The geochemistry of selenium in groundwaters in Finland

The Science of the Total Environment 162 (1995) 93-103

The geochemistry of selenium in groundwaters in Finland Georg Alfthan” a, Dacheng Wang”, Antti Area, Jouko Soverib aDepanment of Nutrition, ‘National Board

National Public Health Institute, Mannerheimintie 166, FIN-00300 of Waters and the Entironment, P.O. Box 250, FIN-00101 Helsinki,

Helsinki, Finland

Finland

Received 16 December 1993; accepted 27 July 1994

Abstract The backgroundlevel, seasonalvariation, speciation,and geographicaldistribution of selenium(Se) were studied in dug and drilled wells and in natural groundwatersin Finland. The total Se concentration of well water samples (n = 256) ranged from 4.1 to 2720rig/l, with a medianof 68.4 rig/l. The medianSe concentration of groundwater from locationsin natural surroundingswas51.5 rig/l (n = 41) and 50.5rig/l (n = 39) in 1989and 1990,respectively. The seasonhad only a minor effect on both well and groundwater Se concentrations. Selenatewas the most abundant speciesin groundwater.Locally, high concentrationsof groundwater Se may originate f’rom bedrock. The proportions of both selenite and Se associatedwith humic substanceswere lessthan 8 and 15%, respectively. The median Se concentration of 37 infiltration water sampleswas 28.6 rig/l (range 6.1-137 rig/l). The annual net depositionof Se from precipitation into soilswas estimatedto be 0.49 g/ha in the southern and 0.33 g/ha in the northern parts of Finland. Se from precipitation had no impact on groundwater Se. The effect of agricultural activities including Se supplementedfertilization on the Se concentration of groundwaterin cultivated areascould not be demonstrated. Keywords: Selenium;Seleniumspecies;Groundwater; Geochemistry

1. Introduction Mortality from cardiovascular diseasesand cancer have been associated with a low serum Se concentration in Finland [1,2]. In order to in-

crease the dietary Se intake of the entire population, sodium selenate has been added to fertilizers and used nationwide since 1985 [3,4]. The

*Corresponding 004&9697/95/$09.50 SSDI

author.

serum Se concentration of Finns has consequently improved remarkably [5]. However, being both an essential and toxic element, addition of Se to fertilizers could result in adverse effects on the environment. Even without addition of Se, the use of fertilizers still has an impact on the Se concentration and mobilization in groundwater [6,71. Nitrate, phosphate and sulphate may increase the solubility of native and appiied Se in the soil [8], promoting leaching of Se into groundwaters.

0 1995 Elsevier Science BV. All rights reserved.

0048-9697(95)04436-Q

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G. Alfthan et al. / Sci. Total En hon. 162 (1995) 93-l 03

There is evidence that soluble Se, mainly selenate, is being leached from the geological substrata and transported to the groundwater by irrigation water in Western San Joaquin Valley of California [9]. Se contamination of farmstead wells in Kansas, USA has been reported by Steichen et al. [lo]. The high Se levels were likely due to naturally occurring soil and rock formations, such as exposed Cretaceous shales. Rain and snow contain significant amounts of Se [ll-131, and are important sources of soil Se [14,15]. High sulphate concentrations and low pH in meltwater and precipitation can change the chemistry of the lower mineral soil and groundwater due to increased ion exchange and weathering [16]. Acid precipitation also has an impact on groundwater Se. Laboratory experiments have shown that a moderate increase in acidity of certain soil types actually decreases Se mobilization by metal hydrous oxides and also enhances reduction of selenite to the elemental form [17]. Groundwater constituted 51% of the municipal mains water in Finland in 1988, and 7% of households used well water as a source of drinking water [16]. Except for a preliminary study with limited data on groundwater Se in Finland by the authors [18], there is no information on groundwater Se available. The aims of this study were to survey the background level of groundwater Se concentration in well water and in groundwater observation stations located in agriculturally non-affected areas, to study the relationships between Se and other chemical components, and to evaluate the impact of precipitation and human activities on the concentration of Se in groundwaters. 2. Experimental 2.1. Sampling of well waters The well water samples were part of a larger study comprising 1421 well water samples collected throughout Finland and analyzed for 17 elements and chemical parameters. Details of the selection of wells and sampling are described elsewhere [19]. Briefly, in the autumn of 1990,256 well water samples from stone-lined, concrete ring, drilled wells and springs were taken, one

