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Regional distribution of halogens in Norwegian forest soils
Geoderma, 16 (1976) 317--325
317
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
R E G I O N A L DISTRIBUTION OF HALOGENS IN NORWEGIAN F O R E S T SOILS
J. L/~G and E. STEINNES
Department of Soil Science, Agricultural University of Norway, ~s-NLH (Norway) Institute for Atomenergy, Isotope Laboratories, Kjeller (Norway) (Received January 26, 1976; accepted June 3, 1976)
ABSTRACT L~g, J. and Steinnes, E., 1976. Regional distribution of halogens in Norwegian forest soils. Geoderma, 16: 317--325. Regional distribution of chlorine, bromine, and iodine in humus layers of Norwegian forest soils has been studied by means of neutron activation analysis. The halogen concentrations show a rapid decrease at increasing distances from the ocean, indicating that the supply of these elements to the soil is mainly through precipitation. Strong correlations between Cl and exchangeable Na ÷ and Mg2+, indicate atmospheric input to soils for these two cations as well.
INTRODUCTION
Discussions related to the chemical composition of soils have most often been based on the composition and properties of soil-forming rocks. For a long time the influence of the chemical composition o f precipitation on the soil chemistry was to a great extent neglected. One of the first to realize the significance of atmospheric inputs of chemical elements to soils seems to have been V. M. Goldschmidt. In his posthumous t e x t b o o k (Goldschmidt, 1954, p. 590) he points out that the supply through atmospheric precipitation is an important process in the cycle of the halogens, as well as sulphate, sodium, magnesium and some potassium. As this process goes on year by year and adds these substances to the uppermost soil horizons, it must, according to Goldschmidt's opinion, be a very important process in the evolution of soils, especially in coastal regions. More recent authors have expressed somewhat different views in this matter. According to Eriksson (1960) the amount of chlorine supplied to the rivers by natural processes represents predominantly oceanic constituents carried into the continents by the atmosphere. Correns (1956), on the other hand, in an extensive treatment of the geochemistry of the halogens, admits that a pDrtion of the halogens found in soil is contributed by rain water but concludes on the basis of available research that the major
318
share of the halogens in soils comes from rock weathering. In the survey by Vinogradov (1959) the significance of atmospheric inputs to soils is clearly emphasized in the case of iodine and to a lesser extent of bromine. During the last two decades, a large-scale investigation of forest soils in Norway has been carried out by the Agricultural University of Norway and the National Forest Survey. The programme of this investigation includes registration of important properties of soil material and soil profiles in most of the productive forest areas of the country. Besides the field registration work, humus samples for chemical analysis were collected in the counties of Nord-Trondelag, Oppland, and Buskerud (Fig. 1) during the period 1960---1964. About 3,000 samples were collected in the three counties. As will be explained later, this material offered a unique opportunity to study the influence of elements supplied through precipitation on the chemical composition of soils. An investigation of the exchangeable cations, Na ÷, Mg2÷, and Ca2+, in these soils (L~g, 1968b) indicated a geographical distribution pattern showing good correspondence with observations on the chemical composition of the
Fig. 1. Sampled areas in Norway.
319
precipitation. The cations, Na ÷ and Mg2÷, which are abundant in sea water, were f o u n d to be enriched relative to Ca 2÷ by a factor about 5 in podzol soils from the coastal districts of Nord-TrSndelag as compared to softs from central eastern Norway. This evidence of the importance of airborne supply with regard to the c o n t e n t of exchangeable cations in the soils led the present authors to focus attentian on the halogens, in which case there might also be reasons to assume distinct trends in regional distribution, because of the well-known association of these elements with the marine environment. EXPERIMENTAL
Collection of samples The humus samples were obtained from sample plots of 78.5 m 2 or 100 m 2 , regularly distributed in forest areas with an annual normal production exceeding 0.12 m3/1,000 m 2. The samples were taken from the central part of the humus layer if its thickness was 10 cm or less; in cases of thicker humus layers the depth of sampling was 5--10 cm. At least ten separate portions were collected at spots regularly distributed over the sample plot and mixed thoroughly to form one sample for analysis. The total number of samples taken in the general programme was about 1,000 in each of the counties of Nord-Tr6ndelag, Oppland, and Buskerud. For the present investigation, every fifth sample was taken for analysis. In all three counties, the collected samples predominantly represent Podzol profiles.
