Zinc and Lead in Background and Contaminated Sediments from Spencer Gulf, South Australia Paul Dossis and Leonard J. Warren* CSlRO Division of Mineral Chemistry, P.O. Box 124, Port Melbourne, Victoria 3207, Australia
Sediments were collected from relatively “clean” areas of Spencer Gulf, South Australia, outside the influence of a lead-zinc smelter at Port Pirie. The sediments were separated into size fractions and density subfractions and were analyzed chemically for their heavy-metal contents. The concentration and distribution patterns of zinc and lead in the main components of these clean sediments were then compared with similar data for contaminated sediments nearer the smelter, where the heavy-metal concentrations were some 30 times higher. The increase was not spread uniformly over all components. For lead, the concentration ratios (contaminated/ background) decreased in the order aragonitic shells > calcitic shells = heavy minerals > conglomerate particles > organic debris. The shells had a large capacity for heavy metals and were the main metal reservoir in many samples. The lead-zinc smelter at Port Pirie, on the shores of Spencer Gulf, South Australia, has been in operation since 1889. Emissions of lead, zinc, and cadmium over the years have raised the concentrations of these metals in the surrounding soils (1) and sediments ( 2 , 3 ) . We have previously reported on the nature and distribution of heavy metals in contaminated sediments,near the smelter ( 3 ) .Our objective in this paper is to compare the concentrations and distributions of heavy metals in the contaminated sediments with those in relatively “clean” sediments outside the influence of the smelter.
Experimental Section Three sediment samples (J, K, and L) were selected as representative of the clean, or background, sediments. All three samples were taken as IO-cm surficial sediment cores. Sample J was obtained, at a mean water depth of 7.0 m, from a sandy area with both live Posidonia sea grass and dead roots. Sample K (mean water depth, 15.1 m) was a gritty sediment with shell fragments but no live animals or plants. Sample L (mean water depth, 22.9 m) was a gritty gray mud with no live animals or plants. All three samples were collected at the same time and were dried in the same way as the contaminated sediment samples A to H described previously ( 3 ) .The locations of all sampling sites are shown in Figure l. The samples were then separated into four size fractions: >lo00 pm (Sl),1000-10 pm (S2), 10-1 pm (S3), and organic debris > conglomerates > calcitic shells > quartz > aragonitic shells. Subfraction S 2 D 6 (heavy minerals) had the highest heavy-metal concentrations, but it comprised only 0.1 wt % of the S2 fraction and contained only a few percent of the total heavy metal, which was probably locked up in detrital grains such as ilmenite (Table 111).Subfraction S 2 D l (organic debris) had the next highest heavy-metal concentrations, accounting for -10% of the total heavy metal in sample J. (Samples K and L were from deeper waters and had little organic debris.) Evidently, in or near sea grass beds, organic debris comprises a significant reservoir of heavy metals a t relatively high concentrations and as such is important in the uptake of heavy metals by benthic detritus feeders. In subfraction S2/D2 (conglomerates) the conglomerate particles were composed of much smaller particles of magnesian calcite, feldspar, and quartz, and their relatively high heavy-metal concentrations can be explained by the fineness of the smaller particles. On the average, subfraction S2/D4 (calcitic shells) contained about 3 times as much zinc and 5 times as much lead as subfraction S2/D5 (aragonitic shells) (Table IV). This preferential accumulation of heavy metals in calcitic shells was also noted in the contaminated samples ( 3 )and has been reported by Sturesson (12, 13) as occurring in individual growing mussel shells. Sturesson suggests that heavy metals may be preferentially accumulated in the organic matrix of the calcitic shells. Alternatively, preferential substitution of heavy-metal cations for calcium may occur in the two different kinds of shells ( 3 ) .In either case, the heavy metals are apparently transferred to the shell primarily via the soft parts of the organism, and there is some evidence to suggest that the
transfer is more efficient for lead than for zinc (14-16). Thus, in some marine gastropods, most of the zinc is stored in the soft parts but most of the lead is stored in the shell (16). Subfraction S2/D3 (quartz) had relatively low heavy-metal concentrations. This is because it consisted of a mixture of quartz and calcitic shell fragments of similar density. The quartz, which normally contains little zinc or lead, had a diluting effect and so lowered the heavy-metal concentrations below those expected for calcitic particles (Table 111). In some sediments, the concentrations of heavy metals are reported to correlate with the concentration of iron (17). Furthermore, the lead concentrations in bivalves living in such sediments are reported to increase linearly with the ratio of sediment lead to sediment iron, which suggests that lead bound to iron oxide is less available than other forms of lead (17).However, in the background samples, we found no relationship between total lead or zinc and total iron (Table 11). Similarly, we found no correlation between the concentrations of heavy metals and the concentration of iron in whole samples of contaminated sediments B, C, D, G, and H. On the other hand, we did find almost a 1:l correlation if the results were plotted in the following way. The heavy-metal concentration [MI in each density subfraction Di of size fraction S2 was divided by the overall heavy-metal concenand that ratio tration in size fraction S2, y = [M]sz/~~/[M]sz, was plotted against the corresponding ratio for iron, 2 = [Fe]sz/~J[Fe]sz.In this way, independent variations in the overall heavy-metal and iron concentrations were taken into account. The result for zinc was y = 1 . 