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Heavy-metal enrichment in surficial sediments in the Oder River discharge area: source or sink for heavy metals?
PII: S0883-2927(97)00102-9
Applied Geochemistry, Vol. 13, pp. 329±337, 1998 # 1998 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0883-2927/98 $19.00 + 0.00
Heavy-metal enrichment in sur®cial sediments in the Oder River discharge area: source or sink for heavy metals? Thomas Neumann* and Thomas Leipe Institut fuÈr Ostseeforschung WarnemuÈnde, Seestr. 15, 18119 Rostock, Germany
Graham Shimmield Department of Geology and Geophysics, University of Edinburgh, West Mains Rd, Edinburgh EH9 3JW, U.K. (Received 9 September 1995; accepted in revised form 18 June 1997) AbstractÐThe Oder river drains a highly polluted industrial area and enters the Baltic Sea through a system of shallow lagoons. Sur®cial sediments in the discharge area of the Oder are highly enriched in heavy metals compared to their preindustrial background levels. Pore-water studies in short sediment cores reveal anoxic environments over the entire sediment column, except for a suboxic layer in the uppermost 5 to 20 mm of the sediment where Mn- and Fe-oxyhydroxides are reduced by organic matter. Heavy metals (such as Cu, Zn and Pb) are mobilized within the suboxic zones in the inner lagoon (represented by the Oder Lagoon) and in the open Baltic (represented by the Arkona Basin). The Achterwasser, located between the Oder Lagoon and the Arkona Basin, is directly aected by sea-level ¯uctuations in the Baltic. Pore-water studies indicate that, in contrast to the situation in the Oder Lagoon and Arkona Basin, sur®cial sediments of the Achterwasser represent a sink for heavy metals. This is associated with the high rate of Fe-sulphide formation occurring there, at least seasonally during saltwater in¯ow. # 1998 Elsevier Science Ltd. All rights reserved
depths of 4 to 5 m where the river load is initially trapped. The Achterwasser is an 8 m deep basin situated between the Oder Lagoon and the open Baltic Sea. It is known to be an area of mixing of river water and Baltic Sea water and is characterized by intermediate salinities (3.3-) (Lampe, 1993). The Arkona Basin is a 45 m deep oshore basin in the open Baltic Sea which is thought to be the ®nal deposition zone for the Oder river load (Neumann et al., 1996) and is characterized by relative high salinities in the deepest areas (up to 25-).
INTRODUCTION
In highly industrialized areas such as the Oder catchment area, rivers become stressed by the anthropogenic input of contaminants. An important source of contaminants in the Oder River is the mining industry. Additional inputs come from domestic sewage, agriculture and atmospheric fallout (HELCOM, 1993). The distribution of heavy metals in the sediments of the Oder Lagoon and its oshore basins re¯ects a clear imprint of heavy-metal pollution on the estuarine and open Baltic environments (Neumann et al., 1996). The Oder estuary is not a real estuarine system because the Oder river water enters the open Baltic Sea through a series of shallow lagoons. This results in dierent biogeochemical conditions within the various components of the Oder system and oers the possibility of investigating processes under distinct biogeochemical and hydrological regimes. In this study, 3 key areas were selected to determine the accumulation and mobilization of heavy metals in the dierent environments of the Oder system. The Oder Lagoon is a shallow brackish water environment with low salinities (2-) and water
METHODOLOGY
*Corresponding author. Present address: Institute of Petrography and Geochemistry, University of Karlsruhe, Kaiserstr. 12, 76131 Karlsruhe, Germany. 329
During cruises of project ODER (Oder dischargeenvironmental response) in May and August 1993, sediment cores were taken in order to investigate the biogeochemical processes of anthropogenic contaminants in the Oder environment (Shimmield et al., 1994). Cores IOW # 18036, 18039 and 18033 were taken in the Oder Lagoon (53849.23'N, 14809.11'E, 5.0 m water depth), Achterwasser (54801.90'N, 13857.97'E, 3.7 m water depth) and southern Arkona Basin (54849.50'N, 13839.20'E, 45 m water depth), respectively. Figure 1 shows the study area and the locations of the cores. Sediments were sampled using a Rumohrlot gravity corer. The corer sampled bottom water, an
330
T. Neumann et al.
Fig. 1. Schematic map showing the locations of the sampling stations within the investigated area.
undisturbed surface sediment layer and between 0.3 and 0.8 m of underlying sediment. The cores were subsampled at 10 or 20 mm intervals for bulk analyses. The samples were stored in polyethylene bottles and frozen. Pore waters were taken immediately after retrieval. Sediment cores within the liners were transferred into an Ar-¯ushed glove box to prevent oxidation, extruded and sliced into 10 mm discs. The slices were stored in precleaned polyethylene centrifuge bottles. After centrifuging, the supernatant water was ®ltered through 0.45 mm nucleopore ®lters. Aliquots were used to determine SO4 by HPLC at the University of Edinburgh. Subsamples of pore water were acidi®ed with suprapure HNO3 and kept frozen until analysis. Sediment samples were freeze-dried and divided into <63 mm and >63 mm fractions by dry sieving. The <63 mm fraction was ground in a porcelain mortar and homogenized for chemical analyses. Weighed splits were totally dissolved using HF/HCl and HClO4/HNO3 and concentrations of Mg, Ca, Al, Fe Mn, Zn, Cu and Pb determined by ICP-AES (Varian Liberty 200). Manganese, Fe, Cu, Zn and Pb were determined in pore waters using operating conditions which were optimized for each element and each type of sample. Data quality was con-
trolled by repeatedly analyzing reference materials ABSS and MBSS (ICES, 1987). Certi®ed multi-element ICP-standard solutions were used for calibration. Analytical precision was generally better than 25%. For S and total organic C (TOC) analysis, an Eltra CS-analyzer was used. 0.1 N HCl was added to the samples (20±150 mg) to remove carbonates. The samples were oxidized at 14008C and measurements made by infra-red adsorption.
