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The content and distribution of cadmium in soils as influenced by the soil properties
the Science of the Total Environment ELSEVIER
The Science of the Total Environment 156 (1994) 183-190
The content and distribution of cadmium in soils as influenced by the soil properties M. S~nchez-Camazano*, M.J. Sanchez-Martin, L.F. Lorenzo Instituto de Recursos Naturalesy Agrobiologla de Salamanca, C.S.I.C., Cordelde Merinas, 40-52, Apdo 257, Salamanca, Spain Received 5 March 1993; accepted 12 January 1994
Abstract
The total and soluble cadmium contents of 29 natural, uncultured, unpolluted soils from the province of Salamanca (Spain) were determined. The two forms of the metal were found to occur at concentrations over the ranges 0.07-0.33 and 0.004-0.061 p.g/g, respectively. The total Cd contents found were somewhat low relative to reported values for worldwide soils in general and European soils in particular. No significant correlation was found between the total and soluble Cd contents or between the former and any chemical parameters of the soils. On the other hand, there was a highly significant correlation (P < 0.001) between soluble Cd and the pH, sum of bases and exchangeable Ca, as well as a significant correlation (P < 0.01) between soluble Cd and the cation exchange capacity, degree of saturation and exchangeable Mg content of the soils. Finally, there were virtually no similarities in the distribution patterns of Cd and the other soil parameters in a given soil profile.
Keywords: Total cadmium; Soluble cadmium; Soil properties; Spain
1. Introduction The growing development of agricultural, industrial and urban activities has given rise to a number of environmental problems because of the presence of heavy metals, in general and cadmium in particular. It has thus become mandatory to develop methods for assessing this metal and elucidating its behaviour in the natural environment in order to be able to predict its behaviour at the increased concentrations resulting from human activities. The potential toxicity of cadmium to plants, animals and humans has encouraged research into the presence of this metal in soils, with
* Corresponding author.
emphasis on soils treated with sewage sludges or phosphate fertilizers [1-4] and those exposed to some Cd polluting source [5-8]. On the other hand, the Cd content of natural, uncultured, unpolluted soils is typically dealt with as a background content and given much less attention. Thus, in a recent review on environmental cadmium in Europe, Jensen and Bro-Rasmussen [9] underscored the lack of information on the Cd content of normal soils in southern European countries. The background Cd content of soils is quite an interesting reference for determining local or regional variations in this element and anticipating potential hazards from polluting sources. The total Cd content of soils should, in principle, be related to its content in the original rock on which the soil developed, as well as to the alter-
0048-9697/94/$07.00 © 1994 Elsevier Science BV. All rights reserved.
SSD1 0048-9697(94)04152-D
M. Sdnchez-Camazano et al. / Sci. Total Environ. 156 (1994) 183 190
184
ations undergone by this staring material. The forms in which Cd may occur in different soil horizons and their distribution depend on the physical, chemical and mineralogical properties of the soil in question. Of such forms, that known as exchangeable or soluble cadmium is of special interest as it determines the actual environmental exposure. In this work, the total and soluble Cd contents of 29 natural (uncultured, unpolluted) soils from the province of Salamanca (Spain) were determined, and the potential relationship between such contents and the soil properties, in addition to the distribution of both Cd forms in terms of depth in six soil profiles were studied.
province of Salamanca (Spain) (Fig. 1). The sampiing sites were chosen not only in terms of their geographical situation, but also in such a way that they encompassed the types of rocks most frequently encountered in the province. The chosen sites were also intended to represent as wide a variety of vegetation, climate and soils as possible. Samples were collected from the soil surface horizon (horizon A), the depth of which ranged between 0-15 and 0-30 cm. As a rule, the original material consisted of granite, slate or clay and sand conglomerates. Soils were from scarcely developed (litho soils or ranker [3,4,8,14,15]) to developed (e.g. vertisols, acrisols and luvisols [2,7,10,21-25,27]), and included intermediately altered soils (e.g. cambisols [1,5,6,9,11-13, 16-20,26,28,29]). The characteristics of these soils are summarized in Table 1. Soil properties varied from sample to sample. Thus, some properties affecting the prevalence and distribu-
2. Material and methods A total of 29 samples of natural, uncultured soils were collected at different places in the
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M. Sdnchez-Camazano et aL / Sci. Total En viron. 156 (1994) 183 - 190
Table 1 Selected properties of the 29 soils studied Sample number
pH
Organic matter
Sand
Silt
Clay
(%)
(%)
(%)
48.1 35.5 50.2 40.5 67.4 75.4 69.2 62.4 43.6 60.2 64.3 46.4 61.8 76.2 54.2 76.8 73.9 83.2 83.1 74.8 75.5 70.7 35.7 88.3 69.6 63.4 69.6 51.7 67.4
