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Total chromium and nickel contents of Scottish soils
Geoderma, 37 ( 1 9 8 6 ) 1 5 - - 2 7 Elsevier Science P u b l i s h e r s B.V., A m s t e r d a m - - P r i n t e d in T h e N e t h e r l a n d s
15
TOTAL CHROMIUM AND NICKEL CONTENTS OF SCOTTISH SOILS
M.L. B E R R O W a n d G.A. R E A V E S
Departments of Speetrochemistry and Statistics, The Macaulay Institute for Soil Research, Aberdeen (Great Britain) (Received M a r c h 21, 1 9 8 5 ; a c c e p t e d a f t e r revision O c t o b e r 29, 1 9 8 5 )
ABSTRACT Berrow, M.L. a n d Reaves, G.A., 1 9 8 6 . T o t a l c h r o m i u m a n d nickel c o n t e n t s o f S c o t t i s h soils. G e o d e r m a , 3 7 : 15--27. T h e f r e q u e n c y d i s t r i b u t i o n s o f c h r o m i u m a n d nickel c o n t e n t s in S c o t t i s h soils are r e p o r t e d a n d t h e r e l a t i o n o f t h e c o n c e n t r a t i o n s of these e l e m e n t s t o s o m e o t h e r soil variables is e x a m i n e d . T h e f r e q u e n c y d i s t r i b u t i o n s o f b o t h e l e m e n t s are a p p r o x i m a t e l y log-normal a n d are d e s c r i b e d in t e r m s o f t h e derived m e a n (x*) a n d t h e " n o r m a l r a n g e " (L--L") w h e r e x * = a n t i l o g x, L = a n t i l o g (x--2s) a n d L" = antilog (x+2s); x a n d s b e i n g t h e m e a n a n d s t a n d a r d d e v i a t i o n , respectively, o f t h e l o g - t r a n s f o r m e d data. T h e derived m e a n f o r t h e full set o f c h r o m i u m d a t a is 62 m g kg -1 a n d t h e n o r m a l range 5 . 4 - - 7 1 0 mg kg -1. T h e c o r r e s p o n d i n g figures for nickel are 27 a n d 3 . 4 - - 2 1 0 m g kg -'. T h e c o n c e n t r a t i o n s o f t h e s e t w o e l e m e n t s show a high degree o f c o r r e l a t i o n (r = 0 . 8 3 ) a n d t h e f o r m s o f t h e r e l a t i o n s h i p s b e t w e e n e l e m e n t c o n t e n t s a n d o t h e r soil variables are very similar. C o n c e n t r a t i o n s o f c h r o m i u m a n d nickel are greater in soils derived f r o m basic a n d i n t e r m e d i a t e i g n e o u s t h a n in t h o s e derived f r o m acid igneous rocks, are h i g h e r in argill a c e o u s t h a n in a r e n a c e o u s s e d i m e n t s , decrease w i t h increasing s a n d a n d organic m a t t e r c o n t e n t s a n d s h o w relatively little v a r i a t i o n w i t h increase in d e p t h b e l o w 10 cm.
INTRODUCTION
In common with many other trace elements, chromium and nickel are of biological importance. Chromium is now recognized to be essential to mammals (Schwartz and Mertz, 1959) but not, so far, to plants (Huffman and Allaway, 1973) and evidence of the nutrient value of nickel to various classes of organisms is accumulating (Dixon et al., 1975; Kirchgessner and Schnegg, 1980). Both elements have been shown to be toxic to plants (N.R.C., 1974; Hunter and Vergnano, 1953) and nickel toxicity arising in the field has been investigated by Hunter and Vergnano (1952) and Anderson et al. (1973). The recent proposal by the Commission of the European Communities for a Directive on the use of sewage sludge in agriculture (C.E.C., 1982)
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16 has led to widespread interest in establishing the background levels of several heavy metals in soils, including chromium and nickel. The Directive seeks to limit the amounts of metals which can be added to soils in sewage sludge and proposes maximum tolerable levels for the total contents of metals in soils. The recommended maxima for chromium and nickel are 50 and 30 mg kg -1, respectively. This paper reports on the chromium and nickel contents of soils derived from a wide range of geological parent materials throughout Scotland and compares the levels with those found in soils from other parts of the world (Ure and Berrow, 1982). MATERIALS AND METHODS
Materials The great majority of the samples analysed were collected during the course of a general-purpose soil survey by the Soil Survey of Scotland and include representative profiles from all the soil series mapped to the present time. In general, all the samples from a given profile were analysed, b u t in a few cases a selection was made; e.g., in some deep profiles only alternate horizons were analysed. Care was taken to avoid chromium and nickel contamination, particularly by stainless steel, at all stages of sampling, storage and preparation for analysis.
Analytical me thods A subsample (approx. 25 g) of the < 2 mm, air-dried fraction of the soil was ground to a fine powder (< 150 pm) in a closed agate ball mill and a 2-g portion dried at 80°C prior to ignition overnight at 450°C. Ignited samples were analysed for chromium and nickel using the direct-current arc spectrographic m e t h o d described in detail by Mitchell (1964a). The concentrations are expressed on an oven-dry basis. The limit of detection for both elements in mineral samples is 3 mg kg -1, b u t it is lower than this for highly organic samples where a concentration factor is introduced as a result of the ashing process. Where a sample value is below the detection limit that sample has been allocated a value equal to half the "less-than" level; e.g., a sample with less than 3 mg kg -1 has been allocated a value of 1.5 mg kg -~. Over the range of contents reported an accuracy of + 20% is obtained.
Processing of analytical data The frequency distributions of trace element concentrations in soils have generally been found to be positively skewed (Berrow and Reaves, 1981; Kinniburgh and Beckett, 1983) and to have an approximately log-
17
normal distribution (Wakatsuki et al., 1978; Davies, 1983). Log-transformation of the chromium and nickel values yields a distribution which approaches normality (Fig. 1). In the results section the distribution of concentrations has been described in terms of the derived mean and "normal range" defined as follows. If x and s are the mean and standard deviation, respectively, of the logtransformed data then the derived mean (x*) = antilog x, and the normal range is that between the limits L and L", where L = antilog (x--2s) and L" = antilog (x+2s). 1200
1000
o.
