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Radiogenic elements in Finnish soils and groundwaters
Applied Geochemistry,Vol. 6, pp. 169-183, 1991
0883--2927/91 $3.00 + .00 O 1991 Pergamon Press plc
Printed in Great Britain.
Radiogenic elements in Finnish soils and groundwaters P. LAHERMO a n d R. JUNTUNEN Geological Survey of Finland, SF-02150Espoo, Finland (Received 19 June 1990; accepted in revised form 15 November 1990) Abstract--In Finland, U and Rn concentrations in groundwaters are highest in the south of the country in
bedrock composed of granites or migmatites with abundant granite or pegmatite veins. The areal distribution patterns of U and Rn in groundwater from bedrock are consistent with U and Th anomalies in the fine fraction of till and with the external gamma dose rate measured on the ground surface. The U and Rn concentrations in groundwater from bedrock are only moderately correlated; hence the elevated U contents do not imply abundant Rn contents. Both elements are practically independent of other dissolved components. The exception is the moderate correlation between U and HCO3 suggesting the occurrence of U-rich carbonates as fracture-coating minerals. In a few areas with U and Rn anomalies, the exceptionally high Rn concentrations in ambient house air pose a considerable health risk, particularly in detached houses founded on pervious sand and gravel deposits. Regardless of the potential health risk no radiological recommendations for U and Rn concentrations in potable water have been officially set in Finland.
INTRODUCTION
ments were made using the standard addition method and a Perkin-Elmer 204 fluorescence spectrophoTHE NATIONWIDE hydrogeochemical mapping of tometer. Later, a direct-reading Perkin-Elmer LS-2 groundwaters carried out by the Geological Survey of B Filter Fiuorimeter device was used without the Finland (GSF) included 3954 U determinations addition method. Radon was analyzed in the FCRNS (LAHERMO et al., 1990). Several thousand Rn deter- by mixing a 10ml water sample with 12 ml of a minations were made by the GSF and the Finnish commercial scintillation cocktail Luma-Gel, in glass Centre for Radiation and Nuclear Safety (FCRNS), vials. The samples were then counted with a Wallac mainly in the south of the country, where the concen- Ultro-Beta 1210 liquid scintillation counter. trations are known to be the highest (ASIKAINEN, The following emphasizes the regional data on U 1982; SALONEN,1987; JUNTUNEN, 1990). The U and and Rn in groundwater, which are compared with the Rn investigations were prompted by the idea that the concentrations of U and Th in soil. The U-isotope elements could be used as pathfinders in U explor- ratios in water and the geomedical aspects of Rn are ation. Later, however, the Rn mapping was intensi- also briefly discussed. fied due to health concerns caused by the unusually high Rn concentrations in groundwater from bedrock wells and house air in some parts of the country. The Nordkalott Project carried out jointly by the OCCURRENCE OF RADIOGENIC ELEMENTS IN Geological Surveys of Finland, Norway and Sweden, ROCKS, TILL AND WATERS analyzed U and Th in the fine fraction (<62/zm) of till, heavy minerals of till (Th only), organic matter of Information about the U concentrations of various stream sediments, and stream moss by XRF and rock types in Finland is scant. It is agreed, however, N A A methods (BOLVIKENet al., 1986). For the Geo- that in the Fennoscandian Shield, in Finland and chemical Atlas of Finland, the clay and silt fractions Sweden, the Proterozoic highly differentiated leuco(<64/~m) of 1054 composite till samples were ana- cratic alkali-granites with relatively low Ca are the lyzed for U and Th as partial concentrations from most uraniferous rocks (WILSON and A,KERBLOM, aqua regia leach by ICP and as total concentrations 1982). Uranium enrichment occurs in rocks of all by NAA. These surveys provide ample regional ages in a variety of geological settings. In Sweden, the material for the evaluation of U cycling in surficial range and average value of U concentrations in 174 deposits and natural waters. granites were 7-24ppm and 12 ppm, respectively Total U and Th concentrations from the fine frac- (WILSON and AKERBLOM,1982). The radioactive gration of till were determined by the N A A method in nites are often enriched in one or more of F, Mo, Sn the Reactor Laboratory of the Technical Research and W. Centre of Finland. Uranium was analyzed from water The highest total U concentrations (4-8 ppm) in by the GSF. The filtered (0.45/~m) and acidified (1 ml the fine fraction of till are found in areas underlain by 10% H2SO 4 into 500 ml of sampling water) samples granites to the north and north-west of the Wiborg were enriched by ion-exchange, and the measure- rapakivi granite batholith and in the north-western 169
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and western parts of the pluton itself (Fig. 1, cf. Fig. 6). Granites, migmatites intermingled with granite veins, and granodiorites in southern and southwestern Finland also show elevated U concentrations. In the part of northern Finland studied by the Nordkalott Project, the anomalous U concen-
trations (3-5 ppm) are encountered in till underlain by granites (BoLVZKEN et al., 1986). This is in good agreement with the well-known fact that the U concentrations in plutonic rocks increase from ultramafic toward felsic rocks ranging from 1-2 to 3-5 ppm, respectively (ROGERS and ADAMS, 1969b).