Determination of chlorine, bromine, and iodine The samples were subjected to neutron activation analysis t%r the determination of chlorine, bromine, and iodine. The fourth member of the halogen group, fluorine, could not easily be determined simultaneously with the available facilities. As the analytical m e t h o d used has been described in detail elsewhere (L~ig and Steinnes, 1972) only a brief account will be given here. Samples of 100--200 mg were irradiated for 5 min in the JEEP-II reactor (Kjeller, Norway) at a thermal neutron flux of 1 . 5 . 1 0 'a n cm -2 s -1, together with a halogen standard solution. The analyses were based on the radionuclides asC1 (t'A = 37.2 min), 8°Br (tlA = 17.6 min), and 12sI (tl/2 = 25.0 min), and a ~/-spectrometer based on a Ge(Li) solid-state detector was used for the activity measurement. For a great number of samples a radiochemical separation had to be carried out in order to remove interfering activities before ~/-spectrometry measurement. The reproducibility of the analyses, including possible inhomogeneous distribution of Halogens within the sample, was of the order of + 5%.
320
SOME GEOGRAPHICAL AND METEOROLOGICAL F E A T U R E S OF S O U T H E R N NORWAY
Before presenting the data, some geographical and meteorological features of the investigated areas are described as relevant to the subsequent discussion. Fig. 1 shows a map of Norway up to a latitude of 65°N, including the forest areas sampled for the present investigation. Nord-TrSndelag is an area without large variability in topography and climate. Only small areas close to the Swedish border reach altitudes higher than 900 m. The climate ranges from a coastal to a slightly more continental type. A certain decrease in mean annual precipitation takes place from areas close to the coast (about 1,200 mm} to areas near the Swedish border (about 700 mm). The air-line distance between coast and border is about 150 km, on average. The precipitation is predominantly supplied through westerly winds. The region of eastern Norway, in which the Oppland and Buskerud counties occur, has a continental-type climate, except for areas close to Oslo fjord. The area is shielded by high mountain chains to the north and west, for which reason the precipitation is to a great extent carried by southern winds. This means that the precipitation falling in the northern part of this area may originate from air masses which have moved quite a long distance after having had contact with the ocean. The mean annual precipitation is about 600 mm in large parts of the area. In the districts along the Oslo fjord it is somewhat higher whereas in the northwestern valleys it is as low as 300 mm in some places. The districts in which the forest soil samples were obtained thus differ considerably in the amounts of precipitation and in distance from the sea. Otherwise, the properties of the Nord-Tr6ndelag soils appear to be similar to those of Oppland and Buskerud (L~ig, 1962, 1965, 1968a). The soil samples therefore seem well suited for studies of regional differences in the supply of chemical constituents to the soil through precipitation. RESULTS AND DISCUSSION
The contents of C1, Br, and I were determined in about 700 humus samples, all of which had been previously analyzed for other characteristics such as pH, loss on ignition, and exchangeable cations. For convenience of discussion, the data from the present study have been divided into 29 sub-groups on a geographical basis, each group comprising 15--30 samples. The location corresponding to each sub-group is indicated in Fig. 1. The numerical order of these sub-areas within each of two major areas has been so arranged as to indicate a decreasing influence of the marine environment as estimated from geographical and meteorological factors. The mean values for the concentrations of C1, Br, and I in the various subgroups of soil, are given in Table I. The C1/Br, Cl/I and Br/I ratios are also
321
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listed in the table, as are data on approximate annual precipitation for the locations in question and mean values for organic matter content, exchangeable Na* and exchangeable Mg 2÷, the latter values having been determined in a previous stage of the project (J. L~g, unpublished work). In Table II, some correlation coefficients calculated from data in Table I are given. TABLE II Some correlation coefficients calculated on the basis of sub-group m e a n values from Table I
CI--Br CI--I Br--I CI--Na CI--Mg
Nord-Tr~ndelag
Eastern Norway
All data
0.85 0.68 0.92 0.92 0.84
0.67 0.77 0.93 0.74 0.52
0.93 0.85 0.94 0.97 0.94
--