800
~- 6 0 0 o z 4OO
200
o 1,0
.
1.2 LN
3.4 5.6 7,8 10.0 TOTAL CHROMIUM CONTENT ( M G / K G )
1800 1600 1400 w 1200 LU <
1000
u. o
800
zd
600
--7
i I I
400 200 O 1
3 5 7 LN TOTAL NICKEL ( M G / K G )
9
Fig. 1, A. Histogram showing the distribution of total chromium contents after logtransformation. B. Histogram showing the distribution of total nickel contents after log-transformation.
18
6
0
I l l l l ~ v
vv~
,iO
O 0 0 0 0 0 0 0
Illllll ¢¢
0 ¢a
..=
.<
0 0
19
The derived mean will not lie in the middle of the normal range, but t h a t range will include approximately 95% of the data and the numbers of data less than and greater than the derived mean will be approximately equal. RESULTS AND DISCUSSION
The range and mean values for the data (untransformed) are presented in Table I which draws together the results of a number of publications on trace elements in large numbers of softs from Finland, Japan, U.S.A. and Scotland and a recent review of data from the world literature. The arithmetic means for chromium and nickel concentrations in Scottish soils are greater than the means for other soils, probably because a larger proportion of Scottish soils are developed on ultra-basic rocks. The arithmetic mean of the chromium content of over 13,000 soil samples tabulated in Table I is 93 mg kg -1 and of over 16,000 samples for nickel is 34 mg kg -1. The form of the d i s t r i b u t i o n of the log-transformed concentrations is shown in Fig. 1. The derived mean for chromium is 62 and the normal range 5.4--710 mg kg -1. The corresponding figures for nickel are 27 and 3.4--210 mg kg -1. The derived mean and normal range (dispersion) for the full set of data and for subsets are presented in Table II and III. TABLE II Chromium contents of various groups of samples (mg kg -1) Sample subset
Number of data
Derived mean, x*
Normal range L' L"
All samples All organic samples (0) All mineral samples (1) All "O" horizon samples (0) All " A " horizon samples (1) All "S" horizon samples (2) All "B" horizon samples (3) All "C" horizon samples (4) Granite and granitic gneiss (0) Intermediate igneous (1) Basic and ultra-basic igneous (2) Metamorphic rocks (3) "ORS" sands and sandstones (4) Fluvio-glacial sands and gravels (5) "Other" sands and sandstones (6) Lower Paleozoie greywackes and shales (7) Silts and clays (8) Mixed acid and basic igneous (9)
2944 326 2618 230 335 398 1080 866 136 134 228 316 49 254 374 133 83 135
62 22 71 13 62 65 77 67 49 100 280 54 30 30 42 190 80 110
5.4 710 0.90 540 7.8 640 0.71 250 4.5 850 11 380 8.9 670 7.2 630 5.8 420 32 350 15 4900 9.5 310 8.5 110 6.6 140 5.0 360 47 750 14 450 22 600
20 T A B L E III Nickel contents of various groups of samples (mg kg -I) Sample subset
Number of data
Derived Normal range mean, L' L" x*
All samples All organic samples (0) All mineral samples (1) All "O" horizon samples (0)
4122 446 3676 321
27 9.9 31 6.6
All "A" horizon samples (1) All "S" horizon samples (2) All "B" horizon samples (3) All "C" horizon samples (4) Granite and granitic gneiss (0) Intermediate igneous (1) Basic and ultra-basic igneous (2) Metamorphic rocks (3) "ORS" sands and sandstones (4) Fluvio-glacial sands and gravels (5) "Other" sands and sandstones (6) Lower Paleozoic greywackes and shales (7) Silts and clays (8) Mixed acid and basic igneous (9)
483 571 1516 1178 198 180 286 367 144 279 514 309 138 174
20 28 33 34 16 42 89 24 19 16 20 53 36 48
3.4 0.75 4.8 0.35 2.5 6.2 5.1 5.3 2.4 17 6.6 6.1 5.6 3.5 3.5 17 6.5 14
210 130 200 130 160 120 210 220 110 110 1200 97 62 76 110 170 200 170
T h e l o g - t r a n s f o r m e d c o n c e n t r a t i o n s o f c h r o m i u m a n d nickel are highly c o r r e l a t e d (r = 0.83, see Fig. 2) a n d as t h e y vary with o t h e r soil variables in a similar w a y t h e t w o elements can be described t o g e t h e r .
Horizon designation The horizon designations used by the Soil Survey of Scotland (Laing, 1976) m a y be summarised as follows: L = undecomposed plant litter; F = partially decomposed litter with recognizable plant remains; H = decomposed organic matter with few or no recognizable plant remains; A = uppermost mineral soil horizon subject to the most influence of climate, plants and animals; S = surface horizon of cultivated soil; B = "subsoil" horizon characterized by a relatively high content of sesquioxides or clay; C = the more~r-less unaltered parent material of the soil. The L, F and H horizons have been dealt with as a single group designated " O " (organic) for convenience. The most notable feature of the derived m e a n concentrations for different horizon types is the difference between the organic " O " horizon samples and the others. In Scottish soils the accumulation of organic matter in the upper horizons of soil profiles normally lowers the concentrations of most trace elements relative to the underlying mineral horizons
21 ~8
~6 bJ
O U
W
~4 z
e#
't"{
Z
-J 2
|
I
I
LN TOTAL CHROMIUM CONTENT (MG/KG) Fig. 2. Relationship b e t w e e n t o t a l c h r o m i u m arid nickel contents showing t r e n d x ~
plots of grouped data in which the range covered by the data for each variate has been subdivided into fifty segments yielding a fifty by fifty matrix. The symbol • indicates the mean of the y-values of the points in the column, and the vertical bar extends above and below the mean position, a distance equal to twice the standard error of these values. because plant residues generally contain considerably lower concentrations of chromium and nickel than soil minerals. The smaller dispersion (normal range) of the S horizon samples is probably the result of cultivation, S horizons being a mixture of O, A and sometimes B horizon material.
