radioactivity in groundwater originating from Finnish bedrock

Transcription

1 Future Groundwater Resources at Risk (Proceedings of the Helsinki Conference, June 1994). IAHSPubl.no. 222, U series radionuclides as a source of increased radioactivity in groundwater originating from Finnish bedrock LAINA SALONEN Finnish Centre for Radiation and Nuclear Safety, PO Box 268, SF Helsinki, Finland Abstract A nationwide survey on natural radioactivity in drinking water has been going on in Finland since the late 1960s. Now more than 7000 private wells have been studied and over analyses and measurements made of 222 Rn, 238 U, 234 U, 226 Ra, 228 Ra, 210 Pb, 210 Po and gross alpha and beta activity. The results indicate that the average and maximum concentrations of 222 Rn in drilled wells are 930 Bq l" 1 and Bq l" 1, but that, respectively, those in wells or springs dug in soil are only 76 Bq l" 1 and 3500 Bq l" 1. The highest concentrations of 222 Rn and uranium (maximum jug 1" 1 ) have occurred in southern Finland in uraniferous granite areas. Of the long-lived radionuclides present in water, 210 Pb and uranium make the highest contribution to the effective dose. INTRODUCTION The Finnish Centre for Radiation and Nuclear Safety (STUK) has conducted a systematic survey of natural radioactivity in drinking water since the late 1960s (Kahlos & Asikainen, 1973,1980; Asikainen& Kahlos, 1979,1980). In the last 15 years special attention has been given to groundwater in bedrock because of the very high concentrations of radon and other radionuclides of the 238 U series found in water from drilled wells (Asikainen, 1981; Salonen, 1988, 1992; Salonen & Saxén, 1991). The aim has been to find areas with anomalously high concentrations and to analyse radionuclide composition to assess the radiation doses. Geologically, Finland is part of the Fennoscandian shield, and the bedrock consists mainly of granitoids. Especially granite rocks contain elevated levels of uranium. In Finland, the relatively high levels of radon and other radionuclides in groundwater seem to come from granite areas in different parts of the country (Fig. 1). Anomalously high concentrations of radon (max Bq l" 1 ) and uranium (max fxg l" 1 ) in bedrock water have been found in the uraniferous granite areas of southern Finland. Bedrock water is used as household water, mainly by individual families in sparsely populated areas without common water supply, but also in schools, military garrisons, hospitals and even in whole small villages. During the last few decades, drilled wells have become quite popular, especially in areas where other groundwater sources are insufficient. Water plants also consider bedrock water a suitable alternative to surface water or to other groundwater sources if they are too far away or inadequate. The use of bedrock groundwater could be increased considerably, since 70% of Finland's groundwater is estimated to lie in bedrock (Lonka et al., 1993).

2 72 Laina Salonen This paper presents the activity concentrations of radon, uranium, radium, 210 Pband 210 Po in groundwater sampled from private wells and water plants, together with the estimated radiation doses. More than 7000 private wells and almost all Finland's water plants were involved in the survey. Altogether, more than measurements or analyses were made. The only water quality parameter measured by STUK was radioactivity. However, the chemical and physical composition of water was studied carefully in those samples that were analysed in conjunction with the Geological Survey of Finland (GSF) or with the National Board of Waters and the Environment. These samples account for more than 60% of the total. The correlations between radioactivity

