- Email: [email protected]
The initial assessment of trace metal pollution in coastal sediments
Marine Pollution Bulletin Maxwell, J. R., Cox, R. E., Ackman, R. G. & Hooper, S. N. (1972). The diagenesis and maturation of phytol. The stereochemistry of 2,6,10,14tetramethylpentadecane from ancient sediment. In Advances in Organic Geochemistry 1971 ( H. R. Gaertner & H. Wehner, eds.), pp. 277-291. Maxwell, J. R., Cox, R. E., Eglinton, G. & Pillinger, C. T. (1973). Stereochemical studies of acyclic isoprenoid c o m p o u n d s - I I . The role of chlorophyll in the derivation of isoprenoid-type acids in a lacustrine sediment. Geochim. Cosmochim. Acta, 37,297-313. McCarty, E. D. & Calvin, M. (1966). The isolation and identification of the C 17isoprenoid satruated hydrocarbon 2,6,10-trimethyl tetradecane:
Squalane as a possible precursor. Preprints. Gen. Papers, Div. Petr. Chem. A.C.S., 11, 3. Patience, R. L., Rowland, S. J. & Maxwell, J. R. (1978). The effect of maturation on the configuration of pristane in sediment and petroleum Geochim. Cosmochim. A cta., 42, 1871-1875. Shlyakhov, A. F., Koreshkova, R. I. & Telkova, M. S. (1975). Gas chromatography of isoprenoid alkanes. J. Chromat., 104, 337-349. Shlyakhov, A. F. & Volkova, L. G. (1977). Stereochemistry of isoprenoid alkanes and possible ways of forming them in sedimentary rocks. Geokhimiya, 8, 1418-1423.
MarinePollutionBulletin. Vol. 12, No. 3, pp. 84-91. 1981. Printed in Great Britain.
0025-326X/8110300844)8$02.0010 © 1981PergamonPressLtd.
The Initial Assessment of Trace Metal Pollution in Coastal Sediments R. CHESTER* and E G. VOUTSINOU -I-
*Department of Oceanography, The University, LiverpoolL 69 3 BX, UK. JrInstitute of Oceanographic and Fishing Research, Agios, Kosmas, Ellinkon, Athens, Greece.
Coastal sediments are important hosts for pollutant trace metals, but analytical difficulties can prevent them being included in routine environmental monitoring programmes. In order to identify a suitable approach to the problem, an established simple technique has been evaluated for the initial assessment of trace metal pollution in coastal sediments. The technique, which involves leaching the samples with cold 0.5 N HCI, has been applied to surface sediments from two Greek gulfs and has been shown to provide a rapid, inexpensive way of initially establishing the gross degree to which a sediment population has been subjected to trace metal pollution from the overlying waters.
In recent years authorities in more and more countries have sought to initiate programmes for the assessment of marine pollution, particularly that involving the trace metals. Some of the highest concentrations of these metals in the marine environment are found in sediments, and in certain regions their emissions have been of sufficient strength to impose 'anthropogenic fingerprints' on the bottom deposits. A knowledge of the concentrations and distributions of trace metals in sediments can therefore play a key role in detecting sources of pollution in aquatic systems (Forstner & Wittmann, 1979), and for this reason sediments should be included among the environmental parameters monitored in major marine pollution programmes. In practice, there are difficulties involved in their inclusion in routine monitoring. These include: the selection of a suitable analytical technique; constraints such as time, cost, number of sediment populations to be monitored; and, the availability of both analytical facilities and trained personnel. Together, these various factors can dictate the extent to which an investigation can proceed, and one of the most important of them is the selection of an analytical technique. The analysis of trace metals in sediments presents 84
a number of complex problems and has resulted in a bewildering variety of techniques being advocated for this purpose in the literature. It is clearly advantageous, therefore, to define as closely as possible the aims of any particular trace metal investigation, and in order to achieve this it is necessary to have some understanding of the mechanisms by which trace metals are incorporated into sediments. There have been various attemplts to classify the elements found in sediments, but perhaps the most useful one from a practical point of view is that which distinguishes between lattice-held, i.e. residual, and non-lattice-held, i.e. nonresidual, trace metals (see e.g.: Chester & Hughes, 1967; Chester & Messiha-Hanna, 19"70; Agemain & Chau, 1976). Residual trace metals are defined as those which are pan of the silicate matrix of the sediment, and which are located mainly in the lattice structures of the component minerals. Non-residual trace metals are not part of the silicate matrix and have been incorporated into the sediment from aqueous solution by processes such as adsorption and organic complexation; i.e. non-residual trace metals include those originating from polluted waters. This simple distinction has been extended by various authors into more detailed classification schemes in which the non-residual fraction has been sub-divided into several categories (see e.g.: Chester & Hughes, 1967; Forstner & Patchineeham, 1976; Chester et al., 1980). There are a large number of techniques described in the literature which have been specifically designed to liberate from a sediment all, or part, of the non-residual trace metals - f o r reviews of this subject see Chester (1978) and Forstner & Wittmann (1979). However, many of the available techniques are complex and time-consuming; especially those involving sequential leaching stages. Further, many of them are primarily of 'academic' interest and may, in fact, provide the enviromental scientist with data he does not require.