The results exhibit a very clear trend of decreasing halogen concentrations with increasing distance from the sea. In Nord-Tr6ndelag a chlorine level of the order of 1,500 ppm is present in soils close to the coast, whereas the level 150 km inland has dropped to about 500 ppm. For bromine, the corresponding levels are 90 ppm and 20 ppm, and for iodine 15 ppm and 6 ppm, respectively. In the areas investigated in eastern Norway, still lower concentrations are observed, but the concentration gradients are much smaller than in Nord-TrSndelag. The chlorine concentration, except for areas near the Oslo fjord, shows only a slight decrease from approximately 300 p p m to 200 ppm in going to areas of lower annual precipitation. Corresponding figures are 7 to 5 ppm for bromine and 5 to 3 ppm for iodine. The possibility exists that the natural halogen distributions could be subject to interference from human activities. Chlorine may be introduced through the use of CaC12 as road salt, and bromine is known to be released through automobile exhaust. Such effects have not been found in the present samples. Another factor that may affect the observed distribution is differing humus contents of the soil samples. As can be seen from Table I, the samples of soils near the coast show a somewhat higher organic matter content (70-80%) than do those from inland regions (50--60%). It has been pointed o u t in the literature that bromine and iodine are concentrated in the humus layer of the soil (see e.g. Correns, 1956; Vinogradov, 1959). If the correlation between the halogens and humus was very strong, this could in part explain the observed distributions. Correlation coefficients calculated on the basis of the data of single samples showed the following results:
323
Nord- Trbndelag
Eastern Norway
Cl - organic matter: 0.54 Br - organic matter: 0.36 I - organic matter: 0.30
C1 - organic matter: 0.62 Br - organic matter: 0.42 I - organic matter: 0.41
As seen, the observed correlations are not strong. It is therefore not likely that differences in humus content between sub-groups has had a decisive influence on the observed halogen distributions. Although there appears to be some connection between the amounts of annual precipitation and the halogen concentrations of humus in the regions studied, the correspondence is not as good as might be expected if one assumes that the halogens are predominantly supplied through precipitation having the same chemical composition. In the Nord-Tr6ndelag area, the decrease in halogen concentrations with increasing distance from the sea by far exceeds the decrease in annual precipitation. This may be explained by a gradual change in the composition of the precipitation inland, resulting in lower contents of salts. This tendency is more pronounced for chlorine and bromine than for iodine, which indicates that iodine may in part be present in one or more chemical form less subject to wash-out from the atmosphere than the main fractions of chlorine and bromine. In eastern Norway, on the other hand, the drop in halogen values is lower than that corresponding to the decrease in annual precipitation, if one excludes areas close to the Oslo fjord. This does not necessarily reflect higher halogen concentrations in the precipitation inland. At the very low levels of precipitation evident in certain parts of this area, a higher equilibrium concentration (lower leaching) of halogens from the humus layer may be assumed. This may hold especially for chlorine, which is presumably present mainly as the chloride anion in the soil. Furthermore, contributions from the local bedrock may form a more significant part of the total concentration in soils of low halogen concentrations (Goldschmidt, 1954; Holtedahl, 1953). From the correlation coefficients in Table II strong interrelationships among the halogens are evident, supporting a theory of a c o m m o n source for these elements. The Br--I relationship is especially strong, which does n o t seem surprising in view of the similarity of chemical properties between these two elements. It is also interesting to note the strong positive correlations between C1 and exchangeable Na ÷ and Mg2 *, respectively. If the halogens are mainly supplied through precipitation, this mechanism could play a significant role in the supply of the major cations Na ÷ and Mg2 ÷, as well. This observation strengthens the conclusion drawn by L~g (1968b) on the basis of data on the chemical composition of the precipitation in relation to exchangeable cation ratios observed in soils. Considering the ratios among halogens, the C1/Br ratio shows only small variations tending to increase from values slightly below 20 in close proximity to the sea u~ to about 40 in central parts of eastern Norway. This corresponds with the range of 17--38 reported by Vinogradov (1959) for the
324
A horizons of podzolic soils. The Br/I-ratio, on the other hand, shows a regular and distinct decrease from a b o u t 6 on the coast of Nord-TrSndelag to a b o u t 1.5 in central eastern Norway. This trend may again be explained b y the presence of iodine c o m p o u n d s in the atmosphere less subject to removal by precipitation than those of bromine and chlorine. This would imply that iodine is more efficiently transported over long distances into the continents or a more effective absorption m a y take place in soils inland. The data o f Vinogradov (1959) show Br/I ratios of less than unity in various soils from the U.S.S.R. Of the three elements, iodine appears to have been most frequently studied in soils in the past. Although a direct comparison with previous work is often difficult because the properties of the softs involved may be considerably different from those studied in Norway, there still seems to be a tendency toward higher iodine values in soils in close proximity to the sea. This is indicated in work from New Zealand (Hercus et al., 1931) as well as in some European investigations (Chilean Iodine Education Bureau, 1956). None o f the previous investigations, however, seem to have been aimed at a systematic study of the role of precipitation in the supply of halogens to soils. Podzolic soils from the Russian Plain were reported to show iodine contents of 0 . 