32.6 5.3 32.1 40.9 18.7 13.0 14.0 22.8 34.0 22.0 21.1 31.6 18.9 12.8 34.0 11.9 11.9 8.1 8.0 11.1 13.9 11.6 13.4 4.7 9.6 6.9 15.5 21.2 12.3
19.3 59.2 17.7 18.6 13.9 11.6 16.8 14.8 22.4 17.8 14.6 22.0 19.3 11.0 11.8 11.3 14.2 8.7 8.9 14.1 10.6 17.7 50.9 7.0 20.8 29.7 14.9 27.1 20.3
(%) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
6.9 6.3 6.0 5.8 5.0 5.3 6.7 4.9 5.1 4.9 4.7 5.0 5.2 5.1 5.6 4.8 5.1 5.3 5.4 5.5 6.1 5.5 7.2 5.7 6.9 7.6 5.9 4.8 5.4
6.90 0.60 5.22 5.40 8.90 5.95 6.68 5.29 5.52 4.24 10.26 6.10 7.28 8.73 3.44 15.40 4.66 1.13 0.36 1.78 1.15 1.42 6.29 4.33 1.75 1.42 1.55 1.92 5.16
Cation exchange capacity (Cmol/kg)
Exchange cations (Cmol/kg) Na +
K+
Mg 2÷
Ca 2+
23.07 26.90 24.72 15.45 18.50 15.25 16.95 12.20 11.95 9.00 25.20 14.20 19.30 24.10 13.45 22.50 13.00 4.5 5.25 9.65 6.60 8.60 42.70 7.0 13.70 22.00 17.40 8.20 12.30
0.41 0.19 0.58 ND 0.08 0.51 0.17 0.17 0.14 0.29 0.18 0.26 0.13 ND 0.23 ND 0.61 0.56 0.10 0.25 0.25 0.17 ND 0.18 0.80 ND 0.10 0.58 0.09
0.39 0.13 0.23 ND 0.10 0.01 0.43 0.07 0.19 0.14 0.13 0.21 0.53 ND 0.19 ND 0.21 0.00 0.06 0.23 0.08 0.14 ND 0.08 0.62 ND 0.53 0.21 0.13
10.42 13.27 11.02 ND 1.38 1.82 5.20 2.24 2.88 2.22 3.59 1.03 2.10 ND 1.40 ND 0.67 1.03 0.81 2.9 2.85 2.69 ND 2.66 7.46 ND 6.29 2.51 4.25
3.64 3.87 1.42 ND 0.37 0.20 1.14 0.57 0.69 0.82 0.69 0.48 0.84 ND 0.72 ND 0.19 0.31 0.30 1.25 0.79 1.05 ND 1.25 1.51 ND 0.82 1.15 1.18
Base Saturation %)
14.86 17.46 13.25 ND 1.93 2.54 6.94 3.05 3.90 3.47 4.59 1.98 3.60 ND 2.54 ND 1.68 1.90 1.27 4.63 3.97 4.05 ND 4.17 9.89 ND 7.74 4.45 5.65
64.41 64.90 53.60 ND 10.43 16.65 40.94 25.00 32.63 38.55 18.21 13.94 18.65 ND 18.88 ND 12.92 42.22 24.19 47.97 60.15 47.09 ND 59.57 72.18 ND 44.48 54.26 45.93
ND, not determined.
tion of the different cadmium forms such as the pH and organic matter and clay contents varied over wide ranges. Samples from six different profiles were also collected in order to determine the vertical distribution of cadmium. Profiles 1 (humic cambisol), 2 (distric cambisol), 3 (humic cambisol) and 4 (humic cambisol) belonged to the same area and were developed on the same type of granite rock. Differences between these profiles arose from differences in their vegetation and moisture content. Such differences lay basically in their clay and organic matter contents. Profiles 5 and 6 were sampled at the same sites as the soil samples 15 and 26, respectively, after the overall Cd content of the samples was determined and found to be anomalously high. Profile 5 was a calcareous
cambisol developed on limestone, whereas profile 6 was a distric regosol lying over slate. The soil samples were sieved through a 2-mm screen. A 2-g portion of the finely powdered sample ( < 75 /~m) was taken from the < 2 mm fraction, placed in a glass beaker and heated for 1 h to 500°C. The sample was dissolved in 2 ml of HCIO4 and 12 ml of HF in a water bath at 90°C, followed by evaporation to dryness. The dissolution of the residue was carried out with 2% HC1 up to 25 ml. Soluble Cd was extracted by shaking 2 g of soil with 15 ml of i N NH4AcO at pH 7 for 1 h. The total and soluble Cd were determined on duplicate samples for all soils. Cadmium was determined by atomic absorption spectrophotometry using a Varian AA1475 instrument equipped with a GTA-95 graphite furnace,
186
M. Sdnchez-Camazano et al. / Sci. Total Environ. 156 (1994) 183-190
also from Varian. Standard solutions containing the same acid matrix as the samples were made at Cd concentrations between 0 and 20 ng/ml. Analyses were performed according to the recommendations given in the Varian Manual for Analytical Methods [10]. Accuracy of the analysis was checked with BCR certified reference materials nos. 141 (calcareous loam soil) and 142 (light sandy soil) and was expressed with a variation coefficient < 10%. Special care was exercised both in preparing the samples and in determining the analytes so as to avoid contamination through deposition of pollutants from the laboratory atmosphere, glassware or potentially contaminated reagents. The precision of the methods were determined by carrying out the experiments 10 times for a soil sample (CV = 3.5% for total Cd determination and CV = 5.4% for soluble Cd determination). 3. Results and discussion
Table 2 shows the total and soluble Cd contents of the analysed soils. The former varied between 0.07 and 0.33/xg/g (average, 0.19/xg/g), while the latter was from 0.004 to 0.061 /zg/g (average, 0.031 ~g/g). Soluble Cd accounted for 2.90-45.4% of the total cadmium content in the soils. On grouping the soils according to starting material it is seen that the variation ranges for Cd content in the three groups, i.e. granites, slates and clay-sand conglomerates, were quite similar: 0.11-0.32, 0.07-0.30 and 0.13-0.33/xg/g, respectively. Consequently, the genesis process of the soils is one other determiner of the total Cd content in addition to the composition of the starting material. According to Tiller [11], many authors have shown that similar starting materials can give rise to soils with widely varying Cd contents. So far, according to the available data, no important Zn or Cd anomaly has been observed in soils, and there is no known mineralization of these elements in the geological materials from the Province of Salamanca. There is only one reference to a Cd geochemical anomaly of slight importance registered at the south-west of the