RESULTS
General characteristics of the sediments The muddy sediments of the investigated areas are characterized by a light brown, ¯uy, oxic layer 10 to 40 mm thick (oxidation zone), overlying a soft, black anoxic layer 200 to 400 mm thick (reduction zone). This then passes into a dark grey± green mud (fermentation zone). The mud is often interbedded with shell layers, particularly in the lagoonal areas and with sand and silt layers in the higher energy regions of the Oder Rinne. Grain-size analysis shows that the bulk of the sediment in the area lies within the 6.3 to 63 mm range correspond-
Heavy-metal enrichment in Oder river discharge area
ing to medium to coarse silt. There is usually no preservation of ®ne scale depositional structures such as lamination within the sediments as a result of intense bioturbation in the upper layer. Scanning electron microscopy (SEM) and X-ray-investigations show biogenic calcite to be present in the oxidation zone, Fe-sulphides in the reduction zone and Fe-rich chlorite and pyrite in the fermentation zone. The high amount of total organic C (TOC) (up to 14 wt% in the lagoonal sediments) re¯ects the very high ¯ux of organic matter into the sediment resulting from the enormous discharge of nutrients into the region (Lampe, 1993; Lippert, 1993). The TOC content of the sur®cial sediments decreases from the lagoon to the open Baltic and has values of about 5% in the Arkona Basin. Concentration pro®les with depth in the sediments reveal contrasting behaviours of TOC and S. TOC is enriched at the sediment surface whereas S increases with depth in the sediment column. Sulphur contents reach values up to 6% in estuarine sediments and 2% in o-shore basin sediments. SEM-investigations suggest that the high sulphur contents result from the presence of Fe-sulphides.
Heavy metals in sediments Heavy-metal concentrations in the sediments display a wide range of values, but their distributions with depth are generally similar throughout the study area (Figs 2±4). Manganese shows highest concentrations in the surface sediments whereas Fe increases towards the lower parts re¯ecting the occurrence of Fe-sulphides in the deeper sediments. Copper, Zn and Pb generally show large and often sharp decreases in concentration 10 to 20 mm below the sediment surface whereas Co and Ni remain almost unchanged. Below this depth, the pro®les display relatively constant values of these elements which are assumed to represent the natural pre-industrial concentrations. Such distribution patterns have been described during earlier investigations of Baltic Sea sediments (Suess and Erlenkeuser, 1975; Szefer and Skwarzec, 1988; BruÈgmann and Lange, 1990; Belmans et al., 1993) and appear typical for muddy sediments throughout the Baltic Sea. Statistical analysis of the sediment compositional data (taken from the appendix in Neumann et al., 1996) reveals that the heavy metals in the sediments are related to certain components such as total organic C (TOC), Al and S (Table 1). Heavy metals with high enrichments in the sur®cial sediments, such as Cu, Zn and Pb, show strong correlations with the organic matter throughout the study area. These metals are taken up biologically, especially by SiO2 in many lagoons and estuaries, as a result
331
by the diatom production during the spring and summer blooms (Kaul and Froelich, 1984; Windom et al., 1991). In addition to these general results, the 3 subareas are distinguished by some typical characteristics which may be described as follows.
Oder lagoon The sediments of the Oder Lagoon contain the highest heavy-metal concentrations within the entire study area (up to 4800 mg/g Mn, 5.9% Fe, 68 mg/g Cu, 865 mg/g Zn and 135 mg/g Pb). There is a high correlation of Fe with Al in the Oder Lagoon sediments, probably because of the high discharge of Fe-rich minerals into it. The positive correlation of Mn and TOC may be explained by early diagenetic processes in which Mn oxides are reduced during oxidation of organic matter. This conclusion is supported by pore-water investigations which show that the dissolved Mn is highest (146 mM Mn) in the uppermost 40 mm of the sediment and decreases with depth to values of about 45 mM Mn (Fig. 2). The dissolved Fe concentration, on the other hand, is very low in the uppermost 100 mm (17.9 mM Fe) but increases with depth to a maximum of about 860 mM Fe at 270 mm. The pore-water SO4 concentration is highest (about 2.0 mM) at the sediment± water interface and decreases to 1.0 mM at a depth of 100 mm. Below 100 mm, the SO4 concentration increases to the level at the sediment surface. Although the Oder Lagoon sediments have the highest heavy-metal concentrations in the entire study area, the pore-water concentrations are much lower than in the Arkona Basin: Cu ranges from 0.03 to 0.30 mM, Zn from 1.93 to 5.19 mM and Pb from <0.01 to 0.09 mM. Pore waters from the Oder Lagoon are enriched in heavy metals, i.e. Zn by a factor of 2 to 3, compared to the bottom-water concentrations of 0.90 mM. This situation is very common and may be explained by the mobilization of heavy metals from organic matter during early diagenetic processes in the suboxic zone in which Fe- and Mn-oxyhydroxides degrade the organic matter.
Achterwasser The heavy-metal contents in the Achterwasser sediments are less than in the Oder Lagoon by factors of 2 to 3 for Cu and Pb and 5 for Mn and Zn. Iron, on the other hand, occurs in concentrations of up to 6.1% in the sediments (Fig. 3). Generally, Fe is positively correlated with S as a consequence of the occurrence of Fe-sulphides in the anoxic sediments. Cobalt and Ni are also positively correlated with Fe and S in Achterwasser sediments re¯ecting
332
T. Neumann et al.
Fig. 2. Al, TOC, S, Fe, Mn, Cu, Zn and Pb concentrations in the sediments (W) and dissolved sulphate, Mn, Fe, Cu, Zn and Pb concentrations in the pore waters (r) in core 18036 located in the Oder Lagoon.
Heavy-metal enrichment in Oder river discharge area
Fig. 3. Al, TOC, S, Fe, Mn, Cu, Zn and Pb concentrations in the sediments (W) and dissolved sulphate, Mn, Fe and Zn concentrations in the pore waters (r) in core 18039 located in the Achterwasser.
333
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T. Neumann et al.
Fig. 4. Al, TOC, S, Fe, Mn, Cu, Zn and Pb concentrations in the sediments (W) and dissolved Mn, Fe, Cu, Zn and Pb concentrations in the pore waters (r) in core 18033 located in the Arkona Basin.
