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Use of geochemical databases to delineate risk areas for contaminated groundwater
ELSEVIER
Journal of Geochemical Exploration 64 (1998) 177–184
Use of geochemical databases to delineate risk areas for contaminated groundwater Timo Tarvainen Ł , Tarja Paukola Geological Survey of Finland, P.O. Box 96, FIN-02151 Espoo, Finland Accepted 29 May 1998
Abstract Areas of high risk for contamination of groundwater were delineated on the basis of element distributions in nation-wide geochemical maps of till, organic stream sediments and stream waters. These areas were then compared with those in which concentrations of the same elements in groundwater wells were above water quality guidelines. All data sets are from the geochemical databases of the Geological Survey of Finland. Concentrations of several elements (e.g. Cr, Cu, Mn, Ni and Zn) in the fine fraction of till were anomalously high in five provinces. In many groundwater samples from these provinces, the concentrations of Mn and Ni in drilled bedrock wells, and the concentrations of Ni and Zn in dug wells, exceeded the permissible limits. Concentrations of Cr and Cu in groundwater were slightly elevated in the same areas, but the recommended limits for these elements were not exceeded. Areas with high concentration of As in drilled bedrock wells correlated with areas of high risk delineated on the basis of both till geochemistry and organic stream sediments. Anomalies of F were similar in stream water and groundwater maps. High geogenic concentrations of potentially harmful elements in groundwater commonly occur in areas that can be delineated from geochemical maps based on till, stream water and stream sediment sampling. However, not all of the wells with poor quality groundwater were identified from geochemical maps at reconnaissance scale. 1998 Elsevier Science B.V. All rights reserved. Keywords: environmental geology; groundwater; geochemistry; pollution; databases
1. Introduction Drinking water is the most common pathway through which people are exposed to harmful geogenic elements. At present, about 20% of the Finnish population, and particulary the population in rural areas, rely on their own water supply. Till is the most common aquifer material for these private wells. Compared with those in areas of sedimentary rock, for example in continental Europe, Finnish aquifers are small. During the 1980s, the Geological Ł Corresponding
author. Tel.: C358 2055020; Fax: C358 2055012; E-mail: [email protected]
Survey of Finland (GSF) carried out a hydrogeochemical survey of shallow groundwaters in reconnaissance scale covering the whole country with a relatively even sampling grid and density of 1 sample per 50 km2 (Lahermo et al., 1990). This nation-wide mapping project was succeeded by denser sampling programmes carried out by the GSF. Dug wells, captured springs and wells drilled into the bedrock were included. According to the results, the natural concentrations of arsenic and fluoride, as well as some other elements, exceed the recommended guidelines for water quality in Finland (Anonymous, 1994a,b) in several parts of the country. Besides groundwaters, geochemical mapping of
c 1998 Elsevier Science B.V. All rights reserved. 0375-6742/98/$19.00 PII: S 0 3 7 5 - 6 7 4 2 ( 9 8 ) 0 0 0 3 1 - 4
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Finland includes several other sample types: till (Koljonen, 1992; Salminen, 1995), lake sediments (Tenhola, 1988), organic stream sediments and stream water (Lahermo et al., 1996). Organic stream sediments refer to organic matter in stream sediments. Stream sediment samples were collected using a scoop net with a mesh diameter of 0.06 mm. A composite sample was scooped within a distance of 250 m by stirring the bottom deposits. The sediment primarily consisted of plant and animal detritus and fresh and variably decayed plant fragments mixed with varying proportions of mineral material. In this work, we used the nation-wide geochemical maps of till, organic stream sediments and stream waters to delineate areas of high-risk with respect to contamination of groundwater. The results are compared with an assessment of groundwater state based on measurement of concentrations of the same elements in groundwater wells. All data sets are from the geochemical databases of the Geological Survey of Finland (Ahlsved et al., 1991; Salminen and Tarvainen, 1995).