Mineral vs organic samples Highly organic surface horizons are c o m m o n l y found in uncultivated Scottish soils such as the peaty podzol and peaty gley groups. For the purposes of this investigation samples with less than 80% ash have been classified as organic and the remainder as mineral. The derived mean concentrations of chromium and nickel are lower and the dispersion (normal range) is greater for organic samples than for mineral samples (Tables II and III). Similar results were obtained previously for copper in Scottish soils (Reaves and Berrow, 1984a) but contrasting results were found for lead (Reaves and Berrow, 1984b). The situation for lead is complicated by the general enrichment of surface horizons which are mostly organic.
22
Parent material
Ten distinct groups have been defined and numbered in the lower parts of Tables II and III on the basis of the parent material descriptions given in various publications of the Soil Survey of Scotland. As superficial accumulations of organic matter are presumed to be unrelated to parent material the figures in groups 0 to 9 are for mineral samples only. The parent material groups 4, 5 and 6 (Tables II and III) which include soils developed from sands, gravels and sandstones have the lowest derived mean contents of 30 to 42 mg Cr kg -1 and 16 to 20 mg Ni kg -1. Parent material groups 0 and 3 developed from granitic and metamorphic rocks, respectively, have slightly higher derived means of a b o u t 50 mg Cr kg -1 and 20 mg Ni kg -~. Groups 8, 1 and 9 which include soils derived from fine textured silts and clays, intermediate igneous rocks and mixed tills, respectively, have increasingly greater derived mean contents within the ranges 80-110 mg Cr kg -1 and 36--48 mg Ni kg -~. Finally, groups 7 and 2 developed from Lower Palaeozoic rocks and basic and ultra-basic igneous rocks have the greatest derived mean contents for both chromium and nickel. The dispersion (normal range) for element contents is greater for the group-2 softs than for all the other groups, the lowest and highest values differing by factors of the order of 200--300. The minerals in which chromium and nickel are most abundant in rocks are commonly, but not always, identical. Olivine, a c o m m o n mineral in basic and ultra-basic rocks is generally poor in chromium b u t pyroxenes, amphiboles and micas can be relatively rich. Chromium is most sensitive to magmatic fractionation during igneous rock crystallization and a wide range of chromium contents therefore occurs in different rock types ranging between an average of 2600 mg kg -~ in ultra-basic rocks to 1--30 mg kg -1 in granites and rhyolites (Shiraki, 1978). Olivine is, however, a major host mineral for nickel in basic igneous rocks b u t it enters other ferromagnesian minerals such as pyroxenes and amphiboles in other igneous rocks, isomorphously substituting for iron or magnesium. The concentrations of chromium and nickel in Scottish softs therefore correspond closely with those found in the rock types from which their parent materials are derived (c.f. Shiraki, 1978, and Turekian, 1978). Soils derived from sandstones, granites and quartzites containing high proportions of quartz and feldspar are at the low end of the range, soils from basic and ultrabasic rich in ferromagnesian minerals are at the higher end while those derived from intermediate and mixed acid and basic igneous rocks are in the middle. The close correlation between the total chromium and nickel contents in Scottish soils derived from a wide range of parent materials (Fig. 2) suggests that most of the chromium and nickel in these softs is probably still present in unweathered primary minerals as isomorphous substituents of iron and magnesium in crystal lattices.
23
Sampling depth The form of variation in concentration of nickel with depth is shown in Fig. 3 and the variation with depth for chromium is similar. The most notable feature is the relatively low contents of both elements in surface horizons (0--6 cm) where organic matter contents are high. There are relatively small changes in total chromium and nickel at depths below 6 cm.
~6 L5
E _J uJ _u 4 z
i.-
~2
0 0
510
I~)0 SAMPLING
150 DEPTH
200
(CM)
Fig. 3. Relationship between total nickel content and sampling depth showing trend. See caption Fig. 2.
Ash content Over the greater part of the range of ash contents (determined by ignition at 450°C) concentrations of nickel and also of chromium increase gradually and fairly uniformly with ash content (Fig. 4). For highly organic soils with low (0--10%) ash, however, the concentrations of chromium and nickel are low. At the other end of the scale, over the range of 95--100% ash, there is a clear decrease in chromium and nickel contents. A very similar pattern was observed for copper (Reaves a n d Berrow, 1984a) and is due to the fact that samples with ash contents in the range of 95--100% are largely composed of siliceous-sand-containing low concentrations of heavy metals.
24
~6 <9 E LU U4 z
-I
t o
<
S z, 2~
o
#o
4b @/o A S H
6'o
8'o
100
CONTENT
Fig. 4. Relationship between total nickel content and ash content showing trend. See caption Fig. 2.
~6 J L)
-J4 <
5
o
2'o
4'o 6'0 ~o ./. AGE S*ND CONTENT
~oo
Fig. 5. Relationship between total nickel content and sand content showing trend. See caption Fig. 2.
25
Texture
The form of variation of concentrations of chromium and nickel with sand content is shown in Fig. 5 for nickel and a similar variation was obtained for chromium. There is little change in total chromium and nickel contents over the range of 0--50% sand but from 50% to 100% the concentrations of both elements fall steadily. The general decrease in concentration with increase in sand content reflects the fact that the bulk of the sand-size particles (20--2000 /~m) are quartz or feldspar which contain only small amounts of chromium and nickel (Mitchell, 1964b). SUMMARY AND CONCLUSIONS
The data reported in Table I show that soils from other parts of the world contain amounts of chromium and nickel similar to those found in Scotland. The chromium and nickel contents of Scottish soils are approximately log-normally distributed. Therefore the derived means and normal ranges as defined here afford a more satisfactory m e t h o d of describing the distributions than the means and standard deviations of the untransformed variates. The derived mean for the full set of chromium data is 62 mg kg -1 and the normal range 5.4--710 mg kg-' and corresponding values for nickel are 27 and 3.4--210 mg kg -1. Chromium and nickel concentrations are strongly correlated and their relationships with other soil variables are very similar. The concentrations of both elements are lower in organic than in mineral material and this is reflected in the variation of concentration with ash content and depth. Both elements are more abundant in mafic than in felsic minerals and as a consequence greater amounts are found in softs derived from basic than acidic igneous rocks and concentrations are inversely related to sand content. Comparing the derived means for chromium and nickel contents of Scottish softs with the maxima of 50 mg Cr kg -~ and 30 mg Ni kg -~ recommended in the proposal of the Commission of the European Communities (C.E.C.), 1982, disposal of sludges containing chromium would be largely prohibited and disposal of sludges containing nickel would be limited to softs of granitic and sandstone origin only. The high natural concentrations of total chromium and nickel in soils emphasise the need to reassess the maxima proposed in the C.E.C. Directive. The total metal content provides a useful b u t only preliminary indication of the risk of toxicity as plant uptake depends on the availability of the metal concerned, soil type and plant species. ACKNOWLEDGEMENTS
We are grateful to members of staff of the Soil Survey of Scotland who sampled and described the soil profiles and to the Department of Mineral Soils for soft texture analyses.