Heavy-metal enrichment in Oder river discharge area
335
Table 1. Correlation coecients of elements in the <63 mm-fraction of sediments of the Oder Lagoon, Achterwasser and Arkona Basin. The correlation coecients were calculated from the total data from each of the cores. Signi®cant positive correlations of elements with Al, TOC and S are printed in bold Mn
Fe
Co
Ni
Cu
Zn
Pb
Al
TOC
S
1.00 ÿ0.62 0.71 ÿ0.24
1.00 ÿ0.45 0.69
1.00 ÿ0.45
1.00
1.00 ÿ0.66 0.51 ÿ0.70
1.00 ÿ0.96 0.78
1.00 ÿ0.76
1.00
1.00 0.79 0.84 ÿ0.33
1.00 0.71 ÿ0.57
1.00 ÿ0.45
1.00
Oder Lagoon (18036-9), n = 13
Mn Fe Co Ni Cu Zn Cd Pb Al TOC S
1.00 ÿ0.71 0.91 0.92 0.90 0.90 0.91 0.90 ÿ0.49 0.77 ÿ0.34
Mn Fe Co Ni Cu Zn Cd Pb Al TOC S
1.00 ÿ0.88 ÿ0.88 ÿ0.88 0.65 0.90 0.80 0.80 ÿ0.83 0.80 ÿ0.86
Mn Fe Co Ni Cu Zn Cd Pb Al TOC S
1.00 0.05 ÿ0.34 ÿ0.72 ÿ0.79 ÿ0.77
1.00 0.70 0.38 0.43 0.44
1.00 0.76 0.70 0.72
1.00 0.91 0.90
1.00 0.99
1.00
ÿ0.78 ÿ0.81 ÿ0.57 0.54
0.40 0.29 0.45 0.39
0.68 0.53 0.68 0.09
0.88 0.78 0.91 ÿ0.38
0.99 0.84 0.89 ÿ0.41
0.99 0.80 0.87 ÿ0.34
1.00 ÿ0.72 ÿ0.74 ÿ0.85 ÿ0.85 ÿ0.83 ÿ0.85 0.75 ÿ0.36 0.42
1.00 0.99 0.97 0.96 0.96 0.97 ÿ0.45 0.76 ÿ0.15
1.00 0.98 0.98 0.97 0.98 ÿ0.54 0.82 ÿ0.26
1.00 0.99 0.99 0.99 ÿ0.62 0.71 ÿ0.25
1.00 0.99 0.99 ÿ0.63 0.71 ÿ0.26
Achterwasser (18039-7), n = 15 1.00 0.97 0.84 ÿ0.62 ÿ0.93 ÿ0.78 ÿ0.76 0.87 ÿ0.83 0.98
1.00 0.82 ÿ0.49 ÿ0.91 ÿ0.78 ÿ0.70 0.91 ÿ0.90 0.94
1.00 ÿ0.57 ÿ0.80 ÿ0.80 ÿ0.76 0.69 ÿ0.66 0.83
1.00 0.73 0.73 0.89 ÿ0.52 0.35 ÿ0.55
1.00 0.88 0.87 ÿ0.89 0.82 ÿ0.85
Arkona Basin (18033-4), n = 19
the incorporation of these metals into the sulphide phases. The core is characterized by very low pore-water Fe concentrations of up to 3.1 mM with a maximum concentration of 11.8 mM at a depth of 170 mm. Manganese displays its highest pore-water concentrations (47.6 mM Mn) in the uppermost 20 mm and decreases to a value of about 22.3 mM at a depth of 90 mm. This suggests that early diagenetic processes reduce Mn oxides during the oxidation of organic matter in the uppermost part of the sediment column. The pore-water SO4 pro®le shows highest concentrations (1.8 mM) at the sediment±water interface which decrease with depth to values of about 0.3 mM. Sediments of the Achterwasser are distinguished by having the lowest pore-water heavy-metal concentrations in the entire study area. Copper and Pb are below the determination limit of about 0.01 mM, and Zn ranges from 0.14 to 0.93 mM. By contrast to the Oder Lagoon and Arkona Basin, Zn is present
in higher concentrations in the bottom water (1.10 mM) than in the pore water (0.70 mM) in the Achterwasser. Arkona Basin The lowest concentrations of heavy metals occur in the sediments of the Arkona Basin (up to 450 mg/ g Mn, 3.7% Fe, 38 mg/g Cu, 200 mg/g Zn and 94 mg/g Pb) (Fig. 4). There is a strong relationship between these metals and Al indicating the association of these metals with clay minerals and suggesting that the metals change from being bound mainly to organic matter in the lagoonal system to being bound mainly to clay minerals in the open Baltic Sea. Pore waters from Arkona Basin sediments display the highest Mn concentrations of 81.9 mM in sur®cial sediments at 0±20 mm depth. This concentration decreases exponentially to a value of about
336
T. Neumann et al.
2.7 mM Mn at a depth of 400 mm. Because elevated levels of Mn occur only when O2 is completely consumed (Jahnke et al., 1989; Rutgers van der Loe, 1990), these data indicate that the suboxic zone lies within the uppermost 30 mm of the sediment column. The Fe pro®le is similar to that of Mn (from 237 to 1.8 mM Fe), except that dissolved Mn appears at a shallower depth than Fe. Based on free energy considerations, Mn reduction precedes Fe reduction by organic matter (e.g. Froelich et al., 1979). These data can therefore be considered to represent ``normal'' pore-water pro®les in¯uenced by early diagenetic processes. Despite having the lowest heavy metal concentrations in the sediments, the pore-water heavy-metal concentrations are the highest in the entire study area. Copper ranges from 0.13 to 0.58 mM, Zn from 6.32 to 8.81 mM and Pb from 0.31 to 0.54 mM.