Radiogenic elements in Finnish soils and groundwaters The geochemistry of U is intimately associated with that of Th. Both elements show virtually t h e same interdependence with bedrock composition. The Th concentrations in rocks and till are generally higher than those of U. In 174 Swedish radioactive granites the range and average Th concentrations were 34-61 ppm and in 41 ppm, respectively (WILsoN and fikKERBLOM,1982). The corresponding U/TH ratio ranges from 0.1 to 0.4. In rapakivi granite and ordinary granite areas in Finland, partial Th concentrations (analyzed from A R leach) and total Th concentrations in the fine fraction of till are 10-20ppm and 10-30ppm, respectively (Fig. 2). Owing to its low mobility, Th tends to remain immobile whereas U enters into solution, a reason for the partial separation of both elements in weathering and the higher Th concentrations remained in till (cf. ROGERS and ADAMS, 1969a). The U/Th ratio in till, ranging typically from 0.3 to 0.4 (Fig. 3), is of the same order of magnitude, or slightly higher, than that in granites. In general, high U concentrations also mean high Th concentrations in till. At moderate or low U concentrations the variation in the U/Th ratio seems to be greater. The ratio is lowest (from 0.2 to 0.3) in the areas composed of granulites and granitic basement gneisses in northern and eastern Finland where the U concentrations in till are lowest. The areal distributons of the U and Th values follow roughly the same pattern in organic stream sediments as in till, but are more varied in other geochemical materials such as stream moss. Uranium is slightly soluble, occurring in water mainly as uranyl-ion or UO~-, which is mobile in an oxidizing environment. The key control on dissolved U concentrations appears to be redox potential (GAsCOYNE, 1989). Concentrations of U in rock, and the residence time and composition of groundwater seem to be less important. Colloidal forms and organic complexes of U are insignificant (GAsCOYNE, 1989). In the pH range 8-10, which is occasionally met in deep groundwater in crystalline bedrock (cf. NURMl et al., 1988; BLOMQV1STet al., 1989) U may occur as carbonate complexes. At a lower pH, phosphate complexes may occur (cf. GASCOYNE, 1989). The availability of dissolved CO~ enhances the dissolution of U. The maximum U solubility in groundwater from a typical granite under CO2-rich, oxidizing conditions is - 1 g/l (PAQUETTEand LEMIRE,1981). In Finland the highest recorded (ER,g,METS.g,, 1968; ASIKAINEN,1982; JUNTUNEN,1990) concentrations in bedrock groundwater are 13mg/l, 15mg/1 and 20 mg/1, respectively. Uranium concentrations in the groundwaters of the overburden and surface waters containing small amounts of dissolved solids are very low (<0.5 or 1.0/~g/l) except in some areas with local, mostly tiny, secondary U mineralization. As an example, a spring draining water from a weathered uraniferous granite in Sodankyl/i, northern Finland, shows a U concentration of 380/~g/1; in water dis-
171
charged into a nearby exploration pit excavated in till the value is 530/~g/1 (PEURANIEMI,written commun.). The hydrogeochemical mapping of Finnish groundwaters carried out by the GSF shows that the median U concentrations in spring waters and in waters from wells dug into overburden or drilled into bedrock are all < 1 ~g/l (LAnERMO et al., 1990). The arithmetic means are somewhat higher because of occasional higher concentrations; 1.4/~g/l (1294 samples), 1.6/~g/1 (3553 samples) and 21.2/~g/l (1027 samples), respectively. In the somewhat larger population of bedrock wells (1388 samples) covering southern Finland only (includes partly the same material), the median and arithmetic means are 5 and 73/~g/l, respectively (JuNTUNEN, 1990). The few elevated U concentrations in water from springs and dug wells occur mainly in the southern part of the country, which is largely composed of granites and migmatites (Fig. 4). This regional pattern is even more conspicuous in more U-rich bedrock groundwaters (Fig. 5). In areas of uraniferous granites, the U concentrations in bedrock groundwater are often of the order of 100-100011g/l, whereas the highest values recorded in Finland are in the range 15-20 mg/l (ASIKAINEN, 1982; JUNTUNEN, 1990). The 14 highest concentrations of U in groundwater from 308 drilled wells in the Helsinki area, which is mainly composed of migmatites with abundant granite veins, are in the range 1-14 mg/l (AsIKAINEN, 1982). The U concentrations in groundwater from bedrock and the total U concentrations in the fine fraction of till show a fairly good correlation (cf. Figs 1 and 5). Uranium has an affinity to organic material, which in Finland is manifested by the high secondary U concentrations in the marginal and bottom layers of peat deposits. Uranium is loosely bound to peat and is usually much more abundant than Th. Hence, the U/Th ratio in peat is often quite high (see BOYLE, 1982; ER~METS~ and YLIRUOKANEN, 1971; YLIRUOKANEN, 1975). YL1-KYNY (1989) reported low values of U in water-rich, weakly humified peat. When the peat is rich in Th, the respective U concentrations are generally low (YLI-KvvNY, 1989). The U deposition is enhanced by the reducing conditions typical of water-saturated; strongly humified peat. Although spring waters and seeping groundwaters in discharge zones, which are the sources of secondary U enrichment, usually exhibit very low U concentrations, over time appreciable U concentrations accumulate close to groundwater discharges. Also, fracture zones in bedrock with abundant circulating groundwater favor the accumulation of U. Because of the higher mobility of the light 234U isotope compared with 23SU, a state of disequilibrium prevails between the isotopes 234Uand 238Uin natural waters (see e.g. HESS et al., 1985). The 234U/238U activity ratio is generally >1.0, reaching up to 3.0 (AS!KAINEN, 1982). In deep-seated fresh bedrock groundwater collected from a 1 km-deep experimen-
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FiG. 2. Total Th concentrations in the fine till fraction (<64pm) analyzed by the NAA method (1054 composite samples). The symbols refer to the smoothed values in a window area, 10 or 15 km in diameter, plotted on an imaginary regular sample grid. tal hole drilled into a granite pluton at Lavia, the average ratio of disequilibrium ranges from 1.5 to 2.3 (WICKSTR(~Mand LAMPI~N,1986). The ratio in upper fresh and lower brackish water columns of the 84- and 208 m-deep drill holes intersecting uraniferous grani-
tic dykes in gneisses and migmatites at Palmottu, Pusula, in southern Finland, ranges from 1.2 to 3.7, increasing down the water column ( J A A K K O L A et al., NIIN] et al., 1990). In the deepest drill hole (1175 m) in Finland, located in Outokumpu, eastern Finland,
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which cuts mica gneisses, black schists and, in the lower part of the hole, some sizeable serpentinite bodies, the disequilibrium ratios in the stratified fresh-saline water column range from 1.2 to 1.8. T h e r e is no systematic trend in the ratio as a function of the depth of occurrence and degree of mineralization of the groundwater (cf. NURMI et al., 1988).
According to GASCOYNE (1989), high 2 3 4 U / 2 3 8 U activity ral~ios in groundwater and a low ratio in altered rocks indicate active and recent leaching of U by groundwater. R a d o n is generally the most abundant dissolved radioactive c o m p o n e n t of Finnish groundwaters. The Rn contents in dilute surface waters and shallow
174
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groundwaters are mostly < 100 Bq/l. In a survey conducted by the FCRNS, the arithmetic mean of 961 samples from springs and dug wells was 92 Bq/l, and that of 2065 samples from drilled bedrock wells 1020 Bq/1) (SALONEN, 1987). The comparatively high Rn concentrations in the bedrock groundwater of this
set of data are partly due to the fact that most of the samples were collected from the anomalous southern part of the country. In a study carried out by the GSF, which also focused on southern Finland, the median and arithmetic mean values of Rn concentrations of 1050 bedrock well samples were 215 and 631 Bq/1,
Radiogenic elements in Finnish soils and groundwaters
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Some 20 samples gave concentrations >5000 Bq/1, the maximum being 24,000 Bq/1. The highest Rn concentrations were found in groundwater from granites and migmatites rich in granite veins to the north and north-west of the Wiborg rapakivi granite batholith in south-eastern
Finland (median 349 Bq/l, 298 samples); they were slightly lower in the rapakivi granite area proper (Fig. 6). The bedrock groundwater in granodiorite areas also exhibited somewhat elevated Rn concentrations, whereas the groundwater in areas composed of mica gneisses and phyllites, migmatitic gneisses, amphibolites, quartz-feldspar schists and sandstones
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FIG. 7. Uranium and Rn concentrations in bedrock groundwater (as arithmetic means) grouped by rock type. The columns are arranged in the order of decreasing U concentrations shown by the left hand columns. (N) Refers to the number of analyses. Data according to JUNTtJNEN(1990). showed lower concentrations, diminishing in this order. As shown by Fig. 7, Rn correlates only moderately with U concentrations (r = 0.27), and there is practically no correlation with any other dissolved components except the H C O 3 and N O 3 concentrations. The correlation of U with H C O 3 alkalinity suggests that dissolved U is causally linked with U-rich carbonates occurring as fracture minerals. This is supported by the study on the Palmottu U - T h deposit in
southern Finland, where the bulk of the U was found as loosely bound forms in calcitic fracture coatings (NUN] et al., 1990). The dating of the calcite coatings yielded ages between 80 and >300 Ka. The correlation of U with the N O 3 concentrations refer to contamination by human and animal sewage, cattle dung and artificial fertilizers, etc., suggesting that U mobilization may be p r o m o t e d by biogenic compounds dissolved in water. The high U concentrations in water do not in-
Radiogenic elements in Finnish soils and groundwaters
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FIG. 8. The frequency distribution of U and Rn concentrations in water from bedrock wells (1388 and 1050 samples, respectively). The scale is logarithmic with the base/2.
