Source and occurrence of radionuclides in geothermal water. With focus on the Margretheholm geothermal plant

This report contains the results of the study “Kilder til radionuklider i geotermisk vand” (Sources of radionuclides in geothermal water) conducted in 2018 by GEUS for HOFOR A/S.

Related to a major cleanup of the injection well (MAH1A) in 2015 at the geothermal demon-stration plant at Margretheholm, a number of analyses were obtained in the period 2015–2017 to identify causes of the increasing injection pressure occurring in MAH1A since its commissioning in 2005. A detailed description of the injectivity history is given in Appendix 1.

The analyses suggest that radionuclides (designated as NORM; naturally occurring radioac-tive material) accumulate, essentially 210Pb, which precipitates as metallic lead in the geo-thermal surface facilities and in the wells due to a galvanic process. The presence of 210Pb requires a source of this short-lived isotope (22.2 years half-life), as shown in Appendix 2. Clogging of the injection well as well as precipitation of NORM creates technical and safety challenges in ensuring stable operation and thus ensuring sufficient demonstration value from the plant within an operationally reasonable financial framework.

As part of the licensing program, Hovedstadsområdets Geotermiske Samarbejde (HGS) has conducted an analysis of the economy in an expansion with small geothermal plants (EUDP Pilot Hole 1b, WP4). This study is based on the Gassum Formation as the primary geother-mal reservoir, as opposed to existing wells at the Margretheholm plant, which utilize the Bunter Formation.
The primary purpose of the present project is therefore to evaluate the risk of whether geo-thermal production from the Gassum Formation reservoir in the Copenhagen area may in-clude similar challenges with NORM as the Bunter Formation does.
Hence, the project is designed to:

1) Investigate the origin and extent of radionuclides in geothermal water from the Bunter Formation reservoir in Margretheholm by comparing water chemistry with an assessment of possible radionuclide sources in the reservoir rock.
2) Assess the risk that similar radionuclide problems may occur for Gassum Formation by comparing the chemistry of geothermal water from Gassum Formation with the occurrence and amount of possible sources of radionuclides in the sandstone res-ervoir based on relevant locations (Margretheholm, Thisted and Sønderborg).