DISCUSSION
The high biological productivity in the Oder Lagoon is demonstrated by the very high TOC content in the dried sediment samples (up to 14 wt%). The positive correlations of Cu, Zn, Cd and Pb to TOC re¯ect the high biological uptake of these elements, especially in the Oder Lagoon. The degradation of organic matter in the sediments is well illustrated by the decreasing TOC content with depth in all 3 areas (Neumann et al., 1996) and the maximum dissolved Mn concentrations in the uppermost sediments. The degradation of the organic matter can be considered to be responsible for the remobilization of heavy metals from the organic matter into the pore water. These metals could then diuse either into the bottom water or into the deeper anoxic sediments. In anoxic sediments containing H2S, heavy metals are usually ®xed as sulphides. In general, Fe-sulphide formation is limited by the amount and reactivity of organic matter buried in the sediments, and the availability of reactive Fe and of SO2ÿ 4 necessary for the production of H2S (Emeis and Morse, 1993). In the brackish waters of the Oder Lagoon with salinities of about 2-, the SO2ÿ 4 content is very low and may be the limiting factor in Fe-sulphide formation. Inspite of this, the S content of the Oder Lagoon sediments is relatively high ranging from 1 to 3%, although considerably lower than in the Achterwasser sediments (upto 6%). Higher amounts of sulphide in the sediments of the Achterwasser re¯ect the higher salinity of the waters (3.3-). According to Froelich et al. (1979) and Thamdrup et al. (1994), diagenetic processes lead to the formation of Fe-sulphides below the suboxic/ anoxic boundary. This occurs at a depth of 20 mm in the lagoon sediments and 40 mm in the Arkona
Basin sediment. This situation is not observed in the Fe and S pro®les of the lagoonal areas of the Oder Lagoon and Achterwasser where Fe and S concentrations in the sediments increase signi®cantly with depth. Historical changes of the sedimentary environment as well as more recent processes, such as leaching of the sur®cial sediments by O2-rich waters or advective ¯ux of water through the sediment column, could explain this observation. Rapidly changing salinity levels are characteristic only of the northern part of the Oder lagoonal system including the Achterwasser. Lampe (1993) documented changes in the salinity of the Achterwasser waters which are directly aected by sea-level ¯uctuations in the Baltic. The increase of salinity in the Achterwasser takes place over a short period (i.e. a few days) followed by a slow decrease over a longer period (i.e. a few months) corresponding to a slow increase in the fresh-water contribution. The in¯ow of the more saline Baltic Sea water into the Achterwasser increases the SO4 content of the waters and may result in pyrite formation. During times of intense Fe-sulphide formation in the Achterwasser, various heavy metals such as Co and Ni are scavanged from the pore water and ®xed in sulphides. This leads to heavy-metal concentrations in the pore waters which are signi®cantly lower than those found in the Oder Lagoon and Arkona Basin. Iron-sulphide formation in the Oder Lagoon and Arkona Basin seems to be much weaker than in the Achterwasser and the heavy metals can diuse into the bottom waters after their mobilization from the organic matter during early diagenetic processes. For these reasons, we suppose that the sur®cial sediments represent a potential source of heavy metals in the Oder Lagoon and Arkona Basin, but a sink for heavy metals in the Achterwasser, at least seasonally during salt-water in¯ow.
AcknowledgementsÐThe authors would like to thank all colleagues of Project ODER for very constructive cooperation and discussions. We are also grateful to Dagmar Benisch and Reinhild Rosenberg (Institute of Baltic Sea Research at WarnemuÈnde, IOW) for assistance in sample collection and to Tim Brand (University of Edinburgh) for determining the sulphate contents of pore waters. The captains and crews of F. S. Alkor, F. K. BornhoÈft, F. S. A. v. Humboldt and F. S. Professor A. Penck are thanked for their help. This research was supported ®nancially by the European Commission (ENVIRONMENT PROGRAMME, Grant #PL-910398) and by the BMBF (Project#03F0101A). Editorial handling: G. P. Glasby and P. Szefer.
Heavy-metal enrichment in Oder river discharge area REFERENCES Belmans F., van Grieken R. and BruÈgmann L. (1993) Geochemical characterization of recent sediments in the Baltic Sea by bulk and electron microprobe analysis. Marine Chemistry 42, 223±236. BruÈgmann L. and Lange D. (1990) Metal distribution in sediments of the Baltic Sea. Limnologica 20, 15±28. Emeis K. C. and Morse J. W. (1993) On the systematics of carbon±sulphur±iron ratios in sediments of upwelling areas. Geol. Rundsch. 82, 604±618in German. Froelich P. N., Klinkhammer G. P., Bender M. L., Luedtke N. A., Heath G. R., Cullen D., Dauphin P., Hammond D., Hartmann B. and Maynard V. (1979) Early oxidation of organic matter in pelagic sediments of the eastern equatorial Atlantic: Suboxic diagenesis. Geochim. Cosmochim. Acta 43, 1075±1090. HELCOM (1993) Second Baltic Sea pollution load compilation. Balt. Sea Environ. Proc., Vol. 45, 161 pp. ICES (1987) Report on the results of the Baltic sediment intercalibration exercise. ICES Coop. Res. Rep., Vol. 147, 92 pp. Jahnke R. A., Emerson S. R., Reimers C. E., Schuert J., Ruttenberg K. and Archer D. (1989) Benthic recycling of biogenic debris in the eastern tropical Atlantic Ocean. Geochim. Cosmochim. Acta 53, 2947±2960. Kaul L. W. and Froelich P. N. (1984) Modeling estuarine nutrient geochemistry in a simple system. Geochim. Cosmochim. Acta 48, 1417±1433.
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Lampe R. (1993) Environmental state and material ¯ux in the western part of the Oder river estuary: Results and consequences. Petermanns Geogr. Mitt. 137, 275±282. Lippert, C. (1993) Discharge of nutrients in the Oder estuary. Diploma Thesis, University of Greifswald (in German). Neumann T., Leipe T., Brand T. and Shimmield G. (1996) Accumulation of heavy metals in the Oder estuary and its o-shore basins. Chemie der Erde 56, 207±222. Rutgers van der Loe M. M. (1990) Oxygen in porewaters of deep-sea sediments. Philos. Trans. R. Soc. London 331, 69±84. Shimmield, G. and Oder-Project-Members (1994) Project ODER-Interim Report, January 1994. EC Environment Programme PL 910398, Brussels, 127 pp. Suess E. and Erlenkeuser H. (1975) History of metal pollution and carbon input in the Baltic Sea sediments. Meyniana 27, 63±75. Szefer P. and Skwarzec B. (1988) Distribution and possible sources of some elements in the sediment cores of the Southern Baltic. Mar. Chem. 23, 109±129. Thamdrup B., Fossing H. and Jorgensen B. B. (1994) Manganese, iron and sulphur cycling in a coastal marine sediment, Aarhus Bay, Denmark. Geochim. Cosmochim. Acta 58, 5115±5129. Windom H., Byrd J., Smith R., Hungspreugs M., Dharmvanij S., Thumntrakul W. and Yeats P. (1991) Trace metal±nutrient relationships in estuaries. Mar. Chem. 32, 177±194.