3 U series radionuclides as a source of increased radioactivity in groundwater 73 and other water parameters, and the influence of the different rock and soil types on the radionuclide concentrations in groundwater will be analysed in the future. SAMPLING AND ANALYTICAL TECHNIQUES The samples have been collected by several people: mainly geologists or employees of STUK and GSF, municipal health inspectors, managers of water plants or owners of private wells. Since the sampling and analytical methods are described in detail in previous papers, they are discussed only briefly here (Asikainen & Kahlos, 1979,1980; Salonen, 1988). All water samples arriving for radioactivity measurements are investigated for radon (= 222 Rn) and nowadays also for gross alpha and beta activity. The latter measurements are used to monitor the long-lived alpha emitting (uranium, mainly isotopes 238 U and 234 U, radium = 226 Ra and 210 Po) and beta emitting ( 40 K, 210 Pb and 228 Ra) radionuclides in water. Depending on the results of these measurements, some of the samples are subsequently analysed by radiochemical methods. Radiochemical analysis and measurement techniques have been developed considerably over the years. The gamma-spectrometric method of measuring radon in water has given way to liquid scintillation counting (LSC) (Asikainen, 1981; Salonen, 1993). The measurement of gross alpha and beta activity with a ZnS(Ag) scintillation counter and with a low background beta counter has also changed over to LSC (Salonen, 1989, 1993). Both LSC techniques are time-saving and enable the automatic measurement of large numbers of samples. Radiochemical and alpha-spectrometric methods have been applied for determining uranium, radium, 210 Pb and 210 Po in water. The present method for uranium involves co-precipitation of the uranium with iron hydroxide, purifying it by ion exchange and finally precipitation with cerium fluoride (Sill & Williams, 1981). 226 Ra and 228 Ra are separated from the other radionuclides by the barium sulphate method (Goldin, 1961), and their activities measured with the Quantulus, a low background LSC spectrometer. The analysis of 210 Pb and 210 Po is based on the spontaneous deposition of 210 Po on silver (Hâsânen, 1977). RESULTS Water plants Table 1 shows that the radioactivity in water distributed by water plants is lower than that in private groundwater wells, largely because the raw water at plants comprises both surface water and groundwater. The proportion of groundwater is at the moment 54%, but was 31% in 1970, which was when studies on water plants started. The treatment of raw water has reduced radioactivity levels by removing some of the radionuclides. The average radon concentration in water distributed by the plants is 40 Bq l" 1 but in their raw water 60 Bq l" 1. The source of the raw water greatly influences the radioactivity level of the water supplied by these plants (Table 2). The concentrations of radon and long-lived alpha and beta-emitting radionuclides in surface water are very low, and close to, or even less than

4 74 Laina Salonen Table 1 Comparison of results from water plants and private groundwater wells. Water plants (768) 222 Rn Gross alpha Gross beta 226 Ra. mean cone. (Bql 1 ) Max. cone. (Bq l" 1 ) Wells and springs in soil (2950) 222 Rn Gross alpha Gross beta 226 Ra Wells drilled in bedrock (4051) 222 Rn Gross alpha Gross beta 226 Ra (Number of plants or wells in parenthesis) 1 the population-weighed average Table 2 Results from water plants using raw water from different sources. Raw water sources Surface water Groundwater in soil Artificial groundwater Groundwater in bedrock Number of plants Rn (Bql" 1 ) < Gross alpha (Bql 1 ) Gross beta (Bql 1 ) Ra (Bql" 1 ) the detection limits of the methods used. In groundwaters, the concentrations are highest in bedrock water however, the highest concentrations measured (max. radon 1630 Bq T 1, max. gross alpha 3.0 Bq l" 1 ) have been in a water plant using groundwater from the soil. The radioactivity of artificial groundwater is between that of surface water and groundwater. The concentrations of radionuclides in artificial groundwater depend on the proportion and radioactivity of the original groundwater used. Gross alpha activity in Finnish groundwaters usually originates from uranium. This can be easily confirmed by comparing the average gross alpha activity with the radium or 210 Po concentration. In water from water plants, the gross alpha activity is about ten times higher than the concentration of radium (Tables 1 and 2) or 210 Po. The results of 210 Po analyses are not presented here, but they indicate that the average concentration is lower than Bq l' 1. The origin of the beta activity has not been investigated in detail. The beta-emitting radionuclides from fallout have also increased the gross activity in surface waters but not in groundwater, where concentrations have been under the detection limits of the methods used. Elevated beta activities in groundwater usually seem to be caused by 40 K, as can be seen in the beta spectra of the samples measured with the Quantulus. The same conclusion can be drawn from the concentrations of potassium in groundwater samples

