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.
As early as in the 1960s, extensive heavy-mineral concentrations containing zircon, monazite, and xenotime were discovered in the Lemme region of south-western Estonia. These concentrations contribute to the elevated radioactivity levels of the enclosing sediments. The near shore sands of the Litorina Sea contain up to 10-cm-thick interlayers with a heavy mineral content of up to 80%. These anomalous layers were formed during the transgressive phase and result from a complicated cross-and alongshore migration of sedimentary material, derived mainly from local Devonian bedrock. Radioactivity level in the study area is higher relative to the majority of the Devonian plateau. The Lemmeoja buried soil has 13 radiocarbon dates in an area of renewed interest for the investigation of the Baltic Sea history.
A drinking water treatment plant in Viimsi, Estonia was monitored over three years for iron, manganese, radium-226, radium-228, and their daughter nuclides in order to determine the efficiency of the treatment process, get an insight of the removal mechanisms and interactions between radium, iron, and manganese, and assess the overall longevity and performance of the technology and possible build-up of NORM from the treatment process. During the study, samples were collected from raw water, first and second stage filtrate, consumer water, backwash water, and filter materials. The results show a consistent removal efficiency for iron and manganese, as well as an average of over 85% removal for radium with a slight decline with time. Backwash process has been optimized for maximum radium removal from the filters, while keeping the radium concentrations in the backwash water below exemption levels. However, accumulation of radium and thorium occurs in the filter material, exceeding exemption levels in the top layer of the filter columns in less than a year. By the end of the observation period, activity concentrations in the top layer of the columns were above 30 000 Bq/kg for Ra-226 and Ra-228, and around 15 000 Bq/kg for Th-228. Radionuclides are not homogenously distributed in the filter columns. In order to estimate the average activity concentrations in the filter media, the height distribution of radionuclides has to be accounted for. Two years and two months after commissioning of the treatment plant average activity concentrations of Ra isotopes in the filter columns were in a range of 10 000 Bq/kg while Th-228 activity concentration was roughly 3500 Bq/kg.
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