Applied Geochemistry, Vol. 13, pp. 329±337, 1998 # 1998 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0883-2927/98 $19.00 + 0.00
Heavy-metal enrichment in sur®cial sediments in the Oder River discharge area: source or sink for heavy metals? Thomas Neumann* and Thomas Leipe Institut fuÈr Ostseeforschung WarnemuÈnde, Seestr. 15, 18119 Rostock, Germany
Graham Shimmield Department of Geology and Geophysics, University of Edinburgh, West Mains Rd, Edinburgh EH9 3JW, U.K. (Received 9 September 1995; accepted in revised form 18 June 1997) AbstractÐThe Oder river drains a highly polluted industrial area and enters the Baltic Sea through a system of shallow lagoons. Sur®cial sediments in the discharge area of the Oder are highly enriched in heavy metals compared to their preindustrial background levels. Pore-water studies in short sediment cores reveal anoxic environments over the entire sediment column, except for a suboxic layer in the uppermost 5 to 20 mm of the sediment where Mn- and Fe-oxyhydroxides are reduced by organic matter. Heavy metals (such as Cu, Zn and Pb) are mobilized within the suboxic zones in the inner lagoon (represented by the Oder Lagoon) and in the open Baltic (represented by the Arkona Basin). The Achterwasser, located between the Oder Lagoon and the Arkona Basin, is directly aected by sea-level ¯uctuations in the Baltic. Pore-water studies indicate that, in contrast to the situation in the Oder Lagoon and Arkona Basin, sur®cial sediments of the Achterwasser represent a sink for heavy metals. This is associated with the high rate of Fe-sulphide formation occurring there, at least seasonally during saltwater in¯ow. # 1998 Elsevier Science Ltd. All rights reserved
depths of 4 to 5 m where the river load is initially trapped. The Achterwasser is an 8 m deep basin situated between the Oder Lagoon and the open Baltic Sea. It is known to be an area of mixing of river water and Baltic Sea water and is characterized by intermediate salinities (3.3-) (Lampe, 1993). The Arkona Basin is a 45 m deep oshore basin in the open Baltic Sea which is thought to be the ®nal deposition zone for the Oder river load (Neumann et al., 1996) and is characterized by relative high salinities in the deepest areas (up to 25-).
INTRODUCTION
In highly industrialized areas such as the Oder catchment area, rivers become stressed by the anthropogenic input of contaminants. An important source of contaminants in the Oder River is the mining industry. Additional inputs come from domestic sewage, agriculture and atmospheric fallout (HELCOM, 1993). The distribution of heavy metals in the sediments of the Oder Lagoon and its oshore basins re¯ects a clear imprint of heavy-metal pollution on the estuarine and open Baltic environments (Neumann et al., 1996). The Oder estuary is not a real estuarine system because the Oder river water enters the open Baltic Sea through a series of shallow lagoons. This results in dierent biogeochemical conditions within the various components of the Oder system and oers the possibility of investigating processes under distinct biogeochemical and hydrological regimes. In this study, 3 key areas were selected to determine the accumulation and mobilization of heavy metals in the dierent environments of the Oder system. The Oder Lagoon is a shallow brackish water environment with low salinities (2-) and water
METHODOLOGY
*Corresponding author. Present address: Institute of Petrography and Geochemistry, University of Karlsruhe, Kaiserstr. 12, 76131 Karlsruhe, Germany. 329
During cruises of project ODER (Oder dischargeenvironmental response) in May and August 1993, sediment cores were taken in order to investigate the biogeochemical processes of anthropogenic contaminants in the Oder environment (Shimmield et al., 1994). Cores IOW # 18036, 18039 and 18033 were taken in the Oder Lagoon (53849.23'N, 14809.11'E, 5.0 m water depth), Achterwasser (54801.90'N, 13857.97'E, 3.7 m water depth) and southern Arkona Basin (54849.50'N, 13839.20'E, 45 m water depth), respectively. Figure 1 shows the study area and the locations of the cores. Sediments were sampled using a Rumohrlot gravity corer. The corer sampled bottom water, an
330
T. Neumann et al.
Fig. 1. Schematic map showing the locations of the sampling stations within the investigated area.
undisturbed surface sediment layer and between 0.3 and 0.8 m of underlying sediment. The cores were subsampled at 10 or 20 mm intervals for bulk analyses. The samples were stored in polyethylene bottles and frozen. Pore waters were taken immediately after retrieval. Sediment cores within the liners were transferred into an Ar-¯ushed glove box to prevent oxidation, extruded and sliced into 10 mm discs. The slices were stored in precleaned polyethylene centrifuge bottles. After centrifuging, the supernatant water was ®ltered through 0.45 mm nucleopore ®lters. Aliquots were used to determine SO4 by HPLC at the University of Edinburgh. Subsamples of pore water were acidi®ed with suprapure HNO3 and kept frozen until analysis. Sediment samples were freeze-dried and divided into <63 mm and >63 mm fractions by dry sieving. The <63 mm fraction was ground in a porcelain mortar and homogenized for chemical analyses. Weighed splits were totally dissolved using HF/HCl and HClO4/HNO3 and concentrations of Mg, Ca, Al, Fe Mn, Zn, Cu and Pb determined by ICP-AES (Varian Liberty 200). Manganese, Fe, Cu, Zn and Pb were determined in pore waters using operating conditions which were optimized for each element and each type of sample. Data quality was con-
trolled by repeatedly analyzing reference materials ABSS and MBSS (ICES, 1987). Certi®ed multi-element ICP-standard solutions were used for calibration. Analytical precision was generally better than 25%. For S and total organic C (TOC) analysis, an Eltra CS-analyzer was used. 0.1 N HCl was added to the samples (20±150 mg) to remove carbonates. The samples were oxidized at 14008C and measurements made by infra-red adsorption.