2. Materials and methods 2.1. Groundwater The 9347 groundwater samples were from a hydrogeochemical mapping of shallow groundwater in reconnaissance scale covering the whole country at a density of 1 sample per 50 km2 carried out in 1978–1982, and from subsequent denser sampling programmes carried out by the GSF. The samples were collected from springs, captured springs, dug wells and wells drilled into bedrock. More than 40 elements or ionic species were determined at the chemical laboratory of the GSF. The concentration of F was measured by the ion specific electrode or ion chromatographic method and the concentrations of As, Cr, Cu, Mn, Ni and Zn were determined by ICP–AES and ICP–MS. 2.2. Till Reconnaissance-scale till sampling (1 sample=300 km2 ) was carried out in 1984. In total, 1057 till samples were collected from a sampling depth of 0.5–1.5 m, and 43 elements were determined in the
<0.06 mm fraction. The total concentration of As, analysed by instrumental neutron activation analysis, was investigated in this study. Regional-scale till sampling (1 sample=4 km2 ) was completed in 1991. The entire country was covered with over 80,000 samples. Sampling depth was 1.5 to 2.5 m, and each sample was a composite of 3–5 subsamples. The <0.06 mm fraction was decomposed using hot aqua regia, and about 30 elements were determined by ICP–AES. Aqua regia leachable concentrations of Al, Cr, Cu, Fe, Mn, Ni and Zn were investigated in this study. 2.3. Streams A total of 1165 water and organic stream sediment samples from headwater streams were collected at the mean sampling density of one sample per 300 km2 during August–September 1990. The sampling points were selected to represent a drainage basin of ca. 30 km2 . Both sample types were analysed for more than 40 elements or ionic species at the chemical laboratory of the GSF. The concentrations of F in stream waters, which were investigated in this study, were analysed by ion chromatography. Organic stream sediment samples were collected with a scoop net with mesh diameter of 0.06 mm. A composite sample was scooped within a distance of 250 m while bottom deposits were stirred. Samples were dried, milled and sieved, and the <2 mm fraction was leached with conc. HNO3 in a microwave oven. The leachate was analysed by ICP–AES and ICP–MS. The concentration of As was investigated in this study. 2.4. Map production The till, stream sediment and stream water quality parameters were interpolated and smoothed into a regular 800 m ð 800 m grid, where the grid values were determined with the aid of a moving weighted median in a circular window (radius 12 km for till and 120 km for streams; Gustavsson et al., 1997). The interpolated and smoothed values of the selected variables were presented on maps using colour or grey tone scales. The water quality parameters of dug wells and drilled wells were then superimposed on the maps as ball symbols.
T. Tarvainen, T. Paukola / Journal of Geochemical Exploration 64 (1998) 177–184
179
Table 1 Groundwaters in dug wells Element
Al As As Cr Cu F Fe Mn Ni Zn
Compared media
till till ss till till sw till till till till
Limit (µg=l)
200 10 10 50 1000 1500 500 200 20 3000
Number of samples
Median (till=stream)
Group 1
Group 2
Group 1
Group 2
947 822 822 1757 5523 5412 4980 5012 5180 5491
256 6 6 0 1 114 549 498 314 33
1.14% 4.0 ppm 4.6 ppm 27.2 ppm 24.2 ppm <0.2 mg=l 1.68% 170 ppm 17.5 ppm 36.2 ppm
1.04% 3.3 ppm 4.6 ppm – 33.7 ppm 1.1 mg=l 1.79% 184 ppm 20.1 ppm 40.3 ppm
Z
Sig.
1.934 0.861 0.407 – – 8.422 1.854 2.017 2.134 0.810
0.001 0.449 0.996 – – 0.000 0.002 0.001 0.000 0.529
Limit D upper permissible limit. Group 1 D concentration of element X in groundwater is lower than upper permissible limit. Group 2 D concentration in groundwater exceeds the limit value. Median D median of corresponding till, stream sediment or stream water data. SS D stream sediment, SW D stream water. Z D test value in the Kolmogorov–Smirnov test. Sig. D asymptotic significance (2-tailed).
2.5. Statistical testing The values of chemical parameters of the groundwaters were geographically correlated with the corresponding values of the till and stream grid cells. For each element, the groundwater samples were divided into two groups: (1) samples with concentration exceeding the national drinking water limit value for the particular element, and (2) samples with lower than the standard value of that element. The distributions of the same element in the other sample media (till, stream water or stream sediment) were then compared by applying the two-sample Kolmogorov–Smirnov test. If, for example, the concentration of Ni in till accurately delineates areas
of Ni-contaminated groundwaters, the distributions of Ni in till should differ for the two groundwater groups and the Ni-contaminated wells should correlate with anomalies of Ni in the geochemical maps based on till samples.
3. Results and discussion The similarity between fluoride anomalies in stream and groundwaters is pronounced (Fig. 1a,b). The distributions of fluoride concentrations in stream waters are significantly different in the areas where F concentration in groundwaters exceeds and falls below the upper permissible value (Tables 1 and 2).
Table 2 Wells drilled into bedrock (for explanations see Table 1) Element
Al As As Cr Cu F Fe Mn Ni Zn
Compared media
till till ss till till sw till till till till
Limit (µg=l)
200 10 10 50 1000 1500 500 200 20 3000
Number of samples
Median (till=stream)
Group 1
Group 2
Group 1
658 301 301 1271 2808 2516 2374 2274 2759 2773
23 27 27 1 1 295 435 533 38 37
1.26% 4.21 ppm 5.46 ppm 22.7 ppm 21.9 ppm <0.2 mg=l 1.68% 170 ppm 16.1 ppm 33.5 ppm
Z
Sig.
