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Combined studies of soil and groundwater chemistry, southwest Sweden, focusing on Al and SO4
Appl~d Geochemistry. Suppl. Issue No.2. pp. 281-283. 1993 Printed in Great Britain
088>-2927193 $6.00+ .00
© 1992PergamonPressUd
Combined studies of soil and groundwater chemistry, southwest Sweden, focusing on Al and S04 JAN SJOSTROM
University of Uppsala, Department of Quaternary Geology and Hydrogeology, Box 555, 75122 Uppsala, Sweden Abstract-The study concerns samples from 22 acidic ground/drinking waters and 12 podzolic soil profiles in southwest Sweden. Exchangeable S04 and Al in the B horizon are positively correlated. A plot of ionic activities in a solubility diagram suggests that basaluminite may control Al and S04 concentrations in the groundwaters. It seems most probable that adsorption/desorption is the regulating mechanism because basic AI-sulphates have not been found in the soils using X-ray diffraction.
INTRODUCTION
SOILS AND groundwaters in Halland County, on the west coast of Sweden, are among the most acidified in the country. The major cause is the airborne acidic load (i.e. wet plus dry deposition) which emanates from central Europe. In combination with the crystalline bedrock and the often thin cover of glacial till, the load has resulted in the wide occurrence of acidic groundwater. Consequently, in many groundwater wells deteriorating water quality has become a common problem both for drinking and other uses, i.e. particularly in shallow private wells m). From the point of view of environment and health, the content of soluble inorganic Al in ground/drinking water deserves special attention. The metal may be toxic to plant roots, or to the biota when it is leached out into surface waters. There are also discussions about possible health effects of high Al concentrations in drinking water (e. g. GUNDERSEN and RAsMUSSEN, 1988; NATIONAL SWEDISH ENVIRONMENTAL PRoTEcnON BOARD, 1986). A soil precipitate of Al and S04 could serve as a potential source of H 2S04 and AI in the future if the S04 concentration in soil/ groundwater is lowered, i.e . atmospheric S deposition is cut down. The primary objectives of this study were : (1) to find the relation between extractable AI and S04 in the soil; (2) to model groundwater chemistry with respect to AI-S0 4 compounds; and (3) to evaluate features of the soil minerals and weathering. Previous results from a groundwater acidification monitoring programme within the area indicated that S04 and Al may be retained in the soil (SJOSTROM, 1990). From the programme, two wells and ten springs were selected for comparative studies of soil and groundwater chemistry (Fig. 1). Sites 33 and 56 are situated on sandy glaciofluvial sediment and the other sites on sandy-silty glacial till. The sites are located on forested land except site 20 which is situated in a meadow. The soil type is Orthic Podzol,
«5
AI sUPP zoa
FIG. 1. A map of Halland County showing the sites selected for investigation of soil and groundwater chem istry.
except for sites 1, 20 and 85 where it is classified as podzolized Cambisol. SAMPLES AND METHODOLOGY The soil analysis concerns twelve A 2 horizon samples , five B. (upper) horizon samples, twelve B2 (lower) plus B horizon samples, and two C horizon samples . The chemical analyses were carried out on air-dried soil <0.5 mm. Aluminium and S04 were measured in a l M ammonium acetate extraction (pH 4.8; 4 ml ammonium acetate per gram of dry soil) . Atomic absorption spectrometry (AAS) was used for determination of AI, and inductively coupled plasma (ICP) for S (S04-S). The ICP method gave the total S content in the extract which may include some organically bound S. This organically bound portion can be estimated
281
282
J. Sjostrom
