Methods to characterize type, relevance, and interactions of organic matter and microorganisms in fluids along the flow path of a geothermal facility

Abstract

Abstract. Dissolved organic matter (DOM) and microorganisms were characterized along the flow path of a geothermal facility that produces water from a deep (2800 m) carbonate rock reservoir for energy provision. A variety of analytical techniques were employed to distinguish between natural and synthetic organic matter, determine the composition of the microbial community, and evaluate the role of microorganisms in the operation of the geothermal site in Bad Blumau, Austria. Ion chromatography (IC), liquid chromatography with organic carbon detection (LC-OCD), and Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) in negative electrospray ionization (ESI(−)) and positive atmospheric pressure photoionization (APPI(+)) mode were applied to the fluid samples for the purpose of characterizing the composition of DOM and distinguishing natural DOM from a chemical inhibitor used to prevent scaling. The concentrations of dissolved organic carbon (DOC) ranged from 8.5 to 10.4 mg C L−1. The chemical scaling inhibitor contributes approximately 1 mg C L−1 of DOC to the produced fluids. Depending on the applied ionization mode, the FT-ICR-MS results show that between 31 % and 65 % of the macromolecular formulas (150–1000 Da) detected in the fluid samples appear to originate from the inhibitor. However, the DOM is primarily composed of low-molecular-weight acids (LMWA), with acetate being the most prevalent, reaching up to 7.4 mg C L−1. To assess the diversity of the bacterial communities, targeted amplification of the 16S rRNA gene was conducted. The composition of the microbial community exhibited variation along the flow path, with Firmicutes, Proteobacteria, and Thermotogae representing the dominant bacterial phyla. Based on the community composition, metabolic pathways associated with the presence of acetate in the samples were predicted. Microorganisms may produce acetate through diverse fermentation processes, including those involving lysine, pyruvate, and hexitol. Assessing the presence and interaction of organic compounds and microorganisms in geothermal fluids can provide a broader understanding of processes within the geothermal facility. This understanding could be beneficial for efficient operation of a geothermal power plant.