Influence of redox conditions and organic matter in the partitioning and mobility of naturally-occurring radionuclides (NORs) in geothermal systems

Geothermal energy is a clean and renewable energy source being studied in Europe, with the aim to contribute to a sustained, stable supply of both electricity and heat. One type of geothermal system that exist in Europe is the hot sedimentary aquifer (HSA). The production of geothermal energy of such low enthalpy geothermal systems generally uses a doublet configuration, where the hot geothermal fluid is extracted using one or more deep production wells which then pass through heat exchangers. After heat extraction, the fluid is reinjected back into the reservoir and reheated by the natural geothermal gradient, which makes the system cyclical and sustainable. Previous studies in the Balmatt geothermal site in Mol, Belgium and other similar deep geothermal systems within Europe demonstrated that the fluids are typically hypersaline (TDS ~ 100 to 270 g/L), contain high gas contents (at Balmatt 2 - 2.5 Nm3 gas/m3), and are dominantly Na-Cl in composition (Vandenberghe et al., 2000; Regenspurg et al., 2010; Sanjuan et al., 2010; Pauwels et al 2021). This hypersaline and gas-bearing fluid composition increases the propensity of mineral scale formation (i.e. galena, laurionite, carbonates, and amorphous silica etc.) driven by changes in physicochemical parameters such as temperature and pressure leading to shifts in chemical equilibria. In the Balmatt installation, the heat exchangers and pipelines are the most affected by mineral scaling. Unsurprisingly, the Balmatt brine also contain high levels of naturally occurring radionuclides (NORs) from the 238U, 232Th, and 235U decay chains, reaching activities of 120 Bq/L (226Ra) and 10 Bq/L (228Ra). The mineral scales are also enriched in 210Pb and 210Po (Pauwels et al 2021). The occurrence of mineral scales and their radionuclide activities may influence the sustainability of the geothermal plant operations. Although the effects of pressure and temperature on the partitioning systematics and element transport in hydrothermal and geothermal environments are well studied, the role of redox reactions and conditions involving redox-sensitive components (i.e. Fe, Mn, As, Cu, etc.) is not yet clear. Furthermore, the influence of organic matter of natural origin and artificial organic compounds from scaling and corrosion inhibitors to the fractionation of elements is also not yet known. These two aspects will be investigated in this doctoral study. The goal is to better understand the behavior of radionuclides (focusing on 210Pb and 210Po) in this geothermal environment and the mineral scaling phenomenon. Understanding the geochemical processes will likely lead to practical solutions to address the problem of mineral scaling and their radioactivity. References: Pauwels, J., S. Salah, M. Vasile, B. Laenen, & V. Cappuyns (2021). Characterization of scaling material obtained from the geothermal power plant of the Balmatt Site, Mol. Geothermics, 94, 102090. https://doi.org/10.1016/j.geothermics.2021.102090 Regenspurg, S., Wiersberg, T., Brandt, W., Huenges, E., Saadat, A., Schmidt, K. and Zimmermann, G. (2010): Geochemical properties of saline geothermal fluids from the in-situ geothermal laboratory Gross Schönebeck., Chemie der Erde, 70, 3-12. Sanjuan, B., Millot, R., Dezayes, C., and Brach, M. (2010): Main characteristics of the deep geothermal brine (5 km) at Soultz-sous-Forêts (France) determined using geochemical and tracer test data. C.R. Géocscience, 342, 546-559. Vandenberghe, N., Dusar, M., Boonen, P., Fan, L.S., Voets, R., Bouckaert, J. (2000): The Merksplas-Beerse thermal well (17W265) and the Dinantian reservoir. Geologica Belgica, 3 (3-4), 349-367.