THE SIGNIFICANCE OF COARSE SEDIMENTS IN METALPOLLUTION STUDIES IN THE COASTAL ZONE

MARIA ALOUPI and MICHAEL O. ANGELIDIS∗Department of Environmental Studies, University of the Aegean, Karadoni 17, 81100 Mytilene,

Greece(∗ author for correspondence)

(Received 28 September 1999; accepted 14 December 2000)

Abstract. The role of coarse material (sand fraction) in the distribution of metals in polluted marinesediments was investigated in the harbors and the coastal zone of Mytilene, island of Lesvos, AegeanSea. It was found that sand fraction contains a relatively significant proportion of the anthropogenicmetals and therefore it cannot be neglected in metal pollution studies of coastal sediments. Also thedistribution of the anthropogenic metals (Cd, Cu, Pb and Zn) in both silt+clay and sand fractionsfollow the same pattern indicating similar pollution sources. In the bulk sediment (clay + silt + sandfraction) all anthropogenic metals had a significant amount (> 50%) in the acid-extractable (andpotentially bio-available) fraction.

Keywords: Aegean Sea, coarse sediments, island, metals, pollution, sand

1. Introduction

One of the major factors that affect the metal concentrations in aquatic sedimentsis the size of the sediment particles. Higher concentrations are found in the finerparticles (Goldberg, 1954; Krauskopf, 1956; Gibbs, 1977; Horowitz and Elrick,1987) due to a synergetic action of physical (surface area) and chemical (min-eralogy) characteristics of the particles in each size fraction. First, the smallerparticles have a greater specific surface area than the larger ones. Second, mostfine particles consist mainly of clay minerals, which have more metal binding sitesthan the silicate or carbonate minerals, which are the major components of coarsermaterial in marine sediments (Campbell et al., 1988). As a result, finer particlesoffer much more sites than the coarser ones for chemical reactions. Therefore,although high metal concentrations can be occasionally found in coarse sedimentmaterial (Filipek et al., 1981; Robinson, 1982), the silt and clay fractions usuallycontain higher metal concentrations than the sand fraction (Förstner and Wittmann,1983). That is the main reason why metal pollution studies are often carried on thefiner fraction (less than 63 µm), suggesting that the coarser material is an ‘inertdiluent’ without any significant metal contribution.

However, in the coastal zone, the coarse material is an important constituentof the sediment and may represent more than 50% of the bulk sediment mass.Therefore it cannot be neglected when assessing the pollution impact on the coastal

Water, Air, and Soil Pollution 133: 121–131, 2002.© 2002 Kluwer Academic Publishers. Printed in the Netherlands.

122 M. ALOUPI AND M. O. ANGELIDIS

sediments. Furthermore, the vicinity of land based pollution sources which maydispose off large particles with relatively high metal content (urban and indus-trial effluents and deposits) may lead to enhanced metal concentrations in coarsesediment material (Horowitz, 1991).

The present study is an attempt to investigate the relative importance of coarsesediments as metal carriers in a coastal area affected by land-based pollution sour-ces, compared to fine grained. As total metal concentrations in sediments are theresultant of both geological (i.e. natural) and anthropogenic inputs in the marineenvironment, we used the extraction with 0.5 N HCl to isolate the anthropogeniceffect. This technique removes the non-residual, i.e. non-lattice held metals fromsediment grains. These metals are not part of the silicate matrix and have beenincorporated into the sediments from aqueous solution by processes such as ad-sorption and organic complexation, including, thus, metals originating from an-thropogenic sources (Chester and Voutsinou, 1981). The non-residual metals areconsidered to represent the bio-available part of total metal content of marine sed-iments (Loring, 1992). In order to compare the amounts of metals accumulated toboth granulometric fractions, we examined separately the silt + clay (i.e. < 63 µm)and sand (63 µm - 1 mm) fractions.

The study area is the harbor and the coastal area of Mytilene (pop. 25,000),island of Lesvos, Aegean Sea, Greece, which receives untreated urban effluentsthrough 25 sewage outfalls located along the town’s coastline (see Figure 1). Ingeneral, the urban effluents of Mytilene, though not including any significant in-dustrial effluents, contained important loads of organic matter and anthropogenicmetals (Angelidis, 1995).

