Abstract. First comprehensive studies of the radionuclides' concentration and its composition in Estonian ground water began in 1990s. It was quickly realized that the activity concentration of 226 Ra and 228 Ra in Cambrian-Vendian (Cm-V) aquifer are the highest contributors to the total indicative dose (TID). About 22% of the population in Estonia uses Cm-V ground water for drinking water supply. In recent study [1], it was concluded, that about 91% of Cm-V aquifer consumers (20% of the Estonian population) obtain higher TID doses (TID exceeding 0.1 mSv/y) than set by European Commission and national regulations. Over the years, a good representation level by the number of measured samples for 226 Ra and 228 Ra from Cm-V aquifer has been obtained. However, for other widely used aquifers -Ordovician-Cambrian, SilurianOrdovician and Devonian -only poor data is available. In several southern counties, only few percent of the water supplies have been surveyed. Data for other natural radionuclides ( 234,238 U, 210 Po and 210 Pb) is also poor. Scarce data has been partly due to the lack of analysis techniques available in Estonian nuclear analysis laboratories. This has been the motivation in the development of gamma spectrometric and liquid scintillation (LSC) counting techniques for ground and drinking water applications.
In northern Estonia, higher levels of Ra-226 and Ra-228 concentrations give rise to problems with water treatment plants (WTPs) using Cambrian-Vendian (Cm-V) aquifer. During water treatment processes, radionuclides accumulate in the filter material to a level where the material could be classified as a radioactive material. In order to understand the scope of the problem, a nation-wide survey was carried out among the Cm-V fed treatment plants. Filter material and water samples were collected from 18 Estonian WTPs to analyse the activity concentration of Ra-226 and Ra-228 in filter materials and also the radium treatment efficiency was determined. Sixteen WTPs exceeded the exemption levels of 1 kBq * kg−1 set by the EC directive 2013/59/Euratom. The quantity of filter material exceeding the exemption levels was approximately 300 tons, average concentrations values for Ra-226, Ra-228 and Th-228 were, accordingly, 7.6 kBq * kg−1, 8.0 kBq * kg−1 and 5.6 kBq * kg−1. That includes WTPs not designed for radium removal. The results suggest a need to investigate further NORM generation in WTPs, as uncontrollable disposal of NORM may occur.
The Viimsi peninsula is located north-east of Tallinn, capital of Estonia. The Cambrian-Vendian (Cm-V) aquifer system is a sole source of drinking water in the area. Historically, the groundwater exploitation has led to freshening of groundwater in the peninsula, but in recent years an increase in chloride concentrations and enrichment in δ18O values has been detected, but in recent years hydrochemical parameters indicate an increasing influence of a saline water source. The exact origin of this saline water has remained unclear. The aim of the current study is to elucidate whether the increase in Cl− concentrations is related to seawater intrusion or to the infiltration of saline water from the underlying crystalline basement. To identify the source of salinity, chemical composition of the groundwater and the isotope tracers (e.g., δ18O and radium isotopes) were studied in the Viimsi peninsula in the period from 1987 to 2018. Our results show that chemical composition of Cm-V groundwater in the peninsula is clearly controlled by three-component mixing between glacial palaeogroundwater, saline water from the underling crystalline basement and modern meteoric water. The concentrations of Ra are also significantly affected by the mixing, but the spatial variation of radium isotopes (226Ra and 228Ra) suggests the widespread occurrence of the U in the surrounding sedimentary sequence. Our hypothesis is that, in addition to U originating from the crystalline basement, some U could be associated with secondary U deposits in sedimentary rocks. The formation of these secondary U deposits could be related to glacial meltwater intrusion in the Pleistocene. Although the results suggest that the infiltration of saline groundwater from the underlying crystalline basement as the main source of salinity in the study area, the risk of seawater intrusion in the future cannot be ruled out. It needs to be highlighted that the present groundwater monitoring networks may not be precise enough to detect the potential seawater intrusion and subsequent changes in water quality of the Cm-V aquifer system in the Viimsi peninsula.
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