sample per municipality out of a total 464 municipalities in Finland. From these 256 wells, 74 samples were obtained in the spring and summer of 1991 selected to represent geological formations and land use in six areas into which the country was divided. Water samples were taken into 250-ml acidwashed polyethylene (PE) bottles. If the water was led to the household by a pipeline, the sample was taken after letting the water run for at least five minutes. 2.2. Sampling of groundwater and in$ltration waters The groundwater observation stations were located in areas where the groundwater quality has not been considerably affected by local disturbances. The stations were established in regions with different types of soil and climatological conditions, and the groundwater basin is well confined at each station. The size of the catchments investigated varied between 0.2 and 3.0 km2. The area of agricultural land totals only 4% of the catchment areas [20]. Groundwater samples were taken from the permanent observation stations either from a spring or from PVC sampling tubes fixed in the soil. Samples from springs were taken directly in PE bottles and from tubes they were taken by pumps when the water was clear. Infiltration water samples were collected by lysimeters. Lysimeters are galvanized iron containers coated with plastic with a diameter of 1596 mm and depth of 1700 mm. They were placed vertically in the soil profile. Both the soil type and surface vegetation in the containers correspond to those prevailing in the area. Samples were collected directly in PE bottles. Rain samples were collected into a plastic bag supported by a cardboard box with an opening of 25 x 35 cm. The snow samples were collected into a plastic bag and melted at room temperature. Rain and snow samples were filtered through a 0.4 -pm Nuclepore polycarbonate filter into PE bottles. 2.3. Storage of water samples All water samples were stored at +4”C in PE bottles without acid addition. At Se concentra-

G. A&than et al. /Sci. Total En&on. 162 (1995) 93-103

tions ranging from 45-138 rig/l, groundwaters can be stored at +4” or 20°C for up to 13 months without significant losses of Se, and snow melt water can be stored for up to 15 days at +4”C [21]. Well water samples were stored for less than 2 months before being analyzed for Se. Groundwater, infiltration water, rain and snow melt water were stored for less than 2 weeks. The Se speciation analyses were performed within 5 days after having been collected. 2.4. AnaJysysis The total Se concentration of all water samples was analyzed in duplicate by a fluorometric method after preconcentration by evaporation [22]. The precision of the method for natural water samples was 2.1 CV%. The detection limit of the method (blank + 2S.D.) was 0.35 ng Se corresponding to 7 rig/l Se in a 50 ml water sample. Reference water samples with certified or recommended Se concentrations at rig/l levels were not available, therefore the accuracy of the method was verified by recovery tests. The mean recovery of selenite to river water was 99.9% (n = 201. The within-series precision of the method for river, tap and snow melt water with total Se concentrations from 28-115 rig/l were 1.5-2.1% (n = 81, and the between-series for river, tap, ground and snow melt water with total Se concentrations from 44-137 rig/l were O-8-3.2% (n = S-20, 4-10 series). Selenite and selenate in water samples were separated through a Dowex AG2-X8 column, and humic substances were separated by an XAD-8 column. The eluate was preconcentrated and Se measured as total. The detection limit for these Se species was lower than 1.0 ng [22]. Color, pH, turbidity, alkalinity, KMnO, consumption of organic matter, F, NO,, NO,, Al, NH,, Cl, Mn, Fe, SO.,, K and Na of a separate aliquote of well water samples were determined by Vesi-Hydro Ltd and Suunnittelukeskus Ltd, Helsinki. Turbidity, alkalinity, pH, NO,, NH,, PO,, Cl, Mn, Al, Fe, F, K, Ca, Cu, Pb, Mg, Na, Zn, SO,, Cd, Ni and Hg of groundwater samples were analyzed by the laboratories of the Water and the Environment Districts or Institute of the National Board of Waters and the Environment,

