The accumulation of naturally occurring radionuclides in solid residues was investigated from
groundwater treatment facilities (GTFs) in Finland. Natural radionuclides U-238, Ra-226, Pb-210
and Ra-228 were found in various precipitates, sludges and filters at concentrations exceeding the
general clearance level of 1 kBq kg−1 used for solid materials in the European directive
2013/59/Euratom. The accumulation of natural radionuclides in different solid residues was
observed even when the activity concentrations in the untreated groundwater were relatively low,
and when there was no measurable change in the concentrations between raw and treated
groundwater within analytical uncertainties. Based on mass and activity balance considerations
this is thought to be due to the large volumes of treated water per year. The exposure of workers to
natural radiation from solid residues in the regular use of a groundwater facility was found not to
be likely to exceed 0.3 mSv a−1 if the activity concentrations are <10 kBq kg−1 for U-238, Ra-226,
Pb-210 and Ra-228. The worker exposure from solid residues is therefore likely to remain below
the reference level of 1 mSv a−1, and indoor radon is more of a concern for the radiation protection
of workers at GTFs. However, the natural radionuclide content in the different solid residues from
groundwater treatment needs to be characterised properly to be able to ensure safety in the final
use of the residues with respect to the potential exposure of the public