2.213 3.102 1.488 – – 8.847 1.584 1.905 1.760 0.456
0.000 0.000 0.024 – – 0.000 0.013 0.001 0.004 0.986
Group 2 0.98% 5.56 ppm 5.62 ppm 14.9 ppm 20.7 ppm 0.51 mg=l 1.80% 190 ppm 20.8 ppm 32.7 ppm
180 T. Tarvainen, T. Paukola / Journal of Geochemical Exploration 64 (1998) 177–184
Fig. 1. Distribution of F in stream waters (a) and groundwaters (b) in Finland and the occurrence of rapakivi granites (c).
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181
Fig. 2. Delineation of areas with high concentration of natural arsenic, based on geochemical mapping of the fine fraction of till (a) and organic stream sediments (b) in reconnaissance scale. Drilled wells with As concentration above the limit value 10 µg=l are shown as ball symbols superimposed on the grey tone maps.
Fluoride, together with chloride and bromide, are among the most conservative anions. The concentration of F in surface and groundwaters is strongly controlled by lithology, in southeastern and southwestern Finland by the occurrence of rapakivi granites. The rapakivi granites of southern Finland form four large batholiths and several subsidiary plutons and stocks (Fig. 1c). The age of the anorogenic rapakivi granites ranges from 1.69 to 1.54 Ga (Vaasjoki, 1977). Rapakivi granites are characterized by higher abundances of SiO2 , K2 O, K=Na, Fe=Mg, F, Rb, Ga, Zr, Th, U and REE (except Eu) than granites on average (Haapala and Ehlers, 1989). The upper permissible limit for As in drinking water is 10 µg=l. The concentration of As is higher than 10 µg=l in many wells drilled in bedrock in the vicinity of Tampere, southwestern Finland. This
area was delineated in the analysis of geochemical maps based on till and organic stream sediments (Fig. 2, Tarvainen, 1996). According to the statistical testing, the till geochemical data reliably outline areas at risk for As contamination. In southwestern Finland, regional till geochemical maps have already been used to select municipalities for more detailed risk mapping. The risk maps at municipality level are based on denser groundwater sampling and the assessment of geological, geophysical and geochemical maps (Idman, 1996, 1997). In dug well water samples, the concentration of As seldom exceeds the upper permissible limit. Anomalously high concentrations of Ni in groundwaters tend to coincide with Ni anomaly areas of till geochemical mapping along the Tampere schist belt, in the eastern part of the Lake Ladoga–
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Fig. 3. Distribution of Ni in the fine fraction of till with the place names referred to in the text. LL–BB D Lake Ladoga–Bothnian Bay zone. PPSB D Pera¨pohja schist belt. The area shown in Fig. 4. is outlined.
Bothnian Bay zone and in central Lapland (Figs. 3 and 4). Statistical testing showed the distribution of Ni concentrations in till to differ significantly in areas where Ni concentrations in dug well water exceed and are below the upper permissible limit. Some elevated Ni concentrations in dug well waters were also correlated with Ni anomalies in stream water in the vicinity of Vaasa on the west coast. However, the high Ni concentrations in shallow groundwaters in eastern Finland did not show up in the analysis of geochemical data based on other sampling materials. This is attributed to the fact that those anomalous samples were taken during 1978–1979 with a Ruttner type sampler which apparently in some cases caused Ni contamination.
The highest concentrations of Fe and Mn in groundwater in dugwells are found in the western and southwestern coastal areas where dissolved organic material in water causes oxygen depletion, leading to reducing conditions. According to Lahermo et al. (1990) the presence of peat, fine-grained sediments and pollution contribute more to the concentrations of Fe and Mn than do lithology and geochemistry of the overburden. Some of the high Mn concentrations in groundwater were related to the Mn anomalies in till in the Tampere schist belt, the Lake Ladoga–Bothnian Bay zone, the Pera¨pohja schist belt and central Lapland. The upper permissible limit of Cr (50 µg=l) was exceeded in only one dug well sample, suggesting that chromium does not cause regional contamination problems in Finnish groundwaters, although further sampling would be necessary to confirm this. The same is true of Cu. The highest Cr and Cu concentrations in shallow groundwaters (dug wells and springs) tend to be located in areas coinciding with Cr and Cu till geochemical anomalies within the Tampere schist belt and along the Lake Ladoga– Bothnian Bay zone. Total Al analyses are lacking for the earliest groundwater geochemical data and the coverage of the data set was not fully satisfactory for statistical testing. Al concentrations in groundwaters are often elevated in the rapakivi granite areas in southeastern ˚ land islands. and southwestern Finland and in the A The concentration of Al in till in these areas is lower than average, but the high Al concentration in groundwaters can be explained by acidity and the formation of soluble Al–F complexes (cf. Driscoll, 1985). Peat bogs are common in central western Finland and the elevated Al concentrations in shallow groundwater there reflect the presence of strong acids, but principally greater acidity. The upper permissible limit for Zn (3.0 mg=l) was exceeded in 33 dug wells and 37 drilled wells. Some of the contaminated dug wells were located in the Zn anomaly zones of till geochemical mapping, but the statistical testing did not support the possibility of using regional till geochemical maps to delineate areas at risk for Zn contamination. Anomalous Zn concentrations in groundwater samples can partly be explained by contamination from water pipes and pails.