177
that of Rn. In one case the U concentration of 20,000/~g/1 measured in a bedrock well was two orders of magnitude lower when remeasured. The distribution of Ra in water is a function of the U and Th contents and texture of the aquifer material, the flow conditions and residence time of groundwater and the half-lives of the respective isotopes (cf. Hess etal., 1985). The Ra concentrations in groundwater are largely controlled by its strong tendency to become adsorbed by the mineral matter. Radium concentrations are, on average, lower than Rn concentrations by four orders of magnitude, and their correlations with dissolved U concentrations are negligible (AsIKAINEN, 1982). Uranium concentrations are generally higher, as implied by the 238U/226Ra ratio, which ranges from 1 to 20 (HEss et al., 1985).
GEOMEDICAL ASPECTS variably mean high Rn activities. This finding is understandable, because extremely mobile, gaseous 222Rn is very short-lived (3.82 d) compared with its immediate predecessor in the same radioactive decay series, 226Ra (1620 a), and the parent of the series, 23~U, and its daughter nuclides 234U and 23°Th, whose half-lives from 475 Ma to 80 Ka (see e.g. ASIKAINEN, 1982; HESS et al,, 1985). It is therefore not surprising that there is a discrepancy between U and Rn values in waters of the Wiborg rapakivi granite area, where conspicuous anomalous areas of dissolved U do not show up as elevated Rn concentrations in groundwater (cf. Fig. 6). It is noteworthy that anomalous U and Rn concentrations in groundwater, and to a lesser extent in till, constitute a distinct aureole-type zone coinciding with the granite areas surrounding the Wiborg rapakivi granite batholith in southeastern Finland (Fig. 7). Elsewhere in granite environments U concentrations tend to be conspicuously lower. The difference is not so conspicuous in the U concentrations of till possibly due to the glacial transport of drift from northwest to southeast. Figure 8 shows the frequency distribution of U and Rn concentrations in bedrock groundwater throughout Finland. The distribution of Rn concentrations is almost symmetrical, whereas that of U is strongly skewed and truncated because the values below or slightly above the detection limit constitute a large part of the sample. The spatial fluctuations of U and Rn concentrations are large. Even in adjacent wells the concentrations may differ by several orders of magnitudes. They are also strongly time-dependent. Fifteen bedrock wells were sampled twice at an interval of 8 months (JUNTUNEN, 1990). The U and Rn concentrations either increased or decreased from the values measured earlier. In places the U and Rn concentrations changed reciprocally. The amplitude of fluctuations in the U concentrations was larger than in
Apart from their economic aspects, U enrichments in peat bogs may pose an environmental threat when peat is utilized as fuel. Further, the oxidation of peat deposits due to land reclamation measures, such as ditching of peat bogs, may mobilize U into surface waters and groundwaters. The mobilization may also be triggered by acid rain (cf. CULBERTand LEIGHTON, 1988). In Finland no upper permissible limits of U, Ra and Rn have been set for drinking water. The directive concerning water quality standards issued by the National Board of Health states that the total alphaand beta-radiation level in natural waters should not generally exceed 0.1 Bq/1 and 0.8 Bq/l, respectively (AnoN, 1985). Before a new water utility is joined to the public water supply network, it must be studied to ensure that the water does not contain exceptionally high levels of radioactive elements. With respect to radiation in general, if the gross radioactivity of tap water >8000 Bq/1 (Rn not included) the measurement of Rn in ambient house air is recommended. The permissible U concentration in drinking water has not been set. In Canada, where geological conditions in crystalline areas are generally similar to those in Finland, the upper permissible U concentration has been set as low as 20pg/1 (Anon, 1978). Only 2% of the 1367 investigated springs in Finland exceeded the 5 pg/l U concentration level, and only 10 samples were >10pg/1. In groundwater from wells, 2% of 1367 cases exceeded 10/~g/1 and 19 cases were higher than the upper Canadian limit of 20pg/1 U. Water from bedrock wells exhibited the highest U concentrations: 15% of the 2385 sites investigated exceeded 20 pg/l and 5% exceeded 100 pg/1 (Fig. 9). The maximum acceptable concentration of Ra in drinking water in Canada is 1 Bq/1 (AnoN, 1978). An investigation of radiological properties of bedrock groundwater in the Helsinki region revealed that the median, arithmetic mean and maximum values in Ra
178
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of 50 samples were 0.06 Bq/1, 0.3 Bq/1 and 9.5 Bq/l, respectively (ASIKAINEN and KAnLOS, 1979; ASlgAINEN, 1982). In this area 22% of the wells exceeded the maximum concentration acceptable in Canada. The Helsinki region is anomalous due to the high U, Rn and Ra concentrations in water of bedrock wells,
the central and northern parts of the country showing much lower concentrations. M o r e than one fifth of the rural population of Finland, or about 1-1.2 million people, use their own drinking water sources. An estimated 5-20% of the private water intakes are from bedrock wells, where
Radiogenic elements in Finnish soils and groundwaters
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excessive U concentrations are common. They are particularly frequent in some granite areas in southern Finland, where high U and Rn concentrations in groundwater constitute a notable health problem. Nonetheless, the main pathway of radioactive radiation to man is through Rn released into the air. Glaciofluvial formations such as eskers and esker deltas and the Salpausselk~i ice-marginal formations,
which are largely composed of pervious sand and gravel, act as conduits and reservoirs for Rn gas emanating from deeper fracture systems in the bedrock. Hence these deposits are the most risky foundations for houses in uraniferous and Rn-afflicted areas (A in Fig. 10). Till deposits (B) and bedrock outcrops (C) are generally safer, although the situation depends on the in-situ permeability of the material and on local U mineralization. Impervious
Radiogenic elements in Finnish soils and groundwaters
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clay and silt (D) do not allow Rn to flow through the deposits, because gas penetrates the fine-grained deposits only by diffusion. Water may contribute conspicuously to radiation doses (generally <10%); basement filling or construction material (concrete, bricks) generally play only a minor role. The intensity of the Rn influx into dwellings fluctuates markedly in the course of a single day or year, depending on hydrometeorological con-
