Working Report, 11/2006 – Radioactive isotopes in Danish drinking water

Radioactive isotopes in Danish drinking water

3 Description of Investigation

In Denmark there are approximately 3000 water works of which about 10% were selected for this investigation. The selected water works were primarily the major producers of drinking water across the country but included also water works from all over the country. Further locations, which for geological reasons could give rise to elevated levels of natural radioactivity in drinking water, were included. Most of the Danish drinking water (99%) is obtained from groundwater, the rest from surface water. Furthermore, 10 locations from a long-term routine monitoring programme operated by Risø National Laboratory on radioactivity in groundwater were included in the investigation.

Total alpha and beta radioactivity for water samples collected in 2001 were measured on dry solids from evaporated samples while the subsequent analyses during 2002 and 2003 were based on liquid samples concentrated by evaporation. If the measured concentrations of total alpha or total beta radioactivity exceeded the screening levels of concentrations of total alpha (0.1 Bq/l) and beta (1 Bq/l) radioactivity in drinking water new samples were taken and nuclide-specific analyses carried out. These analyses included determination of uranium and radium isotopes (²³⁴U, ²³⁸U and ²²⁶Ra). In addition, analyses of radon in water were carried out on samples from water works on the island Bornholm where elevated levels could be expected and on Zealand.

By investigation of the dry solids of waters, isotopes like tritium and radon evaporate and are not included in the analyses. Specific analyses of tritium in drinking water were not considered relevant due to the absence of significant anthropogenic sources of tritium in Denmark. Determination of tritium in surface water and precipitation is covered by Risø's routine monitoring programme in Denmark and shows presently typical environmental levels of a few becquerels (Bq) per litre. Earlier in the 1960's, the levels of tritium in precipitation reached 100-300 Bq/l. The groundwaters extracted today may originate from precipitation fallen in the sixties. But monitoring in 1990 - 1995 of tritium in groundwater of all the 960 borings of the Danish Groundwater Monitoring Programme at different depths showed that concentrations of tritium were far below 10 Bq/l except for one boring at 12 Bq/l. The radiation dose per becquerel by ingestion is relatively low for tritium, so even a concentration of 300 Bq/l in drinking water causes only an annual dose of 0.004 mSv.