95

using standard methods published by the Finnish Standardization Association. 2.5. Statistical analysis Differences between means were calculated by the Student’s t-test if normally distributed, otherwise using the Median Test. Spearman correlation was used for the chemical parameters and analysis of variance to calculate trend in relation between Se and pH @AS). 3. Results and discussion 3.1. Background concentration of Se in waters The Se concentrations of well water, groundwater and infiltration water samples are shown in Table 1. The median Se concentration of well waters was of the same level as that found in surface waters in Finland [23,24]. The distribution frequency of the Se concentration in well waters sampled in 1990 was skewed to the right (Fig. 1). Of the 256 wells, 75% (192) had an Se concentration less than 168 rig/l. Only 10% (26) exceeded 326 rig/l. No samples exceeded the health-based limit of 10 pg/l set for drinking waters [25]. The Se concentration of groundwater samples was lower than that of well waters (P < 0.05) (Table 1). The groundwater observation stations are background stations in a nearly natural state, but the groundwater in rural wells might be influenced by various human activities, resulting in higher Se concentrations. Very little reliable data on the Se concentration of groundwaters is available from other countries except from endemic Se areas in USA. The Se concentrations of groundwaters in USA are considerably higher than the Finnish groundwaters. The Se concentration in samples from nine of 103 farmstead wells in Kansas, USA, a high Se area, exceeded the 10 kg/l limit, due to naturally occurring soil and rock formations [lo]. In 151 irrigation and stock wells in the southern coast range of California, the 10 pg/l limit was exceeded by 26 wells [26]. In a survey of Finnish tap waters in the winter of 1990 [18] a high level of Se, 750 rig/l, was found in a sample from the town of Hameenlinna (population 43,098) in southern Finland. Determi-

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G. Alfthan et al. /Sk

Total Entiron. 162 (1995) 93-103

Table 1 Selenium background in well water, groundwater and infiltration water Sample

Date (month.year)

Well water Well water Well water Groundwater Groundwater Infiltration

g-10.90 3-4.91 7-8.91 11.89-3.90 10-12.90 3-6.92

n

Mean (rig/O

SD.

Median (rig/l)

Range (rig/l)

256 74 74 41 39 37

153 132 120 91.2 83.2 38.8

250 244 195 116 103 31.4

68.4 60.3 64.8 51.5 50.5 28.6

4.1-2720 3.8-1920 5.0-1470 9.3-588 8.2-499 6.1-137

nation of Se on new samples (May 1990) of untreated groundwater, groundwater (man-made) originating from. lake water passed through a morenic ridge, and treated tap water confirmed the first result (Table 2). Suspicion arose over an 60 n-255

abandoned town dump being the source of the high Se, but determination of Cr, Ni, Cu, As and Cd, indicators of industrial waste, showed only natural levels (Joutti, A., personal communication). Further samples were obtained in 1992 from the raw water source Lake Alajarvi, during passage of the water through a morenic ridge and finally the man-made groundwater well, (Table 21, from which the final drinking water was purified. Seven additional well water samples from Himeenlinna that were included in the main study were analyzed. The mean Se concentration of these eight samples was 270 rig/l, range 89-533 rig/l. The results together suggest that the high Se concentration originates in the bedrock, because obvious anthropogenic sources of Se other than the dump have not been identified. Black schists have been reported to contain as high levels as 37 mg Se/kg in Finland [27]. The distribution of these schists may cause very locally elevated Se concentrations in groundwater such as suggested in the town of HImeenlinna in the present study. 3.2. Seasonal miation

0

300

600

900

1200

Total Se (rig/l) Fig. 1. The distribution frequency of selenium concentrations in well water. One sample with selenium concentration of 2720 rig/l not shown.