5 U series radionuclides as a source of increased radioactivity in groundwater 75 collected around the country (Lahermo et al, 1990). The other natural beta-emitting nuclides, 210 Pb and 228 Ra, seem to contribute very little to the gross beta-activity. The concentrations of 210 Pb are lower than Bq l" 1, and 228 Ra has not been observed in waters obtained from water plants. 40 K in water does not usually add to the radiation dose because the potassium content of the body is controlled biologically and potassium intake occurs mainly through food. Private wells The average levels of radionuclides in groundwater from private wells are shown in Table 1. The results of the radiochemical analyses are in Table 3. The average concentrations of different radionuclides are not equally representative because the number of analyses vary a great deal and the different analyses have not necessarily been carried out on the same water samples. In some cases, waters with elevated radionuclide levels have been selected for further analysis. For this reason the nuclide ratios, when calculated on the basis of the concentrations presented in Table 3, are not necessarily typical. More representative nuclide ratios can be obtained from the concentrations presented in Table 4, which contains the results only from those water samples for which all radionuclides were determined separately. The results indicate that the radioactivity of water from drilled wells is more than ten times that of groundwater in soil. It can also be concluded that there exists a great disequilibrium between the uranium series radionuclides. Concentrations of radon in Table 3 Results from private groundwater wells. Wells and springs in soil Activity concentration (Bq l" 1 ) Number of wells Arith. mean Median Max. cone. 222 Rn Gross alpha Gross beta 23 8JJ 234TJ 226 Ra 228 Ra 210p b 210p o Wells drilled in bedrock Activity concentration (Bq l" 1 ) Number of wells Arith. mean Median Max. cone. 222 Rn Gross alpha Gross beta 238TJ 234 U 22S Ra 228 Ra 210p b 210p o

6 76 Laina Salonen Table 4 Average concentration of radionuclides in water samples analysed separately for 222 Rn, 238 U, 234 U, 226 Ra, 21(f Pband 210 Po. Nuclide Wells drilled i in bedrock (174 samples) Arith. mean (Bq l" 1 ) Wells and springs in soil (8 samples) Arith. mean (Bq l" 1 ) 222 Ra 238TJ 234TJ 226 Ra 210p b 210p o TJ/238rj = U:Ra = 26 U:Pb = 22 U:Po = bedrock water are usually more than 1000 times those of long-lived radionuclides, the difference being a little smaller for the groundwater in soil. The concentrations of radionuclides conform to log normal distribution. According to present data, the proportions of drilled wells in which radon exceeds 100, 300, 1000, 2000, 4000 and Bq l" 1 are 68, 43, 19, 10, 4.3 and 1.1%. The proportions of soil wells and springs in which radon exceeds 10,50, 100, 300 and 1000 Bq l" 1 are 70,27, 16, 5.3 and 0.8%. Of the long-lived radionuclides, uranium is the most soluble. The concentrations of uranium are usually tens of times higher than those of other nuclides. One reason for the high solubility of uranium is the chemical composition of groundwaters. Soil groundwater is typically soft, rich in C0 2, and often also acidic (Hiisvirta, 1991; Lahermo et al., 1990). In coastal areas, groundwater also contains high concentrations of chloride and sulphate. Bedrock water is more alkaline, containing calcium bicarbonate and calcium sulphate, changing towards sodium chloride type and becoming more saline at greater depths (Juntunen, 1991; Lahermo & Juntunen, 1991). In such water uranium can form soluble complexes, e.g. with chloride, sulphate, phosphate and, under oxidizing conditions, with carbonate. Carbonate complexes appear to prevail in Finnish groundwaters which are mainly bicarbonate waters, even though the minerals in the bedrock are almost entirely silicates (Hyyppâ, 1984). In such water radium behaves in the opposite way to uranium. It forms very insoluble compounds with sulphate and carbonate and precipitates on the surfaces of carbonate minerals. Radium also co-precipitates with iron or manganese oxides or hydroxides. Our results indicate that radium very rarely occurs in groundwater at equal or higher concentrations than uranium. Such groundwater has been taken from boreholes from a depth of hundreds of metres, where conditions are oxygen-deficient and reducing. The amount of dissolved 226 Ra in such water has been higher than that of uranium. The salinity of these waters has also been found to be high. The maximum concentration of uranium in bedrock water samples has been 440 Bq l' 1 (= ^g I" 1, 234 TJ/ 238 U activity ratio 1.9). In those samples studied by GSF it has been even higher ( tig I" 1 ) (Juntunen, 1991). The ^U/ 238!! activity ratio has varied from 0.3 to 8.2. The mean ratio for all analyses is 1.6, which is the same as that obtained for the samples in Table 4. Radium, 210 Pb and 210 Po are usually less soluble in groundwater than uranium (Durrance, 1986). This is because these nuclides are easily removed from water by precipitation or by adsorption onto clay or rock minerals. They are also adsorbed onto organic particles, and are transported and precipitated along with them.