Volume 12/Number3/March 1981 For the assessment of trace metal pollution in sediments it is the non-residual elements which are of prime interest, and the analysis of the non-residual fractions will often yield more data on the extent of trace metal pollution than will that of the total sediment which includes the residual, or non-polluted, fraction and so may mask the relationships sought (see e.g.: Agemaln & Chau, 1976; Forstner & Wittmann, 1979). There are a number of techniques which have been proposed for the analysis of pollutant trace metals in sediments, some of which have been experimentally assessed by Agemain & Chau (1976). One of the techniques was that outlined by Chester & Hughes (1967) for the study of trace metal partitioning in pelagic deep-sea sediments, which involved leaching the samples with a mixed NH2OH.HC1 + CH3COOH reagent. However, this technique is not directly applicable to those coastal and estuarine sediments which contain appreciable quantities of organic carbon, and for these an additional (e.g. EDTA) stage is required in order to completely solubilize organically complexed trace metals (see e.g.: Agemain & Chau, 1976; Chester et al., 1980). However, according to Duinker et al. (1974), Agemain & Chau (1976) and the I.W.D. (1979) dilute HC1 will release both inorganic and organicassociated non-residual trace metals from sediments without materially affecting the silicate matrix. More specialised techniques should be used for certain sediments, e.g. those containing appreciable quantities of sulphites or coal. The initial aim of many monitoring programmes, with respect to sediments, is simply to establish the degree (if any) to which a population has been subjected to trace metal pollution; more detailed work can then follow concentrating on those sediments for which the pollution has been shown to be potentially hazardous. In order to meet this initial aim, the present work was undertaken to assess the field usefulness of the simple, rapid, inexpensive and widely applicable cold dilute HCI leaching technique, based on those originally investigated by Duinker et al. (1974) and Agemain & Chau (1976) and outlined by the I.W.D. (1979), for the investigation of trace metal pollution in coastal sediments. Many of the techniques described in the literature have been tested on only a limited number of samples, and this can be a considerable disadvantage for the environmental scientist who is confronted with a variety of such techniques and has to choose between them. Further, there is often no assessment made of ways of interpreting the data derived from the application of the various techniques. For reasons such as this, the present study was carried out on sediments from two contrasting 'real' populations, and also included methods of data interpretation.
Sediment Samples Sediments were collected by grab sampler from two Greek coastal localities; the Gulf of Thermaikos, for which trace metal pollution was suspected; and the Gulf of Pagassitikos, an area which was not expected to suffer from trace metal pollution. The sediments had a wide variety of particle sizes and in order to make a viable inter-sediment comparison trace element analyses were carried out on the < 61/an fractions which were separated by wet sieving. The
separates were oven dried and a representative sample subsequently taken for analysis.
Analytical Procedure About 5 g of each representative sample was accurately weighed and placed in a 100 ml wide neck plastic bottle. 75 ml of 0.5 N HC1 was added (evolution of CO2 being allowed where necessary) and the bottles shaken mechanically for approximately 16 h. The solutions were allowed to stand for a few minutes then suction filtered, and the filtrates sprayed directly into an atomic absorption instrument using appropriate lamps and settings. A blank was run omitting the sample. For full details see I. W. D. (1979). The following elements were determined: Mn, Ni, Co, Cr, Cu, Cd, Pb and Zn. Careful checks showed that there were no sample matrix effects for any of the elements and standard curves were therefore constructed, using appropriate dilutions of stock solutions, in 0.5 N HCI. Five replicates of one sample were run and the following coefficients of variation found: Mn, 2.8%; Ni, 3.3%; Co, 2.4%; Cr, 6.3%; Cu, 3.8%; Cd, 5.6%; Pb, 3.5%; Zn, 5.0%.
Results and Discussion The locations of the surface samples in the gulfs of Thermalkos and Pagassitikos are illustrated in Figs 1 and 2 respectively, and the concentrations of the non-residual trace metals in them are listed in Table 1 (a) and (b). There are a number of ways in which the data obtained from a survey of the distribution of non-residual trace metals in surface sediment populations can be interpreted. Two of the most commonly employed involve; (1) the evaluation of spatial elemental variations in a particular sediment population, and (2) the assessment of 'baseline' concentrations by the comparison of polluted and nonpolluted sediment populations. The present investigation was designed so that the usefulness of both of these approaches could be evaluated, and each is discussed individually below. The evaluation of spatial elemental variations in a particular sediment population It is evident from the data in Table l(a) that there are considerable variations in the non-residential trace metal concentrations of some elements, e.g. Ni, Cd, Zn, Pb, in the surface sediments of Thermaikos Gulf. One method of illustrating such variations is by constructing elemental concentration maps, and although this is essentially a crude procedure which is extremely subjective when carried out manually on relatively few samples, it does permit regions of enhanced elemental concentrations to be identified. This can be illustrated with reference to the surface sediment nonresidual distributions of Cd, Pb, Zn, Cu and Ni in Thermaikos G u l f - see Fig. 3. It is evident from this figure that all these elements have their highest concentrations in the northern portion of the gulf; i.e. there are concentration 'fingerprints' of these metals in the surface sediments. Contouring the non-residual trace metals in this manner can also provide information on their sources, and thus play a key role in the assessment of aquatic pollution. For example, the highest non-residual concentrations of Cd, Pb and Zn 85
Marine Pollution Bulletin
TABLE 1
The concentrations of non-residual trace metals in. the surface sediments of Thermaikos and Pagassitikos Gulfs (values in ppm). Thermaikos Gulf Sample No. Mn 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
554 651 860 755 1118 1437 797 796 950 856 922 711 602 1147 1853 977 1441 1310 950 1136 774 1236 1196 1171 220 204 845 489 285
Ni
Co
42 44 36 48 51 53 47 64 61 55 60 61 66 68 92 94 68 69 160 86 68 127 83 99 72 35 77 36 44
13 13 12 14 14 14 13 13 14 13 14 14 10 14 16 17 14 14 20 16 12 18 18 14 12 7.1 13 7.9 7.1
Cr 107 75 69 68 61 72 56 78 83 70 75 73 61 82 73 94 69 73 103 74 60 86 70 69 61 36 96 31 40
Cu 37 31 21 22 20 20 16 29 31 25 25 26 25 26 21 23 23 21 22 21 18 21 17 20 14 10 13 6.8 4.0
Pb
Zn
Cd
228 147 102 80 60 79 58 110 112 80 85 81 40 88 62 63 81 73 40 60 54 47 53 42 38 27 36 25 13
158 153 104 92 97 239 65 144 87 100 104 299 74 74 80 85 80 84 61 70 62 63 80 53 50 31 24 28 23
1.4 0.54 0.41 0.34
n.d. n.d. n.d. 3.0 2.2 0.95 0.64 1.1 1.8 0.91 0.29 0.42
0.69 0.31 0.32 n.d. n.d. n.d. n.d. n.d. n.d. 0.29 n.d. n.d. n.d.
n.d. not detected.