5 6 - 4 . 4 ppm, averaging 2.5 ppm (Vinogradov, 1959, p. 53). This is even lower than the values from eastern Norway, a fact which is not surprising considering the greater distance from the sea. For chlorine and bromine, for which most of the available literature appears to be from the U.S.S.R., Vinogradov (1959) reported 20--100 ppm CI and 1--10 p p m Br in most samples from A horizons of soil profiles. This is substantially lower than our values for chlorine, b u t only slightly lower than for bromine in eastern Norway. The interpretation that halogens in soils are mainly derived from the ocean suggests comparisons of the halogen ratios of sea water to those of soils. In the oceans, the C1/Br ratio is about 300, as compared to approximately 30 in the humus layer of Norwegian forest soils. This difference may be accounted for by a tenfold enrichment of bromine relative to chlorine by humic substances in the soil, and thus does not require any large separation of these two elements at the sea surface or during atmospheric transport. The great difference in the C1/I ratio, showing a value of 3- 105 in ocean water whereas the corresponding value for soils is a b o u t 102 , can hardly be explained solely by separation in the soil. It has been known for some time, however, that the I/C1 ratio in precipitation samples collected in maritime air is in general 102--103 times higher than the same ratio in sea water (Seto and Duce, 1972). Different theories for the nature of the separation causing this iodine enrichment have been presented. Miyake and Tsunogai (1963) proposed a photochemical oxidation of iodine to free iodine in the surface layer of the sea. On the other hand, the observation that iodine in the atmosphere is to a great extent present in organic form (H. I. Svensson and E. Eriksson, cited in Bolin, 1959) led Dean (1963) to suggest that iodine
325 m a y b e e n r i c h e d in s u r f a c e - a c t i v e m a t e r i a l p r e s e n t o n t h e s e a s u r f a c e , a n d correspondingly enriched on the sea-salt particles formed when bubbles break through the surface. So far, no definite conclusion with regard to the separation mechanism involved seems to have been reached (Seto and Duce, 1972).
REFERENCES Bolin, B., 1959. Note on the exchange of iodine between the atmosphere, land and sea. Int. J. Air Poll., 2: 127--131. Chilean Iodine Education Bureau, 1956. Geochemistry of Iodine. Chilean Iodine Education Bureau, Stone House, London, 150 pp. Correns, C. W., 1956. The geochemistry of the halogens. Phys. Chem. Earth, 1: 181233. Dean, G. A., 1963. The iodine content of some New Zealand drinking waters with a note on the contribution from sea spray to the iodine in rain. N.Z.J. Sci., 6, 208--214. Eriksson, E., 1960. The yearly circulation of chloride and sulfur in nature; meteorological, geochemical and pedological implications, II. Tellus, 12: 63--109. Goldschmidt, V. M., 1954. Geochemistry. Clarendon Press, Oxford, 730 pp. Hercus, C. E., Aitken, H. A. A., Thomson, H. M. S. and Cox, G. H., 1931. Further observations on the occurrence of iodine in relation to endemic goitre in New Zealand and on iodine metabolism. J. Hygiene, 31 : 493--522. Holtedahl, O., 1953. N~rges geologi. Norges Geol. Unders., 1 6 4 : 1 1 1 8 pp. L~tg, J., 1962. Undersbkelse av skogjorda i Nord-TrSndelag ved Landsskogtakseringens markarbeid sommeren 1960. (English summary). Medd. norske Skogfors.Vesen, 64: 107--160. L~g, J., 1965. UndersSkelse av jorda i skogene i Buskerud fylke. Taksering av Norges Skoger, Buskerud fylke, pp. 73--74, 147--154. L~g, J., 1968a. UndersSkelse av skogjorda i Oppland red Landsskogtakseringens markarbeid somrene 1962 og 1963 (English summary). Medd. norske Skogfors, Vesen, 91 : 331--393. L~g, J., 1968b. Relationships between the chemical composition of the precipitation and the content of exchangeable ions in the humus layer of natural soils. Acta Agric. Scand., 18: 148--152. L~g, J. and Steinnes, E., 1972. Distribution of chlorine, bromine and iodine in Norwegian forest soils studied by neutron activation analysis. In: Isotopes and Radiation in SoilPlant Relationships including Forestry. International Atomic Energy Agency Commission, Vienna, pp. 383--395. Mayers, J. L. and Duce, R. A., 1972. Gaseous and particulate iodine in the marine atmosphere. J. Ge0Phys. Res., 77: 5229--5238. Miyake, Y. and Tsunogai, S., 1963. Evaporation of iodine from the ocean. J. Geophys. Res., 68: 3989--3993. Vinogradov, A. P., 1959. The Geochemistry of Rare and Dispersed Chemical Elements in Soils. Consultants Bureau, New York, N. Y., 2nd ed., 209 pp.