ditions (e.g. the amount of rainfall, the related soil moisture contents, and the rising or lowering groundwater table), the pumping rates of groundwater, particularly in drilled bedrock wells, the season (e.g. the occurrence of ground frost and snow cover), indoor and outdoor temperature differences and the heating and ventilation of houses. A rising groundwater table may induce a higher Rn influx into dwellings. In general, Rn concentrations are higher
182
P. Lahermo and R. Juntunen
in w i n t e r t h a n in s u m m e r owing to the confining effect of frost a n d snow cover a n d t o the prevailing u n d e r p r e s s u r e in houses caused by high o u t d o o r i n d o o r t e m p e r a t u r e differences a n d v e n t i l a t i o n ("stuck or suction effect", cf. CASTREN et al., 1987). T h e r e is a clear c o r r e l a t i o n b e t w e e n the regional distribution of U in till and b e d r o c k g r o u n d w a t e r (Figs 1 a n d 5), e x t e r n a l g a m m a dose rate (Fig. 11) and R n c o n c e n t r a t i o n s in a m b i e n t h o u s e air (Fig. 12). O n all these maps, the most a n o m a l o u s areas are in the municipality of Lapinjhrvi, which is located in the granite a n d m i g m a t i t e area of e a s t e r n U u s i m a a province. H e n c e , the geophysical a n d geochemical mapping of soil and n a t u r a l waters for radiogenic elem e n t s a n d c o m p o u n d s offers a valuable tool in the regional p l a n n i n g of residential areas. In some Rnafflicted areas i n t e n d e d for d e t a c h e d or semid e t a c h e d housing, the c o n c e n t r a t i o n s of R n were m e a s u r e d by the F C R N S and Helsinki University of T e c h n o l o g y (VoUTILAINEN et al., 1987). Analysis of the R a c o n c e n t r a t i o n s of till soil p r o v e d feasible, because the c o n c e n t r a t i o n s of b o t h radioactive comp o n e n t s correlate fairly well with each o t h e r . R a d o n has b e e n c o n s i d e r e d a h e a l t h risk if it occurs in a b u n d a n c e in inhaled h o u s e a i r . T h e u p p e r recomm e n d e d R n c o n c e n t r a t i o n s in a m b i e n t h o u s e air were previously 800 B q / m 3 but for new h o u s e s they are currently 200 B q / m 3, values which were e x c e e d e d in 1.4% a n d 11% of the cases, respectively, in 4450 m e a s u r e m e n t s carried out by the F C R N S in the most heavily R n afflicted areas in s o u t h e r n F i n l a n d (CAsTREN et al., 1987). A l t h o u g h excess R n is k n o w n to e x a c e r b a t e the risk of lung cancer, the real causal link has not b e e n e s t a b l i s h e d despite the unusually high radiation activity exposures in parts of s o u t h e r n Finland (the Lapinj~irvi area in particular), even if an e s t i m a t e d 10% to 30% of lung c a n c e r cases (200-600 annually) in F i n l a n d are due to R n exposure. N e i t h e r the statistics of clinical investigations n o r a comparison of c a n c e r incidence maps (cf. ANON, 1987) with those showing the distribution of R n in g r o u n d w a t e r or in h o u s e air suggest a c o r r e l a t i o n b e t w e e n R n e m a n a t i o n s a n d h e a l t h conditions. Editorial handling: L. K. Kauranne.
REFERENCES
ANON (1978) Ministry of National Health and Welfare. Guidelines for Canadian Water Quality. Ottawa, Canada. ANON (1985) Talousveden terveydellisen laadun valvonta. Overvakning av hushSllsvattnets hygieniska kvalitet. LhiikintOhaUituksen Yleiskirje--Medieinalstyrelsens Cirkulhr Nro/nr 1862 (in Finnish). ANON (1987) Suomen SyOpiikartasto, Atlas o f Cancer Incidence in Finland, 1953-1982. Suomen Sy6phrekisteri, Finnish Cancer Registry, Helsinki (in Finnish). ASIKAINEN U. and KAHLOS H. (1979) Anomalously high concentrations of uranium, radium and radon in water
from drilled wells in the Helsinki region. Geochim. Cosmochim. Acta 43, 1681-1686. ASIKAINEN W. (1982) Natural radioactivity of ground water and drinking water in Finland. Instit. Radiat. Protect., Helsinki. STL-A39. BOYLE D. R. (1982) Geochemical Prospecting for Thorium and Uranium Deposits. Elsevier. BLOMQVIST R., LAHERMOP., LAHTINEN R. and HALONEN S.
(1989) Geochemical profiles of deep groundwater in Precambrian bedrock in Finland. In Exploration '89. Proc. Applications of Geophys. and Geochem. Third Decennial Int. Conf. on Geophys. and Geochem. Expor. (ed. G. D. GARLAND).Geol. Surv. Ontario. Spec. Vol. 3, pp. 7746-757. BOLVIKEN B., BERGSTROM J., BJORKLUND A.. KONTIO M., LEHMUSPELTOP., LINDHOLMT., MAGNUSSONJ., OT1 ESEN
R-T., STEENFELTA. and VOLDEN T. (1986) Geochemical Atlas of northern Fennoscandia. Scale 1: 4, 000, 000. Geol. Surv. Finland, Norway and Sweden in cooperation with the Swedish Geological Survey. Supported by the Nordic Council of Ministers. CAS'I'REN0 . , M~KEI,,~INEN 1., W~NQVlSlK and Vou rlLAINEN A. (1987) Indoor radon measurements in Finland: A status report. In Radon and Its Deeay Products (ed. P. K. HOeKE), pp. 97--103 (Chapter 8). American Chemical Society. CULBERTR. R. and LEIGHTOND. G. (1988) Young uranium. In Ore Geology Reviews 3. Special Issue: Unconventional Uranium deposits (ed. J. W. GABELMAN),pp. 313-331). Elsevier. ERS,METSX O. (1968) Malminkadun uraanipitoisesta kaiw)vedesth. Suomen Kemistilehti A41,255-258 (in Finnish). ERAMETSA0 . and YLIRUOKANENI. (1971) Niobium, molybdenum, hafnium, tungsten, thorium and uranium in lichens and mosses. Suomen Kemistilehti B44,372-374. GASCOYNE M. (1989) High levels of uranium and radium in groundwaters at Canada's Underground Research Laboratory, Lac du Bonnet, Manitoba, Canada. Appl. Geochem. 4,577-591. HESS C. T., MICHELJ., nORTONT. R., PRITCHARDH. M. and CONIGLIOW. A. (1985) The occurrence of radioactivity in public water supplies in the United States. Health Phys. 48,553-586. JAAKKOLAT., SUKSIJ., SUUTARINENN. H., RUSKEENIEMIY., SODERHOLM B., VESTERINEN U . , BLOMQVISTR., HALONEN
S. and LINDBERG A. (1989) The behavior of natural radionuclides in and around uranium deposits. 2. Rcsuhs of investigations at the Palmottu analogue study site, SW Finland. Geol. Surv. Finland, Rept. YST-64. JUNTUNEN R. 11990) Etel~i-Suomen kallioporakaivojcn uraani- ja radontutkimukset 1982-86. Summary: The 1_1 and Rn investigations of wells drilled into bedrock in southern Finland. Unpub. manuscript (in Finnish). LAHERMOP., ILMASTI U., JUNTUNEN R and TAKA M. (1991)) The hydrogeochemical mapping of Finnish groundwater. The Geochemical Atlas of" Finhmd, Part 1. Gcol. Surv. Finland. LEMMEL~, H. (1984) The Survey of" External Background Radiation in Finland. Rept. STUK-B-VALO-32. Finnish Centre for Radiation and Nuclear Safety, Helsinki (in Finnish). NIINI n . , RUSKEENIEMI T., SODERHOLMB., VESTERINEN M., BLOMQVIST R., AHONEN L., HALONEN S., LINDBERG A., JAAKKOLAT., SUKS! J. and SUUTARINENU. (19911) Radio-
nuklidien k~iyttfiyytyminen uraaniesiint ymfi-ymp~iristOss~i. 3. Tuloksia Palmotun analogia-tutkimuksesta vuonna 1989. Abstract: The behavior of natural radionuclides in and around uranium deposits. 3. Results of the investigations of the Palmottu analogue study in 1989. Geol. Surv. Finland. Nuclear Waste Disposal Res. Rept. YST-71. NURMI P., KUKKONENI. and LAHERMOP. (1988) Geochem-