roughly in the range from -0 to 19 f.lWg dry soil for the present samples (0.58 x Loss on Ignition; based on data from l'ROBDSSON and NYKVIST, 1973; and NOMMIK et al., 1988). For samples taken from the soil profiles at sites 23 and 54, the <0.002 mm fraction was separated by sedimentation in deionized distilled water and vacuum filtration. "Total" samples consisted of soil <0.5 mm which was pulverized in an agate mortar. Analysis by X-ray diffraction (XRD) of these fractions and microscopic examination of bulk samples, were used for mineralogical identification. Samples from shallow groundwater at 22 sites within the monitoring programme were used, which include 9 of the 12 sites above having detectable Al in groundwater. Aluminium and S04 in groundwater were analysed by a HACH DR/3 Spectrophotometer, and pH by a pHM 82 Standard pH meter. No organically bound Al was considered to occur in these samples because they are "clean" drinking waters. Aluminium complexation with F was neglected because previous analyses had shown very low or undetectable total F concentrations in the groundwater. Potassium was determined by emission in an atomic absorption spectrometer. The WATEQX model (VAN GAANS, 1989) was used for calculation of ionic speciation and ionic activities of the groundwaters (applying the Mean Salt Method based on Pitzers parameters). Input for the model consisted of averages of pH and S04 for samples taken in June 1987-1991 and averages of Al and K for June 1990and 1991; plus the following supplementary input: Si02 (June 1991), CI (June 1987), Na (June 1990-1991), and N03, HC03, Fe, Mn, Ca and Mg (June 1987-1991). Soilswere sampled in June 1989. The ICP analyses were performed by the Swedish Geological Company in Lulea and all other analyses at the Institute of Geology, University of Uppsala.
2llOO
V. ·137.07 +... X· o.olllIX'
'iii 1500
!
~
•
• 1000
As shown in Fig. 2, there is a positive correlation between extractable AI and S04 for the 31 soils sampled (from all horizons). Separate correlations for each soil horizon show a significant (p < 0.(03) positive AI-S04 correlation within the B horizons (i.e. B.. B2 and B). In Fig. 3 the ionic activity products modeled are put into relation with the solubility lines for AI-S04 compounds, and AI(OHh (data extracted from the literature). The groundwaters are undersaturated with respect to jurbanite and oversaturated with respect to alunite. The data seem to fit the basaluminite equilibria lines best. The XRD-analyses of the soil profiles at sites 23 and 54 showed that the soils are dominated by quartz and feldspars, both in the <0.002 mm and "total" fractions; these represent -1-3% and -70%, respectively, of soil <20 mm. The most abundant clay mineral is chlorite-vermiculite (up to -18% of quartz in the <0.002 mm fraction). Amphiboles (hornblende) are present in both fractions (-2-8% of quartz), and also some micas. To be detected by XRD, a minimum amount of -3% of basic aluminium sulphate (crystalline phase) in the total sample is needed. This means that the absence of basic aluminium sulphate spikes could result from either too Iowa content «2-3%) or the absence of these compounds, or too amorphous phases.
•
m •
•
• •
500
00
100
200
300
,.0. c 0.0001. n. 31 eoo 400 500
so. (JIM)
2llOO~--------------,
v..... + 7.• X. O.ooexa
2llOO
• I'
'iii 1500
!
I
•
•
~ 1000 500
100
RESULTS AND DISCUSSION
•
., •
2llOO
200
300
,.0.78, c 0.0003, n .17 400 500 eoo
SO._) FIG. 2. Extractable Al vs S04' I. A, Band C horizons; II. B horizons.
31r----------------.
FIG. 3. Solubility diagram (25°C, 1 atm) for: (A) alunite (KA13 (S04MOH)6), pK. = 83.4 and 85.6 (NORDSTROM, 1982), pK+ = 4.38, i.e, average for the samples; (G) gibbsite (aluminium hydroxide, AI(OH)3), pK. = 31.2 (STUMM and MORGAN, 1981), 32.3 (SILL~ and MARTELL, 1964) and 33.9 (MAY et al., 1979); (B) basaluminite (AI. (S04) (OH)1O·5(H20», pK. = 116 and 117.7 (ADAMS and RAWAIFlH, 1977) and 117.3 (SINGH, 1969); (1) jurbanite (AIS040H·5(H20», pK. = 17 and 18 (KHANNA et al., 1987). Plotted X's represent groundwaters from Halland County.