317
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
R E G I O N A L DISTRIBUTION OF HALOGENS IN NORWEGIAN F O R E S T SOILS
J. L/~G and E. STEINNES
Department of Soil Science, Agricultural University of Norway, ~s-NLH (Norway) Institute for Atomenergy, Isotope Laboratories, Kjeller (Norway) (Received January 26, 1976; accepted June 3, 1976)
ABSTRACT L~g, J. and Steinnes, E., 1976. Regional distribution of halogens in Norwegian forest soils. Geoderma, 16: 317--325. Regional distribution of chlorine, bromine, and iodine in humus layers of Norwegian forest soils has been studied by means of neutron activation analysis. The halogen concentrations show a rapid decrease at increasing distances from the ocean, indicating that the supply of these elements to the soil is mainly through precipitation. Strong correlations between Cl and exchangeable Na ÷ and Mg2+, indicate atmospheric input to soils for these two cations as well.
INTRODUCTION
Discussions related to the chemical composition of soils have most often been based on the composition and properties of soil-forming rocks. For a long time the influence of the chemical composition o f precipitation on the soil chemistry was to a great extent neglected. One of the first to realize the significance of atmospheric inputs of chemical elements to soils seems to have been V. M. Goldschmidt. In his posthumous t e x t b o o k (Goldschmidt, 1954, p. 590) he points out that the supply through atmospheric precipitation is an important process in the cycle of the halogens, as well as sulphate, sodium, magnesium and some potassium. As this process goes on year by year and adds these substances to the uppermost soil horizons, it must, according to Goldschmidt's opinion, be a very important process in the evolution of soils, especially in coastal regions. More recent authors have expressed somewhat different views in this matter. According to Eriksson (1960) the amount of chlorine supplied to the rivers by natural processes represents predominantly oceanic constituents carried into the continents by the atmosphere. Correns (1956), on the other hand, in an extensive treatment of the geochemistry of the halogens, admits that a pDrtion of the halogens found in soil is contributed by rain water but concludes on the basis of available research that the major
318
share of the halogens in soils comes from rock weathering. In the survey by Vinogradov (1959) the significance of atmospheric inputs to soils is clearly emphasized in the case of iodine and to a lesser extent of bromine. During the last two decades, a large-scale investigation of forest soils in Norway has been carried out by the Agricultural University of Norway and the National Forest Survey. The programme of this investigation includes registration of important properties of soil material and soil profiles in most of the productive forest areas of the country. Besides the field registration work, humus samples for chemical analysis were collected in the counties of Nord-Trondelag, Oppland, and Buskerud (Fig. 1) during the period 1960---1964. About 3,000 samples were collected in the three counties. As will be explained later, this material offered a unique opportunity to study the influence of elements supplied through precipitation on the chemical composition of soils. An investigation of the exchangeable cations, Na ÷, Mg2÷, and Ca2+, in these soils (L~g, 1968b) indicated a geographical distribution pattern showing good correspondence with observations on the chemical composition of the
Fig. 1. Sampled areas in Norway.
319
precipitation. The cations, Na ÷ and Mg2÷, which are abundant in sea water, were f o u n d to be enriched relative to Ca 2÷ by a factor about 5 in podzol soils from the coastal districts of Nord-TrSndelag as compared to softs from central eastern Norway. This evidence of the importance of airborne supply with regard to the c o n t e n t of exchangeable cations in the soils led the present authors to focus attentian on the halogens, in which case there might also be reasons to assume distinct trends in regional distribution, because of the well-known association of these elements with the marine environment. EXPERIMENTAL
Collection of samples The humus samples were obtained from sample plots of 78.5 m 2 or 100 m 2 , regularly distributed in forest areas with an annual normal production exceeding 0.12 m3/1,000 m 2. The samples were taken from the central part of the humus layer if its thickness was 10 cm or less; in cases of thicker humus layers the depth of sampling was 5--10 cm. At least ten separate portions were collected at spots regularly distributed over the sample plot and mixed thoroughly to form one sample for analysis. The total number of samples taken in the general programme was about 1,000 in each of the counties of Nord-Tr6ndelag, Oppland, and Buskerud. For the present investigation, every fifth sample was taken for analysis. In all three counties, the collected samples predominantly represent Podzol profiles.
