Joonas Pärn, a hydrogeologist at the Geological Survey of Estonia, said that European groundwater reservoirs are studied from two angles where qualitative research looks into its chemical composition, while quantitative analysis reveals the volume of underground water reservoirs. "Considering the climate zone in which Estonia is located, it is fair to say that, qualitatively speaking, Estonia is doing very well, while the chemical status of our groundwater is assessed as being relatively good," said Pärn.

Siiri Salupere, a research fellow of radiation protection at the Institute of Physics at the University of Tartu, said that Estonia has a special aquifer (i.e., an underground reservoir of water) for drinking water that is high in naturally dissolving radium isotopes.

These naturally occurring radioactive elements are the reason why the groundwater could be said to be naturally radioactive. The old Cambrian-Vendian aquifer is the main source of the groundwater used in the northern parts of Estonia, especially in the areas along the coastline.

How this naturally occurring radioactivity in drinking water affects people is difficult to evaluate, said Salupere. Humans have been living for a very long time in the presence of ionizing radiation; there exists no individual who has not been exposed to it to a greater or lesser extent. "However, there is now a widespread belief in the scientific community that even in low doses radiation increases the likelihood of developing cancer," he said.

Our ancestors were not exposed to radioactive drinking water to the same extent as we are now: they did not know how to drill wells as deep as we do today. Cambrian-Venetian groundwater wells in northern Estonia are for the most part at least 80 meters deep, but can go down even to 300 meters.

To monitor the situation, the World Health Organization and the European Commission have decided to determine the levels of radioactivity that can be present in drinking water. When the radiation dose exceeds 0.1 millisievert per year the radiation should be then reduced to the agreed minimum. By way of comparison, two breast cancer screenings cause the same amount of radiation as the yearly maximum.

It is good to keep in mind that the high costs of removing radiation from water might outweigh the costs that might be associated with other ways of reducing minimal cancer risks.

When the dose is less than 0.1 millisieverts per year, the risk is rather low and there is thought to be no need to remove radioactive elements from the water. When using the groundwater from the Cambrian-Vendi reservoir, the radiation doses are often between 0.3 and 0.5 millisieverts per year.

Salupere said that over a year, the average person receives a dose of three millisieverts of Ionizing radiation. For people in Estonia, however, it is radon — the naturally occurring radioactive gas in the air we breath indoors — that causes the highest cancer risks. In Estonia, the annual dose from that source is about 2.1 millisieverts.

Radioactivity can increase

However, the naturally occurring radioactivity in groundwater may increase over time. Salupere said that a five-year study which examined wells of the Viimsi peninsula showed that the more water from the old Cambrian-Vendi reservoir was taken, the higher the radioactivity levels became. At the same time, the chloride content increased as well, i.e. the water became saltier.

The geologists were able to determine that both chloride and radium particles came from fractures in the crystalline bedrock. When the water on top of the crystalline bedrock is used continuously, the water that accumulates in bedrock's fractures containing radioactive elements, seeps into the aquifer. "It is possible that in future we will have to pay more attention to such issues as removing radionuclides and chlorides from our drinking water," Salupere said.

Banning the drilling of deep wells is not the optimal solution, because there are not many other ways to get water, especially on the north coast. "It's still better there to use the water we have," Salupere said.

A total of 31 wells have been studied to assess the quality of groundwater in Estonia. Based on data collected in 2020, eight of them were in poor condition, said Pärna. The reasons for this varied. For example, in Ida-Virumaa the groundwater wells were in poor condition, because of the quarries and mines located in the area need to have aquifers to be emptied to some extent. Also the mining activities themselves have an impact on the groundwater quality.

The groundwater in Pandivere upland and its surrounding is affected by diffused agricultural pollution, where the nutrient and pesticide residues from the agricultural activities are degrading the chemical composition of the groundwater. "Several aspects have come together in the area. On the one hand, this land is known for its fertile soil and active farming. On the other, the geological structure of the area accounts for rapid water absorption into the ground," said Pärn.

The effects of the climate change

The most interesting phenomenon, said Pärn, is that there are also groundwater wells with poor water quality, which does not seem to have any direct connection to human activities. Changes in water quality in these wells may be linked to climate change.

Unless there will be a dramatic increase in water collection from deep groundwater wells, it is unlikely that the levels of naturally occurring radioactivity will increase to any significant extent in the foreseeable future. However, if the collection of water intensifies and the levels of radioactivity rise, then it is likely that some reservoirs will have to be closed and only the groundwater that is located closer to the surface will be in use.

"As far as I know, this has already been done in some places in Ida-Viru County. It's not just a problem of naturally occurring radioactivity, it is equally related to the levels of chloride and salinity of the groundwater that are on a rise as well".

"That is why it has been decided to use the groundwater which is closer to the surface, where those problems do not occur," he said.

All in all, things are rather good in Europe, said Siiri Salupere, and added that there is no reason to panic, but rather we should focus the attention of water supply plant operators at how they are dealing with the old Cambrian-age supply of groundwater, and whether there are new technologies that could be used to reduce the levels of radionuclides in water.

In addition to Estonia, the groundwater with higher than normal levels of radioactive elements has been found also in France, Portugal and Sweden.

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