Characterization of natural and synthetic organic matter and naturally-occurring radionuclides in the hypersaline geothermal fluids of Balmatt site, Mol, Belgium: preliminary results

Low enthalpy geothermal energy utilization is currently of great interest in Europe as a clean and sustainable alternative energy source, due to its potential to meet the demand for both electricity and space-heating. The increased interest presently accelerates research and development particularly for deep-seated, hot sedimentary aquifers all over Europe. Among the least understood aspects is the presence of naturally-occurring radionuclides (NORs)  such as ²²⁶Ra, ²²⁸Ra, ²²²Rn, ²¹⁰Po, and ²¹⁰Pb and the underlying geochemical processes and constraints affecting their mobility. This study aims to characterize natural organic matter (NOM) and synthetic scaling and corrosion inhibitors and their by-products as a first step to determine their role in the partitioning and mobility of these radionuclides. A two-step dead-end filtration at 0.45 and 0.22 μm pore sizes was conducted for the hypersaline brine from the production and injection wells (TDS= 100-270 g/L) of the Balmatt geothermal site in an anoxic environment, to initially determine the total organic carbon (TOC) and radiochemistry per filtrate fraction. The TOC content of filter residues was measured by combustion catalytic oxidation method, while the filtrates were excluded due to their known low organic carbon content yet hypersaline nature, requiring large dilution factors. UV-VIS spectrophotometry of all brine filtrates using various wavelengths (i.e. 204, 220, 254, 280, 365, 400, 436, 665 nm) shows very low absorbance ranges (e.g. A₂₅₄ = 0.03-0.07,  A₂₈₀ = 0.01-0.05), corroborating with  low organic carbon contents. On the other hand, the absorbance values of the scaling and corrosion inhibitors are higher (e.g. A₂₅₄ = 6-10 , A₂₈₀= 5-8)  correlating to higher TOCs of 64 mg C/L and 180 mg C/L, respectively. This suggests that the synthetic inhibitors may be an additional source of organic compounds possibly affecting the partitioning and mobility of NORs. Initial results of the radiochemical analyses by liquid scintillation and alpha spectrometry also show that total activity of ²¹⁰Pb and ²¹⁰Po are relatively higher at the production side (²¹⁰Pb =0.20 Bq/L, ²¹⁰Po=37 mBq/L) than the injection side (²¹⁰Pb <0.09 Bq/L, ²¹⁰Po=0.9 mBq/L), possibly suggesting their fractionation from the fluid to the known mineral scales such as galena. Other NORs measured using gamma spectroscopy indicate high activities of ²²²Rn (12.2 Bq/L), ²²⁶Ra (124-136 Bq/L) and ²²⁸Ra (9.3 Bq/L). In the future, leaching experiments will be conducted using reservoir rock cuttings from the geothermal wells to possibly distinguish between NOMs and synthetic inhibitor by-products. Size exclusion chromatography per filter fraction using a tangential flow setup will also be conducted to further characterize the organic matter. Reverse osmosis and solid phase separation are being considered in sample preparation to resolve the issue of low TOC but very high salt content of the fluids of the Balmatt geothermal system.