RESULTS
General characteristics of the sediments The muddy sediments of the investigated areas are characterized by a light brown, ¯uy, oxic layer 10 to 40 mm thick (oxidation zone), overlying a soft, black anoxic layer 200 to 400 mm thick (reduction zone). This then passes into a dark grey± green mud (fermentation zone). The mud is often interbedded with shell layers, particularly in the lagoonal areas and with sand and silt layers in the higher energy regions of the Oder Rinne. Grain-size analysis shows that the bulk of the sediment in the area lies within the 6.3 to 63 mm range correspond-
Heavy-metal enrichment in Oder river discharge area
ing to medium to coarse silt. There is usually no preservation of ®ne scale depositional structures such as lamination within the sediments as a result of intense bioturbation in the upper layer. Scanning electron microscopy (SEM) and X-ray-investigations show biogenic calcite to be present in the oxidation zone, Fe-sulphides in the reduction zone and Fe-rich chlorite and pyrite in the fermentation zone. The high amount of total organic C (TOC) (up to 14 wt% in the lagoonal sediments) re¯ects the very high ¯ux of organic matter into the sediment resulting from the enormous discharge of nutrients into the region (Lampe, 1993; Lippert, 1993). The TOC content of the sur®cial sediments decreases from the lagoon to the open Baltic and has values of about 5% in the Arkona Basin. Concentration pro®les with depth in the sediments reveal contrasting behaviours of TOC and S. TOC is enriched at the sediment surface whereas S increases with depth in the sediment column. Sulphur contents reach values up to 6% in estuarine sediments and 2% in o-shore basin sediments. SEM-investigations suggest that the high sulphur contents result from the presence of Fe-sulphides.
Heavy metals in sediments Heavy-metal concentrations in the sediments display a wide range of values, but their distributions with depth are generally similar throughout the study area (Figs 2±4). Manganese shows highest concentrations in the surface sediments whereas Fe increases towards the lower parts re¯ecting the occurrence of Fe-sulphides in the deeper sediments. Copper, Zn and Pb generally show large and often sharp decreases in concentration 10 to 20 mm below the sediment surface whereas Co and Ni remain almost unchanged. Below this depth, the pro®les display relatively constant values of these elements which are assumed to represent the natural pre-industrial concentrations. Such distribution patterns have been described during earlier investigations of Baltic Sea sediments (Suess and Erlenkeuser, 1975; Szefer and Skwarzec, 1988; BruÈgmann and Lange, 1990; Belmans et al., 1993) and appear typical for muddy sediments throughout the Baltic Sea. Statistical analysis of the sediment compositional data (taken from the appendix in Neumann et al., 1996) reveals that the heavy metals in the sediments are related to certain components such as total organic C (TOC), Al and S (Table 1). Heavy metals with high enrichments in the sur®cial sediments, such as Cu, Zn and Pb, show strong correlations with the organic matter throughout the study area. These metals are taken up biologically, especially by SiO2 in many lagoons and estuaries, as a result
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by the diatom production during the spring and summer blooms (Kaul and Froelich, 1984; Windom et al., 1991). In addition to these general results, the 3 subareas are distinguished by some typical characteristics which may be described as follows.
Oder lagoon The sediments of the Oder Lagoon contain the highest heavy-metal concentrations within the entire study area (up to 4800 mg/g Mn, 5.9% Fe, 68 mg/g Cu, 865 mg/g Zn and 135 mg/g Pb). There is a high correlation of Fe with Al in the Oder Lagoon sediments, probably because of the high discharge of Fe-rich minerals into it. The positive correlation of Mn and TOC may be explained by early diagenetic processes in which Mn oxides are reduced during oxidation of organic matter. This conclusion is supported by pore-water investigations which show that the dissolved Mn is highest (146 mM Mn) in the uppermost 40 mm of the sediment and decreases with depth to values of about 45 mM Mn (Fig. 2). The dissolved Fe concentration, on the other hand, is very low in the uppermost 100 mm (17.9 mM Fe) but increases with depth to a maximum of about 860 mM Fe at 270 mm. The pore-water SO4 concentration is highest (about 2.0 mM) at the sediment± water interface and decreases to 1.0 mM at a depth of 100 mm. Below 100 mm, the SO4 concentration increases to the level at the sediment surface. Although the Oder Lagoon sediments have the highest heavy-metal concentrations in the entire study area, the pore-water concentrations are much lower than in the Arkona Basin: Cu ranges from 0.03 to 0.30 mM, Zn from 1.93 to 5.19 mM and Pb from <0.01 to 0.09 mM. Pore waters from the Oder Lagoon are enriched in heavy metals, i.e. Zn by a factor of 2 to 3, compared to the bottom-water concentrations of 0.90 mM. This situation is very common and may be explained by the mobilization of heavy metals from organic matter during early diagenetic processes in the suboxic zone in which Fe- and Mn-oxyhydroxides degrade the organic matter.
Achterwasser The heavy-metal contents in the Achterwasser sediments are less than in the Oder Lagoon by factors of 2 to 3 for Cu and Pb and 5 for Mn and Zn. Iron, on the other hand, occurs in concentrations of up to 6.1% in the sediments (Fig. 3). Generally, Fe is positively correlated with S as a consequence of the occurrence of Fe-sulphides in the anoxic sediments. Cobalt and Ni are also positively correlated with Fe and S in Achterwasser sediments re¯ecting
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Fig. 2. Al, TOC, S, Fe, Mn, Cu, Zn and Pb concentrations in the sediments (W) and dissolved sulphate, Mn, Fe, Cu, Zn and Pb concentrations in the pore waters (r) in core 18036 located in the Oder Lagoon.
Heavy-metal enrichment in Oder river discharge area
Fig. 3. Al, TOC, S, Fe, Mn, Cu, Zn and Pb concentrations in the sediments (W) and dissolved sulphate, Mn, Fe and Zn concentrations in the pore waters (r) in core 18039 located in the Achterwasser.
333
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Fig. 4. Al, TOC, S, Fe, Mn, Cu, Zn and Pb concentrations in the sediments (W) and dissolved Mn, Fe, Cu, Zn and Pb concentrations in the pore waters (r) in core 18033 located in the Arkona Basin.
