: 19241),

Main lessons learned

Synthetic polymers have been extensively used to fabricate the equipment of the Porewater Chemistry (PC) experiment. The main reason of this choice was to avoid any contact of water with metals, such as C-steel or stainless steel, which hamper the delicate measurements of redox potential (Eh) in porewater with low levels of dissolved electro-active species due to the presence of metals such as Fe, Ni and Cr (Fernandez, 2003).

In addition to its good mechanical and corrosion resistance, the main

Conclusions

The results of the leaching tests performed on three polymers (polyethylene, PE; polyurethane, PU; and polyamide, PA) selected from the plastic materials used for the fabrication of the equipment of the Porewater Chemistry (PC) experiment have shown that for a pure polymer, such as polyethylene (addition polymerisation) leaching of DOC into water is insignificant, while for condensation polymers containing plasticizers a high release of organic compounds is exhibited. Polyethylene used for the

Acknowledgements

This work is undertaken in close co-operation with the operator of the rock laboratory (BWG/Swisstopo) and the project management team (Geotechnical Institute until 2006), namely Paul Bossart and Christophe Nussbaum, and with the financial support of the Mont Terri Consortium. The fruitful discussions on organic contamination in the PC experiment with Suzanne Mettler (SolExperts), Simcha Stroes-Gascoyne (AECL), Urs Mäder (Uni Bern), Joe Pearson (GWG), Christophe Tournassat (BRGM) and Lorenz

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