26 REFERENCES Anderson, A.J., Meyer, D.R. and Mayer, F.K., 1973. Heavy metal toxicities: levels of nickel, cobalt and chromium in the soil and plants associated with visual symptoms and variation in growth of an oat crop. Aust. J. Agric. Res., 24: 557--571. Berrow, M.L. and Reaves, G.A., 1981. Trace elements in Scottish soils developed on greywackes and shales: variability in the total contents of basal horizon samples. Geoderma, 26: 157--164. Boerngen, J.G. and Tidball, R.R., 1981. Chemical analyses of selected agricultural soils of Missouri. U.S. Geol. Surv. Open-File Report 81-842,125 pp. Commission of the European Communities (C.E.C.), 1982. Proposal for a Council Directive on the Use of Sewage Sludge in Agriculture. Commission of the European Communities, ENV/102/80-EN (Rev. 6) Brussels. Davies, B.E., 1983. A graphical estimation of the normal lead content of some British soils. Geoderma, 29: 67--75. Dixon, N.E., Gazzola, C., Blackeley, R.L. and Zerner, B., 1975. Jack bean urease (EC. 3.5.1.5.). A metalloenzyme. A simple biological role for nickel? J. Am. Chem. Soc., 97: 4131--4133. Gough, L.P., Peard, J.L., Severson, R.C., Shacklette, H.T., Tompkins, M.L., Stewart, K.C. and Briggs, P.H., 1984. Chemical analyses of soils and other surficial materials, Alaska. U.S. Geol. Surv. Open-File Report 84-423, 77 pp. Huffman, E.W.D. and Allaway, W.H., 1973. Growth of plants in solution culture containing low levels of chromium. Plant Physiol., 52: 72--75. Hunter, J.G. and Vergnano, O., 1952. Nickel toxicity in plants. Ann. Appl. Biol., 39: 279--284. Hunter, J.G. and Vergnano, O., 1953. Trace element toxicities in oat plants. Ann. Appl. Biol., 40: 761--777. Iimura, K., 1981. Background contents of heavy metals in Japanese soils. In: K. Kitagishi and I. Yamane (Editors), Heavy Metal Pollution in Soils of Japan. Japan Scientific Societies Press, Tokyo, pp. 19--26. Kinniburgh, D.G. and Beckett, P.H.T., 1983. Geochemical mapping in Oxfordshire: a comparison of stream sediment and soil sampling. J. Soil Sci., 34: 183--203. Kirchgessner, M. and Schnegg, A., 1980. Biochemical and physiological effects of nickel deficiency. In: J.O. Nriagu (Editor), Nickel in the Environment. Wiley, New York, N.Y., pp. 635--652. Laing, D., 1976. The soils of the country round Perth, Arbroath and Dundee. Memoirs of the Soil Survey of Great Britain -- Scotland. H.M.S.O., 192 pp. Mitchell, R.L., 1964a. The spectrochemical analysis of soils, plants and related materials. Commonwealth Bureau of Soils, Harpenden, Tech. Comm. No. 44A, 225 pp. Mitchell, R.L., 1964b. Trace elements in soils. In: F.E. Bear (Editor), Chemistry of the Soil. 2nd ed. Reinhold, New York, N.Y., pp. 320--368. National Research Council (N.R.C.), 1974. Committee on biologic effects of atmospheric pollutants, 1974: Chromium. U.S. National Academy of Sciences, Washington D.C., 155 pp. Reaves, G.A. and Berrow, M.L., 1984a. Total copper contents of Scottish soils. J. Soil Sci., 35: 583--592. Reaves, G.A. and Berrow, M.L., 1984b. Total lead concentrations in Scottish soils. Geoderma, 32: 1--8. Schwartz, K. and Mertz, W., 1959. Chromium(III) and the glucose tolerance factor. Arch. Biochem. Biophys., 85: 292--295. Shacklette, H.T. and Boerngen, J.G., 1984. Element concentrations in soils and other surficial materials of the conterminous United States. U.S. Geol. Surv. Prof. Pap., 1270, 105 pp.
27 Shiraki, K., 1978. Chromium. In: K.H. Wedepohl (Editor), Handbook of Geochemistry. Springer-Verlag, Berlin, Heidelberg, II-3, ch. 24, 73 pp. Turekian, K.K., 1978. Nickel. In: K.H. Wedepohl (Editor), Handbook of Geochemistry. Springer-Verlag, Berlin, Heidelberg, II-3, ch. 28, 51 pp. Ure, A.M. and Berrow, M.L., 1982. The Elemental Constituents of Soils. In: H.J.M. Bowen (Editor), Environmental Chemistry, 2. A specialist Periodical Report. R. Soc. Chem., Lond., pp. 94--204. Vuorinen, J., 1958. On the amounts of minor elements in Finnish soils. Maataloust. Aikak., 30: 30--35. Wakatsuki, T., Matsuo, Y. and Kyuma, K., 1978. Natural background of element distribution in soil (1). Pb, Zn, Cu, Ni, Cr and V in paddy soils in Japan. J. Sci. Soil Manure, Japan, 49: 507--512.