Determination of chlorine, bromine, and iodine The samples were subjected to neutron activation analysis t%r the determination of chlorine, bromine, and iodine. The fourth member of the halogen group, fluorine, could not easily be determined simultaneously with the available facilities. As the analytical m e t h o d used has been described in detail elsewhere (L~ig and Steinnes, 1972) only a brief account will be given here. Samples of 100--200 mg were irradiated for 5 min in the JEEP-II reactor (Kjeller, Norway) at a thermal neutron flux of 1 . 5 . 1 0 'a n cm -2 s -1, together with a halogen standard solution. The analyses were based on the radionuclides asC1 (t'A = 37.2 min), 8°Br (tlA = 17.6 min), and 12sI (tl/2 = 25.0 min), and a ~/-spectrometer based on a Ge(Li) solid-state detector was used for the activity measurement. For a great number of samples a radiochemical separation had to be carried out in order to remove interfering activities before ~/-spectrometry measurement. The reproducibility of the analyses, including possible inhomogeneous distribution of Halogens within the sample, was of the order of + 5%.
320
SOME GEOGRAPHICAL AND METEOROLOGICAL F E A T U R E S OF S O U T H E R N NORWAY
Before presenting the data, some geographical and meteorological features of the investigated areas are described as relevant to the subsequent discussion. Fig. 1 shows a map of Norway up to a latitude of 65°N, including the forest areas sampled for the present investigation. Nord-TrSndelag is an area without large variability in topography and climate. Only small areas close to the Swedish border reach altitudes higher than 900 m. The climate ranges from a coastal to a slightly more continental type. A certain decrease in mean annual precipitation takes place from areas close to the coast (about 1,200 mm} to areas near the Swedish border (about 700 mm). The air-line distance between coast and border is about 150 km, on average. The precipitation is predominantly supplied through westerly winds. The region of eastern Norway, in which the Oppland and Buskerud counties occur, has a continental-type climate, except for areas close to Oslo fjord. The area is shielded by high mountain chains to the north and west, for which reason the precipitation is to a great extent carried by southern winds. This means that the precipitation falling in the northern part of this area may originate from air masses which have moved quite a long distance after having had contact with the ocean. The mean annual precipitation is about 600 mm in large parts of the area. In the districts along the Oslo fjord it is somewhat higher whereas in the northwestern valleys it is as low as 300 mm in some places. The districts in which the forest soil samples were obtained thus differ considerably in the amounts of precipitation and in distance from the sea. Otherwise, the properties of the Nord-Tr6ndelag soils appear to be similar to those of Oppland and Buskerud (L~ig, 1962, 1965, 1968a). The soil samples therefore seem well suited for studies of regional differences in the supply of chemical constituents to the soil through precipitation. RESULTS AND DISCUSSION
The contents of C1, Br, and I were determined in about 700 humus samples, all of which had been previously analyzed for other characteristics such as pH, loss on ignition, and exchangeable cations. For convenience of discussion, the data from the present study have been divided into 29 sub-groups on a geographical basis, each group comprising 15--30 samples. The location corresponding to each sub-group is indicated in Fig. 1. The numerical order of these sub-areas within each of two major areas has been so arranged as to indicate a decreasing influence of the marine environment as estimated from geographical and meteorological factors. The mean values for the concentrations of C1, Br, and I in the various subgroups of soil, are given in Table I. The C1/Br, Cl/I and Br/I ratios are also
321
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÷
¢.)
..=
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~
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~
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322
listed in the table, as are data on approximate annual precipitation for the locations in question and mean values for organic matter content, exchangeable Na* and exchangeable Mg 2÷, the latter values having been determined in a previous stage of the project (J. L~g, unpublished work). In Table II, some correlation coefficients calculated from data in Table I are given. TABLE II Some correlation coefficients calculated on the basis of sub-group m e a n values from Table I
CI--Br CI--I Br--I CI--Na CI--Mg
Nord-Tr~ndelag
Eastern Norway
All data
0.85 0.68 0.92 0.92 0.84
0.67 0.77 0.93 0.74 0.52
0.93 0.85 0.94 0.97 0.94
--