Previous investigation on the contamination of surface sediments in the vicinityof the town, revealed highly enriched concentrations of Cd, Cu, Pb and Zn, as wellas organic carbon, in the inner and most of the outer harbor. Some enrichment wasalso recorded in a part of the sediments of ancient harbor located at the northernpart of the town (stations 19 and 21), whereas no signs of metal contamination weredetected in the wider coastal zone (Angelidis and Aloupi, 1997). The coastal areais characterized by fine grain sediments in the inner and outer harbor (stations 1–12, Figure 1) turning into coarser texture seawards (stations 13–18). The sedimentsof the ancient harbor are sandy with very small amounts of fine-grained material(stations 19–21).

2. Materials and Methods

Surface sediment samples were collected from 21 stations in the harbor and thecoastal area of Mytilene, island of Lesvos, Greece, during July 1995 (Figure 1).Grain size distribution was measured by wet sieving and the following fractionswere determined: silt+clay (< 63 µm), sand (63 µm < × < 1 mm) and gravel (> 1mm).

SIGNIFICANCE OF COARSE SEDIMENTS IN METAL POLLUTION 123

Figure 1. Study area and sampling stations.

The total metal (Cd, Cr, Cu, Fe, Mn, Pb and Zn) content of the samples wasmeasured after total decomposition of 200 mg of dried ground sediment (combinedclay + silt + sand (< 1 mm) sample) with 1 mL of aqua regia and 6 mL of HF, inSavillex Teflon bombs heated in a microwave oven (Loring and Rantala, 1992).The most-available metal content of the sediments was determined separately inthe silt + clay fraction and the sand fraction of the sediments using a one- stepweak acid extraction method (0.5 N HCl at room temperature [approximately 20◦C] overnight - Agemian and Chau, 1976). The metals extracted with weak acidrepresent the most biologically available fraction (Luoma and Bryan, 1981) and

124 M. ALOUPI AND M. O. ANGELIDIS

may be used as an indicator of the anthropogenic contamination of the sediments(Chester and Voutsinou, 1981).

Metal determinations were performed using a Perkin-Elmer 5100ZL AtomicAbsorption Spectrometer with Zeeman background correction. Chromium, Cu, Fe,Mn, Pb and Zn were determined by FAAS, while Cd was determined by GFAASwith a mixture of 50 µg NH2H2PO4 and 3 µg Mg(NO3)2 as matrix modifier(Aloupi, 1999). The quality assurance (accuracy and precision) of the analyticalresults for the total metal concentrations was controlled with replicate analysis (n= 6) of Reference Materials certified by NRCC (BCSS-1 marine sediment, PACS-1harbor sediment) and IAEA (SDM2TM marine sediment) (Aloupi and Angelidis,2001). Since no certified Reference Material is available for the acid extractablemetals, only precision was tested (analysis of 10 sub-samples) and the followingrelative standard deviations were calculated: Cd 9.9%, Cr 1.5%, Cu 1.9%, Fe 2.1%,Mn 0.6%, Pb 3.8%, Zn 1.5%. In the determination of both total and extractablemetal concentrations the 20% of the samples were analysed in duplicates. Fieldhomogeneity was also tested by duplicate sampling in 5 stations.

Statistical analysis of data was performed with SPSS Ver. 7.5 for Windowssoftware.

3. Results and Discussion

The total metal concentrations and the silt + clay content in the bulk sediments(clay + silt + sand fraction) of the study area are presented in Table I. Table IIshows the acid extractable concentrations of the metals in the silt + clay and sandfractions of the sediments.

As in the case of total concentrations (Angelidis and Aloupi, 1997), acid ex-tractable concentrations of the anthropogenic metals (Cd, Cu, Pb and Zn) werehigher in the sediments of the harbor of the town (stations 1–12) compared to thesediments of the wider coastal area, for both the fine (silt + clay) and coarse (sand)fractions. For all metals (with the exception of Mn), concentrations in the silt + clayfraction were higher than in the sand, as expected. Statistical analysis with the useof paired samples t-test showed statistically significant differences in the means ofmetal concentrations between the two grain size fractions (Table III). However, thesand fraction had also important metal concentrations, especially in the stationsfrom the harbor of Mytilene (Table II). Metal enrichments in coarse sedimentshave already been recorded in polluted areas (Krumgalz et al., 1992) and may beexplained by the formation of coatings on the surface of the particles. Carbonates,hydroxy-oxides and organic matter, play an important role in the development ofthe specific surface area of particles that may lead to the increase of their adsorptioncapacity (Horowitz and Elrick, 1987). On the other hand, coating formation maylead to completely different results when occurring in fine sediments. In such casescoatings may lead to the consolidation of fine particles and to the formation of