15
TOTAL CHROMIUM AND NICKEL CONTENTS OF SCOTTISH SOILS
M.L. B E R R O W a n d G.A. R E A V E S
Departments of Speetrochemistry and Statistics, The Macaulay Institute for Soil Research, Aberdeen (Great Britain) (Received M a r c h 21, 1 9 8 5 ; a c c e p t e d a f t e r revision O c t o b e r 29, 1 9 8 5 )
ABSTRACT Berrow, M.L. a n d Reaves, G.A., 1 9 8 6 . T o t a l c h r o m i u m a n d nickel c o n t e n t s o f S c o t t i s h soils. G e o d e r m a , 3 7 : 15--27. T h e f r e q u e n c y d i s t r i b u t i o n s o f c h r o m i u m a n d nickel c o n t e n t s in S c o t t i s h soils are r e p o r t e d a n d t h e r e l a t i o n o f t h e c o n c e n t r a t i o n s of these e l e m e n t s t o s o m e o t h e r soil variables is e x a m i n e d . T h e f r e q u e n c y d i s t r i b u t i o n s o f b o t h e l e m e n t s are a p p r o x i m a t e l y log-normal a n d are d e s c r i b e d in t e r m s o f t h e derived m e a n (x*) a n d t h e " n o r m a l r a n g e " (L--L") w h e r e x * = a n t i l o g x, L = a n t i l o g (x--2s) a n d L" = antilog (x+2s); x a n d s b e i n g t h e m e a n a n d s t a n d a r d d e v i a t i o n , respectively, o f t h e l o g - t r a n s f o r m e d data. T h e derived m e a n f o r t h e full set o f c h r o m i u m d a t a is 62 m g kg -1 a n d t h e n o r m a l range 5 . 4 - - 7 1 0 mg kg -1. T h e c o r r e s p o n d i n g figures for nickel are 27 a n d 3 . 4 - - 2 1 0 m g kg -'. T h e c o n c e n t r a t i o n s o f t h e s e t w o e l e m e n t s show a high degree o f c o r r e l a t i o n (r = 0 . 8 3 ) a n d t h e f o r m s o f t h e r e l a t i o n s h i p s b e t w e e n e l e m e n t c o n t e n t s a n d o t h e r soil variables are very similar. C o n c e n t r a t i o n s o f c h r o m i u m a n d nickel are greater in soils derived f r o m basic a n d i n t e r m e d i a t e i g n e o u s t h a n in t h o s e derived f r o m acid igneous rocks, are h i g h e r in argill a c e o u s t h a n in a r e n a c e o u s s e d i m e n t s , decrease w i t h increasing s a n d a n d organic m a t t e r c o n t e n t s a n d s h o w relatively little v a r i a t i o n w i t h increase in d e p t h b e l o w 10 cm.
INTRODUCTION
In common with many other trace elements, chromium and nickel are of biological importance. Chromium is now recognized to be essential to mammals (Schwartz and Mertz, 1959) but not, so far, to plants (Huffman and Allaway, 1973) and evidence of the nutrient value of nickel to various classes of organisms is accumulating (Dixon et al., 1975; Kirchgessner and Schnegg, 1980). Both elements have been shown to be toxic to plants (N.R.C., 1974; Hunter and Vergnano, 1953) and nickel toxicity arising in the field has been investigated by Hunter and Vergnano (1952) and Anderson et al. (1973). The recent proposal by the Commission of the European Communities for a Directive on the use of sewage sludge in agriculture (C.E.C., 1982)
0016-7061/86/$03.50
© 1 9 8 6 Elsevier Science P u b l i s h e r s B.V.
16 has led to widespread interest in establishing the background levels of several heavy metals in soils, including chromium and nickel. The Directive seeks to limit the amounts of metals which can be added to soils in sewage sludge and proposes maximum tolerable levels for the total contents of metals in soils. The recommended maxima for chromium and nickel are 50 and 30 mg kg -1, respectively. This paper reports on the chromium and nickel contents of soils derived from a wide range of geological parent materials throughout Scotland and compares the levels with those found in soils from other parts of the world (Ure and Berrow, 1982). MATERIALS AND METHODS
Materials The great majority of the samples analysed were collected during the course of a general-purpose soil survey by the Soil Survey of Scotland and include representative profiles from all the soil series mapped to the present time. In general, all the samples from a given profile were analysed, b u t in a few cases a selection was made; e.g., in some deep profiles only alternate horizons were analysed. Care was taken to avoid chromium and nickel contamination, particularly by stainless steel, at all stages of sampling, storage and preparation for analysis.
Analytical me thods A subsample (approx. 25 g) of the < 2 mm, air-dried fraction of the soil was ground to a fine powder (< 150 pm) in a closed agate ball mill and a 2-g portion dried at 80°C prior to ignition overnight at 450°C. Ignited samples were analysed for chromium and nickel using the direct-current arc spectrographic m e t h o d described in detail by Mitchell (1964a). The concentrations are expressed on an oven-dry basis. The limit of detection for both elements in mineral samples is 3 mg kg -1, b u t it is lower than this for highly organic samples where a concentration factor is introduced as a result of the ashing process. Where a sample value is below the detection limit that sample has been allocated a value equal to half the "less-than" level; e.g., a sample with less than 3 mg kg -1 has been allocated a value of 1.5 mg kg -~. Over the range of contents reported an accuracy of + 20% is obtained.
Processing of analytical data The frequency distributions of trace element concentrations in soils have generally been found to be positively skewed (Berrow and Reaves, 1981; Kinniburgh and Beckett, 1983) and to have an approximately log-
17
normal distribution (Wakatsuki et al., 1978; Davies, 1983). Log-transformation of the chromium and nickel values yields a distribution which approaches normality (Fig. 1). In the results section the distribution of concentrations has been described in terms of the derived mean and "normal range" defined as follows. If x and s are the mean and standard deviation, respectively, of the logtransformed data then the derived mean (x*) = antilog x, and the normal range is that between the limits L and L", where L = antilog (x--2s) and L" = antilog (x+2s). 1200
1000
o.