Pagassitikos Gulf Sample No. Mn
Ni
Co
Cr
Cu
Pb
Zn
Cd
19 19 13 24 18 2.9 47 10 27 29 21 30
6.9 7.8 8.8 lO 2.1 13 6.4 7.1 9.0 9.0 7.9 9.7
21 21 22 20 19 1.6 30 12 25 25 21 32
16 12 7.2 14 8.3 1.0 12 5.3 9.3 10 7.8 10
23 22 25 21 17 5.3 23 16 21 22 17 18
30 25 26 20 17 2.2 26 17 25 25 22 24
n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.
9.0 9.7 8.8
33 27 27
11 10 11
21 23 18
25 26 24
n.d. n.d. n.d.
9.6 9.7 7.1 9.5
25 27 16 23
18 19 16 23
25 24 17 22
n.d. n.d. n.d. n.d.
12
24
29
n.d.
22 32 30 32
13 13 9.3 11
25 21 21 21
24 24 21 24
n.d. n.d. n.d. n.d.
36 38 37 37 38 34 36
11 13 12 12 10 12 10
24 24 22 22 26 28 23
24 25 25 27 26 25 23
n.d. n.d. n.d. n.d. n.d. n.d. n.d.
1 2 3 4 5 6 7 8 9 10 11 12
226 232 326 444 256 313 1611 327 360 364 398 398
13 14 15
470 406 405
31 32 32
16 17 18 19
364 470 458 1312
28 33 15 32
20 2317
51
14
31
21 22 23 24
2334 1711 770 1865
37 50 37 51
14 12 9.6 12
25 26 27 28 29 30 31
889 1466 723 822 664 1114 883
65 62 64 68 59 56 66
13 13 12 12 13 13 12
32
626
5.6
2.7
1.6
1.5
33 34 35 36
464 489 570 514
4.6 4.1 100 114
2.6 2.0 12 12
1.7 1.0 41 43
1.1 0.5 10 14
n.d. not detected.
86
7.9 9.8 4.3 7.8
12 11 6.4 22 21
2.7 2.4 1.1 22 22
n.d. n.d. n.d. n.d. n.d.
are all found in sediments around Thessaloniki and the industrialized area of the northern gulf; in particular in the region of the outflow of the Axios River, which directly drains an industrialized catchment. Contour profiles of this kind suggest that the 'fingerprint' distributions of the nonresidual Cd, Pb and Zn have been imposed on the sediments from anthropogenic sources. The non-residual distribution of Ni also exhibits welldefined contours in the surface sediments of Thermaikos Gulf. However, in contrast to the distributions of Cd, Pband Zn, that of Ni shows the highest concentrations around the outflow of the Alaikmon River, which does not drain the most industrialized regions of the gulf coast. Patterns in the non-residual distribution of Cu are less well-defined than those of Cd, Pb, Zn and Ni. However, there is a general trend for the highest Cu concentrations to be found in the sediments of the northern portion of the gulf, although the overall concentration range is considerably less than that found for Cd, Pb, Zn and Ni. The distributions of the non-residual trace metals in the surface sediments of the Gulf of Pagassitikos are in extreme contrast to those found in Thermaikos. The average concentrations, and ranges, of Cd, Pb and Zn are smaller in Pagassitikos, and there are no discernible patterns in their surface distributions in the sediments. There are patterns in the distributions of some elements, e.g. Mn, Ni, but these are not related to any potential pollution sources and are probably dependent on natural geochemical processes. It may be concluded, therefore, that those elements, such as the heavy metals, which are expected to show the strongest indications of anthropogenic influences have not had their distributions materially affected in this manner in the sediments of the Gulf of Pagassitikos; i.e. this gulf may be regarded as being unpolluted with respect to Cd, Pb, Zn and Cu.
The assessment of
"baseline" concentrations by the
comparisonofpollutedandnon-polluted sedimentpopulations One approach which may be used to interpret the e x t e n t of anomalous trace metal concentations in sediments is t o assess their magnitude in relation to baseline, or 'natural', levels. However, this is particularly difficult for c o a s t a l sediments because they can be deposited under such a wide variety of environments. For example, factors such as redox conditions, which c a n e x e r t a considerable influence on the geochemistry of a sediment, and dilution by t r a c e m e t a l poor components, ssuch as calcium carbonate, can vary greatly from place to place. Forstner & Wittmann (1979) have considered the problems inherent in the selection of a sediment baseline material for environmental studies and have proposed a number of criteria which should be fulfilled in order to best satisfy the principal requirements. These criteria include: (1) t h e b a s e l i n e m a t e r i a l s h o u l d h a v e a grain size, material composition and origin w h i c h a r e s i m i l a r t o r e c e n t deposits, (2) it should b e u n c o n t a m i n a t e d by anthropogenic influences, and (3) a large number of analyses of the material should be available. In practicel it is almost impossible to adequately fulfill all these criteria in a single baseline material, b u t F o r s t n e r & W i t t m a n n (1979) have concluded that shales offer the best compromise for a generally acceptable baseline sediment. There a r e , however, even more critical problems involved in the
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Fig. 3 The distributions of some non-residual trace metals in the surface sediments of the Gulf of Thermaikos. There are a number of sophisticated (e.g. computerized) techniques which can be used to construct contour maps. However, such techniques are not always available to the analyst, and for the present work therefore the contours have deliberately been drawn manually with no attempt to adopt a statistical approach. This procedure is an extremely subjective one, particularly with such a relatively small number of samples• Although it is a useful preliminary data treatment, the maps given here are simply meant to offer a general indication of sediment metal distributions in order to identify possible metal sources.