Heavy-metal enrichment in Oder river discharge area
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Table 1. Correlation coecients of elements in the <63 mm-fraction of sediments of the Oder Lagoon, Achterwasser and Arkona Basin. The correlation coecients were calculated from the total data from each of the cores. Signi®cant positive correlations of elements with Al, TOC and S are printed in bold Mn
Fe
Co
Ni
Cu
Zn
Pb
Al
TOC
S
1.00 ÿ0.62 0.71 ÿ0.24
1.00 ÿ0.45 0.69
1.00 ÿ0.45
1.00
1.00 ÿ0.66 0.51 ÿ0.70
1.00 ÿ0.96 0.78
1.00 ÿ0.76
1.00
1.00 0.79 0.84 ÿ0.33
1.00 0.71 ÿ0.57
1.00 ÿ0.45
1.00
Oder Lagoon (18036-9), n = 13
Mn Fe Co Ni Cu Zn Cd Pb Al TOC S
1.00 ÿ0.71 0.91 0.92 0.90 0.90 0.91 0.90 ÿ0.49 0.77 ÿ0.34
Mn Fe Co Ni Cu Zn Cd Pb Al TOC S
1.00 ÿ0.88 ÿ0.88 ÿ0.88 0.65 0.90 0.80 0.80 ÿ0.83 0.80 ÿ0.86
Mn Fe Co Ni Cu Zn Cd Pb Al TOC S
1.00 0.05 ÿ0.34 ÿ0.72 ÿ0.79 ÿ0.77
1.00 0.70 0.38 0.43 0.44
1.00 0.76 0.70 0.72
1.00 0.91 0.90
1.00 0.99
1.00
ÿ0.78 ÿ0.81 ÿ0.57 0.54
0.40 0.29 0.45 0.39
0.68 0.53 0.68 0.09
0.88 0.78 0.91 ÿ0.38
0.99 0.84 0.89 ÿ0.41
0.99 0.80 0.87 ÿ0.34
1.00 ÿ0.72 ÿ0.74 ÿ0.85 ÿ0.85 ÿ0.83 ÿ0.85 0.75 ÿ0.36 0.42
1.00 0.99 0.97 0.96 0.96 0.97 ÿ0.45 0.76 ÿ0.15
1.00 0.98 0.98 0.97 0.98 ÿ0.54 0.82 ÿ0.26
1.00 0.99 0.99 0.99 ÿ0.62 0.71 ÿ0.25
1.00 0.99 0.99 ÿ0.63 0.71 ÿ0.26
Achterwasser (18039-7), n = 15 1.00 0.97 0.84 ÿ0.62 ÿ0.93 ÿ0.78 ÿ0.76 0.87 ÿ0.83 0.98
1.00 0.82 ÿ0.49 ÿ0.91 ÿ0.78 ÿ0.70 0.91 ÿ0.90 0.94
1.00 ÿ0.57 ÿ0.80 ÿ0.80 ÿ0.76 0.69 ÿ0.66 0.83
1.00 0.73 0.73 0.89 ÿ0.52 0.35 ÿ0.55
1.00 0.88 0.87 ÿ0.89 0.82 ÿ0.85
Arkona Basin (18033-4), n = 19
the incorporation of these metals into the sulphide phases. The core is characterized by very low pore-water Fe concentrations of up to 3.1 mM with a maximum concentration of 11.8 mM at a depth of 170 mm. Manganese displays its highest pore-water concentrations (47.6 mM Mn) in the uppermost 20 mm and decreases to a value of about 22.3 mM at a depth of 90 mm. This suggests that early diagenetic processes reduce Mn oxides during the oxidation of organic matter in the uppermost part of the sediment column. The pore-water SO4 pro®le shows highest concentrations (1.8 mM) at the sediment±water interface which decrease with depth to values of about 0.3 mM. Sediments of the Achterwasser are distinguished by having the lowest pore-water heavy-metal concentrations in the entire study area. Copper and Pb are below the determination limit of about 0.01 mM, and Zn ranges from 0.14 to 0.93 mM. By contrast to the Oder Lagoon and Arkona Basin, Zn is present
in higher concentrations in the bottom water (1.10 mM) than in the pore water (0.70 mM) in the Achterwasser. Arkona Basin The lowest concentrations of heavy metals occur in the sediments of the Arkona Basin (up to 450 mg/ g Mn, 3.7% Fe, 38 mg/g Cu, 200 mg/g Zn and 94 mg/g Pb) (Fig. 4). There is a strong relationship between these metals and Al indicating the association of these metals with clay minerals and suggesting that the metals change from being bound mainly to organic matter in the lagoonal system to being bound mainly to clay minerals in the open Baltic Sea. Pore waters from Arkona Basin sediments display the highest Mn concentrations of 81.9 mM in sur®cial sediments at 0±20 mm depth. This concentration decreases exponentially to a value of about
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T. Neumann et al.
2.7 mM Mn at a depth of 400 mm. Because elevated levels of Mn occur only when O2 is completely consumed (Jahnke et al., 1989; Rutgers van der Loe, 1990), these data indicate that the suboxic zone lies within the uppermost 30 mm of the sediment column. The Fe pro®le is similar to that of Mn (from 237 to 1.8 mM Fe), except that dissolved Mn appears at a shallower depth than Fe. Based on free energy considerations, Mn reduction precedes Fe reduction by organic matter (e.g. Froelich et al., 1979). These data can therefore be considered to represent ``normal'' pore-water pro®les in¯uenced by early diagenetic processes. Despite having the lowest heavy metal concentrations in the sediments, the pore-water heavy-metal concentrations are the highest in the entire study area. Copper ranges from 0.13 to 0.58 mM, Zn from 6.32 to 8.81 mM and Pb from 0.31 to 0.54 mM.