SIGNIFICANCE OF COARSE SEDIMENTS IN METAL POLLUTION 125

TABLE I

Total metal concentrations and silt + clay content in surface sediments from Mytilene coastal area

Stations Silt + Clay Cd Cr Cu Fe Mn Pb Zn

% µg g−1 µg g−1 µg g−1 % µg g−1 µg g−1 µg g−1

H1 85.79 0.364 150 66.0 2.69 258 82.5 173

H2 71.29 0.395 144 63.5 2.39 266 73.6 184

H3 95.37 0.495 154 86.2 2.81 257 93.0 230

H4 70.75 0.166 108 43.1 2.19 242 52.0 112

H5 96.66 0.409 146 67.6 2.66 260 78.2 191

H6 82.30 0.232 138 48.8 2.56 262 58.0 128

H7 90.06 0.206 103 41.5 2.34 248 63.3 134

H8 11.04 0.173 86.3 21.0 1.43 360 32.7 83.5

H9 67.42 0.130 111 34.5 2.71 271 38.1 74.1

H10 66.88 0.142 120 41.3 2.74 259 54.3 112

H11 66.73 0.173 116 42.0 2.58 263 56.5 116

H12 68.86 0.170 83.0 30.5 2.00 249 51.7 95.1

H13 64.80 0.131 122 27.3 2.40 280 44.2 69.1

H14 87.80 0.085 132 23.4 2.70 300 39.8 73.1

H15 75.96 0.082 120 30.5 2.75 302 42.5 72.9

H16 41.42 0.119 86.5 24.0 1.65 222 39.0 76.7

H17 42.00 0.076 81.8 16.7 1.74 229 34.5 45.8

H18 35.45 0.062 97.5 20.4 1.74 261 37.1 55.0

H19 3.20 0.125 71.2 24.6 1.34 228 44.7 83.0

H20 0.40 0.052 44.6 9.39 1.29 223 30.5 38.8

H21 5.95 0.048 52.6 14.9 0.77 201 54.9 45.1

larger particles with smaller specific surface area and, thus, smaller adsorptioncapacity. The particle consolidation may also lead to an ‘artificial’ enhancement ofmetal concentrations in the coarser sediment fractions, if the consolidated particlesare not disaggregated during grain size separation, and therefore analyzed togetherwith the coarse particles.

High correlation coefficients were calculated for the human-related metals Cd,Cu, Pb and Zn, in each grain-size fraction separately (Table IV), indicating similardistribution processes. Also, the concentrations of each of the anthropogenic metalsin the silt+clay and sand fraction were highly correlated (correlation coefficientsfor Cd: 0.86, Cu: 0.66, Pb: 0.64 and Zn: 0.84, all significant at 0.05 level). Suchcorrelations suggest that the metal enhancement in the sediments of a polluted areais simultaneously recorded in both silt + clay and sand fractions.

126 M. ALOUPI AND M. O. ANGELIDIS

TAB

LE

II

0.5

NH

Cl

extr

acta

ble

met

alco

ncen

trat

ions

(in

µg

g−1

exce

ptfo

rF

ein

%)

inth

esi

lt+

clay

and

sand

frac

tion

sof

surf

ace

sedi

men

tsfr

omM

ytil

ene

coas

tala

rea

Sta

tion

sC

dC

rC

uF

eM

nP

bZ

n

silt

+sa

ndsi

lt+

sand

silt

+sa

ndsi

lt+

sand

silt

+sa

ndsi

lt+

sand

silt

+sa

nd

clay

clay

clay

clay

clay

clay

clay

H1

0.38

80.

195

18.0

9.36

42.0

19.8

0.38

30.

184

107

117

54.0

40.4

110

64.0

H2

0.49

20.

322

17.7

11.4

44.2

23.8

0.37

20.

281

108

120

61.3

48.7

137

92.7

H3

0.51

30.

156

19.3

6.58

58.9

11.5

0.42

30.

114

99.7

125

81.8

38.6

156

58.6

H4

0.18

80.

106

15.8

3.22

30.7

3.22

0.37

40.

018

111

107

51.1

5.72

75.0

28.5

H5

0.42

00.

202

18.8

8.48

44.1

23.4

0.40

80.

258

110

108

65.8

58.6

131

77.1

H6

0.23

60.

146

17.1

7.82

30.4

14.1

0.38

60.

179

114

110

48.4

28.9

82.1

40.5

H7

0.21

90.

103

16.4

2.50

24.6

1.19

0.37

20.

008

133

112

52.7

0.40

79.8

18.6

H8

0.29

90.