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183
Fig. 4. Distribution of Ni in the fine fraction of till (grey scale) and in dug wells where Ni concentration exceeds the limit value 20 µg=l.
The geochemistry of till explains the groundwater distribution of Cr, Cu, Mn, Ni and Zn, which have been derived from the weathering of bedrock and the overburden. In coastal areas, seawater has affected the quality of groundwaters, causing elevation of SO24 concentrations for example. Sulphates may be released to groundwater during post-glacial uplift of marine silts and clays rich in sulphides and sulphates, from relict seawater trapped in fine-grained sediments or from aerosols in rainfall. Besides contaminants deriving from natural sources, groundwater resources are threatened by anthropogenic pollution. Lahermo et al. (1990) have reported that NO3 concentrations are often elevated in the central and southeastern parts of the country presumably mainly due to the transport of nutrients in fertilizers into farm wells. The elevated levels of Al, Fe, and to a lesser extent Mn, are related to the acidity of groundwater and the concentration of organic anions in groundwater. There would be little value in plotting the regional distribution of these contaminants because the concentration of humic substances is mostly controlled by the condition of the well. Furthermore, Finnish aquifers are small and the quality of groundwater may differ from well to well.
4. Conclusions Regional geochemical maps of till, stream sediments and stream water can be used to delineate areas where concentrations of elements derived from bedrock and overburden may exceed the healthbased risk limits for drinking water. The presence of F contamination can be predicted most effectively from stream water chemistry, whereas areas of As and Ni contamination were better delineated on the basis of till geochemical data. In contrast, the occurrence of anthropogenic contaminants such as NO3 in groundwaters could not be predicted from regional geochemical mapping data sets because these do not present the distributions of anthropogenic elements and compounds so much as the natural distributions in the overburden and surface environment. Analysis of regional geochemical maps can suggest areas where geogenic groundwater contamination is probable and where risk analyses based on groundwater sampling and geological and geophysical mapping at municipality level should be carried out.
References Ahlsved, C., Lampio, E., Tarvainen, T., 1991. ALKEMIA — a VAX minicomputer database and program package for geochemical exploration. J. Geochem. Explor. 41, 23–28.
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Anonymous, 1994a. The quality criteria and control research of drinking water, a decision issued by the Ministry of Social Affairs and Health on January 21, 1994 (serial number 74=1994, in Finnish). Suomen sa¨a¨do¨skokoelma 74: 246–254. Anonymous, 1994b. Soveltamisopas sosiaali- ja terveysministerio¨n pa¨a¨to¨kseen talousveden laatuvaatimuksista ja valvontatutkimuksista (the quality criteria and control research of drinking water, application guide; in Finnish). Vesi- ja viema¨rilaitosyhdistys, Helsinki. Driscoll, C.T., 1985. Aluminum in acidic surface waters: chemistry, transport, and effects. Environ. Health Perspect. 63, 93– 104. Gustavsson, N., Lampio, E., Tarvainen, T., 1997. Visualization of geochemical data on maps at the Geological Survey of Finland. J. Geochem. Explor. 59, 197–207. Haapala, I., Ehlers, C., 1989. The rapakivi granites and postorogenic granites of southern Finland. In: Ehlers, C., Haapala, I. (Eds.), Symposium Precambrian Granitoids. Petrogenesis, Geochemistry and Metallogeny, August 14–17, 1989, Helsinki, Finland. Excursion A 1: rapakivi granites and postorogenic granites of southwestern Finland. Geologian tutkimuskeskus, Opas – Geological Survey of Finland, Guide 27, pp. 5–11. Idman, H., 1996. Luonnollisen arseenipitoisuuden vaihtelu pohjavesissa¨ ja siihen vaikuttavat tekija¨t (Naturally occurring arsenic in groundwaters and factors affecting its concentration) (in Finnish with English abstract). In: Nyste´n, T., Suokko, T., Tarvainen, T. (Eds.), Ympa¨risto¨geologian sovelluksia. Suomen ympa¨risto¨ 71, 17–22. Idman, H., 1997. Naturally occurring arsenic in groundwaters in Finland, and factors affecting its concentration. GAC=MAC
Annual Meeting, May 19–21, 1997, Ottawa ’97, Abstract Volume, A63–A64. Koljonen, T. (Ed.), 1992. The Geochemical Atlas of Finland, Part 2. Till. Geological Survey of Finland, Espoo, 218 pp. Lahermo, P., Ilmasti, M., Juntunen, R., Taka, M., 1990. The Geochemical Atlas of Finland, Part 1. The Hydrogeochemical Mapping of Finnish Groundwater. Geological Survey of Finland, Espoo, 66 pp. Lahermo, P., Va¨a¨na¨nen, P., Tarvainen, T., Salminen, R., 1996. The Geochemical Atlas of Finland, Part 3. Environmental Geochemistry. Stream Waters and Sediments. Geological Survey of Finland, Espoo, 149 pp. Salminen, R. (Ed.), 1995. Alueellinen geokemiallinen kartoitus Suomessa vuosina 1982–1994 (English summary: Regional geochemical mapping in Finland in 1982–1994). Geological Survey of Finland, Report of Investigation 130, 49 pp., 24 app. pages. Salminen, R., Tarvainen, T., 1995. Geochemical mapping and databases in Finland. J. Geochem. Explor. 55, 321–327. Tarvainen, T., 1996. Environmental Application of Geochemical Databases in Finland. Synopsis. Geological Survey of Finland, Espoo, 75 pp. Tenhola, M., 1988. Regional geochemical mapping based on lake sediments in eastern Finland (in Finnish, with English summary). Geological Survey of Finland, Report of Investigation 78, 42 pp. Vaasjoki, M., 1977. Rapakivi granites and other postorogenic rocks in Finland: their age and the lead isotopic composition of certain associated mineralizations. Geological Survey of Finland, Bulletin 294, 64 pp.