Soil and groundwater chemistry, southwest Sweden
Microscopic examination of thin sections from the upper soil horizons at sites 23 and 54 revealed weathering features on feldspar, amphibole, and biotite grains as clay mineral formation under replacement by Fe- and Ti-oxides along fractures and cleavage plains. Within the A z and B 1 horizons (-11 and 16 em average depth, respectively), clay minerals and Fe- and Ti-oxides were present as aggregates which often included organic matter, e.g. root fragments. Thin sections from the Bz and C horizons (- 37 and 83 em average depth, respectively) show much less weathered soil material.
CONCLUSIONS
Exchangeable concentrations of AI and S04 show a positive correlation, particularily in the B horizon. As AI is released from minerals due to weathering in the eluvial horizon (A z), it leaches downward and accumulates naturally in the enrichment horizon (B), along with Fe and humus. The major part of S04 comes from atmospheric deposition. The weathering intensity appears to be considerably higher in the A z and B 1 horizons. Within these layers organic matter should play an important role for aggregate forming and coating of soil minerals. The plot of ionic activity products of the groundwaters in the solubility diagram (Fig. 3) suggests basaluminite as a possible controlling compound. An adsorption of exchangeable Al and S04 in the soil (B horizon) could then be an initializing step for solid precipitation. If so, such a compound should occur in the soils-which has not been positively proved so far on the basis of results from the XRD analysis. However, basic aluminium sulphates have not yet been revealed in any Swedish forest soils. This favors the theory that adsorption/desorption processes regulate the presence of Al and S04 in the studied groundwaters. Editorial handling: Ron Fuge.
283
REFERENCES ADAMS F. and RAWAJFIH Z. (1977) Basaluminite and alunite: a possible cause of sulfate retention by acid soils. Soil. Sci. Soc. Am. J. 41,686-692. VAN GAANS P. F. M. (1989) WATEQX-A restructured, generalized, and extended Fortran 77 computer code and database format for the WATEQX aqueous chemical model for element speciation and mineral saturation, for use on personal computers or mainframes. Comput. Geosci. 15,843--887. GUNDERSEN P. and RASMUSSEN L. (1988) Nitrification, acidification and aluminium release in forest soils. In Critical Loads for Sulphur and Nitrogen. Nordic Council of Ministers. Environmental Rept 1988:15, 225-268. KHANNA P. K., PRENZEL J., MElWES K. J., ULRICH B. and MATZNER E. (1987) Dynamics of sulfate retention by acid forest soils in an acidic deposition environment. Soil. Sci. Soc. Am. J. 51,446-452. MAy H. M., HELMKE P. A. and JACKSON M. L. (1979) Gibbsite solubility and thermodynamic properties of hydroxy-aluminium ions in aqueous solution at 25°C. Geochim. cosmochim. Acta 43,861--868. NATIONAL SWEDISH ENVlRONMENTAL PROTECTION BOARD (1986) Monitor 1986-Sura och forsurade vatten. ("Acidic and acidified waters"; in Swedish.) NORDSTROM D. K. (1982) The effect of sulfate on aluminium concentrations in natural waters: some stability relations in the system Ah03-S0rH20 at 298 K. Geochim. cosmochim. Acta 46, 681~92. NOMMIK H., POPOVIC B., LARSSON K., NILSSON A. and CLEGG S. (1988) Sulphate stores and sulphate adsorption capacity of the spodic B horizon of Swedish forest soils. National Swedish Environmental Protection Board. Rept 3520. SILLEN L. G. and MARTELL A. E. (1964) Stability Constants of Metal-ion Complexes. Chern. Soc. London. Spec. Pub. 17. SINGH S. S. (1969) Basic aluminum sulfate formed as a metastable phase and its transformation to gibbsite. Can. J. Soil. Sci. 49, 383-388. SJOSTROM J. (1990) Grundvatten och forsurning i Hallands Ian 1986--1989. ("Groundwater and acidification in Halland 1986--1989": with an English summary.) Report from the Community Association of Halland. STUMM W. and MORGAN J. J. (1981) Aquatic Chemistry. Wiley-Interscience. TROEDSSON T. and NYKVIST N. (1973) Marklara och markvard, ("Soil science and soil protection"; in Swedish.) Almqvist and Wiksell, Uppsala, Sweden.