selection of a baseline material against which to compare the concentrations of non-residual trace metals in sediments. Shale itself cannot be used directly because the analyses given in the literature refer to the total sediment samples. This applies to most other sediments which have been proposed as baseline materials, and it is clear that another approch is required in order to assess the degree of trace metal pollution in the non-residual fractions of recent sediments. One such approach, which is at least potentially rewarding, involves the direct comparison of the nonresidual trace metal fractions of 'polluted' and 'nonpolluted sediment populations. If the initial aim of the investigation is simply to assess whether or not a sediment 90
population has suffered trace metal pollution, this approach has the great advantage that it is not necessary to restrict the comparison of the non-residual fractions to sediments having similar major mineral compositions. This is because it does not matter, in this instance, if a sediment has ,,, 5 °70or -~ 95°-/o of trace metal-poor components (such as calcium carbonate) if the only information required is how polluted the actual sediment, as deposited, is. The present study was designed to include sediment populations from both a 'polluted' and a 'non-polluted' region and permits two possible methods of obtaining non-residual trace metal baseline concentrations to be investigated. (1) Under certain circumstances, it may be possible to
Volume 12/Number 3/March 1981
select baseline sediments from particular parts of the region which is under investigation. For example, it was shown above that there are distinct geographical patterns in the non-residual distributions of Cd, Pb, Zn, Ni and Cu in the surface sediments of Thermaikos Gulf. In general, the concentrations of these trace metals decrease markedly towards the southern portion of the gulf and there the distribution patterns become far less distinct, or disappear altogether. The southern sediments, therefore, offer a potential background material against which to assess the extent to which those in the northern region have suffered pollution. The average non-residual trace metal concentrations in surface sediments from the northern and southern regions of the Gulf of Thermaikos are given in Table 2. It can be seen from this table that the northern deposits are indeed considerably enriched in non-residual trace metals relative to those in the southern region. However, evidence of this kind is not, in itself, sufficient to characterize the southern sediments as being non-polluted baseline material because anthropogenic emissions may have played a part, albeit a smaller one, in controlling the levels of non-residual trace metals in them. (2) Another approach is to choose baseline sediments from other regions which have not undergone trace metal pollution. In the present study, Pagassitikos Gulf was selected for this reason, and it was shown above that there are no indications that the sediments deposited there have been affected by anthropogenic factors. The average nonresidual trace metal concentrations in the surface sediments from Pagassitikos are also given in Table 2, from which it is apparent that the concentrations of those trace metals (i.e. Cd, Zn, Pb) whose surface distributions in Thermaikos suggested pollutant sources are all higher in the southern region of Thermaikos than in Pagassitikos. This confirms the suspicion that the southern Thermaikos sediments, although impoverished in trace metals relative to those of the northern gulf, are unsuitable as baseline material. In contrast, the sediments of Pagassitikos appear to be useful, at least tentatively, as baseline sediments against which to compare the non-residual trace metal concentrations in other coastal sediment populations. However, their use, like that of any other baseline material, must be tempered with great caution, and in order to establish reliable non-residual trace metal baseline values data from many more coastal regions are required.
Conclusions Sediments are important hosts for pollutant trace metals and as such should be included in environmental monitoring
TABLE 2
The average concentrations of non-residual trace metals in the surface sediments of the northern and southern regions of Thermaikos Gulf and Passitikos Gulf (values in ppm). Element
Northern Thermaikos
Southern Thermaikos
Mn Ni Co Cr Cu Pb Zn
966 70 14 73 24 82 107
536 61 10 56 11 30 39
Pagassitikos 754 38 9.5 26 9.5 20 21
programmes. To this end, a simple technique in which the sediment samples are leached with cold dilute HCI provides a rapid and inexpensive way of initially establishing the gross degree to which a sediment population has been subjected to trace metal polltuion from the overlying waters. The technique is capable of identifying 'anthropogenic fingerprints' in the sediments, and the results of the present study lead to the conclusion that the spatial contouring of surface non-residual trace metal concentrations offers one of the most readily interpretable data presentations. A knowledge of baseline non-residual trace metal levels is required, but it is evident that great care must be taken in selecting the best sediment propulations from which to establish such levels.
Agemain, H. & Chau, A. S. Y. (1976). Evaluation of extraction techniques for the determination of metals in aquatic sediments. TheAnalyst, 101, 761-767. Chester, R. (1978). The partitioning of trace elements in sediments. Comitata Nazionale Energia Nucleare, (Fiascherino), pp. 28. Chester, R. & Hughes, M. J. (1967). A chemical technique for the separation of ferro-manganese minerals, carbonate minerals and adsorbed trace elements from pelagic sediments. Chem. Geol., 2,249-263. Chester, R. & Messiha-Hanna. (1970). Trace element partition patterns in North Atlantic deep-sea sediments. Geochim. Cosmochim. Acta, 34, 1121, 1128. Chester, R., Saydam, J. & Towner, J. (1980). A chemical technique for the study of trace metal partitioning in estuarine and coastal sediments. In Prep. Duinker, J. C., Van Eck, G. T. M. & Nolting, R. F. (1974). On the behaviour of copper, zinc, iron and manganese, and evidence for mobilization processes in the Dutch Wadden Sea. Neth J. Sea Res., 8, 214-239. Forstner, U. & Patchineelam, S. R. (1976). Bindung und Mobi|isation von Schwermmetallen in fluviatilen Sedimenten. Chem. Z. 100, 49-57. Forstner, U. & Wittmann, G. T, W. (1979). Metal Pollution in the Aquatic Environment, Springer-Verlag, Berlin, pp. 486. I.W.D. Analytical Methods Manual (1979). Inland Waters Directorate, Water Quality Branch, Ottawa, Canada; Part Four.