DISCUSSION
The high biological productivity in the Oder Lagoon is demonstrated by the very high TOC content in the dried sediment samples (up to 14 wt%). The positive correlations of Cu, Zn, Cd and Pb to TOC re¯ect the high biological uptake of these elements, especially in the Oder Lagoon. The degradation of organic matter in the sediments is well illustrated by the decreasing TOC content with depth in all 3 areas (Neumann et al., 1996) and the maximum dissolved Mn concentrations in the uppermost sediments. The degradation of the organic matter can be considered to be responsible for the remobilization of heavy metals from the organic matter into the pore water. These metals could then diuse either into the bottom water or into the deeper anoxic sediments. In anoxic sediments containing H2S, heavy metals are usually ®xed as sulphides. In general, Fe-sulphide formation is limited by the amount and reactivity of organic matter buried in the sediments, and the availability of reactive Fe and of SO2ÿ 4 necessary for the production of H2S (Emeis and Morse, 1993). In the brackish waters of the Oder Lagoon with salinities of about 2-, the SO2ÿ 4 content is very low and may be the limiting factor in Fe-sulphide formation. Inspite of this, the S content of the Oder Lagoon sediments is relatively high ranging from 1 to 3%, although considerably lower than in the Achterwasser sediments (upto 6%). Higher amounts of sulphide in the sediments of the Achterwasser re¯ect the higher salinity of the waters (3.3-). According to Froelich et al. (1979) and Thamdrup et al. (1994), diagenetic processes lead to the formation of Fe-sulphides below the suboxic/ anoxic boundary. This occurs at a depth of 20 mm in the lagoon sediments and 40 mm in the Arkona
Basin sediment. This situation is not observed in the Fe and S pro®les of the lagoonal areas of the Oder Lagoon and Achterwasser where Fe and S concentrations in the sediments increase signi®cantly with depth. Historical changes of the sedimentary environment as well as more recent processes, such as leaching of the sur®cial sediments by O2-rich waters or advective ¯ux of water through the sediment column, could explain this observation. Rapidly changing salinity levels are characteristic only of the northern part of the Oder lagoonal system including the Achterwasser. Lampe (1993) documented changes in the salinity of the Achterwasser waters which are directly aected by sea-level ¯uctuations in the Baltic. The increase of salinity in the Achterwasser takes place over a short period (i.e. a few days) followed by a slow decrease over a longer period (i.e. a few months) corresponding to a slow increase in the fresh-water contribution. The in¯ow of the more saline Baltic Sea water into the Achterwasser increases the SO4 content of the waters and may result in pyrite formation. During times of intense Fe-sulphide formation in the Achterwasser, various heavy metals such as Co and Ni are scavanged from the pore water and ®xed in sulphides. This leads to heavy-metal concentrations in the pore waters which are signi®cantly lower than those found in the Oder Lagoon and Arkona Basin. Iron-sulphide formation in the Oder Lagoon and Arkona Basin seems to be much weaker than in the Achterwasser and the heavy metals can diuse into the bottom waters after their mobilization from the organic matter during early diagenetic processes. For these reasons, we suppose that the sur®cial sediments represent a potential source of heavy metals in the Oder Lagoon and Arkona Basin, but a sink for heavy metals in the Achterwasser, at least seasonally during salt-water in¯ow.
AcknowledgementsÐThe authors would like to thank all colleagues of Project ODER for very constructive cooperation and discussions. We are also grateful to Dagmar Benisch and Reinhild Rosenberg (Institute of Baltic Sea Research at WarnemuÈnde, IOW) for assistance in sample collection and to Tim Brand (University of Edinburgh) for determining the sulphate contents of pore waters. The captains and crews of F. S. Alkor, F. K. BornhoÈft, F. S. A. v. Humboldt and F. S. Professor A. Penck are thanked for their help. This research was supported ®nancially by the European Commission (ENVIRONMENT PROGRAMME, Grant #PL-910398) and by the BMBF (Project#03F0101A). Editorial handling: G. P. Glasby and P. Szefer.
Heavy-metal enrichment in Oder river discharge area REFERENCES Belmans F., van Grieken R. and BruÈgmann L. (1993) Geochemical characterization of recent sediments in the Baltic Sea by bulk and electron microprobe analysis. Marine Chemistry 42, 223±236. BruÈgmann L. and Lange D. (1990) Metal distribution in sediments of the Baltic Sea. Limnologica 20, 15±28. Emeis K. C. and Morse J. W. (1993) On the systematics of carbon±sulphur±iron ratios in sediments of upwelling areas. Geol. Rundsch. 82, 604±618in German. Froelich P. N., Klinkhammer G. P., Bender M. L., Luedtke N. A., Heath G. R., Cullen D., Dauphin P., Hammond D., Hartmann B. and Maynard V. (1979) Early oxidation of organic matter in pelagic sediments of the eastern equatorial Atlantic: Suboxic diagenesis. Geochim. Cosmochim. Acta 43, 1075±1090. HELCOM (1993) Second Baltic Sea pollution load compilation. Balt. Sea Environ. Proc., Vol. 45, 161 pp. ICES (1987) Report on the results of the Baltic sediment intercalibration exercise. ICES Coop. Res. Rep., Vol. 147, 92 pp. Jahnke R. A., Emerson S. R., Reimers C. E., Schuert J., Ruttenberg K. and Archer D. (1989) Benthic recycling of biogenic debris in the eastern tropical Atlantic Ocean. Geochim. Cosmochim. Acta 53, 2947±2960. Kaul L. W. and Froelich P. N. (1984) Modeling estuarine nutrient geochemistry in a simple system. Geochim. Cosmochim. Acta 48, 1417±1433.
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Lampe R. (1993) Environmental state and material ¯ux in the western part of the Oder river estuary: Results and consequences. Petermanns Geogr. Mitt. 137, 275±282. Lippert, C. (1993) Discharge of nutrients in the Oder estuary. Diploma Thesis, University of Greifswald (in German). Neumann T., Leipe T., Brand T. and Shimmield G. (1996) Accumulation of heavy metals in the Oder estuary and its o-shore basins. Chemie der Erde 56, 207±222. Rutgers van der Loe M. M. (1990) Oxygen in porewaters of deep-sea sediments. Philos. Trans. R. Soc. London 331, 69±84. Shimmield, G. and Oder-Project-Members (1994) Project ODER-Interim Report, January 1994. EC Environment Programme PL 910398, Brussels, 127 pp. Suess E. and Erlenkeuser H. (1975) History of metal pollution and carbon input in the Baltic Sea sediments. Meyniana 27, 63±75. Szefer P. and Skwarzec B. (1988) Distribution and possible sources of some elements in the sediment cores of the Southern Baltic. Mar. Chem. 23, 109±129. Thamdrup B., Fossing H. and Jorgensen B. B. (1994) Manganese, iron and sulphur cycling in a coastal marine sediment, Aarhus Bay, Denmark. Geochim. Cosmochim. Acta 58, 5115±5129. Windom H., Byrd J., Smith R., Hungspreugs M., Dharmvanij S., Thumntrakul W. and Yeats P. (1991) Trace metal±nutrient relationships in estuaries. Mar. Chem. 32, 177±194.