Table 2 Total and soluble Cadmium contents (/,~g/g) in natural soils from Salamanca (Spain) Sample number
Total Cd
Soluble Cd
Soluble fraction (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
0.18 0.13 0.20 0.24 0.24 0.11 0.13 0.24 0.23 0.09 0.07 0.13 0.22 0.22 0,30 0.32 0.14 0.11 0.13 0,11 0.16 0.14 0.15 0.23 0,33 0,30 0.19 0.15 0.19
0.050 0.046 0.050 0.022 0.033 0.035 0.059 0.038 0.022 0.030 0.018 0.013 0,019 0.016 0.016 0.035 0,029 0.004 0.015 0.031 0.025 0.017 0.017 0.061 0.039 0.046 0.056 0.017 0.038
27.8 35.4 25,0 9.2 13.8 31.8 45.4 15.8 9.6 33.3 25.7 10,0 8,6 7.3 5.3 10.9 20.7 2.9 11.5 28,2 15.6 12.1 11,3 26.5 11.8 15,3 29,5 11.3 20.0
Province of Salamanca [12]. This anomaly is not reflected in soils 5 and 6 from this work, which belong to this zone. The soluble Cd content was found to be more sensitive to the type of rock than was the total Cd content; in fact, it increased in the following sequence: granite (0.004-0.035 /~g/g) < slate (0.013-0.05 /xg/g) < clay and sand conglomerates (0.017-0.061 /~g/g). Total Cd contents can be considered somewhat low compared with reported values for other normal soils. Thus, our variation ranges are narrower and our average values lower than those reported by Adriano (0.1-11 /~g/g) [13], and by Garcia Sfinchez (0.1-2 /~g/g) [14] for worldwide soil samples. Our average values are also less than
M. Sdnchez-Camazano et al. /Sci. Total En t~ron. 156 (1994) 183-190
those reported by Jensen and Bro-Rasmussen [9] for Belgian (0.40/~g/g), Dutch (0.40/.~g/g), German (0.30 /~g/g) and English (0.50 /xg/g) soils, and very close to those reported by the same authors for French (0.20 /xg/g) and Danish (0.20 /~g/g) soils. Our values of soluble Cd, which is potentially more hazardous on account of its high mobility and availability, were also somewhat low. Attempts at finding simple correlations between the total and soluble Cd contents were unsuccessful, which is consistent with previous findings [15]. Accordingly, the total Cd content is a poor indicator for soluble Cd. Therefore, the former cannot be used to predict potential adverse effects in terms of soluble Cd mobility and bioavailability. Nor was a significant correlation found between the total Cd content and any of the soil parameters studied. This was to be expected since the total Cd content of soils must be determined by the original material and its subsequent alteration. Some authors [16,17] found some significant correlations, particularly with the organic matter content of soils from the same area which developed over the same type of rock but varied somehow in their chemical properties. However, if the soils are classified into small groups, some relationships can indeed be established. There are four especially high total Cd contents relative to the rest, i.e. those of soils 15, 16, 25 and 26. All four soils possess some special feature related to accumulation of some Cd form in the soil [13]. In fact, soil 16, which featured the highest organic matter content of all (15%), contained 0.32/.~g/g of total Cd of which only 10.9% was exchangeable Cd; consequently, the metal must have accumulated as a chelate with the organic matter present in the soil. Soil 15, with a Cd content of 0.30 /~g/g, was a scarcely developed litho soil supported on slate. Soils 25 and 26, which contained 0.33 and 0.30 p~g Cd/g, respectively, were both calcareous luvisols (according to the results of some studies on the speciation of Cd in soils, the metal precipitates in the presence of free CaCO3). On the other hand, soils 10, 11 and 12, which developed over the same type of slate, all fea-
187
tured similar, low total Cd contents. Soils 16 and 17, on the same type of granite rock but featuring rather different organic matter contents, also had rather disparate total Cd contents, i.e. 0.32 and 0.14 ~g/g, respectively. Finally, the most well developed soils (vertisols, acrisols and luvisols, i.e. 2, 7, 10, 22 and 23) had low total Cd contents, whereas the opposite held true with poorly developed soils (litho soils and rankers, i.e. 3, 4, 8, 14 and 15). This is consistent with previous findings of Tiller [18] in studying the concentrations of heavy metals in a number of Australian soil groups. He found appreciable differences between highly altered soils (e.g. vertisols) and podsols; however, most soil groups featured similar total and soluble heavy metal concentration ranges. Table 3 gives the simple correlation coefficients between soluble Cd and the soil properties. The former was found to be highly significantly correlated (P < 0.001) with the pH, sum of bases and exchangeable Ca, and significantly correlated (P < 0.01) with the cation exchange capacity, degree of saturation and exchangeable Mg content of the soils. On the other hand, there was no correlation between soluble Cd and the clay or organic matter content of the soils, even though these two fractions determine such properties as the exchange capacity, sum of bases, etc. This is a result of the nature of such fractions rather than their contents being the actual determiner. The Table 3 Simple correlation coefficients between soluble Cd and soil characteristics Soil parameter pH Organic matter Clay Cation exchange capacity Na ÷ K÷ Ca 2+ Mg 2+ E bases Base saturation * Significant at 0.05-0.01 level. ** Significant at 0.01-0.001 level.