Radiogenic elements in Finnish soils and groundwaters istry and origin of saline groundwaters in the Fennoscandian Shield. Appl. Geochem. 3, 185-203. PAQUETrE J. and LEMIRE R. J. (1981) A description of the chemistry of aqueous solutions of uranium and plutonium to 200°C using potential-pH diagrams. Nucl. Sci. Engin. 79, 26-48. ROGERSJ. J. W. and ADAMS,J. A. S. (1969a) Thorium, 90B900. In Handbook of Geochemistry (ed: K. H. WEDEeOHL), Vol. I1/5. Springer 1978. ROGERS J. J. W. and ADAMS J, A. S. (1969b) Uranium, 92B920. In Handbook of Geochemistry (ed. K. H. WEDEpone), Vol. II/5. Springer 1978. SALONENL. (1987) Natural radionuclides in ground water in Finland. Fourth Int. Symp. on the Natural Radiation in the Environment. 7-11 Dec. 1987. Lisbon, Portugal. S1MONENA. (198(/) The Precambrian in Finland. Bull. Geol. Surv. Finland, 304. VOUaqLAINENA., HECtN M., TORPPAT., HOLKKOJ., LEHTOVIITA T., VANHANEN, A. and VILJANEN M. (1987) Rado-
183
nin merkitys talonrakennustekniikassa. Moreenipohjaisen pientaloalueen radontitkimus. Abstract: Radon in building technology. Radon research of till soil housing area. Helsinki Univ. Technol. Fac. Cir. Eng. Surv. Rept. 97. WICKSTROM P. and LAMP~N P. (1986) Lavian koereik~i-yhteenveto pohjavesitutkimuksista 1984-1985. English summary: The Lavia test hole--summary of the groundwater studies 1984-1986. Nuclear Waste Comm. of Finnish Power Gompanies. Rep. YJT-86-11. WILSON M. R. and AKERBLOM G. V. (1982) Geological setting and geochemistry of uranium-rich granites in the Proterozoic of Sweden. Min. Mag. 46, 233-245. Yu-KvvNY K. (1989) Nuoret uraanirikastumat: Esitutkimuksen tulokset. Geol. Surv. Finland, Rept. M19/2732/89/1/60 (in Finnish). YLIRUOKANEN I. (1975) Uranium, thorium, lanthanoids and yttrium in some plants growing on granitic and radioactive rocks. Bull, Geol. Soc. Finland 47, 71-78.
0883--2927/91 $3.00 + .00 O 1991 Pergamon Press plc
Printed in Great Britain.
Radiogenic elements in Finnish soils and groundwaters P. LAHERMO a n d R. JUNTUNEN Geological Survey of Finland, SF-02150Espoo, Finland (Received 19 June 1990; accepted in revised form 15 November 1990) Abstract--In Finland, U and Rn concentrations in groundwaters are highest in the south of the country in
bedrock composed of granites or migmatites with abundant granite or pegmatite veins. The areal distribution patterns of U and Rn in groundwater from bedrock are consistent with U and Th anomalies in the fine fraction of till and with the external gamma dose rate measured on the ground surface. The U and Rn concentrations in groundwater from bedrock are only moderately correlated; hence the elevated U contents do not imply abundant Rn contents. Both elements are practically independent of other dissolved components. The exception is the moderate correlation between U and HCO3 suggesting the occurrence of U-rich carbonates as fracture-coating minerals. In a few areas with U and Rn anomalies, the exceptionally high Rn concentrations in ambient house air pose a considerable health risk, particularly in detached houses founded on pervious sand and gravel deposits. Regardless of the potential health risk no radiological recommendations for U and Rn concentrations in potable water have been officially set in Finland.
INTRODUCTION
ments were made using the standard addition method and a Perkin-Elmer 204 fluorescence spectrophoTHE NATIONWIDE hydrogeochemical mapping of tometer. Later, a direct-reading Perkin-Elmer LS-2 groundwaters carried out by the Geological Survey of B Filter Fiuorimeter device was used without the Finland (GSF) included 3954 U determinations addition method. Radon was analyzed in the FCRNS (LAHERMO et al., 1990). Several thousand Rn deter- by mixing a 10ml water sample with 12 ml of a minations were made by the GSF and the Finnish commercial scintillation cocktail Luma-Gel, in glass Centre for Radiation and Nuclear Safety (FCRNS), vials. The samples were then counted with a Wallac mainly in the south of the country, where the concen- Ultro-Beta 1210 liquid scintillation counter. trations are known to be the highest (ASIKAINEN, The following emphasizes the regional data on U 1982; SALONEN,1987; JUNTUNEN, 1990). The U and and Rn in groundwater, which are compared with the Rn investigations were prompted by the idea that the concentrations of U and Th in soil. The U-isotope elements could be used as pathfinders in U explor- ratios in water and the geomedical aspects of Rn are ation. Later, however, the Rn mapping was intensi- also briefly discussed. fied due to health concerns caused by the unusually high Rn concentrations in groundwater from bedrock wells and house air in some parts of the country. The Nordkalott Project carried out jointly by the OCCURRENCE OF RADIOGENIC ELEMENTS IN Geological Surveys of Finland, Norway and Sweden, ROCKS, TILL AND WATERS analyzed U and Th in the fine fraction (<62/zm) of till, heavy minerals of till (Th only), organic matter of Information about the U concentrations of various stream sediments, and stream moss by XRF and rock types in Finland is scant. It is agreed, however, N A A methods (BOLVIKENet al., 1986). For the Geo- that in the Fennoscandian Shield, in Finland and chemical Atlas of Finland, the clay and silt fractions Sweden, the Proterozoic highly differentiated leuco(<64/~m) of 1054 composite till samples were ana- cratic alkali-granites with relatively low Ca are the lyzed for U and Th as partial concentrations from most uraniferous rocks (WILSON and A,KERBLOM, aqua regia leach by ICP and as total concentrations 1982). Uranium enrichment occurs in rocks of all by NAA. These surveys provide ample regional ages in a variety of geological settings. In Sweden, the material for the evaluation of U cycling in surficial range and average value of U concentrations in 174 deposits and natural waters. granites were 7-24ppm and 12 ppm, respectively Total U and Th concentrations from the fine frac- (WILSON and AKERBLOM,1982). The radioactive gration of till were determined by the N A A method in nites are often enriched in one or more of F, Mo, Sn the Reactor Laboratory of the Technical Research and W. Centre of Finland. Uranium was analyzed from water The highest total U concentrations (4-8 ppm) in by the GSF. The filtered (0.45/~m) and acidified (1 ml the fine fraction of till are found in areas underlain by 10% H2SO 4 into 500 ml of sampling water) samples granites to the north and north-west of the Wiborg were enriched by ion-exchange, and the measure- rapakivi granite batholith and in the north-western 169
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and western parts of the pluton itself (Fig. 1, cf. Fig. 6). Granites, migmatites intermingled with granite veins, and granodiorites in southern and southwestern Finland also show elevated U concentrations. In the part of northern Finland studied by the Nordkalott Project, the anomalous U concen-
trations (3-5 ppm) are encountered in till underlain by granites (BoLVZKEN et al., 1986). This is in good agreement with the well-known fact that the U concentrations in plutonic rocks increase from ultramafic toward felsic rocks ranging from 1-2 to 3-5 ppm, respectively (ROGERS and ADAMS, 1969b).