7 U series radionuclides as a source of increased radioactivity in groundwater 11 Our results indicate that the average concentration of 210 Pb in bedrock waters is a little higher than that of radium and 210 Po, which seem to be present in equal concentrations. The concentrations of radium and 210 Pb in groundwater from soil are the same but the concentration of 210 Po is lower. It was nevertheless unexpected to find that the concentration of 210 Pb is higher than that of radium. This increases the radiation dose since 210 Pb is more radiotoxic than radium. The number of 2I0 Pb analyses is still much lower than that of radium, but the future analyses need not necessarily change the Fig. 2 Communal averages of radon concentration in drilled wells in Finland.

8 78 Laina Salonen situation. One explanation for the higher concentrations of 210 Pb than radium may be the considerably lower solubility of radium than radon. In bedrock waters the radium/radon ratio is usually 1/ Pb is generated by the radioactive decay of radon. The low solubility of radium was attributed to the chemical composition of the groundwaters. Also, the correlation between radon and 210 Pb (Pearson's correlation coefficient = 0.80) is higher than between radon and radium (0.53). The occurrence of 228 Ra in groundwater has been studied mainly in bedrock water (Asikainen, 1981). 228 Ra belongs to the decay series of thorium ( 232 Th). The concentration of 228 Ra has been found to be independent of the amount of 226 Ra. The concentrations of 228 Ra have been nearly the same over the whole range of 226 Ra concentrations. It should be noted, however, that the concentrations of 226 Ra have increased distinctly, while the concentrations of 228 Ra have increased only slightly. The 226 Ra/ 228 Ra activity ratio has varied between 0.3 and 26, but in most samples has been near to 1. In any case, the activity levels of 228 Ra are usually low. Only some beta spectra of those 2000 samples now measured by the Quantulus indicate that some 228 Ra exists in bedrock water but none in groundwater from soil. The occurrence of radon in water from drilled wells in Finland is shown in Fig. 2 and the gross alpha activity in Fig. 3. The latter figure also illustrates the occurrence of uranium, because most gross alpha activity in bedrock water is due to uranium. The geographical distribution of gross alpha activity, shown in Fig. 3, is identical to that of uranium obtained from GSF, where uranium was determined fluorometrically. The high concentrations of radon and uranium in groundwater occur mainly in those areas where the concentrations of radon in indoor air are also high. The areas with high concentrations of radon and uranium in bedrock water seem to be in the two granitic rock areas in southern Finland and in the granitoids of central Lapland (Fig. 1). The granites in southern Finland are very heterogeneous and form migmatites with other rock types. The uranium content of the rocks also varies a lot, and there are several uranium deposits in the granitic rock areas. The highest communal average of radon ( Bq l" 1 ) in drilled wells (27) was found in a village located in such an area. However, no anomalously high concentrations of radon or uranium in groundwater have been found in the Vyborg rapakivi massif area of southeastern Finland; although the average concentrations of uranium in rocks there are the highest, they fluctuate within narrower limits. The small concentrations in water are also lacking in this area. Therefore the geometric mean (370 Bq l" 1 ) and median (350 Bq l" 1 ) of radon in water are highest in the rapakivi area, whereas the arithmetic mean (610 Bq l" 1 ) is lower than in the other granite areas (1 200 Bq l" 1 ) in the southern part of the country. RADIATION DOSES The average annual doses to the Finnish population caused by household water are presented in Table 5. The doses due to radon have been calculated using the mean concentrations presented in Table 1. The dose from radon ingested with water is directed at the stomach. The calculation of this dose is based on an assumption that a radon concentration of 1 Bq l" 1 leads to an annual effective dose of msv (Kendall et al., 1988). Radon is released from water to the air, from where it enters the lungs through inhalation. The amount of radon released from water to air depends a lot on the

9 U series radionuclides as a source of increased radioactivity in groundwater 79 circumstances. The same air/water transfer factor 1/ has been used for all houses. The calculation of the dose to the lungs is based on an assumption that a radon concentration of 1 Bq m" 3 in indoor air causes an annual effective dose of msv (ICRP Publication 65, the International Commission on Radiological Protection). The doses given for the long-lived radionuclides are based on the results of the gross alpha and beta activity measurements. The proportions of different radionuclides giving rise to these gross activities have been calculated using the nuclide ratios obtained from Fig. 3 Communal averages of gross alpha activity in drilled wells in Finland.