0.60* * 0.12 0.14 0.40* -0.11 0.34 0.61"* 0.47* 0.59'* 0.45*
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M. Sdnchez-Camazano et al. / Sci. Total Environ. 156 (1994) 183-190
correlations obtained suggest that some soil parameters can be used to predict soluble Cd in soils. Thus, they can be used to determine the Cd critical load a given soil can bear under external supplies of this heavy metal from agricultural, industrial or mining sources. Fig. 2 shows the distribution of total and solu-
ble Cd in the six profiles used to determine their distribution in terms of depth. Fig. 3 shows the distribution of organic matter in the above-mentioned profiles. The distribution of total Cd in soil profiles has been scarcely studied. The few available data suggest that the metal either occurs quite evenly
Depth (cm)
Depth (cm)
0
0
20
20
40 40
60
60
80 100
80
120 1O0
140 160
0
I
I
I
0.1
0.2
0.3
0.4
120
0
I
i
I
0.1
0.2
0.3
Total Cd (IJg/g)
0.4
Total Cd (pg/g) Depth (cm)
Depth (cm) 0
0
YX;
2O 4O
20 40
6O
60
8O 100
80
120
,I
140 160
I
0
100 I
I
I
I
0.01 0.02 0.03 0.04 0.05 0.06 Soluble Cd (IJg/g)
120
0
I
I
I
0.01
0.02
0.03
Soluble Cd (IJg/g)
Fig. 2. Distribution of total and soluble Cd in the six soil profiles.
0.04
189
M. Sdnchez-Camazano et aL /Sci. Total En~ron. 156 (1994) 183-190
Depth (cm)
Depth (cm)
0
0
20
2
3
4
1
20
40 so
40
80
60
100
80
120 140 160
100 0
6
10
16
120
t
0
Organic matter (%)
1
I
L
2
3
Organic matter (%)
Fig. 3. Variation of the organic matter content in the six soil profiles. t h r o u g h o u t a given profile [13] or accumulates at the top, in parallel with h u m u s [13,16]; however, there are no similarities between the distribution of total C d and other soil parameters. Virtually no similarities between the Cd distribution patterns and those of other soil p a r a m e ters were f o u n d except for the organic m a t t e r c o n t e n t (Fig. 3). T h e distributions o f total Cd and soluble Cd were s o m e w h a t similar, particularly in profiles 1, 5 and 6. T h e total C d c o n t e n t of all the soils was higher in the top horizon, which contained the most organic m a t t e r of all, except in profile 4. In profiles 1 and 4, which were located in the same area and developed over the same type of granite rock, the total Cd content o f the top horizon increased with an increase in the organic m a t t e r content. However, the distributions of total Cd and organic matter were only similar in profile 1.
Acknowledgements T h a n k s are expressed to C. Perez and V. Nieves for technical assistance. This w o r k was s u p p o r t e d by the ' J u n t a de Castilla y L e 6 n ' (Spain) u n d e r Project No. 0 6 1 1 / 9 0 .
References 1 H.P. Rothbaum, R.L. Goguel, A.E. Johnston and G.E.G. Mattingly, Cadmium accumulation in soils from long-continued application of superphosphate. J. Soil Sci., 37 (1986) 99-107. 2 M.S. Kuhad, R.S. Malik, R. Shing and A. Shing, Studies on mobility and accumulation of heavy metals in agricultural soils receiving sewer water irrigation. J. Indian Soc. Soil Sci., 37 (1989) 290-294. 3 S.P. McGrath and J. Cegarra, Chemical extractability of heavy metals during and after long-term applications of sewage sludge to soil. J. Soil Sci., 43 (1992) 313-321. 4 W. De Boo, Cadmium in agriculture. Toxicol. Environ. Chem., 27 (1990) 55-63. 5 M.G. Hickey and J.A. Kittrick, Chemical partitioning of cadmium, copper, nickel and zinc in soils and sediments containing high levels of heavy metals. J. Environ. Qual., 13 (1984) 372-376. 6 X. Xian, Distribution of cadmium and zinc in field and paddy field soils near a zinc smelter. J. Environ. Sci. Health, A, 23 (1988) 157-167. 7 M.J. Gonzalez, M. Fernandez and L.M. Hernandez, Influence of acid mine water in the distribution of heavy metal in soils of Dofiana National Park. Application of multivariate analysis. Environ. Technol., 11 (1990) 1027-1038. 8 F. Macias, M.T. Barral, C. Monterroso and R.M. Calvo, Metales pesados en las escombreras de lamina Puentes
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9 10
11
12
13
M. Sdnchez-Camazano et al. / Sci. Total Entffon. 156 (1994) 183-190 (La Corufia): influencia de las condiciones En-pH. Suelo Plant., 2 (1992) 139-150. A. Jensen and F. Bro-Rasmussen, Environmental cadmium in Europe. Residue Rev., 125 (1992) 101-182. E. Rothery, Analytical Methods for Graphite Tube Atomizers, Varian Techtron Pty. Limited, Mulgrave, Australia, 1982, 132 pp. K.G. Tiller, Heavy metals in soils and their environmental significance, in B.A. Stewart (Ed.), Advances in Soil Science, Vol. 9, Springer, London, 1989, pp. 113-144. A. Garcla-S~inchez, J.F. Antona and M. Urrutia, Geochemical prospection of cadmium in a high incidence area of prostate cancer, Sierra de Gata, Salamanca, Spain. Sci. Total Environ., 116 (1992) 243-251. D.C. Adriano, Trace Elements in the Terrestrial Environment, Springer, New York, 1986, Ch. 4, pp. 106-155.
14 A. Garcia Sfinchez, Cadmio y medio ambiente. Rev. Tecn. Med. Ambiente, 8 (1989) 105-109. 15 A.E. Boekhold, S.E.A.T.M. Van der Zee and F.A.M. de Haan, Spatial patterns of cadmium contents related to soil heterogeneity. Water Air Soil Pollut., 57-58 (1991) 479-488. 16 A. Andersson, Heavy metals in Swedish soils: on their retention, distribution and amounts. Swed. J. Agric. Res., 7 (1977) 7-20. 17 A.E. Boekhold and E.A.T.M. Van der Zce, Significance of soil chemical heterogeneity for spatial behaviour of cadmium in field soils. Soil Sci. Soc. Am. J., 56 (1992) 747-754. 18 K.G. Tiller, in GSIRO (Ed.), Soils: An Australian Viewpoint, Academic Press, London, 1983, Ch. 25.