Journal of Geochemical Exploration 64 (1998) 177–184
Use of geochemical databases to delineate risk areas for contaminated groundwater Timo Tarvainen Ł , Tarja Paukola Geological Survey of Finland, P.O. Box 96, FIN-02151 Espoo, Finland Accepted 29 May 1998
Abstract Areas of high risk for contamination of groundwater were delineated on the basis of element distributions in nation-wide geochemical maps of till, organic stream sediments and stream waters. These areas were then compared with those in which concentrations of the same elements in groundwater wells were above water quality guidelines. All data sets are from the geochemical databases of the Geological Survey of Finland. Concentrations of several elements (e.g. Cr, Cu, Mn, Ni and Zn) in the fine fraction of till were anomalously high in five provinces. In many groundwater samples from these provinces, the concentrations of Mn and Ni in drilled bedrock wells, and the concentrations of Ni and Zn in dug wells, exceeded the permissible limits. Concentrations of Cr and Cu in groundwater were slightly elevated in the same areas, but the recommended limits for these elements were not exceeded. Areas with high concentration of As in drilled bedrock wells correlated with areas of high risk delineated on the basis of both till geochemistry and organic stream sediments. Anomalies of F were similar in stream water and groundwater maps. High geogenic concentrations of potentially harmful elements in groundwater commonly occur in areas that can be delineated from geochemical maps based on till, stream water and stream sediment sampling. However, not all of the wells with poor quality groundwater were identified from geochemical maps at reconnaissance scale. 1998 Elsevier Science B.V. All rights reserved. Keywords: environmental geology; groundwater; geochemistry; pollution; databases
1. Introduction Drinking water is the most common pathway through which people are exposed to harmful geogenic elements. At present, about 20% of the Finnish population, and particulary the population in rural areas, rely on their own water supply. Till is the most common aquifer material for these private wells. Compared with those in areas of sedimentary rock, for example in continental Europe, Finnish aquifers are small. During the 1980s, the Geological Ł Corresponding
author. Tel.: C358 2055020; Fax: C358 2055012; E-mail: [email protected]
Survey of Finland (GSF) carried out a hydrogeochemical survey of shallow groundwaters in reconnaissance scale covering the whole country with a relatively even sampling grid and density of 1 sample per 50 km2 (Lahermo et al., 1990). This nation-wide mapping project was succeeded by denser sampling programmes carried out by the GSF. Dug wells, captured springs and wells drilled into the bedrock were included. According to the results, the natural concentrations of arsenic and fluoride, as well as some other elements, exceed the recommended guidelines for water quality in Finland (Anonymous, 1994a,b) in several parts of the country. Besides groundwaters, geochemical mapping of
c 1998 Elsevier Science B.V. All rights reserved. 0375-6742/98/$19.00 PII: S 0 3 7 5 - 6 7 4 2 ( 9 8 ) 0 0 0 3 1 - 4
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T. Tarvainen, T. Paukola / Journal of Geochemical Exploration 64 (1998) 177–184
Finland includes several other sample types: till (Koljonen, 1992; Salminen, 1995), lake sediments (Tenhola, 1988), organic stream sediments and stream water (Lahermo et al., 1996). Organic stream sediments refer to organic matter in stream sediments. Stream sediment samples were collected using a scoop net with a mesh diameter of 0.06 mm. A composite sample was scooped within a distance of 250 m by stirring the bottom deposits. The sediment primarily consisted of plant and animal detritus and fresh and variably decayed plant fragments mixed with varying proportions of mineral material. In this work, we used the nation-wide geochemical maps of till, organic stream sediments and stream waters to delineate areas of high-risk with respect to contamination of groundwater. The results are compared with an assessment of groundwater state based on measurement of concentrations of the same elements in groundwater wells. All data sets are from the geochemical databases of the Geological Survey of Finland (Ahlsved et al., 1991; Salminen and Tarvainen, 1995).