0 3 ~ 1.02, with r = 0.96, and the result for lead was y = 1.252 0.04, with r = 0.97. The implication is that zinc and lead have a chemical behavior similar to that of iron in relation to the way that they are incorporated into the density subfractions. In other words, each sediment component has a similar affinity for zinc, lead, and iron, although the affinity is different for different components. Analyses of the density subfractions of contaminated samples A to H showed that discharges of heavy metals from the Port Pirie smelter have increased the concentrations of lead and zinc in sediments within 30 km of the smelter by -30 times (Table IV and Figure 1).Average cadmium concentrations have increased -70 times, from 0.1 to 7.1 pg/g. The analyses also showed that the increase has not been uniform over all sediment components. For example, the coarser particles had higher metal enrichment ratios than the finer particles, and some density subfractions had higher metal enrichment ratios than others (Table IV). For zinc, the largest metal enrichment ratio was for subfraction S2/D6 (heavy minerals), with progressively lower ratios for subfraction S 2 D 5 (aragonitic shells), S 2 D 4 (calcitic shells), S2/D2 (conglomerates), and S2/D1 (organic debris) (Table IV). The order of enrichment ratios for lead was much the same as for zinc, except that the ratio for lead in subfraction S2/D6 was considerably lower and roughly equal to that in subfraction S2/D4 (Table IV). The high zinc enrichment ratio for subfraction S2/D6 can be explained by the presence of particles of sphalerite (ZnS) that, weight for weight, contribute large amounts of zinc. The sphalerite particles were fairly coarse (mostly 10-1000 pm) and crystalline, which suggests that individual particles may have settled through the water onto the seabed after being transported to Spencer Gulf via the smelter plume or fugitive dusts. If the sphalerite particles had been precipitated in situ from biologically generated sulfide ions, they probably would have been finer and semicrystalline (18). Lead enrichment in subfraction S2/D6 probably occurred in a similar fashion, for crystalline galena (PbS) was observed in several samples. However, the enrichment ratio for lead in subfraction S2/D6 is so much lower than for zinc that further
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explanation is required. Galena may be more soluble in the oxidizing marine environment, or lead may be more easily extracted from galena by animals that ingest sulfide particles. Either way, relatively more lead than zinc would be transferred to shells and sea grass. This is consistent with the metal enrichment per gram in subfraction S2/D6 and in the whole sample (Table IV). Whereas there was 8.7 times as much zinc as lead added to subfraction S2/D6, there was only 2.1 times as much added to the whole sample. The zinc enrichment ratios for the shell fragments in subfractions S2/D4 and SZ/D5 were much lower than for the heavy minerals in subfraction S2/D6, but, as can be seen in Table IV, the shells still accumulated an average of -30 times more zinc than they had in their pristine, Uncontaminated state. The zinc enrichment ratio for the aragonitic shells (subfraction S2/D5) was, on the average, nearly 1.5 times as high as for the calcitic shells (subfraction S2/D4). This effect was even more marked for lead the lead enrichment ratio for the aragonitic shells was -2 times as high as for the calcitic shells (Table IV). If the enrichment ratio is assumed to be the best measure of the degree of contamination, then the aragonitic shells can be said to have been contaminated preferentially in relation to the calcitic shells. On the other hand, the background heavy-metal concentrations in the aragonitic shells were so low (Table IV) that only a small fraction of added metal was required to generate the observed high enrichment ratios. In terms of the amount of heavy metal added to the samples by the smelter, calcitic shells took up about twice the weight of heavy metal as the aragonitic shells and reached higher heavy-metal concentrations. It is clear that calcareous shells, no matter whether they are calcitic or aragonitic, have a large capacity for heavy metals and may be the main heavy-metal reservoir where they constitute a large proportion of a contaminated sediment. We estimate that -50% of the added lead and -35% of the added zinc were incorporated into 52 shell fragments. Although the organic debris (subfraction S2/D1) accumulated about the same amount of added metal as the aragonitic shells, the resulting enrichment ratios were lower than for any other sediment component. This is largely because the heavy-metal concentrations in the organic debris in its natural uncontaminated state were relatively high (Table IV). Thus, for example, while the concentration of lead increased only 5.7 times, it reached an absolute value of 647 pg/g, which is relatively high and has obvious significancefor bottom-feeding animals. Metal Availability as Measured by Dilute Acid Extraction. The results in Table V show that a large proportion of the heavy metal in the shell fragments was removed by dilute acid, as might be expected. However, there was a marked difference between the calcitic shells (-88% of the metal removed) and the aragonitic shells (-72% of the metal removed). If dilute HC1 simulates the digestive juices of some animals, then it appears that much of the heavy metal in ingested shell fragments will be released despite the fact that it is held within shells. The difficulty of extracting cadmium from the organic debris, as compared with zinc or lead (Table V), confirms earlier experiments on large fragments of sea grass separated by hand from sample A (3). Particles in subfraction S2/D6 (heavy minerals) were fairly resistant to attack by dilute HC1 (Table V). The results also show that the particular form of sphalerite (with its impurities) in these samples is more soluble in dilute acid than the galena (with its impurities).