10 80 Laina Salonen Table 5 Average annual effective doses from radionuclides in household water. Water source From ingestion of water From inhalation of From ingestion of water radon released from and inhalation of radon 222 Rn Long-lived In all water to room air In all radionuclides (msv year"') (msv year" 1 ) (msv year" 1 ) (msv year" 1 ) (msv year" 1 ) Water plant Wells and springs in soil Wells drilled in bedrock the measurements. The doses would have been too high had they been calculated from the concentrations of the long-lived radionuclides in Table 3. Dose calculations are based on the ICRP Publication 61. Table 5 shows that the average doses due to radionuclides in household water in Finland are quite high for those people who use bedrock water. Their average dose is nearly double the average dose to Finns in general, all sources of radiation taken into account, which is about 4 msv year" 1. Of this, 1.8 msv is caused by radon in indoor air. For other people, the radionuclides in household water make a much smaller contribution to the dose. Table 5 also shows that % of the dose is caused by radon. The maximum dose through radionuclides in household water has been calculated as 197 msv year" 1. Of this, 140 msv is caused by ingested radon, 12 msv by ingested longlived radionuclides and 45 msv by inhaled radon. These dose calculations were based on measurements of radionuclides in water and radon in indoor air (3000 Bq m" 3 ). In Finland, high indoor air radon concentrations may also originate from household water. When the radon concentration in water exceeds 1000 Bq l" 1, its influence on the radon concentration in indoor air can usually be observed. In the worst case, the radon released from water has increased the radon concentration in indoor air by thousands of Bq m" 3. In such houses radon should be removed from water, or else the water source should be changed, as indoor air concentrations cannot be reduced sufficiently by other means. However, radon-rich household water is the main reason for high indoor radon concentrations only in a minority of Finnish houses. Direct influx from the ground to the dwelling is the main source of radon (Castrén et al., 1984; Arvela et al., 1993). Figure 4 compares the contribution of different long-lived radionuclides with the effective dose. Usually, most of the dose is caused by 210 Pb and uranium. Although radium and 210 Po have little influence on the dose, in very rare cases either one of them can actually cause the highest dose. The large contribution of uranium to the dose is due to its high concentrations in water, while 210 Pb, on the other hand, greatly influences the dose because it has the highest radiotoxicity. REGULATIONS AND CONTROL MEASURES The new Finnish Radiation Act came into force on 1 January Because the new Act also requires natural radiation to be monitored, STUK has provided a safety guide concerning the radioactivity in household water (ST-guide 12.3). This guide came into

11 U series radionuclides as a source of increased radioactivity in groundwater 81 Proportion of total dose m., & :!! i 1 i m^ = _ i 1! i 1 BELOW ,000 1,000-2,000 2,000-4,000 4,000-10,000 OVER 10,000-1 Radon concentration Bq 1 U-238 U-234 g Ra-226 H Pb-210 M Po-210 Fig. 4 Long-lived radionuclides as a proportion of the annual effective dose caused by ingested radionuclides in water. The rest of the dose is caused by ingested radon. force on 1 October The guide concerns water distributed by water plants and water used for the production of food or beverages. The safety requirement for household water is that the effective dose due to radionuclides in the water should be lower than 0.5 msv per year. The radon released from water to air is not included. The limits for radon in dwellings (400 Bq m~ 3 for existing buildings and 200 Bq m" 3 for new buildings) have been issued by the Ministry of Social Affairs and Health in Control measures have been established for the measurements of radon, and gross alpha and beta activities in water. The activity index /is calculated in the following way: / = C a + C & + C^/300 where C a, C &, and C^ are the gross alpha activity, gross beta activity and radon concentration of the water, respectively, expressed in Bq l" 1. The safety requirement is fulfilled when the activity index does not exceed 1. If the index exceeds 1, the concentrations of different radionuclides should be determined and used to calculate a new activity index. If this index still exceeds 1, measures must be taken to reduce the radioactivity of the water. The safety guide suggests that the maximum concentration of radon in water should be 300 Bq l" 1 provided no dose is caused by other radionuclides. Because in water radon usually co-exists with other radionuclides, the concentrations of radon and other radionuclides are less than the maximums given for them. The maximum concentration of uranium is set on the basis of its chemical toxicity, which is higher than its radiotoxicity. This latter limit has not yet been set, but it will be in the range fig Ï 1 of uranium.