There was no significant differences between the mean Se concentrations of well water samples obtained in the autumn compared with the spring or summer samples. The high correlation (n = 74, r = 0.970, P < 0.001) between spring and summer samples of the same year indicated that the impact of season or cultivation, etc. on well water Se concentration was minimal (Fig. 2). There was also no significant difference between the Se concentration of groundwaters collected in winter, 1989/1990, or in autumn, 1990 (Fig. 3). Additio-

G. Alfthan et al. /Sci Total Emiron. 162 (1995) 93-103

97

Table 2 Selenium concentration in groundwater in H&meenlinna area (rig/l) Location

Water source

Date

Se concentration

Ahvenisto Kylm%lahti Ahvenisto

Groundwater Groundwatera Tap water

Kylm&xhti

Tap water

Alajiini Morenic ridge KylmZlahti

Lake water Passage water Groundwatera

29.590 29.590 22.590 29.5% 22.590 29.590 2.12.92 2.12.92 2.12.92

838 840 836 839 837 841 102 26 814

‘Man-made

groundwater originating in Lake Alaj%rvi and passed through a morenic ridge.

nally, the variation of groundwater Se concentration in three locations sampled in three consecutive years during different seasons was small (Ta700

ble 3). The annual variation of water Se concentration in groundwater stations was smaller than the variatik in well waters.

u

n-74,

r-0.070,’

p
600 = 33 S

go0 5

cz 400

6 9

n-34,

6oo I

I

500

-

Q ;300

-

is

E3

g 200

p~0.001~

/

-

t400

6300

r-0.841,

d/

-

* 200

100 -

100

0

0 0

100

200

300 sprhg,

400 tee1

500

800 700 Se be/l)

Fig. 2. Seasonal variation of selenium concentrations in well water.

0

100

200

300

400

NOV.,lese

- Mar.,

lee0

500

600

Se Oxt/lf

Fig. 3. Seasonal variation of selenium concentrations in natural groundwater.

98

G. Alfthan et at. / Sci. Total Em&on. 162 (1995) 93-103

Table 3 The variation of groundwater selenium concentration (rig/l) Location

Date

Se

Date

Se

Date

Se

Gag oriplii Joutsa

8.1.90 1.3.90 8.1.90

54.4 220 588

6.11.90 30.1090 6.11.90

50.5 215 499

10.6.93 10.6.93 18.6.93

25.3 216 504

3.3. The geographical distribution of groundwater Se The median Se concentration of well waters did not show any distinct difference between either the six geological areas (Fig. 4) or the 12 provinces (data not shown). In areas I and II a higher Se concentration in well waters was expected due to the heavy agricultural activity and dense population. However, there was no statistically significant association between the Se concentration of well waters and the area of cultivated fields per municipality in the entire well water material [28] (data not shown). Eleven out of the 15 well water samples with a Se concentration exceeding 500 rig/l were located in central Finland, area III (Fig. 4). This area is characterized by lakes, with varying morenic ground, the main bedrock is granodiorite and the main agricultural activity is cattle breeding. Two wells were located near an old copper mine and five were near medium-sized towns. Two wells located near the west coast were in the vicinity of a large copper smelter and refinery factory where about 20 tons of Se are produced every year as a by product.

the drilled wells (n = 30, r = -0.382, P < 0.05) indicated that the deeper wells were less intluenced by Se from surface sources. Wells located outside household areas had the lowest Se concentrations (Table 4). Stone-lined wells were built before concrete was commonly used in Finland, and are generally located in the yards, being susceptible to pollution from cattle shelters and waste water drainage. Also because of their construction, they are more prone to surface water seepage than concrete wells. There was, however, no relationship between the Se concentration of wells (stone-lined + concrete rings) located in the yard and the distance of the well from cattle shelters or other potential polluting sources (data not shown). Wells located in cultivated fields tended to have a higher water Se concentration than wells located in the forest, on a slope and on lowland, but it was not statistically significant. Although sulphate, nitrate and phosphate fertilization has been reported to cause desorption of selenite and selenate from soils [8], it seems that the leaching of Se from soil into groundwater is presently not significant in Finland.