Conclusions (1)Samples from relatively “clean” areas of Spencer Gulf, South Australia, contained about 18 pg/g of zinc, 11p g l g of lead, and 0.1 pg/g of cadmium. Volume 15, Number 12, December 1981
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(2) The zinc and lead concentrations in the six main components of these background samples decreased in the order heavy minerals > organic debris > conglomerate particles > calcitic shell fragments > quartz > aragonitic shell fragments. The various shell fragments comprised the largest single component by weight, and they contained the largest proportion of the total heavy metal in the samples. The heavy metals appeared to be present within the shells rather than on their surfaces. In or near sea grass beds, the organic debris comprised a significant reservoir of heavy metal a t relatively high concentrations, whereas the heavy minerals accounted for only a few percent of the total heavy metal. (3) In the background samples, zinc and lead were distributed between the components in proportions similar to iron. The affinity for the metals differed between components, but for any one component the affinity was similar for zinc, lead, and iron. (4) Samples within -30 km of the Port Pirie smelter but outside the intertidal zone had zinc and lead concentrations -30 times higher, on the average, than the background samples, and they were slightly more enriched in zinc than in lead. (5) The average lead and zinc enrichment ratios in the contaminated sediments were higher for particles in the size range 10-1000 pm than they were for finer particles. (6) Comparison of the contaminated and background samples also showed that the enrichment ratio for zinc in the various density subfractions decreased in the order heavy minerals > aragonitic shells > calcitic shells > conglomerates > organic debris. For Iead, the order was aragonitic shells > calcitic shells = heavy minerals > conglomerates > organic debris. (7) Shell fragments in sediment samples from near the smelter had lead and zinc concentrations -30 times higher than shell fragments ‘from uncontaminated areas. Because of their large capacity for heavy metals and their abundance in the sediments, shells were the main reservoir for heavy metals. (8) On a weight basis, the calcitic shells took up more heavy metal than the aragonitic shells. However, the aragonitic shells were more contaminated in the sense that their metal enrichment ratio was larger. (9) Most of the zinc, lead, and cadmium in the contaminated
shell fragments was removed by 0.15 M HCl. However, only -50% of the cadmium was extracted from the organic debris, compared with -80% of the zinc and lead. Acknowledgment
We thank Dr. K. G. Tiller for supplying samples of the Spencer Gulf sediments and Mr. E. S. Pilkington for performing the AAS cadmium analyses. Literature Cited (1) Cartwright, B.; Merry, R. H.; Tiller, K. G. Aust. J. Soil Res. 1976,
15,69.
( 2 ) Tiller, K. G., CSIRO Division of Soils, personal communication,
1977. (3) Dossis, P.; Warren, L. J. In “Contaminants and Sediments”; Baker, R. A,, Ed.; Ann Arbor Science Publishers: Ann Arbor, MI, 1980; Vol. 1,pp 119-39. (4) Pilkington, E. S.; Warren, L. J. Enuiron. Sci. Technol. 1979.13, 295. (5) Ward, T. J.; Young, P. C . Aust J . Mar. Freshwater Res., in press. (6) Warren, L. J. In “Effects of Heavy Metals on Aquatic Life” (First Annual Progress Report of the CSIRO Heavy Metals Task Force, Jan-Dec 1978);Commonwealth Scientific and Industrial Research Organization: Canberra, Australia, 1979; p 27. (7) Chester, R.; Aston, S. R. In “Chemical Oceanography”, 2nd ed.; Riley, J. P., Chester, R., Eds.; Academic Press: London, 1976;Vol. 6, pp 281-390. (8) Taylor, D. Estuarine Coastal Mar. Sci. 1974,2,417. (9) Calvert, S. E. In “Chemical Oceanography”, 2nd ed.; Riley, J. P., Chester, R., Eds.; Academic Press: London, 1976; Vol. 6, pp 187280. (10) Helmke, P. A.; Koons, R. D.; Schomberg, R. J.; Iskander, I. K. Enuiron. Sci. Technol. 1977,11,984. (11) Forstner, U. In “Interactions Between Sediments and Fresh Water”; Golterman, H. L., Ed.; Junk: The Hague, 1977; pp 94103. (12) Sturesson,U. Ambio 1976,5,253. (13) Sturesson, U. Ambio 1978,7,122. (14) Lande, E. Enuiron. Pollut. 1977,12,187. (15) Phillips, D. J. H. Enuiron. Pollut. 1977,13, 281. (16) Ireland, M. P.; Wooton, R. J. Enuiron. Pollut. 1977,12,27. (17) Luoma, S. N.; Bryan, G. W. J. Mar. Biol. Assoc. U. K.1978,58, 793. (18)