Radiogenic elements in Finnish soils and groundwaters The geochemistry of U is intimately associated with that of Th. Both elements show virtually t h e same interdependence with bedrock composition. The Th concentrations in rocks and till are generally higher than those of U. In 174 Swedish radioactive granites the range and average Th concentrations were 34-61 ppm and in 41 ppm, respectively (WILsoN and fikKERBLOM,1982). The corresponding U/TH ratio ranges from 0.1 to 0.4. In rapakivi granite and ordinary granite areas in Finland, partial Th concentrations (analyzed from A R leach) and total Th concentrations in the fine fraction of till are 10-20ppm and 10-30ppm, respectively (Fig. 2). Owing to its low mobility, Th tends to remain immobile whereas U enters into solution, a reason for the partial separation of both elements in weathering and the higher Th concentrations remained in till (cf. ROGERS and ADAMS, 1969a). The U/Th ratio in till, ranging typically from 0.3 to 0.4 (Fig. 3), is of the same order of magnitude, or slightly higher, than that in granites. In general, high U concentrations also mean high Th concentrations in till. At moderate or low U concentrations the variation in the U/Th ratio seems to be greater. The ratio is lowest (from 0.2 to 0.3) in the areas composed of granulites and granitic basement gneisses in northern and eastern Finland where the U concentrations in till are lowest. The areal distributons of the U and Th values follow roughly the same pattern in organic stream sediments as in till, but are more varied in other geochemical materials such as stream moss. Uranium is slightly soluble, occurring in water mainly as uranyl-ion or UO~-, which is mobile in an oxidizing environment. The key control on dissolved U concentrations appears to be redox potential (GAsCOYNE, 1989). Concentrations of U in rock, and the residence time and composition of groundwater seem to be less important. Colloidal forms and organic complexes of U are insignificant (GAsCOYNE, 1989). In the pH range 8-10, which is occasionally met in deep groundwater in crystalline bedrock (cf. NURMl et al., 1988; BLOMQV1STet al., 1989) U may occur as carbonate complexes. At a lower pH, phosphate complexes may occur (cf. GASCOYNE, 1989). The availability of dissolved CO~ enhances the dissolution of U. The maximum U solubility in groundwater from a typical granite under CO2-rich, oxidizing conditions is - 1 g/l (PAQUETTEand LEMIRE,1981). In Finland the highest recorded (ER,g,METS.g,, 1968; ASIKAINEN,1982; JUNTUNEN,1990) concentrations in bedrock groundwater are 13mg/l, 15mg/1 and 20 mg/1, respectively. Uranium concentrations in the groundwaters of the overburden and surface waters containing small amounts of dissolved solids are very low (<0.5 or 1.0/~g/l) except in some areas with local, mostly tiny, secondary U mineralization. As an example, a spring draining water from a weathered uraniferous granite in Sodankyl/i, northern Finland, shows a U concentration of 380/~g/1; in water dis-
171
charged into a nearby exploration pit excavated in till the value is 530/~g/1 (PEURANIEMI,written commun.). The hydrogeochemical mapping of Finnish groundwaters carried out by the GSF shows that the median U concentrations in spring waters and in waters from wells dug into overburden or drilled into bedrock are all < 1 ~g/l (LAnERMO et al., 1990). The arithmetic means are somewhat higher because of occasional higher concentrations; 1.4/~g/l (1294 samples), 1.6/~g/1 (3553 samples) and 21.2/~g/l (1027 samples), respectively. In the somewhat larger population of bedrock wells (1388 samples) covering southern Finland only (includes partly the same material), the median and arithmetic means are 5 and 73/~g/l, respectively (JuNTUNEN, 1990). The few elevated U concentrations in water from springs and dug wells occur mainly in the southern part of the country, which is largely composed of granites and migmatites (Fig. 4). This regional pattern is even more conspicuous in more U-rich bedrock groundwaters (Fig. 5). In areas of uraniferous granites, the U concentrations in bedrock groundwater are often of the order of 100-100011g/l, whereas the highest values recorded in Finland are in the range 15-20 mg/l (ASIKAINEN, 1982; JUNTUNEN, 1990). The 14 highest concentrations of U in groundwater from 308 drilled wells in the Helsinki area, which is mainly composed of migmatites with abundant granite veins, are in the range 1-14 mg/l (AsIKAINEN, 1982). The U concentrations in groundwater from bedrock and the total U concentrations in the fine fraction of till show a fairly good correlation (cf. Figs 1 and 5). Uranium has an affinity to organic material, which in Finland is manifested by the high secondary U concentrations in the marginal and bottom layers of peat deposits. Uranium is loosely bound to peat and is usually much more abundant than Th. Hence, the U/Th ratio in peat is often quite high (see BOYLE, 1982; ER~METS~ and YLIRUOKANEN, 1971; YLIRUOKANEN, 1975). YL1-KYNY (1989) reported low values of U in water-rich, weakly humified peat. When the peat is rich in Th, the respective U concentrations are generally low (YLI-KvvNY, 1989). The U deposition is enhanced by the reducing conditions typical of water-saturated; strongly humified peat. Although spring waters and seeping groundwaters in discharge zones, which are the sources of secondary U enrichment, usually exhibit very low U concentrations, over time appreciable U concentrations accumulate close to groundwater discharges. Also, fracture zones in bedrock with abundant circulating groundwater favor the accumulation of U. Because of the higher mobility of the light 234U isotope compared with 23SU, a state of disequilibrium prevails between the isotopes 234Uand 238Uin natural waters (see e.g. HESS et al., 1985). The 234U/238U activity ratio is generally >1.0, reaching up to 3.0 (AS!KAINEN, 1982). In deep-seated fresh bedrock groundwater collected from a 1 km-deep experimen-
172
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tic dykes in gneisses and migmatites at Palmottu, Pusula, in southern Finland, ranges from 1.2 to 3.7, increasing down the water column ( J A A K K O L A et al., NIIN] et al., 1990). In the deepest drill hole (1175 m) in Finland, located in Outokumpu, eastern Finland,
173
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which cuts mica gneisses, black schists and, in the lower part of the hole, some sizeable serpentinite bodies, the disequilibrium ratios in the stratified fresh-saline water column range from 1.2 to 1.8. T h e r e is no systematic trend in the ratio as a function of the depth of occurrence and degree of mineralization of the groundwater (cf. NURMI et al., 1988).
