Data interpretation of the Porewater Chemistry (PC) experiment at the Mont Terri Rock Laboratory has led to unexpected observations of anaerobic microbial processes which enacted an important geochemical perturbation 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 methane to 0.5 mg C/L was unexpected and could not be explained without the presence of a carbon source in the system. The organic carbon fuelling the observed microbial activity was so far unknown. Leaching tests were performed on several polymers used for the fabrication of the PC equipment to identify the source of organic matter (OM). According to the analysis results, as well as indicated by the geochemical calculations, the most likely primary organic carbon 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.