12 82 Laina Salonen Table 6 Distribution of annual effective doses to the population from radionuclides in ingested water. Water source Number of Number of people whose dose (in msv year" 1 ) exceeds: people Water plant Well or spring in soil Well drilled in bedrock Table 6 presents estimates for the number of persons whose annual effective doses exceed certain values. The effective doses in this table consider only ingested radionuclides. If the radon released from water to air had also been taken into account the number of persons would have been much bigger. According to the present estimates the safety requirements are not fulfilled at small water plants serving people. The remedial actions would be necessary to lower the radon concentration in the water. No other radionuclide needs be removed at any plant if the maximum concentration of uranium is set to 100 /*g l" 1. Action is also needed, for instance, at schools and other establishments and in certain villages that have their own water supply. If the safety guide were also to consider water from private wells, about people would be using water that is too radioactive, and in a small number of cases highly radioactive. About people use water in which the uranium content exceeds 100 jttg l" 1. Most of these people use water from drilled wells but would prefer to stop using such water. The exact number of these people is not known because the number of drilled wells is not known precisely and only a small number of existing wells have already been studied. Sometimes the only way to help these people is to give advice on how to remove radionuclides from water. In Finland, good results have been achieved in radon removal from water by aeration. Investigations are under way to find methods for other radionuclides too. These methods and their effectiveness should be tested on groundwaters that are similar to those found in Finland. CONCLUSIONS Greater use of bedrock water will increase the radiation exposure of the Finnish population, depending on the radioactivity levels in the water sources employed. Studies on private wells have indicated that the radioactivity of bedrock groundwater is on average more than ten times higher than that of groundwater in soil. However, in most of the drilled wells, radioactivity levels in water are low, and in certain areas no high concentrations have been found. On the other hand, there are a few areas where the risk of getting highly radioactive water from bedrock is great. Such areas should be located and bedrock water should be replaced by common water supply to provide water for entire village or group of households. Acknowledgements I gratefully thank Mrs S. Hâmâlàinen for her assistance in carrying out the analyses and Mrs A. Hâmâlàinen, Mrs I. Mâkelâinen and Mrs A. Voutilainen