2. Materials and methods 2.1. Groundwater The 9347 groundwater samples were from a hydrogeochemical mapping of shallow groundwater in reconnaissance scale covering the whole country at a density of 1 sample per 50 km2 carried out in 1978–1982, and from subsequent denser sampling programmes carried out by the GSF. The samples were collected from springs, captured springs, dug wells and wells drilled into bedrock. More than 40 elements or ionic species were determined at the chemical laboratory of the GSF. The concentration of F was measured by the ion specific electrode or ion chromatographic method and the concentrations of As, Cr, Cu, Mn, Ni and Zn were determined by ICP–AES and ICP–MS. 2.2. Till Reconnaissance-scale till sampling (1 sample=300 km2 ) was carried out in 1984. In total, 1057 till samples were collected from a sampling depth of 0.5–1.5 m, and 43 elements were determined in the
<0.06 mm fraction. The total concentration of As, analysed by instrumental neutron activation analysis, was investigated in this study. Regional-scale till sampling (1 sample=4 km2 ) was completed in 1991. The entire country was covered with over 80,000 samples. Sampling depth was 1.5 to 2.5 m, and each sample was a composite of 3–5 subsamples. The <0.06 mm fraction was decomposed using hot aqua regia, and about 30 elements were determined by ICP–AES. Aqua regia leachable concentrations of Al, Cr, Cu, Fe, Mn, Ni and Zn were investigated in this study. 2.3. Streams A total of 1165 water and organic stream sediment samples from headwater streams were collected at the mean sampling density of one sample per 300 km2 during August–September 1990. The sampling points were selected to represent a drainage basin of ca. 30 km2 . Both sample types were analysed for more than 40 elements or ionic species at the chemical laboratory of the GSF. The concentrations of F in stream waters, which were investigated in this study, were analysed by ion chromatography. Organic stream sediment samples were collected with a scoop net with mesh diameter of 0.06 mm. A composite sample was scooped within a distance of 250 m while bottom deposits were stirred. Samples were dried, milled and sieved, and the <2 mm fraction was leached with conc. HNO3 in a microwave oven. The leachate was analysed by ICP–AES and ICP–MS. The concentration of As was investigated in this study. 2.4. Map production The till, stream sediment and stream water quality parameters were interpolated and smoothed into a regular 800 m ð 800 m grid, where the grid values were determined with the aid of a moving weighted median in a circular window (radius 12 km for till and 120 km for streams; Gustavsson et al., 1997). The interpolated and smoothed values of the selected variables were presented on maps using colour or grey tone scales. The water quality parameters of dug wells and drilled wells were then superimposed on the maps as ball symbols.
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179
Table 1 Groundwaters in dug wells Element
Al As As Cr Cu F Fe Mn Ni Zn
Compared media
till till ss till till sw till till till till
Limit (µg=l)
200 10 10 50 1000 1500 500 200 20 3000
Number of samples
Median (till=stream)
Group 1
Group 2
Group 1
Group 2
947 822 822 1757 5523 5412 4980 5012 5180 5491
256 6 6 0 1 114 549 498 314 33
1.14% 4.0 ppm 4.6 ppm 27.2 ppm 24.2 ppm <0.2 mg=l 1.68% 170 ppm 17.5 ppm 36.2 ppm
1.04% 3.3 ppm 4.6 ppm – 33.7 ppm 1.1 mg=l 1.79% 184 ppm 20.1 ppm 40.3 ppm
Z
Sig.