The results exhibit a very clear trend of decreasing halogen concentrations with increasing distance from the sea. In Nord-Tr6ndelag a chlorine level of the order of 1,500 ppm is present in soils close to the coast, whereas the level 150 km inland has dropped to about 500 ppm. For bromine, the corresponding levels are 90 ppm and 20 ppm, and for iodine 15 ppm and 6 ppm, respectively. In the areas investigated in eastern Norway, still lower concentrations are observed, but the concentration gradients are much smaller than in Nord-TrSndelag. The chlorine concentration, except for areas near the Oslo fjord, shows only a slight decrease from approximately 300 p p m to 200 ppm in going to areas of lower annual precipitation. Corresponding figures are 7 to 5 ppm for bromine and 5 to 3 ppm for iodine. The possibility exists that the natural halogen distributions could be subject to interference from human activities. Chlorine may be introduced through the use of CaC12 as road salt, and bromine is known to be released through automobile exhaust. Such effects have not been found in the present samples. Another factor that may affect the observed distribution is differing humus contents of the soil samples. As can be seen from Table I, the samples of soils near the coast show a somewhat higher organic matter content (70-80%) than do those from inland regions (50--60%). It has been pointed o u t in the literature that bromine and iodine are concentrated in the humus layer of the soil (see e.g. Correns, 1956; Vinogradov, 1959). If the correlation between the halogens and humus was very strong, this could in part explain the observed distributions. Correlation coefficients calculated on the basis of the data of single samples showed the following results:
323
Nord- Trbndelag
Eastern Norway
Cl - organic matter: 0.54 Br - organic matter: 0.36 I - organic matter: 0.30
C1 - organic matter: 0.62 Br - organic matter: 0.42 I - organic matter: 0.41
As seen, the observed correlations are not strong. It is therefore not likely that differences in humus content between sub-groups has had a decisive influence on the observed halogen distributions. Although there appears to be some connection between the amounts of annual precipitation and the halogen concentrations of humus in the regions studied, the correspondence is not as good as might be expected if one assumes that the halogens are predominantly supplied through precipitation having the same chemical composition. In the Nord-Tr6ndelag area, the decrease in halogen concentrations with increasing distance from the sea by far exceeds the decrease in annual precipitation. This may be explained by a gradual change in the composition of the precipitation inland, resulting in lower contents of salts. This tendency is more pronounced for chlorine and bromine than for iodine, which indicates that iodine may in part be present in one or more chemical form less subject to wash-out from the atmosphere than the main fractions of chlorine and bromine. In eastern Norway, on the other hand, the drop in halogen values is lower than that corresponding to the decrease in annual precipitation, if one excludes areas close to the Oslo fjord. This does not necessarily reflect higher halogen concentrations in the precipitation inland. At the very low levels of precipitation evident in certain parts of this area, a higher equilibrium concentration (lower leaching) of halogens from the humus layer may be assumed. This may hold especially for chlorine, which is presumably present mainly as the chloride anion in the soil. Furthermore, contributions from the local bedrock may form a more significant part of the total concentration in soils of low halogen concentrations (Goldschmidt, 1954; Holtedahl, 1953). From the correlation coefficients in Table II strong interrelationships among the halogens are evident, supporting a theory of a c o m m o n source for these elements. The Br--I relationship is especially strong, which does n o t seem surprising in view of the similarity of chemical properties between these two elements. It is also interesting to note the strong positive correlations between C1 and exchangeable Na ÷ and Mg2 *, respectively. If the halogens are mainly supplied through precipitation, this mechanism could play a significant role in the supply of the major cations Na ÷ and Mg2 ÷, as well. This observation strengthens the conclusion drawn by L~g (1968b) on the basis of data on the chemical composition of the precipitation in relation to exchangeable cation ratios observed in soils. Considering the ratios among halogens, the C1/Br ratio shows only small variations tending to increase from values slightly below 20 in close proximity to the sea u~ to about 40 in central parts of eastern Norway. This corresponds with the range of 17--38 reported by Vinogradov (1959) for the
324