160

14.6

6.84

26.7

6.44

0.23

10.

311

120

330

49.2

16.1

105

31.5

H9

0.11

80.

145

15.1

3.35

16.7

3.92

0.44

70.

072

103

73.8

28.2

12.1

27.8

19.4

H10

0.19

40.

119

15.6

9.68

26.9

19.2

0.39

90.

346

101

108

40.8

5.32

54.5

49.3

H11

0.18

60.

126

15.8

6.45

28.1

20.3

0.36

70.

168

104

103

69.6

31.2

72.4

43.1

H12

0.16

00.

139

13.7

2.44

19.4

3.79

0.25

10.

056

120

116

50.8

3.10

61.9

24.7

H13

0.16

40.

105

14.7

6.01

17.5

8.13

0.28

30.

264

95.4

127

35.3

14.1

32.1

17.1

H14

0.10

60.

072

20.5

2.56

10.7

3.96

0.35

20.

003

105

81.7

28.4

1.08

27.9

5.72

H15

0.12

70.

062

16.0

7.18

16.1

6.40

0.37

50.

262

134

131

36.6

11.4

32.5

10.7

H16

0.11

30.

078

14.2

5.84

17.2

4.81

0.26

70.

055

105

119

69.2

22.7

52.4

47.2

H17

0.07

20.

037

15.2

7.21

9.4

2.87

0.34

60.

164

133

121

32.9

8.60

31.1

18.3

H18

0.07

40.

059

13.2

6.93

12.6

3.99

0.31

70.

126

124

180

37.3

13.8

35.7

23.5

H19

0.17

50.

119

13.3

8.81

32.7

10.3

0.23

30.

138

92.9

115

74.5

32.4

89.3

58.3

H20

a–

0.06

4–

5.87

–3.

11–

0.15

6–

92.0

–14

.5–

26.6

H21

0.13

60.

058

10.9

3.07

20.1

3.04

0.17

80.

017

87.7

109

57.3

5.88

68.8

30.9

a10

0%sa

ndy

sedi

men

t.

SIGNIFICANCE OF COARSE SEDIMENTS IN METAL POLLUTION 127

TABLE III

Paired-samples t-test for the comparison of means of metal concen-trations extracted with 0.5 N HCl from silt + clay and sand fractionsof the sediments

Difference p

Pairs of variables of means t df (2-tailed)

Cdsilt+clay – Cdsand 0.098 4.779 17 0.000

Crsilt+clay – Crsand 9.48 12.324 17 0.000

Cusilt+clay – Cusand 16.8 7.017 17 0.000

Fesilt+clay – Fesand 0.191 8.052 17 0.000

Mnsilt+clay – Mnsand -6.94 -1.504 17 ns

Pbsilt+clay – Pbsand 32.1 9.466 17 0.000

Znsilt+clay – Znsand 34.5 6.392 17 0.000

ns: non significant.

TABLE IV

Correlation coefficients of 0.5 HCl extractable metal concentrationsin each of the silt + clay and the sand fractions of the sediments

Cd Cr Cu Fe Mn Pb

Cr silt + clay 0.59a

sand 0.56a

Cu silt + clay 0.94b 0.47a

sand 0.79b 0.77b

Fe silt + clay 0.52a 0.81b ns

sand ns 0.83b 0.69b

Mn silt + clay ns ns ns ns

sand ns ns ns ns

Pb silt + clay 0.57a 0.02 0.72b ns ns

sand 0.74b 0.70b 0.79b ns ns

Zn silt + clay 0.94b 0.42 0.97b ns ns 0.77b

sand 0.84b 0.71b 0.82b ns ns 0.87b

a correlation significant at 0.05 level (2-tailed test).b correlation significant at 0.05 level (2-tailed test).ns: non significant.

128 M. ALOUPI AND M. O. ANGELIDIS

Figure 2. Metals projection in the first three Principal Components for the silt+clay (a) and sand (b)fraction of sediments.