1.934 0.861 0.407 – – 8.422 1.854 2.017 2.134 0.810
0.001 0.449 0.996 – – 0.000 0.002 0.001 0.000 0.529
Limit D upper permissible limit. Group 1 D concentration of element X in groundwater is lower than upper permissible limit. Group 2 D concentration in groundwater exceeds the limit value. Median D median of corresponding till, stream sediment or stream water data. SS D stream sediment, SW D stream water. Z D test value in the Kolmogorov–Smirnov test. Sig. D asymptotic significance (2-tailed).
2.5. Statistical testing The values of chemical parameters of the groundwaters were geographically correlated with the corresponding values of the till and stream grid cells. For each element, the groundwater samples were divided into two groups: (1) samples with concentration exceeding the national drinking water limit value for the particular element, and (2) samples with lower than the standard value of that element. The distributions of the same element in the other sample media (till, stream water or stream sediment) were then compared by applying the two-sample Kolmogorov–Smirnov test. If, for example, the concentration of Ni in till accurately delineates areas
of Ni-contaminated groundwaters, the distributions of Ni in till should differ for the two groundwater groups and the Ni-contaminated wells should correlate with anomalies of Ni in the geochemical maps based on till samples.
3. Results and discussion The similarity between fluoride anomalies in stream and groundwaters is pronounced (Fig. 1a,b). The distributions of fluoride concentrations in stream waters are significantly different in the areas where F concentration in groundwaters exceeds and falls below the upper permissible value (Tables 1 and 2).
Table 2 Wells drilled into bedrock (for explanations see Table 1) Element
Al As As Cr Cu F Fe Mn Ni Zn
Compared media
till till ss till till sw till till till till
Limit (µg=l)
200 10 10 50 1000 1500 500 200 20 3000
Number of samples
Median (till=stream)
Group 1
Group 2
Group 1
658 301 301 1271 2808 2516 2374 2274 2759 2773
23 27 27 1 1 295 435 533 38 37
1.26% 4.21 ppm 5.46 ppm 22.7 ppm 21.9 ppm <0.2 mg=l 1.68% 170 ppm 16.1 ppm 33.5 ppm
Z
Sig.
2.213 3.102 1.488 – – 8.847 1.584 1.905 1.760 0.456
0.000 0.000 0.024 – – 0.000 0.013 0.001 0.004 0.986
Group 2 0.98% 5.56 ppm 5.62 ppm 14.9 ppm 20.7 ppm 0.51 mg=l 1.80% 190 ppm 20.8 ppm 32.7 ppm
180 T. Tarvainen, T. Paukola / Journal of Geochemical Exploration 64 (1998) 177–184
Fig. 1. Distribution of F in stream waters (a) and groundwaters (b) in Finland and the occurrence of rapakivi granites (c).
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181
Fig. 2. Delineation of areas with high concentration of natural arsenic, based on geochemical mapping of the fine fraction of till (a) and organic stream sediments (b) in reconnaissance scale. Drilled wells with As concentration above the limit value 10 µg=l are shown as ball symbols superimposed on the grey tone maps.
Fluoride, together with chloride and bromide, are among the most conservative anions. The concentration of F in surface and groundwaters is strongly controlled by lithology, in southeastern and southwestern Finland by the occurrence of rapakivi granites. The rapakivi granites of southern Finland form four large batholiths and several subsidiary plutons and stocks (Fig. 1c). The age of the anorogenic rapakivi granites ranges from 1.69 to 1.54 Ga (Vaasjoki, 1977). Rapakivi granites are characterized by higher abundances of SiO2 , K2 O, K=Na, Fe=Mg, F, Rb, Ga, Zr, Th, U and REE (except Eu) than granites on average (Haapala and Ehlers, 1989). The upper permissible limit for As in drinking water is 10 µg=l. The concentration of As is higher than 10 µg=l in many wells drilled in bedrock in the vicinity of Tampere, southwestern Finland. This
area was delineated in the analysis of geochemical maps based on till and organic stream sediments (Fig. 2, Tarvainen, 1996). According to the statistical testing, the till geochemical data reliably outline areas at risk for As contamination. In southwestern Finland, regional till geochemical maps have already been used to select municipalities for more detailed risk mapping. The risk maps at municipality level are based on denser groundwater sampling and the assessment of geological, geophysical and geochemical maps (Idman, 1996, 1997). In dug well water samples, the concentration of As seldom exceeds the upper permissible limit. Anomalously high concentrations of Ni in groundwaters tend to coincide with Ni anomaly areas of till geochemical mapping along the Tampere schist belt, in the eastern part of the Lake Ladoga–
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Fig. 3. Distribution of Ni in the fine fraction of till with the place names referred to in the text. LL–BB D Lake Ladoga–Bothnian Bay zone. PPSB D Pera¨pohja schist belt. The area shown in Fig. 4. is outlined.