The Science of the Total Environment 156 (1994) 183-190
The content and distribution of cadmium in soils as influenced by the soil properties M. S~nchez-Camazano*, M.J. Sanchez-Martin, L.F. Lorenzo Instituto de Recursos Naturalesy Agrobiologla de Salamanca, C.S.I.C., Cordelde Merinas, 40-52, Apdo 257, Salamanca, Spain Received 5 March 1993; accepted 12 January 1994
Abstract
The total and soluble cadmium contents of 29 natural, uncultured, unpolluted soils from the province of Salamanca (Spain) were determined. The two forms of the metal were found to occur at concentrations over the ranges 0.07-0.33 and 0.004-0.061 p.g/g, respectively. The total Cd contents found were somewhat low relative to reported values for worldwide soils in general and European soils in particular. No significant correlation was found between the total and soluble Cd contents or between the former and any chemical parameters of the soils. On the other hand, there was a highly significant correlation (P < 0.001) between soluble Cd and the pH, sum of bases and exchangeable Ca, as well as a significant correlation (P < 0.01) between soluble Cd and the cation exchange capacity, degree of saturation and exchangeable Mg content of the soils. Finally, there were virtually no similarities in the distribution patterns of Cd and the other soil parameters in a given soil profile.
Keywords: Total cadmium; Soluble cadmium; Soil properties; Spain
1. Introduction The growing development of agricultural, industrial and urban activities has given rise to a number of environmental problems because of the presence of heavy metals, in general and cadmium in particular. It has thus become mandatory to develop methods for assessing this metal and elucidating its behaviour in the natural environment in order to be able to predict its behaviour at the increased concentrations resulting from human activities. The potential toxicity of cadmium to plants, animals and humans has encouraged research into the presence of this metal in soils, with
* Corresponding author.
emphasis on soils treated with sewage sludges or phosphate fertilizers [1-4] and those exposed to some Cd polluting source [5-8]. On the other hand, the Cd content of natural, uncultured, unpolluted soils is typically dealt with as a background content and given much less attention. Thus, in a recent review on environmental cadmium in Europe, Jensen and Bro-Rasmussen [9] underscored the lack of information on the Cd content of normal soils in southern European countries. The background Cd content of soils is quite an interesting reference for determining local or regional variations in this element and anticipating potential hazards from polluting sources. The total Cd content of soils should, in principle, be related to its content in the original rock on which the soil developed, as well as to the alter-
0048-9697/94/$07.00 © 1994 Elsevier Science BV. All rights reserved.
SSD1 0048-9697(94)04152-D
M. Sdnchez-Camazano et al. / Sci. Total Environ. 156 (1994) 183 190
184
ations undergone by this staring material. The forms in which Cd may occur in different soil horizons and their distribution depend on the physical, chemical and mineralogical properties of the soil in question. Of such forms, that known as exchangeable or soluble cadmium is of special interest as it determines the actual environmental exposure. In this work, the total and soluble Cd contents of 29 natural (uncultured, unpolluted) soils from the province of Salamanca (Spain) were determined, and the potential relationship between such contents and the soil properties, in addition to the distribution of both Cd forms in terms of depth in six soil profiles were studied.
province of Salamanca (Spain) (Fig. 1). The sampiing sites were chosen not only in terms of their geographical situation, but also in such a way that they encompassed the types of rocks most frequently encountered in the province. The chosen sites were also intended to represent as wide a variety of vegetation, climate and soils as possible. Samples were collected from the soil surface horizon (horizon A), the depth of which ranged between 0-15 and 0-30 cm. As a rule, the original material consisted of granite, slate or clay and sand conglomerates. Soils were from scarcely developed (litho soils or ranker [3,4,8,14,15]) to developed (e.g. vertisols, acrisols and luvisols [2,7,10,21-25,27]), and included intermediately altered soils (e.g. cambisols [1,5,6,9,11-13, 16-20,26,28,29]). The characteristics of these soils are summarized in Table 1. Soil properties varied from sample to sample. Thus, some properties affecting the prevalence and distribu-
2. Material and methods A total of 29 samples of natural, uncultured soils were collected at different places in the
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sf"
B,~a, i ,....,I~ ~ I.
c Fig. 1. Location of sampling points.
