238U-series radionuclides in Finnish groundwater-based drinking water and effective doses

The thesis deals with the occurrence of 238U-series radionuclides and particle-bound 210Pb and 210Po in Finnish groundwater-based drinking water, methods used for removing 234U, 238U, 210Pb and 210Po, and the annual effective doses caused by 238U-series radionuclides in drinking water. In order to reduce radiation exposure and avoid high doses, it is important to examine the activity levels of natural radionuclides in groundwater. In this work, the activity concentrations of radon (222Rn), radium (226Ra), uranium (238U and 234U), lead (210Pb) and polonium (210Po) were determined from 472 private wells, which were selected randomly from across Finland. On the basis of the results, the activity concentrations in groundwater and the radiation exposure from drinking water of people living outside the public water supply in Finland was specified. The efficiency of 238U, 234U, 210Pb and 210Po removal from drinking water was examined at ten private homes. In order to obtain accurate results and correct estimates of effective doses, attention was paid to the sampling of 222Rn and 210Pb, and the determination of 210Pb. The results revealed that the median activity concentrations of natural radionuclides were as much as ten times higher in drilled wells than in wells dug in soil. The average activity concentration of 222Rn in drilled wells was 460 Bq/l and in dug wells 50 Bq/l. The highest activity concentrations were found in Southern Finland. In addition, occasional high activity concentrations were found all over Finland. The average activity concentrations of 234U and 238U in drilled wells were 0.35 and 0.26 Bq/l and in dug wells 0.020 and 0.015 Bq/l, respectively. The spatial distribution of 234U, 238U, 210Pb and 210Po was essentially similar to that of 222Rn. In contrast to other natural radionuclides, the highest 226Ra activity concentrations were found in coastal areas, since drilled well water near the sea has a higher salinity than water in drilled wells inland. 210Pb and 210Po occur in untreated groundwater as ions, molecules, complexes and bound to particles of different sizes depending on the chemical quality of the water. In Finnish groundwater, the majority of 210Pb was bound to the large particle fraction. Compared to 210Pb, 210Po was bound more to the intermediate and small particle fraction. Anion exchange was found to be an efficient removal method for 234U and 238U. In most cases, the removal efficiency was over 95%. The removal of 234U and 238U from drinking water reduced the annual effective dose by 0.8 mSv on average, and at maximum by 1.4 mSv. Since the removal efficiency of 210Pb and 210Po varies greatly, their presence in different particle-size fractions was studied. The ion exchange unit more efficiently removed 210Pb and 210Po bound to the small particle fraction than those radionuclides in the large particle fraction. The efficiency of activated carbon units in removing 210Pb and 210Po was independent of the particle size fraction. In Finland, about 10% of the population (500 000) permanently uses water from private wells for daily drinking and household requirements. This group receives nearly half (77.4 manSv) of the annual collective effective dose of all Finnish people due to natural radioactivity in drinking water. The average annual effective dose from natural radionuclides was estimated to be 0.4 mSv for drilled well users and 0.05 mSv for users of wells dug in soil. According to earlier studies, the maximum effective dose due to natural radioactivity in drinking water has been as high as 70 mSv. In this work, the maximum doses were 6.8 mSv for drilled well users and 0.6 mSv for users of wells dug in soil. For the users of drilled wells over 90% of the dose is caused by 222Rn (75%), 210Po (12%) and 210Pb (5%). An efficient way to detect high doses is to perform 222Rn measurement in all drilled wells. In new houses, natural radionuclides in groundwater need to be taken into account when planning household water resources in order to avoid high costs afterwards. In connection with house planning it is also recommended to reserve adequate places for removal equipment in the utility room. (orig.)