3.4. The geomorphic location of the wells The Se concentration of well water was related to well type and location. Water samples from stone-lined wells located in the yard had significantly higher Se concentrations than drilled wells in yard and concrete ring wells in field and in forest (all P values < 0.05) (Table 4). Although large differences between the mean and median Se concentrations of different well types were seen, the differences were not significant due to the large variation between other types of wells except those above mentioned. An inverse correlation between Se concentration and the depth of

3.5. Precipitation and infiltration effect The mean total Se concentration of infiltration waters was 38.8 rig/l with a median of 28.6 rig/l. There was no significant difference in the intiltration water Se concentration between soil types. The mean Se concentration of infihration waters was significantly lower than the median Se concentration of snow (62.6 rig/l, n = 124) and rain (115 rig/l, n = 26) in Finland (P < 0.01 and P < 0.001, respectively), indicating that precipitation may be one source of soil Se. In southern Finland, snowfall represents 15% of the annual precipitation of 621 mm, while in

G. Al’han et al. /Sci. Total Entiron. 162 (1995) 93-103 , 100 km

Se

No.

of

samples:

Symbol size Cumulative frequency ;

. . . . ..A

).

-

253

l

'a f

a .

-

\

VI

J .

. . . .. ‘i\ l-4 ..I.. . .

>

l

c

l

IV .. 00 . .. . . . e a . cr.* . I

L!-J

Fig. 4. Geographical distribution of selenium concentration in well water in Finland, I-VI, geological areas.

99

100

G. A&an

et al. /Sk

Total Emiron. 162 (1995) 93-103

Table 4 Well type, location and water selenium concentration (rig/l) Type and location

n

Mean

SD.

Median

Min.

Max.

Stone-lined in yard Drilled in yard Concrete in yard Concrete in field Concrete in forest Concrete on slope Concrete on lowland

9 31 120 41 22 10 7

263 113 196 120 79.2 89.5 56.6

299 154 320 151 84.3 99.2 33.5

112 49.7 104 72.6 45.4 54.9 44.9

27.9 4.8 4.1 4.2 12.2 4.2 39.2

880 646 2720 913 295 326 132

Total

256

153

2.50

68.4

4.1

2720

the North it is about 40% of 500 mm [28,29]. The annual deposition of Se from precipitation was calculated to be 0.67 g/ha in the South and 0.47 g/ha in the northern parts of the country. The difference between the annual deposition of Se from snow and rain and the amount of Se found in infiltration water was calculated from the median infiltration water Se concentration times annual precipitation. It could be estimated that over 70% of the Se in precipitation was retained in the surface soil. On an annual basis, this is about 0.49 g/ha in the Helsinki area and 0.33 g/ha in Sodankyla (North) which is about 6% of the amount of Se used annually in fertilizers, 8 g/ha applied for cereals [3,41. There was no correlation between the Se concentration of snow [13] and groundwater or infiltration water and groundwater collected from the same stations (data not shown). This suggests that the effects of surfacial environmental conditions, e.g., climate, season and soil type on groundwater Se concentration was minimal. When surface water, such as precipitation or river and lake water, infiltrates through soils into groundwater, most of the Se is retained in the surface soil. Soil has a high capacity of retaining Se. Weres et al. (1990) have shown that in a storage and evaporation facility for Se-contaminated agricultural drain water, very little Se entered the shallow aquifer below the ponds with percolating water. With few localized exceptions, most of the Se was removed from the water and retained in the first decimeter of soil, which was rich in decaying organic matter [30].