91
Squalane as a possible precursor. Preprints. Gen. Papers, Div. Petr. Chem. A.C.S., 11, 3. Patience, R. L., Rowland, S. J. & Maxwell, J. R. (1978). The effect of maturation on the configuration of pristane in sediment and petroleum Geochim. Cosmochim. A cta., 42, 1871-1875. Shlyakhov, A. F., Koreshkova, R. I. & Telkova, M. S. (1975). Gas chromatography of isoprenoid alkanes. J. Chromat., 104, 337-349. Shlyakhov, A. F. & Volkova, L. G. (1977). Stereochemistry of isoprenoid alkanes and possible ways of forming them in sedimentary rocks. Geokhimiya, 8, 1418-1423.
MarinePollutionBulletin. Vol. 12, No. 3, pp. 84-91. 1981. Printed in Great Britain.
0025-326X/8110300844)8$02.0010 © 1981PergamonPressLtd.
The Initial Assessment of Trace Metal Pollution in Coastal Sediments R. CHESTER* and E G. VOUTSINOU -I-
*Department of Oceanography, The University, LiverpoolL 69 3 BX, UK. JrInstitute of Oceanographic and Fishing Research, Agios, Kosmas, Ellinkon, Athens, Greece.
Coastal sediments are important hosts for pollutant trace metals, but analytical difficulties can prevent them being included in routine environmental monitoring programmes. In order to identify a suitable approach to the problem, an established simple technique has been evaluated for the initial assessment of trace metal pollution in coastal sediments. The technique, which involves leaching the samples with cold 0.5 N HCI, has been applied to surface sediments from two Greek gulfs and has been shown to provide a rapid, inexpensive way of initially establishing the gross degree to which a sediment population has been subjected to trace metal pollution from the overlying waters.
In recent years authorities in more and more countries have sought to initiate programmes for the assessment of marine pollution, particularly that involving the trace metals. Some of the highest concentrations of these metals in the marine environment are found in sediments, and in certain regions their emissions have been of sufficient strength to impose 'anthropogenic fingerprints' on the bottom deposits. A knowledge of the concentrations and distributions of trace metals in sediments can therefore play a key role in detecting sources of pollution in aquatic systems (Forstner & Wittmann, 1979), and for this reason sediments should be included among the environmental parameters monitored in major marine pollution programmes. In practice, there are difficulties involved in their inclusion in routine monitoring. These include: the selection of a suitable analytical technique; constraints such as time, cost, number of sediment populations to be monitored; and, the availability of both analytical facilities and trained personnel. Together, these various factors can dictate the extent to which an investigation can proceed, and one of the most important of them is the selection of an analytical technique. The analysis of trace metals in sediments presents 84
a number of complex problems and has resulted in a bewildering variety of techniques being advocated for this purpose in the literature. It is clearly advantageous, therefore, to define as closely as possible the aims of any particular trace metal investigation, and in order to achieve this it is necessary to have some understanding of the mechanisms by which trace metals are incorporated into sediments. There have been various attemplts to classify the elements found in sediments, but perhaps the most useful one from a practical point of view is that which distinguishes between lattice-held, i.e. residual, and non-lattice-held, i.e. nonresidual, trace metals (see e.g.: Chester & Hughes, 1967; Chester & Messiha-Hanna, 19"70; Agemain & Chau, 1976). Residual trace metals are defined as those which are pan of the silicate matrix of the sediment, and which are located mainly in the lattice structures of the component minerals. Non-residual trace metals are not part of the silicate matrix and have been incorporated into the sediment from aqueous solution by processes such as adsorption and organic complexation; i.e. non-residual trace metals include those originating from polluted waters. This simple distinction has been extended by various authors into more detailed classification schemes in which the non-residual fraction has been sub-divided into several categories (see e.g.: Chester & Hughes, 1967; Forstner & Patchineeham, 1976; Chester et al., 1980). There are a large number of techniques described in the literature which have been specifically designed to liberate from a sediment all, or part, of the non-residual trace metals - f o r reviews of this subject see Chester (1978) and Forstner & Wittmann (1979). However, many of the available techniques are complex and time-consuming; especially those involving sequential leaching stages. Further, many of them are primarily of 'academic' interest and may, in fact, provide the enviromental scientist with data he does not require.