f..,~
-
i-'-'J /
185
M. Sdnchez-Camazano et aL / Sci. Total En viron. 156 (1994) 183 - 190
Table 1 Selected properties of the 29 soils studied Sample number
pH
Organic matter
Sand
Silt
Clay
(%)
(%)
(%)
48.1 35.5 50.2 40.5 67.4 75.4 69.2 62.4 43.6 60.2 64.3 46.4 61.8 76.2 54.2 76.8 73.9 83.2 83.1 74.8 75.5 70.7 35.7 88.3 69.6 63.4 69.6 51.7 67.4
32.6 5.3 32.1 40.9 18.7 13.0 14.0 22.8 34.0 22.0 21.1 31.6 18.9 12.8 34.0 11.9 11.9 8.1 8.0 11.1 13.9 11.6 13.4 4.7 9.6 6.9 15.5 21.2 12.3
19.3 59.2 17.7 18.6 13.9 11.6 16.8 14.8 22.4 17.8 14.6 22.0 19.3 11.0 11.8 11.3 14.2 8.7 8.9 14.1 10.6 17.7 50.9 7.0 20.8 29.7 14.9 27.1 20.3
(%) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
6.9 6.3 6.0 5.8 5.0 5.3 6.7 4.9 5.1 4.9 4.7 5.0 5.2 5.1 5.6 4.8 5.1 5.3 5.4 5.5 6.1 5.5 7.2 5.7 6.9 7.6 5.9 4.8 5.4
6.90 0.60 5.22 5.40 8.90 5.95 6.68 5.29 5.52 4.24 10.26 6.10 7.28 8.73 3.44 15.40 4.66 1.13 0.36 1.78 1.15 1.42 6.29 4.33 1.75 1.42 1.55 1.92 5.16
Cation exchange capacity (Cmol/kg)
Exchange cations (Cmol/kg) Na +
K+
Mg 2÷
Ca 2+
23.07 26.90 24.72 15.45 18.50 15.25 16.95 12.20 11.95 9.00 25.20 14.20 19.30 24.10 13.45 22.50 13.00 4.5 5.25 9.65 6.60 8.60 42.70 7.0 13.70 22.00 17.40 8.20 12.30
0.41 0.19 0.58 ND 0.08 0.51 0.17 0.17 0.14 0.29 0.18 0.26 0.13 ND 0.23 ND 0.61 0.56 0.10 0.25 0.25 0.17 ND 0.18 0.80 ND 0.10 0.58 0.09
0.39 0.13 0.23 ND 0.10 0.01 0.43 0.07 0.19 0.14 0.13 0.21 0.53 ND 0.19 ND 0.21 0.00 0.06 0.23 0.08 0.14 ND 0.08 0.62 ND 0.53 0.21 0.13
10.42 13.27 11.02 ND 1.38 1.82 5.20 2.24 2.88 2.22 3.59 1.03 2.10 ND 1.40 ND 0.67 1.03 0.81 2.9 2.85 2.69 ND 2.66 7.46 ND 6.29 2.51 4.25
3.64 3.87 1.42 ND 0.37 0.20 1.14 0.57 0.69 0.82 0.69 0.48 0.84 ND 0.72 ND 0.19 0.31 0.30 1.25 0.79 1.05 ND 1.25 1.51 ND 0.82 1.15 1.18
Base Saturation %)
14.86 17.46 13.25 ND 1.93 2.54 6.94 3.05 3.90 3.47 4.59 1.98 3.60 ND 2.54 ND 1.68 1.90 1.27 4.63 3.97 4.05 ND 4.17 9.89 ND 7.74 4.45 5.65
64.41 64.90 53.60 ND 10.43 16.65 40.94 25.00 32.63 38.55 18.21 13.94 18.65 ND 18.88 ND 12.92 42.22 24.19 47.97 60.15 47.09 ND 59.57 72.18 ND 44.48 54.26 45.93
ND, not determined.
tion of the different cadmium forms such as the pH and organic matter and clay contents varied over wide ranges. Samples from six different profiles were also collected in order to determine the vertical distribution of cadmium. Profiles 1 (humic cambisol), 2 (distric cambisol), 3 (humic cambisol) and 4 (humic cambisol) belonged to the same area and were developed on the same type of granite rock. Differences between these profiles arose from differences in their vegetation and moisture content. Such differences lay basically in their clay and organic matter contents. Profiles 5 and 6 were sampled at the same sites as the soil samples 15 and 26, respectively, after the overall Cd content of the samples was determined and found to be anomalously high. Profile 5 was a calcareous
cambisol developed on limestone, whereas profile 6 was a distric regosol lying over slate. The soil samples were sieved through a 2-mm screen. A 2-g portion of the finely powdered sample ( < 75 /~m) was taken from the < 2 mm fraction, placed in a glass beaker and heated for 1 h to 500°C. The sample was dissolved in 2 ml of HCIO4 and 12 ml of HF in a water bath at 90°C, followed by evaporation to dryness. The dissolution of the residue was carried out with 2% HC1 up to 25 ml. Soluble Cd was extracted by shaking 2 g of soil with 15 ml of i N NH4AcO at pH 7 for 1 h. The total and soluble Cd were determined on duplicate samples for all soils. Cadmium was determined by atomic absorption spectrophotometry using a Varian AA1475 instrument equipped with a GTA-95 graphite furnace,
186
M. Sdnchez-Camazano et al. / Sci. Total Environ. 156 (1994) 183-190
also from Varian. Standard solutions containing the same acid matrix as the samples were made at Cd concentrations between 0 and 20 ng/ml. Analyses were performed according to the recommendations given in the Varian Manual for Analytical Methods [10]. Accuracy of the analysis was checked with BCR certified reference materials nos. 141 (calcareous loam soil) and 142 (light sandy soil) and was expressed with a variation coefficient < 10%. Special care was exercised both in preparing the samples and in determining the analytes so as to avoid contamination through deposition of pollutants from the laboratory atmosphere, glassware or potentially contaminated reagents. The precision of the methods were determined by carrying out the experiments 10 times for a soil sample (CV = 3.5% for total Cd determination and CV = 5.4% for soluble Cd determination). 3. Results and discussion
Table 2 shows the total and soluble Cd contents of the analysed soils. The former varied between 0.07 and 0.33/xg/g (average, 0.19/xg/g), while the latter was from 0.004 to 0.061 /zg/g (average, 0.031 ~g/g). Soluble Cd accounted for 2.90-45.4% of the total cadmium content in the soils. On grouping the soils according to starting material it is seen that the variation ranges for Cd content in the three groups, i.e. granites, slates and clay-sand conglomerates, were quite similar: 0.11-0.32, 0.07-0.30 and 0.13-0.33/xg/g, respectively. Consequently, the genesis process of the soils is one other determiner of the total Cd content in addition to the composition of the starting material. According to Tiller [11], many authors have shown that similar starting materials can give rise to soils with widely varying Cd contents. So far, according to the available data, no important Zn or Cd anomaly has been observed in soils, and there is no known mineralization of these elements in the geological materials from the Province of Salamanca. There is only one reference to a Cd geochemical anomaly of slight importance registered at the south-west of the