088>-2927193 $6.00+ .00
© 1992PergamonPressUd
Combined studies of soil and groundwater chemistry, southwest Sweden, focusing on Al and S04 JAN SJOSTROM
University of Uppsala, Department of Quaternary Geology and Hydrogeology, Box 555, 75122 Uppsala, Sweden Abstract-The study concerns samples from 22 acidic ground/drinking waters and 12 podzolic soil profiles in southwest Sweden. Exchangeable S04 and Al in the B horizon are positively correlated. A plot of ionic activities in a solubility diagram suggests that basaluminite may control Al and S04 concentrations in the groundwaters. It seems most probable that adsorption/desorption is the regulating mechanism because basic AI-sulphates have not been found in the soils using X-ray diffraction.
INTRODUCTION
SOILS AND groundwaters in Halland County, on the west coast of Sweden, are among the most acidified in the country. The major cause is the airborne acidic load (i.e. wet plus dry deposition) which emanates from central Europe. In combination with the crystalline bedrock and the often thin cover of glacial till, the load has resulted in the wide occurrence of acidic groundwater. Consequently, in many groundwater wells deteriorating water quality has become a common problem both for drinking and other uses, i.e. particularly in shallow private wells m). From the point of view of environment and health, the content of soluble inorganic Al in ground/drinking water deserves special attention. The metal may be toxic to plant roots, or to the biota when it is leached out into surface waters. There are also discussions about possible health effects of high Al concentrations in drinking water (e. g. GUNDERSEN and RAsMUSSEN, 1988; NATIONAL SWEDISH ENVIRONMENTAL PRoTEcnON BOARD, 1986). A soil precipitate of Al and S04 could serve as a potential source of H 2S04 and AI in the future if the S04 concentration in soil/ groundwater is lowered, i.e . atmospheric S deposition is cut down. The primary objectives of this study were : (1) to find the relation between extractable AI and S04 in the soil; (2) to model groundwater chemistry with respect to AI-S0 4 compounds; and (3) to evaluate features of the soil minerals and weathering. Previous results from a groundwater acidification monitoring programme within the area indicated that S04 and Al may be retained in the soil (SJOSTROM, 1990). From the programme, two wells and ten springs were selected for comparative studies of soil and groundwater chemistry (Fig. 1). Sites 33 and 56 are situated on sandy glaciofluvial sediment and the other sites on sandy-silty glacial till. The sites are located on forested land except site 20 which is situated in a meadow. The soil type is Orthic Podzol,
«5
AI sUPP zoa
FIG. 1. A map of Halland County showing the sites selected for investigation of soil and groundwater chem istry.
except for sites 1, 20 and 85 where it is classified as podzolized Cambisol. SAMPLES AND METHODOLOGY The soil analysis concerns twelve A 2 horizon samples , five B. (upper) horizon samples, twelve B2 (lower) plus B horizon samples, and two C horizon samples . The chemical analyses were carried out on air-dried soil <0.5 mm. Aluminium and S04 were measured in a l M ammonium acetate extraction (pH 4.8; 4 ml ammonium acetate per gram of dry soil) . Atomic absorption spectrometry (AAS) was used for determination of AI, and inductively coupled plasma (ICP) for S (S04-S). The ICP method gave the total S content in the extract which may include some organically bound S. This organically bound portion can be estimated
281
282
J. Sjostrom
roughly in the range from -0 to 19 f.lWg dry soil for the present samples (0.58 x Loss on Ignition; based on data from l'ROBDSSON and NYKVIST, 1973; and NOMMIK et al., 1988). For samples taken from the soil profiles at sites 23 and 54, the <0.002 mm fraction was separated by sedimentation in deionized distilled water and vacuum filtration. "Total" samples consisted of soil <0.5 mm which was pulverized in an agate mortar. Analysis by X-ray diffraction (XRD) of these fractions and microscopic examination of bulk samples, were used for mineralogical identification. Samples from shallow groundwater at 22 sites within the monitoring programme were used, which include 9 of the 12 sites above having detectable Al in groundwater. Aluminium and S04 in groundwater were analysed by a HACH DR/3 Spectrophotometer, and pH by a pHM 82 Standard pH meter. No organically bound Al was considered to occur in these samples because they are "clean" drinking waters. Aluminium complexation with F was neglected because previous analyses had shown very low or undetectable total F concentrations in the groundwater. Potassium was determined by emission in an atomic absorption spectrometer. The WATEQX model (VAN GAANS, 1989) was used for calculation of ionic speciation and ionic activities of the groundwaters (applying the Mean Salt Method based on Pitzers parameters). Input for the model consisted of averages of pH and S04 for samples taken in June 1987-1991 and averages of Al and K for June 1990and 1991; plus the following supplementary input: Si02 (June 1991), CI (June 1987), Na (June 1990-1991), and N03, HC03, Fe, Mn, Ca and Mg (June 1987-1991). Soilswere sampled in June 1989. The ICP analyses were performed by the Swedish Geological Company in Lulea and all other analyses at the Institute of Geology, University of Uppsala.