According to GASCOYNE (1989), high 2 3 4 U / 2 3 8 U activity ral~ios in groundwater and a low ratio in altered rocks indicate active and recent leaching of U by groundwater. R a d o n is generally the most abundant dissolved radioactive c o m p o n e n t of Finnish groundwaters. The Rn contents in dilute surface waters and shallow
174
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groundwaters are mostly < 100 Bq/l. In a survey conducted by the FCRNS, the arithmetic mean of 961 samples from springs and dug wells was 92 Bq/l, and that of 2065 samples from drilled bedrock wells 1020 Bq/1) (SALONEN, 1987). The comparatively high Rn concentrations in the bedrock groundwater of this
set of data are partly due to the fact that most of the samples were collected from the anomalous southern part of the country. In a study carried out by the GSF, which also focused on southern Finland, the median and arithmetic mean values of Rn concentrations of 1050 bedrock well samples were 215 and 631 Bq/1,
Radiogenic elements in Finnish soils and groundwaters
175
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respectively (JUNTUNEN,1990).
Some 20 samples gave concentrations >5000 Bq/1, the maximum being 24,000 Bq/1. The highest Rn concentrations were found in groundwater from granites and migmatites rich in granite veins to the north and north-west of the Wiborg rapakivi granite batholith in south-eastern
Finland (median 349 Bq/l, 298 samples); they were slightly lower in the rapakivi granite area proper (Fig. 6). The bedrock groundwater in granodiorite areas also exhibited somewhat elevated Rn concentrations, whereas the groundwater in areas composed of mica gneisses and phyllites, migmatitic gneisses, amphibolites, quartz-feldspar schists and sandstones
176
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-1000
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FIG. 7. Uranium and Rn concentrations in bedrock groundwater (as arithmetic means) grouped by rock type. The columns are arranged in the order of decreasing U concentrations shown by the left hand columns. (N) Refers to the number of analyses. Data according to JUNTtJNEN(1990). showed lower concentrations, diminishing in this order. As shown by Fig. 7, Rn correlates only moderately with U concentrations (r = 0.27), and there is practically no correlation with any other dissolved components except the H C O 3 and N O 3 concentrations. The correlation of U with H C O 3 alkalinity suggests that dissolved U is causally linked with U-rich carbonates occurring as fracture minerals. This is supported by the study on the Palmottu U - T h deposit in
southern Finland, where the bulk of the U was found as loosely bound forms in calcitic fracture coatings (NUN] et al., 1990). The dating of the calcite coatings yielded ages between 80 and >300 Ka. The correlation of U with the N O 3 concentrations refer to contamination by human and animal sewage, cattle dung and artificial fertilizers, etc., suggesting that U mobilization may be p r o m o t e d by biogenic compounds dissolved in water. The high U concentrations in water do not in-
Radiogenic elements in Finnish soils and groundwaters
70] l
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Rn 10 la 34 04 120 230 420790160027005100 U 2 3.3 9.2 20 42 90 194 410 890 19104100 p,g/'l
FIG. 8. The frequency distribution of U and Rn concentrations in water from bedrock wells (1388 and 1050 samples, respectively). The scale is logarithmic with the base/2.
177
that of Rn. In one case the U concentration of 20,000/~g/1 measured in a bedrock well was two orders of magnitude lower when remeasured. The distribution of Ra in water is a function of the U and Th contents and texture of the aquifer material, the flow conditions and residence time of groundwater and the half-lives of the respective isotopes (cf. Hess etal., 1985). The Ra concentrations in groundwater are largely controlled by its strong tendency to become adsorbed by the mineral matter. Radium concentrations are, on average, lower than Rn concentrations by four orders of magnitude, and their correlations with dissolved U concentrations are negligible (AsIKAINEN, 1982). Uranium concentrations are generally higher, as implied by the 238U/226Ra ratio, which ranges from 1 to 20 (HEss et al., 1985).