Volume 12/Number3/March 1981 For the assessment of trace metal pollution in sediments it is the non-residual elements which are of prime interest, and the analysis of the non-residual fractions will often yield more data on the extent of trace metal pollution than will that of the total sediment which includes the residual, or non-polluted, fraction and so may mask the relationships sought (see e.g.: Agemaln & Chau, 1976; Forstner & Wittmann, 1979). There are a number of techniques which have been proposed for the analysis of pollutant trace metals in sediments, some of which have been experimentally assessed by Agemain & Chau (1976). One of the techniques was that outlined by Chester & Hughes (1967) for the study of trace metal partitioning in pelagic deep-sea sediments, which involved leaching the samples with a mixed NH2OH.HC1 + CH3COOH reagent. However, this technique is not directly applicable to those coastal and estuarine sediments which contain appreciable quantities of organic carbon, and for these an additional (e.g. EDTA) stage is required in order to completely solubilize organically complexed trace metals (see e.g.: Agemain & Chau, 1976; Chester et al., 1980). However, according to Duinker et al. (1974), Agemain & Chau (1976) and the I.W.D. (1979) dilute HC1 will release both inorganic and organicassociated non-residual trace metals from sediments without materially affecting the silicate matrix. More specialised techniques should be used for certain sediments, e.g. those containing appreciable quantities of sulphites or coal. The initial aim of many monitoring programmes, with respect to sediments, is simply to establish the degree (if any) to which a population has been subjected to trace metal pollution; more detailed work can then follow concentrating on those sediments for which the pollution has been shown to be potentially hazardous. In order to meet this initial aim, the present work was undertaken to assess the field usefulness of the simple, rapid, inexpensive and widely applicable cold dilute HCI leaching technique, based on those originally investigated by Duinker et al. (1974) and Agemain & Chau (1976) and outlined by the I.W.D. (1979), for the investigation of trace metal pollution in coastal sediments. Many of the techniques described in the literature have been tested on only a limited number of samples, and this can be a considerable disadvantage for the environmental scientist who is confronted with a variety of such techniques and has to choose between them. Further, there is often no assessment made of ways of interpreting the data derived from the application of the various techniques. For reasons such as this, the present study was carried out on sediments from two contrasting 'real' populations, and also included methods of data interpretation.
Sediment Samples Sediments were collected by grab sampler from two Greek coastal localities; the Gulf of Thermaikos, for which trace metal pollution was suspected; and the Gulf of Pagassitikos, an area which was not expected to suffer from trace metal pollution. The sediments had a wide variety of particle sizes and in order to make a viable inter-sediment comparison trace element analyses were carried out on the < 61/an fractions which were separated by wet sieving. The
separates were oven dried and a representative sample subsequently taken for analysis.
Analytical Procedure About 5 g of each representative sample was accurately weighed and placed in a 100 ml wide neck plastic bottle. 75 ml of 0.5 N HC1 was added (evolution of CO2 being allowed where necessary) and the bottles shaken mechanically for approximately 16 h. The solutions were allowed to stand for a few minutes then suction filtered, and the filtrates sprayed directly into an atomic absorption instrument using appropriate lamps and settings. A blank was run omitting the sample. For full details see I. W. D. (1979). The following elements were determined: Mn, Ni, Co, Cr, Cu, Cd, Pb and Zn. Careful checks showed that there were no sample matrix effects for any of the elements and standard curves were therefore constructed, using appropriate dilutions of stock solutions, in 0.5 N HCI. Five replicates of one sample were run and the following coefficients of variation found: Mn, 2.8%; Ni, 3.3%; Co, 2.4%; Cr, 6.3%; Cu, 3.8%; Cd, 5.6%; Pb, 3.5%; Zn, 5.0%.
Results and Discussion The locations of the surface samples in the gulfs of Thermalkos and Pagassitikos are illustrated in Figs 1 and 2 respectively, and the concentrations of the non-residual trace metals in them are listed in Table 1 (a) and (b). There are a number of ways in which the data obtained from a survey of the distribution of non-residual trace metals in surface sediment populations can be interpreted. Two of the most commonly employed involve; (1) the evaluation of spatial elemental variations in a particular sediment population, and (2) the assessment of 'baseline' concentrations by the comparison of polluted and nonpolluted sediment populations. The present investigation was designed so that the usefulness of both of these approaches could be evaluated, and each is discussed individually below. The evaluation of spatial elemental variations in a particular sediment population It is evident from the data in Table l(a) that there are considerable variations in the non-residential trace metal concentrations of some elements, e.g. Ni, Cd, Zn, Pb, in the surface sediments of Thermaikos Gulf. One method of illustrating such variations is by constructing elemental concentration maps, and although this is essentially a crude procedure which is extremely subjective when carried out manually on relatively few samples, it does permit regions of enhanced elemental concentrations to be identified. This can be illustrated with reference to the surface sediment nonresidual distributions of Cd, Pb, Zn, Cu and Ni in Thermaikos G u l f - see Fig. 3. It is evident from this figure that all these elements have their highest concentrations in the northern portion of the gulf; i.e. there are concentration 'fingerprints' of these metals in the surface sediments. Contouring the non-residual trace metals in this manner can also provide information on their sources, and thus play a key role in the assessment of aquatic pollution. For example, the highest non-residual concentrations of Cd, Pb and Zn 85
Marine Pollution Bulletin
TABLE 1
The concentrations of non-residual trace metals in. the surface sediments of Thermaikos and Pagassitikos Gulfs (values in ppm). Thermaikos Gulf Sample No. Mn 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
554 651 860 755 1118 1437 797 796 950 856 922 711 602 1147 1853 977 1441 1310 950 1136 774 1236 1196 1171 220 204 845 489 285
Ni
Co
42 44 36 48 51 53 47 64 61 55 60 61 66 68 92 94 68 69 160 86 68 127 83 99 72 35 77 36 44
13 13 12 14 14 14 13 13 14 13 14 14 10 14 16 17 14 14 20 16 12 18 18 14 12 7.1 13 7.9 7.1
Cr 107 75 69 68 61 72 56 78 83 70 75 73 61 82 73 94 69 73 103 74 60 86 70 69 61 36 96 31 40
Cu 37 31 21 22 20 20 16 29 31 25 25 26 25 26 21 23 23 21 22 21 18 21 17 20 14 10 13 6.8 4.0
Pb
Zn
Cd
228 147 102 80 60 79 58 110 112 80 85 81 40 88 62 63 81 73 40 60 54 47 53 42 38 27 36 25 13
158 153 104 92 97 239 65 144 87 100 104 299 74 74 80 85 80 84 61 70 62 63 80 53 50 31 24 28 23
1.4 0.54 0.41 0.34
n.d. n.d. n.d. 3.0 2.2 0.95 0.64 1.1 1.8 0.91 0.29 0.42
0.69 0.31 0.32 n.d. n.d. n.d. n.d. n.d. n.d. 0.29 n.d. n.d. n.d.
n.d. not detected.