800
~- 6 0 0 o z 4OO
200
o 1,0
.
1.2 LN
3.4 5.6 7,8 10.0 TOTAL CHROMIUM CONTENT ( M G / K G )
1800 1600 1400 w 1200 LU <
1000
u. o
800
zd
600
--7
i I I
400 200 O 1
3 5 7 LN TOTAL NICKEL ( M G / K G )
9
Fig. 1, A. Histogram showing the distribution of total chromium contents after logtransformation. B. Histogram showing the distribution of total nickel contents after log-transformation.
18
6
0
I l l l l ~ v
vv~
,iO
O 0 0 0 0 0 0 0
Illllll ¢¢
0 ¢a
..=
.<
0 0
19
The derived mean will not lie in the middle of the normal range, but t h a t range will include approximately 95% of the data and the numbers of data less than and greater than the derived mean will be approximately equal. RESULTS AND DISCUSSION
The range and mean values for the data (untransformed) are presented in Table I which draws together the results of a number of publications on trace elements in large numbers of softs from Finland, Japan, U.S.A. and Scotland and a recent review of data from the world literature. The arithmetic means for chromium and nickel concentrations in Scottish soils are greater than the means for other soils, probably because a larger proportion of Scottish soils are developed on ultra-basic rocks. The arithmetic mean of the chromium content of over 13,000 soil samples tabulated in Table I is 93 mg kg -1 and of over 16,000 samples for nickel is 34 mg kg -1. The form of the d i s t r i b u t i o n of the log-transformed concentrations is shown in Fig. 1. The derived mean for chromium is 62 and the normal range 5.4--710 mg kg -1. The corresponding figures for nickel are 27 and 3.4--210 mg kg -1. The derived mean and normal range (dispersion) for the full set of data and for subsets are presented in Table II and III. TABLE II Chromium contents of various groups of samples (mg kg -1) Sample subset
Number of data
Derived mean, x*
Normal range L' L"
All samples All organic samples (0) All mineral samples (1) All "O" horizon samples (0) All " A " horizon samples (1) All "S" horizon samples (2) All "B" horizon samples (3) All "C" horizon samples (4) Granite and granitic gneiss (0) Intermediate igneous (1) Basic and ultra-basic igneous (2) Metamorphic rocks (3) "ORS" sands and sandstones (4) Fluvio-glacial sands and gravels (5) "Other" sands and sandstones (6) Lower Paleozoie greywackes and shales (7) Silts and clays (8) Mixed acid and basic igneous (9)
2944 326 2618 230 335 398 1080 866 136 134 228 316 49 254 374 133 83 135
62 22 71 13 62 65 77 67 49 100 280 54 30 30 42 190 80 110
5.4 710 0.90 540 7.8 640 0.71 250 4.5 850 11 380 8.9 670 7.2 630 5.8 420 32 350 15 4900 9.5 310 8.5 110 6.6 140 5.0 360 47 750 14 450 22 600
20 T A B L E III Nickel contents of various groups of samples (mg kg -I) Sample subset
Number of data
Derived Normal range mean, L' L" x*
All samples All organic samples (0) All mineral samples (1) All "O" horizon samples (0)
4122 446 3676 321
27 9.9 31 6.6
All "A" horizon samples (1) All "S" horizon samples (2) All "B" horizon samples (3) All "C" horizon samples (4) Granite and granitic gneiss (0) Intermediate igneous (1) Basic and ultra-basic igneous (2) Metamorphic rocks (3) "ORS" sands and sandstones (4) Fluvio-glacial sands and gravels (5) "Other" sands and sandstones (6) Lower Paleozoic greywackes and shales (7) Silts and clays (8) Mixed acid and basic igneous (9)
483 571 1516 1178 198 180 286 367 144 279 514 309 138 174
20 28 33 34 16 42 89 24 19 16 20 53 36 48
3.4 0.75 4.8 0.35 2.5 6.2 5.1 5.3 2.4 17 6.6 6.1 5.6 3.5 3.5 17 6.5 14
210 130 200 130 160 120 210 220 110 110 1200 97 62 76 110 170 200 170
T h e l o g - t r a n s f o r m e d c o n c e n t r a t i o n s o f c h r o m i u m a n d nickel are highly c o r r e l a t e d (r = 0.83, see Fig. 2) a n d as t h e y vary with o t h e r soil variables in a similar w a y t h e t w o elements can be described t o g e t h e r .
Horizon designation The horizon designations used by the Soil Survey of Scotland (Laing, 1976) m a y be summarised as follows: L = undecomposed plant litter; F = partially decomposed litter with recognizable plant remains; H = decomposed organic matter with few or no recognizable plant remains; A = uppermost mineral soil horizon subject to the most influence of climate, plants and animals; S = surface horizon of cultivated soil; B = "subsoil" horizon characterized by a relatively high content of sesquioxides or clay; C = the more~r-less unaltered parent material of the soil. The L, F and H horizons have been dealt with as a single group designated " O " (organic) for convenience. The most notable feature of the derived m e a n concentrations for different horizon types is the difference between the organic " O " horizon samples and the others. In Scottish soils the accumulation of organic matter in the upper horizons of soil profiles normally lowers the concentrations of most trace elements relative to the underlying mineral horizons
21 ~8
~6 bJ
O U
W
~4 z
e#
't"{
Z
-J 2
|
I
I
LN TOTAL CHROMIUM CONTENT (MG/KG) Fig. 2. Relationship b e t w e e n t o t a l c h r o m i u m arid nickel contents showing t r e n d x ~
plots of grouped data in which the range covered by the data for each variate has been subdivided into fifty segments yielding a fifty by fifty matrix. The symbol • indicates the mean of the y-values of the points in the column, and the vertical bar extends above and below the mean position, a distance equal to twice the standard error of these values. because plant residues generally contain considerably lower concentrations of chromium and nickel than soil minerals. The smaller dispersion (normal range) of the S horizon samples is probably the result of cultivation, S horizons being a mixture of O, A and sometimes B horizon material.