To further understand the relationships between metal concentrations, reflectedby the correlation coefficients discussed above, Principal Component Analysis wasperformed to the data sets of the metal concentrations in the two granulometricfractions. The three first components of the correlation coefficients matrix, afterVarimax rotation, were used to describe the relationships between the variables.The results are presented in Figure 2. The three first components explained 94%of the total variation in the silt+clay fraction and 92% in the sand fraction. In bothgrain-size fractions, metals were grouped following a similar pattern. The anthro-pogenic metals Cd, Cu, Pb and Zn have their higher loadings in the first PrincipalComponent, which explains the greater part of the total variance (57% for silt +clay and 64% for sand). So, it can be assumed that the first Component representsthe human impact to the system, which is responsible for the greater part of thedata variance. It also seems that this human impact, carried out through the urbaneffluents discharge to the marine environment, is recorded to the coarser as well asto the finer material of the adjacent sediments. The second Principal Componentcarries the higher loading of Cr and Fe. This second Component represents thenatural origin of sediment components. The grouping of Cr with Fe suggests thatCr like Fe is mainly of natural origin. This finding, along with the small extract-able percentage of both metals (see below), is supported by previous results fromthe investigation of surface sediments from the study area (Angelidis and Aloupi,1997). Finally, the third Principal Component carries the higher loading of Mn, anindication of the special behavior of the metal in the marine environment. Unlikeall the other metals, extractable Mn concentrations show a uniform spatial distri-bution in the studied sediments, whereas these concentrations are similar in bothgranulometric fractions (see Tables II and III). The lack of a preference of non-residual forms of Mn for a specific grain-size fraction has been assessed in other

SIGNIFICANCE OF COARSE SEDIMENTS IN METAL POLLUTION 129

Figure 3. Percentage of extractable Cd, Cu, Pb, and Zn from the sand fraction in relation to the meansand content of the sediments of the inner, outer and ancient harbors and the wide coastal area ofMytilene.

marine environments, where coatings of Mn oxides were found on grains of sand(Loring and Nota, 1973).

The acid extractable concentrations in the bulk sediment (silt+clay+sand) werecalculated by summing the partial concentrations weighted to the percentage of therelative granulometric fraction in the sediment. Afterwards the sums were com-pared to the total metal concentrations, to estimate the percentage of the acidextractable, i.e. non-residual, metal.

The non-residual character of the metals follows the order:

Cd » Pb > Zn > Cu > Mn > Fe Cr.

Cadmium appears to be totally extractable and bioavailable (104% ± 17%), in ac-cordance to the literature (Campbell et al., 1988). The other anthropogenic metalshad also high extractable percentages (48% ± 12% for Cu, 69% ± 20% for Pb and55% ± 13% for Zn) and were placed in the first positions of the sequence. On theother hand Fe and Cr were the less acid-extractable metals (12% ± 3% and 11%± 2% respectively), indicating their association mostly to the lattice of minerals ofthe sediments. These results indicate that an important part of the metals accumu-lated in fine, as well as in coarser sediments of the marine coastal environment ofMytilene are potentially bio-available and may be released to the marine trophicchain.

130 M. ALOUPI AND M. O. ANGELIDIS

The percentage of total metal concentrations (Cd, Cu, Pb and Zn) extractedby 0.5 N HCl from the sand fraction in 4 sub-areas (inner harbor, outer harbor,wider coastal area and station 19 of the ancient harbor), were calculated and arepresented in Figure 3. For all metals examined the percentage of total concen-trations extracted from the sand fraction increases with the sand content of thesediments, reflecting the increasing relative importance of the coarser fraction inthe sediment, as expected. Therefore, since contamination is restricted to the harborarea, the real importance of sand as anthropogenic metal carrier seems negligible.However, this is not the case in the sediments of the ancient harbor, which areconsidered contaminated by anthropogenic metals (Aloupi and Angelidis, 2000).In the ancient harbor sediments, sand (at the absence of fine-grained sediments) isthe major anthropogenic metal carrier (92% of Cd, 41% of Cu, 70% of Pb and 68%of Zn). Thus, although the fine material is efficiently dispersed from the ancientharbor by the high hydrological regime of the area, contamination is still recordedin the sandy sediments.

4. Conclusions

The study of the acid extractable metals in the surface sediments of the coastalarea of Mytilene revealed the contamination of the harbor sediments by Cd, Cu, Pband Zn. Although concentrations were higher in the silt + clay fraction than in thesand fraction, percentages of total concentrations extracted from the coarse frac-tion, especially in the sandy northern harbor, showed that sand is also an importantcarrier of extractable metals. The high correlation coefficients found between theacid-extractable concentrations of the metals Cd, Cu, Pb and Zn in both grain-sizefractions, as well as for each metal in the two granulometric fractions, indicatetheir common anthropogenic source. Also, it seems that the metal contaminationis recorded in both silt + clay and sand fractions of the polluted sediments. It istherefore suggested that coarse sediments should not be disregarded in pollutionstudies of the coastal zone, especially in areas where an important part of thesediment material may have a texture characterized as ‘sand’.

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