A horizons of podzolic soils. The Br/I-ratio, on the other hand, shows a regular and distinct decrease from a b o u t 6 on the coast of Nord-TrSndelag to a b o u t 1.5 in central eastern Norway. This trend may again be explained b y the presence of iodine c o m p o u n d s in the atmosphere less subject to removal by precipitation than those of bromine and chlorine. This would imply that iodine is more efficiently transported over long distances into the continents or a more effective absorption m a y take place in soils inland. The data o f Vinogradov (1959) show Br/I ratios of less than unity in various soils from the U.S.S.R. Of the three elements, iodine appears to have been most frequently studied in soils in the past. Although a direct comparison with previous work is often difficult because the properties of the softs involved may be considerably different from those studied in Norway, there still seems to be a tendency toward higher iodine values in soils in close proximity to the sea. This is indicated in work from New Zealand (Hercus et al., 1931) as well as in some European investigations (Chilean Iodine Education Bureau, 1956). None o f the previous investigations, however, seem to have been aimed at a systematic study of the role of precipitation in the supply of halogens to soils. Podzolic soils from the Russian Plain were reported to show iodine contents of 0 . 5 6 - 4 . 4 ppm, averaging 2.5 ppm (Vinogradov, 1959, p. 53). This is even lower than the values from eastern Norway, a fact which is not surprising considering the greater distance from the sea. For chlorine and bromine, for which most of the available literature appears to be from the U.S.S.R., Vinogradov (1959) reported 20--100 ppm CI and 1--10 p p m Br in most samples from A horizons of soil profiles. This is substantially lower than our values for chlorine, b u t only slightly lower than for bromine in eastern Norway. The interpretation that halogens in soils are mainly derived from the ocean suggests comparisons of the halogen ratios of sea water to those of soils. In the oceans, the C1/Br ratio is about 300, as compared to approximately 30 in the humus layer of Norwegian forest soils. This difference may be accounted for by a tenfold enrichment of bromine relative to chlorine by humic substances in the soil, and thus does not require any large separation of these two elements at the sea surface or during atmospheric transport. The great difference in the C1/I ratio, showing a value of 3- 105 in ocean water whereas the corresponding value for soils is a b o u t 102 , can hardly be explained solely by separation in the soil. It has been known for some time, however, that the I/C1 ratio in precipitation samples collected in maritime air is in general 102--103 times higher than the same ratio in sea water (Seto and Duce, 1972). Different theories for the nature of the separation causing this iodine enrichment have been presented. Miyake and Tsunogai (1963) proposed a photochemical oxidation of iodine to free iodine in the surface layer of the sea. On the other hand, the observation that iodine in the atmosphere is to a great extent present in organic form (H. I. Svensson and E. Eriksson, cited in Bolin, 1959) led Dean (1963) to suggest that iodine
325 m a y b e e n r i c h e d in s u r f a c e - a c t i v e m a t e r i a l p r e s e n t o n t h e s e a s u r f a c e , a n d correspondingly enriched on the sea-salt particles formed when bubbles break through the surface. So far, no definite conclusion with regard to the separation mechanism involved seems to have been reached (Seto and Duce, 1972).
REFERENCES Bolin, B., 1959. Note on the exchange of iodine between the atmosphere, land and sea. Int. J. Air Poll., 2: 127--131. Chilean Iodine Education Bureau, 1956. Geochemistry of Iodine. Chilean Iodine Education Bureau, Stone House, London, 150 pp. Correns, C. W., 1956. The geochemistry of the halogens. Phys. Chem. Earth, 1: 181233. Dean, G. A., 1963. The iodine content of some New Zealand drinking waters with a note on the contribution from sea spray to the iodine in rain. N.Z.J. Sci., 6, 208--214. Eriksson, E., 1960. The yearly circulation of chloride and sulfur in nature; meteorological, geochemical and pedological implications, II. Tellus, 12: 63--109. Goldschmidt, V. M., 1954. Geochemistry. Clarendon Press, Oxford, 730 pp. Hercus, C. E., Aitken, H. A. A., Thomson, H. M. S. and Cox, G. H., 1931. Further observations on the occurrence of iodine in relation to endemic goitre in New Zealand and on iodine metabolism. J. Hygiene, 31 : 493--522. Holtedahl, O., 1953. N~rges geologi. Norges Geol. Unders., 1 6 4 : 1 1 1 8 pp. L~tg, J., 1962. Undersbkelse av skogjorda i Nord-TrSndelag ved Landsskogtakseringens markarbeid sommeren 1960. (English summary). Medd. norske Skogfors.Vesen, 64: 107--160. L~g, J., 1965. UndersSkelse av jorda i skogene i Buskerud fylke. Taksering av Norges Skoger, Buskerud fylke, pp. 73--74, 147--154. L~g, J., 1968a. UndersSkelse av skogjorda i Oppland red Landsskogtakseringens markarbeid somrene 1962 og 1963 (English summary). Medd. norske Skogfors, Vesen, 91 : 331--393. L~g, J., 1968b. Relationships between the chemical composition of the precipitation and the content of exchangeable ions in the humus layer of natural soils. Acta Agric. Scand., 18: 148--152. L~g, J. and Steinnes, E., 1972. Distribution of chlorine, bromine and iodine in Norwegian forest soils studied by neutron activation analysis. In: Isotopes and Radiation in SoilPlant Relationships including Forestry. International Atomic Energy Agency Commission, Vienna, pp. 383--395. Mayers, J. L. and Duce, R. A., 1972. Gaseous and particulate iodine in the marine atmosphere. J. Ge0Phys. Res., 77: 5229--5238. Miyake, Y. and Tsunogai, S., 1963. Evaporation of iodine from the ocean. J. Geophys. Res., 68: 3989--3993. Vinogradov, A. P., 1959. The Geochemistry of Rare and Dispersed Chemical Elements in Soils. Consultants Bureau, New York, N. Y., 2nd ed., 209 pp.