13 U series radionuclides as a source of increased radioactivity in groundwater 83 for processing the data. I also wish to thank all the persons who helped with the collection of the samples. REFERENCES Arvela, H., Màkelâinen, Ï. & Castrén, O. (1993) Otantatutkimusasuntojen radonistasuomessa (Residentialradon survey in Finland). Report STUK-A108, Finnish Centre for Radiation and Nuclear Safety, Helsinki. Asikainen, M. (1981) State of disequilibriumbetween 238 U, 234 U, ^Ra and 222 Rn in groundwater from bedrock. Geochim. Cosmochim, Acta 45, Asikainen, M. (1981) Radium contentand the 226 Ra/ 228 Ra activity ratio in groundwater from bedrock. Geochim. Cosmochim. Acta 45, Asikainen, M. & Kahlos, H. (1979) Anomalously high concentrations of uranium, radium and radon in water from drilled wells in the Helsinki region. Geochim. Cosmochim. Acta 43, Asikainen,M. & Kahlos, H. (1980) Natural radioactivity of drinking water in Finland. Health Phys. 34, Castrén, O., Winqvist, K., Màkelâinen, I. & Voutilainen, A. (1984) Radon measurements in Finnish houses. Rod. Prot. Dosim. 7(1-4), Durrance, E. M. (1986) Radioactivity in Geology. Principles and Applications (ed. by D. T. Donovan & J. W. Murray). Halsted Press: a division of John Wiley & Sons, Chichester, UK. Finnish Centre for Radiation and Nuclear Safety (1993) Talousveden radioaktiivisuus (The radioactivity of household waters). ST-Guide Finnish Centre for Radiation and Nuclear Safety, Helsinki. Goldin,A. S. (1961) Determination of dissolved radium. Anal. Chem. 33, Hâsânen, E. (1977) Dating of sediments, based on 210 Po measurements. Radiochem. Radioanal. Lett. 31, Hiisvirta, L. (199 l)suomalainenvesikansainvalisessavertailussa (The quality of drinking water in Finland compared with that in other countries). Vesitalous (Finnish journal of water economy, water technology, hydraulic and agricultural engineering and environmental protection) 32(4), 4-7. Hyyppà, J. (1984) Pohjaveden kemiallinen koostumus Suomen kallioperassa (Chemical composition of ground water in the bedrock of Finland). Report YJT-84-10, Nuclear Waste Commission of Finnish Power Companies, Helsinki. International Commission on Radiological Protection (1991) Annual Limits on Intake of Radionuclides by Workers Based on the 1990 Recommendations. ICRP Publication 61. The International Commission on Radiological Protection, Pergamon Press. International Commission on Radiological Protection (1993) Protection Against Radon-222 at Home and at Work. ICRP Publication 65, The International Commission on Radiological Protection, Pergamon Press. Juntunen, R. (1991) Etelâ-Suomen kallioporakaivojenuraani-ja radontutkimukset (Uranium and radon in wells drilled into bedrock in southern Finland). Report of Investigation, no. 98, Geological Survey of Finland, Espoo. Kahlos, H. & Asikainen, M. (1973) Natural radioactivity of ground water in the Helsinki area. Report SFL-A19, Institute of Radiation Physics, Helsinki. Kahlos, H. & Asikainen, M. (1980) Internal radiation doses from radioactivity of drinking water in Finland. Health. Phys. 39, Kendall, G. M., Fell, T. P. &Phipps, A. W. (1988) A model to evaluatedoses from radon in drinking water. Radiol. Prot. Bull. 97, 7-8. Lahermo, P. & Juntunen, R. (1991) Radiogenic elements in Finnish soils and groundwaters. Appl. Geochem. 6, Lahermo, P., Ilmasti, M., Juntunen, R. & Taka M. (1990) Suomen Geokemian Atlas, Osa 1. Suomen Pohjavesien Hydrokemiallinen Kartoitus (The Geochemical Atlas of Finland, Part 1. The Hydrochemical Mapping of Finnish Groundwater). The Geological Survey of Finland, Espoo. Lonka, H., Leveinen, J. &. Tossavainen, J. (1993) Kalliopohjavesi - Suomen vesihuollon vâh&n kâytetty mahdollisuus (Bedrock groundwater an option seldom used for water supply in Finland). Vesitalous (Finnish journal of water economy, water technology, hydraulic and agricultural engineering and environmental protection) 34(3),7-11. Salonen, L. (1988) Natural radionuclides in groundwater in Finland. Radiât. Prot. Dosim. 24(1/4), Salonen, L. (1989) Simultaneous determination of gross alpha and beta in water by low level liquid scintillation counting. Proc. Second Karlsruhe International Conf. on analytical Chemistry in Nuclear Technology, Karlsruhe, Federal Republic of Germany, June File copy, Finnish Centre for Radiation and Nuclear Safety, Helsinki. Salonen, L. (1992) Talousvesien radioaktiivisuusja sen poistaminen (The radioactivity of domestic water and its removal). Vesitalous (Finnish journal of water economy, water technology, hydraulic and agricultural engineering and environmental protection) 33(6), Salonen, L. (1993) A rapid method for monitoring of uranium and radium in drinking water. Sci. Tot. Environ. 130/131, Salonen, L. (1993) Measurement of low levels of 222 Rn in water with different commercial liquid scintillation counters and pulse shape analysis. Accepted for publication in Radiocarbon, Int. J. of Cosmogenichot. Res. Salonen, L. & Saxén, R. (1991) Talousvesien radioaktiivisuus Suomessa (Radioactivity of household waters in Finland). Kemia-Kemi 18(5),

14 84 Laina Salonen Sill, C. W. & Williams R. L. (1981) Preparation of actinides for a spectrometry without electrodeposition./lna/. Chem. 53,