GEOMEDICAL ASPECTS variably mean high Rn activities. This finding is understandable, because extremely mobile, gaseous 222Rn is very short-lived (3.82 d) compared with its immediate predecessor in the same radioactive decay series, 226Ra (1620 a), and the parent of the series, 23~U, and its daughter nuclides 234U and 23°Th, whose half-lives from 475 Ma to 80 Ka (see e.g. ASIKAINEN, 1982; HESS et al,, 1985). It is therefore not surprising that there is a discrepancy between U and Rn values in waters of the Wiborg rapakivi granite area, where conspicuous anomalous areas of dissolved U do not show up as elevated Rn concentrations in groundwater (cf. Fig. 6). It is noteworthy that anomalous U and Rn concentrations in groundwater, and to a lesser extent in till, constitute a distinct aureole-type zone coinciding with the granite areas surrounding the Wiborg rapakivi granite batholith in southeastern Finland (Fig. 7). Elsewhere in granite environments U concentrations tend to be conspicuously lower. The difference is not so conspicuous in the U concentrations of till possibly due to the glacial transport of drift from northwest to southeast. Figure 8 shows the frequency distribution of U and Rn concentrations in bedrock groundwater throughout Finland. The distribution of Rn concentrations is almost symmetrical, whereas that of U is strongly skewed and truncated because the values below or slightly above the detection limit constitute a large part of the sample. The spatial fluctuations of U and Rn concentrations are large. Even in adjacent wells the concentrations may differ by several orders of magnitudes. They are also strongly time-dependent. Fifteen bedrock wells were sampled twice at an interval of 8 months (JUNTUNEN, 1990). The U and Rn concentrations either increased or decreased from the values measured earlier. In places the U and Rn concentrations changed reciprocally. The amplitude of fluctuations in the U concentrations was larger than in
Apart from their economic aspects, U enrichments in peat bogs may pose an environmental threat when peat is utilized as fuel. Further, the oxidation of peat deposits due to land reclamation measures, such as ditching of peat bogs, may mobilize U into surface waters and groundwaters. The mobilization may also be triggered by acid rain (cf. CULBERTand LEIGHTON, 1988). In Finland no upper permissible limits of U, Ra and Rn have been set for drinking water. The directive concerning water quality standards issued by the National Board of Health states that the total alphaand beta-radiation level in natural waters should not generally exceed 0.1 Bq/1 and 0.8 Bq/l, respectively (AnoN, 1985). Before a new water utility is joined to the public water supply network, it must be studied to ensure that the water does not contain exceptionally high levels of radioactive elements. With respect to radiation in general, if the gross radioactivity of tap water >8000 Bq/1 (Rn not included) the measurement of Rn in ambient house air is recommended. The permissible U concentration in drinking water has not been set. In Canada, where geological conditions in crystalline areas are generally similar to those in Finland, the upper permissible U concentration has been set as low as 20pg/1 (Anon, 1978). Only 2% of the 1367 investigated springs in Finland exceeded the 5 pg/l U concentration level, and only 10 samples were >10pg/1. In groundwater from wells, 2% of 1367 cases exceeded 10/~g/1 and 19 cases were higher than the upper Canadian limit of 20pg/1 U. Water from bedrock wells exhibited the highest U concentrations: 15% of the 2385 sites investigated exceeded 20 pg/l and 5% exceeded 100 pg/1 (Fig. 9). The maximum acceptable concentration of Ra in drinking water in Canada is 1 Bq/1 (AnoN, 1978). An investigation of radiological properties of bedrock groundwater in the Helsinki region revealed that the median, arithmetic mean and maximum values in Ra
178
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of 50 samples were 0.06 Bq/1, 0.3 Bq/1 and 9.5 Bq/l, respectively (ASIKAINEN and KAnLOS, 1979; ASlgAINEN, 1982). In this area 22% of the wells exceeded the maximum concentration acceptable in Canada. The Helsinki region is anomalous due to the high U, Rn and Ra concentrations in water of bedrock wells,
the central and northern parts of the country showing much lower concentrations. M o r e than one fifth of the rural population of Finland, or about 1-1.2 million people, use their own drinking water sources. An estimated 5-20% of the private water intakes are from bedrock wells, where
Radiogenic elements in Finnish soils and groundwaters
179
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excessive U concentrations are common. They are particularly frequent in some granite areas in southern Finland, where high U and Rn concentrations in groundwater constitute a notable health problem. Nonetheless, the main pathway of radioactive radiation to man is through Rn released into the air. Glaciofluvial formations such as eskers and esker deltas and the Salpausselk~i ice-marginal formations,
which are largely composed of pervious sand and gravel, act as conduits and reservoirs for Rn gas emanating from deeper fracture systems in the bedrock. Hence these deposits are the most risky foundations for houses in uraniferous and Rn-afflicted areas (A in Fig. 10). Till deposits (B) and bedrock outcrops (C) are generally safer, although the situation depends on the in-situ permeability of the material and on local U mineralization. Impervious
Radiogenic elements in Finnish soils and groundwaters
181
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Fl6. 12. Radon concentrations in ambient house air. The size of the symbol indicates the local geometric mean of -14,000 measurements in detached and semi-detached houses computed for 284 communities. Measured by the Finnish Centre for Radiation and Nuclear Safety (from CAST~E~e! al., ]987).
clay and silt (D) do not allow Rn to flow through the deposits, because gas penetrates the fine-grained deposits only by diffusion. Water may contribute conspicuously to radiation doses (generally <10%); basement filling or construction material (concrete, bricks) generally play only a minor role. The intensity of the Rn influx into dwellings fluctuates markedly in the course of a single day or year, depending on hydrometeorological con-
ditions (e.g. the amount of rainfall, the related soil moisture contents, and the rising or lowering groundwater table), the pumping rates of groundwater, particularly in drilled bedrock wells, the season (e.g. the occurrence of ground frost and snow cover), indoor and outdoor temperature differences and the heating and ventilation of houses. A rising groundwater table may induce a higher Rn influx into dwellings. In general, Rn concentrations are higher
182
P. Lahermo and R. Juntunen
in w i n t e r t h a n in s u m m e r owing to the confining effect of frost a n d snow cover a n d t o the prevailing u n d e r p r e s s u r e in houses caused by high o u t d o o r i n d o o r t e m p e r a t u r e differences a n d v e n t i l a t i o n ("stuck or suction effect", cf. CASTREN et al., 1987). T h e r e is a clear c o r r e l a t i o n b e t w e e n the regional distribution of U in till and b e d r o c k g r o u n d w a t e r (Figs 1 a n d 5), e x t e r n a l g a m m a dose rate (Fig. 11) and R n c o n c e n t r a t i o n s in a m b i e n t h o u s e air (Fig. 12). O n all these maps, the most a n o m a l o u s areas are in the municipality of Lapinjhrvi, which is located in the granite a n d m i g m a t i t e area of e a s t e r n U u s i m a a province. H e n c e , the geophysical a n d geochemical mapping of soil and n a t u r a l waters for radiogenic elem e n t s a n d c o m p o u n d s offers a valuable tool in the regional p l a n n i n g of residential areas. In some Rnafflicted areas i n t e n d e d for d e t a c h e d or semid e t a c h e d housing, the c o n c e n t r a t i o n s of R n were m e a s u r e d by the F C R N S and Helsinki University of T e c h n o l o g y (VoUTILAINEN et al., 1987). Analysis of the R a c o n c e n t r a t i o n s of till soil p r o v e d feasible, because the c o n c e n t r a t i o n s of b o t h radioactive comp o n e n t s correlate fairly well with each o t h e r . R a d o n has b e e n c o n s i d e r e d a h e a l t h risk if it occurs in a b u n d a n c e in inhaled h o u s e a i r . T h e u p p e r recomm e n d e d R n c o n c e n t r a t i o n s in a m b i e n t h o u s e air were previously 800 B q / m 3 but for new h o u s e s they are currently 200 B q / m 3, values which were e x c e e d e d in 1.4% a n d 11% of the cases, respectively, in 4450 m e a s u r e m e n t s carried out by the F C R N S in the most heavily R n afflicted areas in s o u t h e r n F i n l a n d (CAsTREN et al., 1987). A l t h o u g h excess R n is k n o w n to e x a c e r b a t e the risk of lung cancer, the real causal link has not b e e n e s t a b l i s h e d despite the unusually high radiation activity exposures in parts of s o u t h e r n Finland (the Lapinj~irvi area in particular), even if an e s t i m a t e d 10% to 30% of lung c a n c e r cases (200-600 annually) in F i n l a n d are due to R n exposure. N e i t h e r the statistics of clinical investigations n o r a comparison of c a n c e r incidence maps (cf. ANON, 1987) with those showing the distribution of R n in g r o u n d w a t e r or in h o u s e air suggest a c o r r e l a t i o n b e t w e e n R n e m a n a t i o n s a n d h e a l t h conditions. Editorial handling: L. K. Kauranne.
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