Table 2 Total and soluble Cadmium contents (/,~g/g) in natural soils from Salamanca (Spain) Sample number
Total Cd
Soluble Cd
Soluble fraction (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
0.18 0.13 0.20 0.24 0.24 0.11 0.13 0.24 0.23 0.09 0.07 0.13 0.22 0.22 0,30 0.32 0.14 0.11 0.13 0,11 0.16 0.14 0.15 0.23 0,33 0,30 0.19 0.15 0.19
0.050 0.046 0.050 0.022 0.033 0.035 0.059 0.038 0.022 0.030 0.018 0.013 0,019 0.016 0.016 0.035 0,029 0.004 0.015 0.031 0.025 0.017 0.017 0.061 0.039 0.046 0.056 0.017 0.038
27.8 35.4 25,0 9.2 13.8 31.8 45.4 15.8 9.6 33.3 25.7 10,0 8,6 7.3 5.3 10.9 20.7 2.9 11.5 28,2 15.6 12.1 11,3 26.5 11.8 15,3 29,5 11.3 20.0
Province of Salamanca [12]. This anomaly is not reflected in soils 5 and 6 from this work, which belong to this zone. The soluble Cd content was found to be more sensitive to the type of rock than was the total Cd content; in fact, it increased in the following sequence: granite (0.004-0.035 /~g/g) < slate (0.013-0.05 /xg/g) < clay and sand conglomerates (0.017-0.061 /~g/g). Total Cd contents can be considered somewhat low compared with reported values for other normal soils. Thus, our variation ranges are narrower and our average values lower than those reported by Adriano (0.1-11 /~g/g) [13], and by Garcia Sfinchez (0.1-2 /~g/g) [14] for worldwide soil samples. Our average values are also less than
M. Sdnchez-Camazano et al. /Sci. Total En t~ron. 156 (1994) 183-190
those reported by Jensen and Bro-Rasmussen [9] for Belgian (0.40/~g/g), Dutch (0.40/.~g/g), German (0.30 /~g/g) and English (0.50 /xg/g) soils, and very close to those reported by the same authors for French (0.20 /xg/g) and Danish (0.20 /~g/g) soils. Our values of soluble Cd, which is potentially more hazardous on account of its high mobility and availability, were also somewhat low. Attempts at finding simple correlations between the total and soluble Cd contents were unsuccessful, which is consistent with previous findings [15]. Accordingly, the total Cd content is a poor indicator for soluble Cd. Therefore, the former cannot be used to predict potential adverse effects in terms of soluble Cd mobility and bioavailability. Nor was a significant correlation found between the total Cd content and any of the soil parameters studied. This was to be expected since the total Cd content of soils must be determined by the original material and its subsequent alteration. Some authors [16,17] found some significant correlations, particularly with the organic matter content of soils from the same area which developed over the same type of rock but varied somehow in their chemical properties. However, if the soils are classified into small groups, some relationships can indeed be established. There are four especially high total Cd contents relative to the rest, i.e. those of soils 15, 16, 25 and 26. All four soils possess some special feature related to accumulation of some Cd form in the soil [13]. In fact, soil 16, which featured the highest organic matter content of all (15%), contained 0.32/.~g/g of total Cd of which only 10.9% was exchangeable Cd; consequently, the metal must have accumulated as a chelate with the organic matter present in the soil. Soil 15, with a Cd content of 0.30 /~g/g, was a scarcely developed litho soil supported on slate. Soils 25 and 26, which contained 0.33 and 0.30 p~g Cd/g, respectively, were both calcareous luvisols (according to the results of some studies on the speciation of Cd in soils, the metal precipitates in the presence of free CaCO3). On the other hand, soils 10, 11 and 12, which developed over the same type of slate, all fea-
187
tured similar, low total Cd contents. Soils 16 and 17, on the same type of granite rock but featuring rather different organic matter contents, also had rather disparate total Cd contents, i.e. 0.32 and 0.14 ~g/g, respectively. Finally, the most well developed soils (vertisols, acrisols and luvisols, i.e. 2, 7, 10, 22 and 23) had low total Cd contents, whereas the opposite held true with poorly developed soils (litho soils and rankers, i.e. 3, 4, 8, 14 and 15). This is consistent with previous findings of Tiller [18] in studying the concentrations of heavy metals in a number of Australian soil groups. He found appreciable differences between highly altered soils (e.g. vertisols) and podsols; however, most soil groups featured similar total and soluble heavy metal concentration ranges. Table 3 gives the simple correlation coefficients between soluble Cd and the soil properties. The former was found to be highly significantly correlated (P < 0.001) with the pH, sum of bases and exchangeable Ca, and significantly correlated (P < 0.01) with the cation exchange capacity, degree of saturation and exchangeable Mg content of the soils. On the other hand, there was no correlation between soluble Cd and the clay or organic matter content of the soils, even though these two fractions determine such properties as the exchange capacity, sum of bases, etc. This is a result of the nature of such fractions rather than their contents being the actual determiner. The Table 3 Simple correlation coefficients between soluble Cd and soil characteristics Soil parameter pH Organic matter Clay Cation exchange capacity Na ÷ K÷ Ca 2+ Mg 2+ E bases Base saturation * Significant at 0.05-0.01 level. ** Significant at 0.01-0.001 level.