3.4. Speciatbn of selenium Selenate was the most abundant species in groundwater (Table 5). The proportion of both selenite and the Se associated with humic substances (humic Se) were less than 8 and 15%, respectively. In infiltration water, the proportion of Se species followed the order: humic Se > selenate > selenite (Table 5). In snow, there was more selenate than selenite, and in rain, 60% of the total Se was in the form of selenite. Soil retains selenite more effectively than selenate [8], therefore more selenate penetrates soils into groundwater. This is only a very small fraction compared with the native selenate in groundwater, which may originate from bedrock. On the other hand, humic Se in infiltration water may have a potential impact on the organic Se concentration in groundwater. The highest fraction of humic Se, 14.6% of the total Se was found in a spring in &jala (Table 5). In a natural groundwater in Oriptil the fraction of humic Se was only 0.5%. The sample from Joutsa was from a shallow well with an intermediate level of humic Se (Table 5). Tanzer and Heumann [31] determined selenite and selenate separated by chromatography in three groundwater samples. The selenite ranged from 3.8 to 9.4%, and that of selenate ranged from 89.8 to 96.8% of the total. In western San Joaquin Valley and the Kesterson Reservoir, California, most of the dissolved Se in groundwater was also reported to be in the selenate form [32,33].

G. Alfrhan et al. /Sci. Total Emiron. 162 (1995) 93-153

101

Table 5 The speeiation of selenium in natural waters (rig/l) Selenium species

Sampling date

Total Se

SeO,

SeO,

Humic substances

Infiltration water oripeii Pemiii Siuntio

10.6.93 24.6.93 10.6.93

19.6 33.4 112

2.5(16.8%) 6.4 (19.2%) 10.4 (9.3%)

4x24.5%) 10.X32.0%) 23.1(20.6%)

5x26.0%) 11.X33.2%) 55X49.2%)

;iijglia OripiiLb JoutsaC

10.6.93 10.6.93 18.6.93

25.3 216 504

1.9 (7.5%) 2.0 (0.9%) 7.9 (1.6%)

6.9(27.3%) 194 (89.8%) 418 (82.9%)

3.7(14.6%) 1.0 (0.5%) 16.4 (3.3%)

Rain” Helsinki

24.8.92

43.5

26.3(60.5%)

14.7(33.8%)

NDd

Snow” Helsinki Viikki

9.3.93 9.3.93

50.9 55.9

17.3(34.0%) 16.6(29.7%)

28.5(56.0%) 31.6(56.5%)

ND* NDd

Groundwater

aSpring. bGroundwater sampled through a tube. “Shallow well. dNot determined. ‘Data from Wang et al. (1993).

3.7. The groundwater selenium and other chemical components

In the total well water material (n = 256) Se was significantly correlated with turbidity, F, NO,, CL, Mn, Fe, SO,, K and Na (Table 6). When

calculated separately according to well type, significant correlations with NO,, SO,, K and Na remained in concrete ring wells. Se was significantly correlated with all parameters except Na for drill wells located in the yard. For field wells,

Table 6 Correlation of well water selenium with other parameters Parameters

Depth Turbidity F NO3

Cl Mn Fe SO4 K Na

Spearman correlation coefficient All wells Concrete well (n = 256) in yard (n = 119) - 0.122* - 0.123* 0 431*** 0:200* * - 0.155* - 0.136* 0.359*** 0.277*** 0.247***

*P < 0.05; **p < 0.01; ***p < 0.001.

0.519***

o&39*** 0.264* * 0.325***

Drilled well in yard (n = 30) - 0.382* -O&6* - 0.364* 0.681** 0.511** -0 623*** - 0:399* 0.494** 0.389*