2llOO
V. ·137.07 +... X· o.olllIX'
'iii 1500
!
~
•
• 1000
As shown in Fig. 2, there is a positive correlation between extractable AI and S04 for the 31 soils sampled (from all horizons). Separate correlations for each soil horizon show a significant (p < 0.(03) positive AI-S04 correlation within the B horizons (i.e. B.. B2 and B). In Fig. 3 the ionic activity products modeled are put into relation with the solubility lines for AI-S04 compounds, and AI(OHh (data extracted from the literature). The groundwaters are undersaturated with respect to jurbanite and oversaturated with respect to alunite. The data seem to fit the basaluminite equilibria lines best. The XRD-analyses of the soil profiles at sites 23 and 54 showed that the soils are dominated by quartz and feldspars, both in the <0.002 mm and "total" fractions; these represent -1-3% and -70%, respectively, of soil <20 mm. The most abundant clay mineral is chlorite-vermiculite (up to -18% of quartz in the <0.002 mm fraction). Amphiboles (hornblende) are present in both fractions (-2-8% of quartz), and also some micas. To be detected by XRD, a minimum amount of -3% of basic aluminium sulphate (crystalline phase) in the total sample is needed. This means that the absence of basic aluminium sulphate spikes could result from either too Iowa content «2-3%) or the absence of these compounds, or too amorphous phases.
•
m •
•
• •
500
00
100
200
300
,.0. c 0.0001. n. 31 eoo 400 500
so. (JIM)
2llOO~--------------,
v..... + 7.• X. O.ooexa
2llOO
• I'
'iii 1500
!
I
•
•
~ 1000 500
100
RESULTS AND DISCUSSION
•
., •
2llOO
200
300
,.0.78, c 0.0003, n .17 400 500 eoo
SO._) FIG. 2. Extractable Al vs S04' I. A, Band C horizons; II. B horizons.
31r----------------.
FIG. 3. Solubility diagram (25°C, 1 atm) for: (A) alunite (KA13 (S04MOH)6), pK. = 83.4 and 85.6 (NORDSTROM, 1982), pK+ = 4.38, i.e, average for the samples; (G) gibbsite (aluminium hydroxide, AI(OH)3), pK. = 31.2 (STUMM and MORGAN, 1981), 32.3 (SILL~ and MARTELL, 1964) and 33.9 (MAY et al., 1979); (B) basaluminite (AI. (S04) (OH)1O·5(H20», pK. = 116 and 117.7 (ADAMS and RAWAIFlH, 1977) and 117.3 (SINGH, 1969); (1) jurbanite (AIS040H·5(H20», pK. = 17 and 18 (KHANNA et al., 1987). Plotted X's represent groundwaters from Halland County.