0.60* * 0.12 0.14 0.40* -0.11 0.34 0.61"* 0.47* 0.59'* 0.45*
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M. Sdnchez-Camazano et al. / Sci. Total Environ. 156 (1994) 183-190
correlations obtained suggest that some soil parameters can be used to predict soluble Cd in soils. Thus, they can be used to determine the Cd critical load a given soil can bear under external supplies of this heavy metal from agricultural, industrial or mining sources. Fig. 2 shows the distribution of total and solu-
ble Cd in the six profiles used to determine their distribution in terms of depth. Fig. 3 shows the distribution of organic matter in the above-mentioned profiles. The distribution of total Cd in soil profiles has been scarcely studied. The few available data suggest that the metal either occurs quite evenly
Depth (cm)
Depth (cm)
0
0
20
20
40 40
60
60
80 100
80
120 1O0
140 160
0
I
I
I
0.1
0.2
0.3
0.4
120
0
I
i
I
0.1
0.2
0.3
Total Cd (IJg/g)
0.4
Total Cd (pg/g) Depth (cm)
Depth (cm) 0
0
YX;
2O 4O
20 40
6O
60
8O 100
80
120
,I
140 160
I
0
100 I
I
I
I
0.01 0.02 0.03 0.04 0.05 0.06 Soluble Cd (IJg/g)
120
0
I
I
I
0.01
0.02
0.03
Soluble Cd (IJg/g)
Fig. 2. Distribution of total and soluble Cd in the six soil profiles.
0.04
189
M. Sdnchez-Camazano et aL /Sci. Total En~ron. 156 (1994) 183-190
Depth (cm)
Depth (cm)
0
0
20
2
3
4
1
20
40 so
40
80
60
100
80
120 140 160
100 0
6
10
16
120
t
0
Organic matter (%)
1
I
L
2
3
Organic matter (%)
Fig. 3. Variation of the organic matter content in the six soil profiles. t h r o u g h o u t a given profile [13] or accumulates at the top, in parallel with h u m u s [13,16]; however, there are no similarities between the distribution of total C d and other soil parameters. Virtually no similarities between the Cd distribution patterns and those of other soil p a r a m e ters were f o u n d except for the organic m a t t e r c o n t e n t (Fig. 3). T h e distributions o f total Cd and soluble Cd were s o m e w h a t similar, particularly in profiles 1, 5 and 6. T h e total C d c o n t e n t of all the soils was higher in the top horizon, which contained the most organic m a t t e r of all, except in profile 4. In profiles 1 and 4, which were located in the same area and developed over the same type of granite rock, the total Cd content o f the top horizon increased with an increase in the organic m a t t e r content. However, the distributions of total Cd and organic matter were only similar in profile 1.
Acknowledgements T h a n k s are expressed to C. Perez and V. Nieves for technical assistance. This w o r k was s u p p o r t e d by the ' J u n t a de Castilla y L e 6 n ' (Spain) u n d e r Project No. 0 6 1 1 / 9 0 .
References 1 H.P. Rothbaum, R.L. Goguel, A.E. Johnston and G.E.G. Mattingly, Cadmium accumulation in soils from long-continued application of superphosphate. J. Soil Sci., 37 (1986) 99-107. 2 M.S. Kuhad, R.S. Malik, R. Shing and A. Shing, Studies on mobility and accumulation of heavy metals in agricultural soils receiving sewer water irrigation. J. Indian Soc. Soil Sci., 37 (1989) 290-294. 3 S.P. McGrath and J. Cegarra, Chemical extractability of heavy metals during and after long-term applications of sewage sludge to soil. J. Soil Sci., 43 (1992) 313-321. 4 W. De Boo, Cadmium in agriculture. Toxicol. Environ. Chem., 27 (1990) 55-63. 5 M.G. Hickey and J.A. Kittrick, Chemical partitioning of cadmium, copper, nickel and zinc in soils and sediments containing high levels of heavy metals. J. Environ. Qual., 13 (1984) 372-376. 6 X. Xian, Distribution of cadmium and zinc in field and paddy field soils near a zinc smelter. J. Environ. Sci. Health, A, 23 (1988) 157-167. 7 M.J. Gonzalez, M. Fernandez and L.M. Hernandez, Influence of acid mine water in the distribution of heavy metal in soils of Dofiana National Park. Application of multivariate analysis. Environ. Technol., 11 (1990) 1027-1038. 8 F. Macias, M.T. Barral, C. Monterroso and R.M. Calvo, Metales pesados en las escombreras de lamina Puentes
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13
M. Sdnchez-Camazano et al. / Sci. Total Entffon. 156 (1994) 183-190 (La Corufia): influencia de las condiciones En-pH. Suelo Plant., 2 (1992) 139-150. A. Jensen and F. Bro-Rasmussen, Environmental cadmium in Europe. Residue Rev., 125 (1992) 101-182. E. Rothery, Analytical Methods for Graphite Tube Atomizers, Varian Techtron Pty. Limited, Mulgrave, Australia, 1982, 132 pp. K.G. Tiller, Heavy metals in soils and their environmental significance, in B.A. Stewart (Ed.), Advances in Soil Science, Vol. 9, Springer, London, 1989, pp. 113-144. A. Garcla-S~inchez, J.F. Antona and M. Urrutia, Geochemical prospection of cadmium in a high incidence area of prostate cancer, Sierra de Gata, Salamanca, Spain. Sci. Total Environ., 116 (1992) 243-251. D.C. Adriano, Trace Elements in the Terrestrial Environment, Springer, New York, 1986, Ch. 4, pp. 106-155.
14 A. Garcia Sfinchez, Cadmio y medio ambiente. Rev. Tecn. Med. Ambiente, 8 (1989) 105-109. 15 A.E. Boekhold, S.E.A.T.M. Van der Zee and F.A.M. de Haan, Spatial patterns of cadmium contents related to soil heterogeneity. Water Air Soil Pollut., 57-58 (1991) 479-488. 16 A. Andersson, Heavy metals in Swedish soils: on their retention, distribution and amounts. Swed. J. Agric. Res., 7 (1977) 7-20. 17 A.E. Boekhold and E.A.T.M. Van der Zce, Significance of soil chemical heterogeneity for spatial behaviour of cadmium in field soils. Soil Sci. Soc. Am. J., 56 (1992) 747-754. 18 K.G. Tiller, in GSIRO (Ed.), Soils: An Australian Viewpoint, Academic Press, London, 1983, Ch. 25.