Release and fractionation of naturally occurring radionuclides in geothermal systems and the potential role of organic matter.

Geothermal installations in Europe generate Naturally Occurring Radioactive Material (NORM) as mineral and organic precipitates in wellbore casings, pipelines, heat exchangers, and filters. Naturally occurring radionuclides (NOR) of and progeny are released from reservoir rocks together with natural organic matter, while synthetic organic compounds from scale and corrosion inhibitors add to the total organic content of geothermal brines. NOR release from reservoir rocks and mechanisms of fractionation (i.e. physical and chemical separation of radionuclides to phases of matter resulting to partitioning) leading to NORM formation in geothermal environments are poorly understood. The influence of organic matter on NOR fractionation and mobility is similarly unclear. Documented cases of NORM generation in geothermal installations are site-specific. Despite inadequate characterization, , , and are the predominant NOR found in these NORM. To identify release and fractionation mechanisms of NOR relevant to geothermal environments, other well-studied systems such as oilfields, geologic repositories, and groundwaters were reviewed. However, their applicability to basin-type geothermal systems is limited by differences in temperature, salinity, and brine chemistry. The most relevant NOR release mechanisms are alpha recoil, dissolution and leaching, and less significantly by Rn emanation. Once released into the brine, NOR mobility is controlled by complexation and, to a lesser extent, complexation with low-molecular-weight organics in the dissolved fraction, and by size-dependent physical separation of particle-bound NOR (minerals and organics) in the particulate fraction. In addition to the dynamic conditions of geothermal environments, these fractionation mechanisms explain the persistent radioactive disequilibrium in geothermal fluids, exacerbated by continuous brine extraction and degassing. Co-precipitation and deposition on mineral surfaces also play major roles in NOR fractionation and NORM generation. Overall, mechanical processes, solubility, retention mechanisms (e.g. sorption, surface precipitation, co-precipitation), control NOR mobility in geothermal environments.