Soil and groundwater chemistry, southwest Sweden
Microscopic examination of thin sections from the upper soil horizons at sites 23 and 54 revealed weathering features on feldspar, amphibole, and biotite grains as clay mineral formation under replacement by Fe- and Ti-oxides along fractures and cleavage plains. Within the A z and B 1 horizons (-11 and 16 em average depth, respectively), clay minerals and Fe- and Ti-oxides were present as aggregates which often included organic matter, e.g. root fragments. Thin sections from the Bz and C horizons (- 37 and 83 em average depth, respectively) show much less weathered soil material.
CONCLUSIONS
Exchangeable concentrations of AI and S04 show a positive correlation, particularily in the B horizon. As AI is released from minerals due to weathering in the eluvial horizon (A z), it leaches downward and accumulates naturally in the enrichment horizon (B), along with Fe and humus. The major part of S04 comes from atmospheric deposition. The weathering intensity appears to be considerably higher in the A z and B 1 horizons. Within these layers organic matter should play an important role for aggregate forming and coating of soil minerals. The plot of ionic activity products of the groundwaters in the solubility diagram (Fig. 3) suggests basaluminite as a possible controlling compound. An adsorption of exchangeable Al and S04 in the soil (B horizon) could then be an initializing step for solid precipitation. If so, such a compound should occur in the soils-which has not been positively proved so far on the basis of results from the XRD analysis. However, basic aluminium sulphates have not yet been revealed in any Swedish forest soils. This favors the theory that adsorption/desorption processes regulate the presence of Al and S04 in the studied groundwaters. Editorial handling: Ron Fuge.
283
REFERENCES ADAMS F. and RAWAJFIH Z. (1977) Basaluminite and alunite: a possible cause of sulfate retention by acid soils. Soil. Sci. Soc. Am. J. 41,686-692. VAN GAANS P. F. M. (1989) WATEQX-A restructured, generalized, and extended Fortran 77 computer code and database format for the WATEQX aqueous chemical model for element speciation and mineral saturation, for use on personal computers or mainframes. Comput. Geosci. 15,843--887. GUNDERSEN P. and RASMUSSEN L. (1988) Nitrification, acidification and aluminium release in forest soils. In Critical Loads for Sulphur and Nitrogen. Nordic Council of Ministers. Environmental Rept 1988:15, 225-268. KHANNA P. K., PRENZEL J., MElWES K. J., ULRICH B. and MATZNER E. (1987) Dynamics of sulfate retention by acid forest soils in an acidic deposition environment. Soil. Sci. Soc. Am. J. 51,446-452. MAy H. M., HELMKE P. A. and JACKSON M. L. (1979) Gibbsite solubility and thermodynamic properties of hydroxy-aluminium ions in aqueous solution at 25°C. Geochim. cosmochim. Acta 43,861--868. NATIONAL SWEDISH ENVlRONMENTAL PROTECTION BOARD (1986) Monitor 1986-Sura och forsurade vatten. ("Acidic and acidified waters"; in Swedish.) NORDSTROM D. K. (1982) The effect of sulfate on aluminium concentrations in natural waters: some stability relations in the system Ah03-S0rH20 at 298 K. Geochim. cosmochim. Acta 46, 681~92. NOMMIK H., POPOVIC B., LARSSON K., NILSSON A. and CLEGG S. (1988) Sulphate stores and sulphate adsorption capacity of the spodic B horizon of Swedish forest soils. National Swedish Environmental Protection Board. Rept 3520. SILLEN L. G. and MARTELL A. E. (1964) Stability Constants of Metal-ion Complexes. Chern. Soc. London. Spec. Pub. 17. SINGH S. S. (1969) Basic aluminum sulfate formed as a metastable phase and its transformation to gibbsite. Can. J. Soil. Sci. 49, 383-388. SJOSTROM J. (1990) Grundvatten och forsurning i Hallands Ian 1986--1989. ("Groundwater and acidification in Halland 1986--1989": with an English summary.) Report from the Community Association of Halland. STUMM W. and MORGAN J. J. (1981) Aquatic Chemistry. Wiley-Interscience. TROEDSSON T. and NYKVIST N. (1973) Marklara och markvard, ("Soil science and soil protection"; in Swedish.) Almqvist and Wiksell, Uppsala, Sweden.