Bothnian Bay zone and in central Lapland (Figs. 3 and 4). Statistical testing showed the distribution of Ni concentrations in till to differ significantly in areas where Ni concentrations in dug well water exceed and are below the upper permissible limit. Some elevated Ni concentrations in dug well waters were also correlated with Ni anomalies in stream water in the vicinity of Vaasa on the west coast. However, the high Ni concentrations in shallow groundwaters in eastern Finland did not show up in the analysis of geochemical data based on other sampling materials. This is attributed to the fact that those anomalous samples were taken during 1978–1979 with a Ruttner type sampler which apparently in some cases caused Ni contamination.
The highest concentrations of Fe and Mn in groundwater in dugwells are found in the western and southwestern coastal areas where dissolved organic material in water causes oxygen depletion, leading to reducing conditions. According to Lahermo et al. (1990) the presence of peat, fine-grained sediments and pollution contribute more to the concentrations of Fe and Mn than do lithology and geochemistry of the overburden. Some of the high Mn concentrations in groundwater were related to the Mn anomalies in till in the Tampere schist belt, the Lake Ladoga–Bothnian Bay zone, the Pera¨pohja schist belt and central Lapland. The upper permissible limit of Cr (50 µg=l) was exceeded in only one dug well sample, suggesting that chromium does not cause regional contamination problems in Finnish groundwaters, although further sampling would be necessary to confirm this. The same is true of Cu. The highest Cr and Cu concentrations in shallow groundwaters (dug wells and springs) tend to be located in areas coinciding with Cr and Cu till geochemical anomalies within the Tampere schist belt and along the Lake Ladoga– Bothnian Bay zone. Total Al analyses are lacking for the earliest groundwater geochemical data and the coverage of the data set was not fully satisfactory for statistical testing. Al concentrations in groundwaters are often elevated in the rapakivi granite areas in southeastern ˚ land islands. and southwestern Finland and in the A The concentration of Al in till in these areas is lower than average, but the high Al concentration in groundwaters can be explained by acidity and the formation of soluble Al–F complexes (cf. Driscoll, 1985). Peat bogs are common in central western Finland and the elevated Al concentrations in shallow groundwater there reflect the presence of strong acids, but principally greater acidity. The upper permissible limit for Zn (3.0 mg=l) was exceeded in 33 dug wells and 37 drilled wells. Some of the contaminated dug wells were located in the Zn anomaly zones of till geochemical mapping, but the statistical testing did not support the possibility of using regional till geochemical maps to delineate areas at risk for Zn contamination. Anomalous Zn concentrations in groundwater samples can partly be explained by contamination from water pipes and pails.
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Fig. 4. Distribution of Ni in the fine fraction of till (grey scale) and in dug wells where Ni concentration exceeds the limit value 20 µg=l.
The geochemistry of till explains the groundwater distribution of Cr, Cu, Mn, Ni and Zn, which have been derived from the weathering of bedrock and the overburden. In coastal areas, seawater has affected the quality of groundwaters, causing elevation of SO24 concentrations for example. Sulphates may be released to groundwater during post-glacial uplift of marine silts and clays rich in sulphides and sulphates, from relict seawater trapped in fine-grained sediments or from aerosols in rainfall. Besides contaminants deriving from natural sources, groundwater resources are threatened by anthropogenic pollution. Lahermo et al. (1990) have reported that NO3 concentrations are often elevated in the central and southeastern parts of the country presumably mainly due to the transport of nutrients in fertilizers into farm wells. The elevated levels of Al, Fe, and to a lesser extent Mn, are related to the acidity of groundwater and the concentration of organic anions in groundwater. There would be little value in plotting the regional distribution of these contaminants because the concentration of humic substances is mostly controlled by the condition of the well. Furthermore, Finnish aquifers are small and the quality of groundwater may differ from well to well.
4. Conclusions Regional geochemical maps of till, stream sediments and stream water can be used to delineate areas where concentrations of elements derived from bedrock and overburden may exceed the healthbased risk limits for drinking water. The presence of F contamination can be predicted most effectively from stream water chemistry, whereas areas of As and Ni contamination were better delineated on the basis of till geochemical data. In contrast, the occurrence of anthropogenic contaminants such as NO3 in groundwaters could not be predicted from regional geochemical mapping data sets because these do not present the distributions of anthropogenic elements and compounds so much as the natural distributions in the overburden and surface environment. Analysis of regional geochemical maps can suggest areas where geogenic groundwater contamination is probable and where risk analyses based on groundwater sampling and geological and geophysical mapping at municipality level should be carried out.
References Ahlsved, C., Lampio, E., Tarvainen, T., 1991. ALKEMIA — a VAX minicomputer database and program package for geochemical exploration. J. Geochem. Explor. 41, 23–28.
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