Mineral vs organic samples Highly organic surface horizons are c o m m o n l y found in uncultivated Scottish soils such as the peaty podzol and peaty gley groups. For the purposes of this investigation samples with less than 80% ash have been classified as organic and the remainder as mineral. The derived mean concentrations of chromium and nickel are lower and the dispersion (normal range) is greater for organic samples than for mineral samples (Tables II and III). Similar results were obtained previously for copper in Scottish soils (Reaves and Berrow, 1984a) but contrasting results were found for lead (Reaves and Berrow, 1984b). The situation for lead is complicated by the general enrichment of surface horizons which are mostly organic.
22
Parent material
Ten distinct groups have been defined and numbered in the lower parts of Tables II and III on the basis of the parent material descriptions given in various publications of the Soil Survey of Scotland. As superficial accumulations of organic matter are presumed to be unrelated to parent material the figures in groups 0 to 9 are for mineral samples only. The parent material groups 4, 5 and 6 (Tables II and III) which include soils developed from sands, gravels and sandstones have the lowest derived mean contents of 30 to 42 mg Cr kg -1 and 16 to 20 mg Ni kg -1. Parent material groups 0 and 3 developed from granitic and metamorphic rocks, respectively, have slightly higher derived means of a b o u t 50 mg Cr kg -1 and 20 mg Ni kg -~. Groups 8, 1 and 9 which include soils derived from fine textured silts and clays, intermediate igneous rocks and mixed tills, respectively, have increasingly greater derived mean contents within the ranges 80-110 mg Cr kg -1 and 36--48 mg Ni kg -~. Finally, groups 7 and 2 developed from Lower Palaeozoic rocks and basic and ultra-basic igneous rocks have the greatest derived mean contents for both chromium and nickel. The dispersion (normal range) for element contents is greater for the group-2 softs than for all the other groups, the lowest and highest values differing by factors of the order of 200--300. The minerals in which chromium and nickel are most abundant in rocks are commonly, but not always, identical. Olivine, a c o m m o n mineral in basic and ultra-basic rocks is generally poor in chromium b u t pyroxenes, amphiboles and micas can be relatively rich. Chromium is most sensitive to magmatic fractionation during igneous rock crystallization and a wide range of chromium contents therefore occurs in different rock types ranging between an average of 2600 mg kg -~ in ultra-basic rocks to 1--30 mg kg -1 in granites and rhyolites (Shiraki, 1978). Olivine is, however, a major host mineral for nickel in basic igneous rocks b u t it enters other ferromagnesian minerals such as pyroxenes and amphiboles in other igneous rocks, isomorphously substituting for iron or magnesium. The concentrations of chromium and nickel in Scottish softs therefore correspond closely with those found in the rock types from which their parent materials are derived (c.f. Shiraki, 1978, and Turekian, 1978). Soils derived from sandstones, granites and quartzites containing high proportions of quartz and feldspar are at the low end of the range, soils from basic and ultrabasic rich in ferromagnesian minerals are at the higher end while those derived from intermediate and mixed acid and basic igneous rocks are in the middle. The close correlation between the total chromium and nickel contents in Scottish soils derived from a wide range of parent materials (Fig. 2) suggests that most of the chromium and nickel in these softs is probably still present in unweathered primary minerals as isomorphous substituents of iron and magnesium in crystal lattices.
23
Sampling depth The form of variation in concentration of nickel with depth is shown in Fig. 3 and the variation with depth for chromium is similar. The most notable feature is the relatively low contents of both elements in surface horizons (0--6 cm) where organic matter contents are high. There are relatively small changes in total chromium and nickel at depths below 6 cm.
~6 L5
E _J uJ _u 4 z
i.-
~2
0 0
510
I~)0 SAMPLING
150 DEPTH
200
(CM)
Fig. 3. Relationship between total nickel content and sampling depth showing trend. See caption Fig. 2.
Ash content Over the greater part of the range of ash contents (determined by ignition at 450°C) concentrations of nickel and also of chromium increase gradually and fairly uniformly with ash content (Fig. 4). For highly organic soils with low (0--10%) ash, however, the concentrations of chromium and nickel are low. At the other end of the scale, over the range of 95--100% ash, there is a clear decrease in chromium and nickel contents. A very similar pattern was observed for copper (Reaves a n d Berrow, 1984a) and is due to the fact that samples with ash contents in the range of 95--100% are largely composed of siliceous-sand-containing low concentrations of heavy metals.
24
~6 <9 E LU U4 z
-I
t o
<
S z, 2~
o
#o
4b @/o A S H
6'o
8'o
100
CONTENT
Fig. 4. Relationship between total nickel content and ash content showing trend. See caption Fig. 2.
~6 J L)
-J4 <
5
o
2'o
4'o 6'0 ~o ./. AGE S*ND CONTENT
~oo
Fig. 5. Relationship between total nickel content and sand content showing trend. See caption Fig. 2.
25
Texture
The form of variation of concentrations of chromium and nickel with sand content is shown in Fig. 5 for nickel and a similar variation was obtained for chromium. There is little change in total chromium and nickel contents over the range of 0--50% sand but from 50% to 100% the concentrations of both elements fall steadily. The general decrease in concentration with increase in sand content reflects the fact that the bulk of the sand-size particles (20--2000 /~m) are quartz or feldspar which contain only small amounts of chromium and nickel (Mitchell, 1964b). SUMMARY AND CONCLUSIONS
The data reported in Table I show that soils from other parts of the world contain amounts of chromium and nickel similar to those found in Scotland. The chromium and nickel contents of Scottish soils are approximately log-normally distributed. Therefore the derived means and normal ranges as defined here afford a more satisfactory m e t h o d of describing the distributions than the means and standard deviations of the untransformed variates. The derived mean for the full set of chromium data is 62 mg kg -1 and the normal range 5.4--710 mg kg-' and corresponding values for nickel are 27 and 3.4--210 mg kg -1. Chromium and nickel concentrations are strongly correlated and their relationships with other soil variables are very similar. The concentrations of both elements are lower in organic than in mineral material and this is reflected in the variation of concentration with ash content and depth. Both elements are more abundant in mafic than in felsic minerals and as a consequence greater amounts are found in softs derived from basic than acidic igneous rocks and concentrations are inversely related to sand content. Comparing the derived means for chromium and nickel contents of Scottish softs with the maxima of 50 mg Cr kg -~ and 30 mg Ni kg -~ recommended in the proposal of the Commission of the European Communities (C.E.C.), 1982, disposal of sludges containing chromium would be largely prohibited and disposal of sludges containing nickel would be limited to softs of granitic and sandstone origin only. The high natural concentrations of total chromium and nickel in soils emphasise the need to reassess the maxima proposed in the C.E.C. Directive. The total metal content provides a useful b u t only preliminary indication of the risk of toxicity as plant uptake depends on the availability of the metal concerned, soil type and plant species. ACKNOWLEDGEMENTS
We are grateful to members of staff of the Soil Survey of Scotland who sampled and described the soil profiles and to the Department of Mineral Soils for soft texture analyses.