Pagassitikos Gulf Sample No. Mn
Ni
Co
Cr
Cu
Pb
Zn
Cd
19 19 13 24 18 2.9 47 10 27 29 21 30
6.9 7.8 8.8 lO 2.1 13 6.4 7.1 9.0 9.0 7.9 9.7
21 21 22 20 19 1.6 30 12 25 25 21 32
16 12 7.2 14 8.3 1.0 12 5.3 9.3 10 7.8 10
23 22 25 21 17 5.3 23 16 21 22 17 18
30 25 26 20 17 2.2 26 17 25 25 22 24
n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.
9.0 9.7 8.8
33 27 27
11 10 11
21 23 18
25 26 24
n.d. n.d. n.d.
9.6 9.7 7.1 9.5
25 27 16 23
18 19 16 23
25 24 17 22
n.d. n.d. n.d. n.d.
12
24
29
n.d.
22 32 30 32
13 13 9.3 11
25 21 21 21
24 24 21 24
n.d. n.d. n.d. n.d.
36 38 37 37 38 34 36
11 13 12 12 10 12 10
24 24 22 22 26 28 23
24 25 25 27 26 25 23
n.d. n.d. n.d. n.d. n.d. n.d. n.d.
1 2 3 4 5 6 7 8 9 10 11 12
226 232 326 444 256 313 1611 327 360 364 398 398
13 14 15
470 406 405
31 32 32
16 17 18 19
364 470 458 1312
28 33 15 32
20 2317
51
14
31
21 22 23 24
2334 1711 770 1865
37 50 37 51
14 12 9.6 12
25 26 27 28 29 30 31
889 1466 723 822 664 1114 883
65 62 64 68 59 56 66
13 13 12 12 13 13 12
32
626
5.6
2.7
1.6
1.5
33 34 35 36
464 489 570 514
4.6 4.1 100 114
2.6 2.0 12 12
1.7 1.0 41 43
1.1 0.5 10 14
n.d. not detected.
86
7.9 9.8 4.3 7.8
12 11 6.4 22 21
2.7 2.4 1.1 22 22
n.d. n.d. n.d. n.d. n.d.
are all found in sediments around Thessaloniki and the industrialized area of the northern gulf; in particular in the region of the outflow of the Axios River, which directly drains an industrialized catchment. Contour profiles of this kind suggest that the 'fingerprint' distributions of the nonresidual Cd, Pb and Zn have been imposed on the sediments from anthropogenic sources. The non-residual distribution of Ni also exhibits welldefined contours in the surface sediments of Thermaikos Gulf. However, in contrast to the distributions of Cd, Pband Zn, that of Ni shows the highest concentrations around the outflow of the Alaikmon River, which does not drain the most industrialized regions of the gulf coast. Patterns in the non-residual distribution of Cu are less well-defined than those of Cd, Pb, Zn and Ni. However, there is a general trend for the highest Cu concentrations to be found in the sediments of the northern portion of the gulf, although the overall concentration range is considerably less than that found for Cd, Pb, Zn and Ni. The distributions of the non-residual trace metals in the surface sediments of the Gulf of Pagassitikos are in extreme contrast to those found in Thermaikos. The average concentrations, and ranges, of Cd, Pb and Zn are smaller in Pagassitikos, and there are no discernible patterns in their surface distributions in the sediments. There are patterns in the distributions of some elements, e.g. Mn, Ni, but these are not related to any potential pollution sources and are probably dependent on natural geochemical processes. It may be concluded, therefore, that those elements, such as the heavy metals, which are expected to show the strongest indications of anthropogenic influences have not had their distributions materially affected in this manner in the sediments of the Gulf of Pagassitikos; i.e. this gulf may be regarded as being unpolluted with respect to Cd, Pb, Zn and Cu.
The assessment of
"baseline" concentrations by the
comparisonofpollutedandnon-polluted sedimentpopulations One approach which may be used to interpret the e x t e n t of anomalous trace metal concentations in sediments is t o assess their magnitude in relation to baseline, or 'natural', levels. However, this is particularly difficult for c o a s t a l sediments because they can be deposited under such a wide variety of environments. For example, factors such as redox conditions, which c a n e x e r t a considerable influence on the geochemistry of a sediment, and dilution by t r a c e m e t a l poor components, ssuch as calcium carbonate, can vary greatly from place to place. Forstner & Wittmann (1979) have considered the problems inherent in the selection of a sediment baseline material for environmental studies and have proposed a number of criteria which should be fulfilled in order to best satisfy the principal requirements. These criteria include: (1) t h e b a s e l i n e m a t e r i a l s h o u l d h a v e a grain size, material composition and origin w h i c h a r e s i m i l a r t o r e c e n t deposits, (2) it should b e u n c o n t a m i n a t e d by anthropogenic influences, and (3) a large number of analyses of the material should be available. In practicel it is almost impossible to adequately fulfill all these criteria in a single baseline material, b u t F o r s t n e r & W i t t m a n n (1979) have concluded that shales offer the best compromise for a generally acceptable baseline sediment. There a r e , however, even more critical problems involved in the
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Fig. 3 The distributions of some non-residual trace metals in the surface sediments of the Gulf of Thermaikos. There are a number of sophisticated (e.g. computerized) techniques which can be used to construct contour maps. However, such techniques are not always available to the analyst, and for the present work therefore the contours have deliberately been drawn manually with no attempt to adopt a statistical approach. This procedure is an extremely subjective one, particularly with such a relatively small number of samples• Although it is a useful preliminary data treatment, the maps given here are simply meant to offer a general indication of sediment metal distributions in order to identify possible metal sources.