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G. Alfhan et al. /Sci. Total Environ. 162 (1995) 93-103

only K remained significantly correlated with Se (n = 41, r = 0.344, P < 0.05). Field wells seem not to be influenced by Se-containing fertilizers, as the only association was found for K and not for other soluble fertilizer constituents (Table 6). This indicates that leaching of Se into field well waters due to selenate added to the fertilizers is not the main source of Se. In groundwaters the Se concentration was negatively correlated with Mn (n = 48, I = -0.296, P < 0.051, and positively correlated with SO, (n = 47, r = 0.349, P < 0.05), and F (n = 49, r = 0.365, P < 0.01). These results suggest that Se along with the typical agricultural and household pollutants, nitrate, sulphate, potassium and sodium tend to appear together, i.e., they may originate from the same source. In the combined concrete ring and stone-lined well material, the Se concentrations were not significantly different in samples with a pH below 7.0 or the median, 6.5 compared with those exceeding the mean or median, respectively. Also no significant trend was seen dividing the wells into pH classes < 5.5 to > 7.5 at intervals of 0.5 pH-units. An increased mobilization of Se was observed in river water in Venezuela at pH values above 7 [34]. However, similar relations were not seen in groundwater. Weres et al. [30] also found that Se was associated with nitrate in groundwater from wells in California. The turbidity of groundwater is caused by suspended matters, mostly clay particles. Both inorganic and organic Se species may be adsorbed on the surface of clay minerals and precipitate [35-371. Negative correlations between aqueous Se and Fe, Mn, and H,S in groundwater at the Kesterson Reservoir, California, have been reported [33]. Several studies have indicated that Mn, Fe and Al oxides strongly adsorb selenite and selenate and remove them completely from the water environment [30,36,38]. The negative correlations between Se and Mn and Fe in the present study support previous studies. 4. Conclusion The Se concentration of Finnish well waters and groundwaters was low, comparable with surface waters in Finland. No sample exceeded the

limit set for drinking water. Locally, high concentrations of groundwater Se may originate from bedrock. Selenate was the most abundant species in groundwaters. Anthropogenic sources may influence groundwater Se concentration in populated areas. However, the effect of agricultural activities including Se supplemented fertilization on the Se concentration of groundwater in cultivated areas could not be demonstrated. Acknowledgements This study was supported by the Ministry of Agriculture and Forestry of Finland. Margit Kettunen and Markku Myllyla are kindly thanked for arranging the sample collection in I-Gmeenlinna. References [ll J.T. Salonen, G. Alfthan, J.K. Huttunen, J. Pikkarainen and P. Puska, Association between cardiovascular death and myocardial infarction and serum selenium in a matched-pair longitudinal study. Lancet, ii (1982) 175-179. 121 J.T. Salonen, G. Alfthan, J.K. Huttunen and P. Puska, Association between serum selenium and the risk of cancer. Am. J. Epidemiol., 120 (1984) 342-349. 131 Ministry of Agriculture and Forestry, Addition of selenium into fertilizers-review and proposal for actions. Working Group Report, Helsinki, 1983 (in Finnish). and Forestry, Proposal for [41 Ministry of Agriculture amounts of selenium to be added into fertilizers. Working Group Report. No. 7, Helsinki, 1984 (in Finnish). P. Varo, G. Alfthan, J.K. Huttunen and A. Aro, NationEl wide selenium supplementation in Finland - effects on diet, blood and tissue levels, and health, in R.F. Burk (Ed.), Selenium in Biology and Human Health. Springer-Verlag, New York, 1994, pp. 198-218. b51E. Sabbioni and G. Bignoli, Heavy metals in phosphate fertilizers: potential impact on groundwater quality. Eur. Appl. Res. Rep. Environ. Natl. Res. Sect., 1 (1980) 141-180. [71 G. Bignoli and E. Sabbioni, Long term prediction of the potential impact of heavy metals on groundwater quality as a result of fertilizer use, in W. van Duijvenbooden, P. Glasbergen and H. van Lelyveld (Eds), Quality of Groundwater, Proceedings of an International Sympo sium, Noordwijkerhout, The Netherlands, 23-27, March 1981. Studies in Environmental Science, 17 (1981) 857-862. 181 E.E. Cary and G. Gissel-Nielsen, Effect of fertilizer anions on the solubility of native and applied selenium in soil. Soil Sci. Sot. Am. Proc., 37 (1973) 590-593.

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