26 REFERENCES Anderson, A.J., Meyer, D.R. and Mayer, F.K., 1973. Heavy metal toxicities: levels of nickel, cobalt and chromium in the soil and plants associated with visual symptoms and variation in growth of an oat crop. Aust. J. Agric. Res., 24: 557--571. Berrow, M.L. and Reaves, G.A., 1981. Trace elements in Scottish soils developed on greywackes and shales: variability in the total contents of basal horizon samples. Geoderma, 26: 157--164. Boerngen, J.G. and Tidball, R.R., 1981. Chemical analyses of selected agricultural soils of Missouri. U.S. Geol. Surv. Open-File Report 81-842,125 pp. Commission of the European Communities (C.E.C.), 1982. Proposal for a Council Directive on the Use of Sewage Sludge in Agriculture. Commission of the European Communities, ENV/102/80-EN (Rev. 6) Brussels. Davies, B.E., 1983. A graphical estimation of the normal lead content of some British soils. Geoderma, 29: 67--75. Dixon, N.E., Gazzola, C., Blackeley, R.L. and Zerner, B., 1975. Jack bean urease (EC. 3.5.1.5.). A metalloenzyme. A simple biological role for nickel? J. Am. Chem. Soc., 97: 4131--4133. Gough, L.P., Peard, J.L., Severson, R.C., Shacklette, H.T., Tompkins, M.L., Stewart, K.C. and Briggs, P.H., 1984. Chemical analyses of soils and other surficial materials, Alaska. U.S. Geol. Surv. Open-File Report 84-423, 77 pp. Huffman, E.W.D. and Allaway, W.H., 1973. Growth of plants in solution culture containing low levels of chromium. Plant Physiol., 52: 72--75. Hunter, J.G. and Vergnano, O., 1952. Nickel toxicity in plants. Ann. Appl. Biol., 39: 279--284. Hunter, J.G. and Vergnano, O., 1953. Trace element toxicities in oat plants. Ann. Appl. Biol., 40: 761--777. Iimura, K., 1981. Background contents of heavy metals in Japanese soils. In: K. Kitagishi and I. Yamane (Editors), Heavy Metal Pollution in Soils of Japan. Japan Scientific Societies Press, Tokyo, pp. 19--26. Kinniburgh, D.G. and Beckett, P.H.T., 1983. Geochemical mapping in Oxfordshire: a comparison of stream sediment and soil sampling. J. Soil Sci., 34: 183--203. Kirchgessner, M. and Schnegg, A., 1980. Biochemical and physiological effects of nickel deficiency. In: J.O. Nriagu (Editor), Nickel in the Environment. Wiley, New York, N.Y., pp. 635--652. Laing, D., 1976. The soils of the country round Perth, Arbroath and Dundee. Memoirs of the Soil Survey of Great Britain -- Scotland. H.M.S.O., 192 pp. Mitchell, R.L., 1964a. The spectrochemical analysis of soils, plants and related materials. Commonwealth Bureau of Soils, Harpenden, Tech. Comm. No. 44A, 225 pp. Mitchell, R.L., 1964b. Trace elements in soils. In: F.E. Bear (Editor), Chemistry of the Soil. 2nd ed. Reinhold, New York, N.Y., pp. 320--368. National Research Council (N.R.C.), 1974. Committee on biologic effects of atmospheric pollutants, 1974: Chromium. U.S. National Academy of Sciences, Washington D.C., 155 pp. Reaves, G.A. and Berrow, M.L., 1984a. Total copper contents of Scottish soils. J. Soil Sci., 35: 583--592. Reaves, G.A. and Berrow, M.L., 1984b. Total lead concentrations in Scottish soils. Geoderma, 32: 1--8. Schwartz, K. and Mertz, W., 1959. Chromium(III) and the glucose tolerance factor. Arch. Biochem. Biophys., 85: 292--295. Shacklette, H.T. and Boerngen, J.G., 1984. Element concentrations in soils and other surficial materials of the conterminous United States. U.S. Geol. Surv. Prof. Pap., 1270, 105 pp.
27 Shiraki, K., 1978. Chromium. In: K.H. Wedepohl (Editor), Handbook of Geochemistry. Springer-Verlag, Berlin, Heidelberg, II-3, ch. 24, 73 pp. Turekian, K.K., 1978. Nickel. In: K.H. Wedepohl (Editor), Handbook of Geochemistry. Springer-Verlag, Berlin, Heidelberg, II-3, ch. 28, 51 pp. Ure, A.M. and Berrow, M.L., 1982. The Elemental Constituents of Soils. In: H.J.M. Bowen (Editor), Environmental Chemistry, 2. A specialist Periodical Report. R. Soc. Chem., Lond., pp. 94--204. Vuorinen, J., 1958. On the amounts of minor elements in Finnish soils. Maataloust. Aikak., 30: 30--35. Wakatsuki, T., Matsuo, Y. and Kyuma, K., 1978. Natural background of element distribution in soil (1). Pb, Zn, Cu, Ni, Cr and V in paddy soils in Japan. J. Sci. Soil Manure, Japan, 49: 507--512.