selection of a baseline material against which to compare the concentrations of non-residual trace metals in sediments. Shale itself cannot be used directly because the analyses given in the literature refer to the total sediment samples. This applies to most other sediments which have been proposed as baseline materials, and it is clear that another approch is required in order to assess the degree of trace metal pollution in the non-residual fractions of recent sediments. One such approach, which is at least potentially rewarding, involves the direct comparison of the nonresidual trace metal fractions of 'polluted' and 'nonpolluted sediment populations. If the initial aim of the investigation is simply to assess whether or not a sediment 90
population has suffered trace metal pollution, this approach has the great advantage that it is not necessary to restrict the comparison of the non-residual fractions to sediments having similar major mineral compositions. This is because it does not matter, in this instance, if a sediment has ,,, 5 °70or -~ 95°-/o of trace metal-poor components (such as calcium carbonate) if the only information required is how polluted the actual sediment, as deposited, is. The present study was designed to include sediment populations from both a 'polluted' and a 'non-polluted' region and permits two possible methods of obtaining non-residual trace metal baseline concentrations to be investigated. (1) Under certain circumstances, it may be possible to
Volume 12/Number 3/March 1981
select baseline sediments from particular parts of the region which is under investigation. For example, it was shown above that there are distinct geographical patterns in the non-residual distributions of Cd, Pb, Zn, Ni and Cu in the surface sediments of Thermaikos Gulf. In general, the concentrations of these trace metals decrease markedly towards the southern portion of the gulf and there the distribution patterns become far less distinct, or disappear altogether. The southern sediments, therefore, offer a potential background material against which to assess the extent to which those in the northern region have suffered pollution. The average non-residual trace metal concentrations in surface sediments from the northern and southern regions of the Gulf of Thermaikos are given in Table 2. It can be seen from this table that the northern deposits are indeed considerably enriched in non-residual trace metals relative to those in the southern region. However, evidence of this kind is not, in itself, sufficient to characterize the southern sediments as being non-polluted baseline material because anthropogenic emissions may have played a part, albeit a smaller one, in controlling the levels of non-residual trace metals in them. (2) Another approach is to choose baseline sediments from other regions which have not undergone trace metal pollution. In the present study, Pagassitikos Gulf was selected for this reason, and it was shown above that there are no indications that the sediments deposited there have been affected by anthropogenic factors. The average nonresidual trace metal concentrations in the surface sediments from Pagassitikos are also given in Table 2, from which it is apparent that the concentrations of those trace metals (i.e. Cd, Zn, Pb) whose surface distributions in Thermaikos suggested pollutant sources are all higher in the southern region of Thermaikos than in Pagassitikos. This confirms the suspicion that the southern Thermaikos sediments, although impoverished in trace metals relative to those of the northern gulf, are unsuitable as baseline material. In contrast, the sediments of Pagassitikos appear to be useful, at least tentatively, as baseline sediments against which to compare the non-residual trace metal concentrations in other coastal sediment populations. However, their use, like that of any other baseline material, must be tempered with great caution, and in order to establish reliable non-residual trace metal baseline values data from many more coastal regions are required.
Conclusions Sediments are important hosts for pollutant trace metals and as such should be included in environmental monitoring
TABLE 2
The average concentrations of non-residual trace metals in the surface sediments of the northern and southern regions of Thermaikos Gulf and Passitikos Gulf (values in ppm). Element
Northern Thermaikos
Southern Thermaikos
Mn Ni Co Cr Cu Pb Zn
966 70 14 73 24 82 107
536 61 10 56 11 30 39
Pagassitikos 754 38 9.5 26 9.5 20 21
programmes. To this end, a simple technique in which the sediment samples are leached with cold dilute HCI provides a rapid and inexpensive way of initially establishing the gross degree to which a sediment population has been subjected to trace metal polltuion from the overlying waters. The technique is capable of identifying 'anthropogenic fingerprints' in the sediments, and the results of the present study lead to the conclusion that the spatial contouring of surface non-residual trace metal concentrations offers one of the most readily interpretable data presentations. A knowledge of baseline non-residual trace metal levels is required, but it is evident that great care must be taken in selecting the best sediment propulations from which to establish such levels.
Agemain, H. & Chau, A. S. Y. (1976). Evaluation of extraction techniques for the determination of metals in aquatic sediments. TheAnalyst, 101, 761-767. Chester, R. (1978). The partitioning of trace elements in sediments. Comitata Nazionale Energia Nucleare, (Fiascherino), pp. 28. Chester, R. & Hughes, M. J. (1967). A chemical technique for the separation of ferro-manganese minerals, carbonate minerals and adsorbed trace elements from pelagic sediments. Chem. Geol., 2,249-263. Chester, R. & Messiha-Hanna. (1970). Trace element partition patterns in North Atlantic deep-sea sediments. Geochim. Cosmochim. Acta, 34, 1121, 1128. Chester, R., Saydam, J. & Towner, J. (1980). A chemical technique for the study of trace metal partitioning in estuarine and coastal sediments. In Prep. Duinker, J. C., Van Eck, G. T. M. & Nolting, R. F. (1974). On the behaviour of copper, zinc, iron and manganese, and evidence for mobilization processes in the Dutch Wadden Sea. Neth J. Sea Res., 8, 214-239. Forstner, U. & Patchineelam, S. R. (1976). Bindung und Mobi|isation von Schwermmetallen in fluviatilen Sedimenten. Chem. Z. 100, 49-57. Forstner, U. & Wittmann, G. T, W. (1979). Metal Pollution in the Aquatic Environment, Springer-Verlag, Berlin, pp. 486. I.W.D. Analytical Methods Manual (1979). Inland Waters Directorate, Water Quality Branch, Ottawa, Canada; Part Four.
91