METALS IN SEDIMENTS FROM THE STOCKHOLM REGION: GEOGRAPHICAL POLLUTION PATTERNS AND TIME TRENDS. 1. Introduction

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1 METALS IN SEDIMENTS FROM THE STOCKHOLM REGION: GEOGRAPHICAL POLLUTION PATTERNS AND TIME TRENDS JOHN STERNBECK and PER ÖSTLUND Swedish Environmental Research Institute (IVL), Stockholm, Sweden ( author for correspondence, john.sternbeck@ivl.se; fax: +46(8) ) (Received 4 February 1999; accepted 31 May 2000) Abstract. The city of Stockholm is intersected and surrounded by water. Freshwater flows eastward and mixes with slightly brackish water in central Stockholm to finally reach the Baltic Sea. In order to estimate the magnitude and geographical impact of Stockholm as a source of metals to the aquatic environments, sediments from 117 stations in the Stockholm region have been analysed for metals (As, Cd, Co, Cr, Cu, Hg, Ni, Pb and Zn). This paper reports on the geographical pollution pattern and on the time trends in metal pollution in Stockholm during the 20th century. Stockholm is a strong source of Cd, Cu, Hg, Pb and Zn whereas the Cr concentrations are moderately perturbed. In contrast, the concentrations of As, Co, and Ni are in general close to preindustrial levels. The concentrations of Cd, Cu, Hg, Pb and Zn are highest in central Stockholm and eastwards (i.e., in the water flow direction). Sediments upstream of Stockholm and in lakes that are not hydrologically connected to central Stockholm show a significantly lower impact. This may be indicative of metals emitted in Stockholm mainly being transported by the water route. Of Cd, Hg, Pb, and Zn, Cd shows the most pronounced decline in accumulation rates over the last two decades. Keywords: metals, sediments, time trends, urban 1. Introduction Metal emissions in urban areas are frequently attributed to traffic, corrosion of buildings and constructions, sewage, waste incineration and coal combustion. The metals are generally primarily emitted to air (e.g. Kowalczyk et al., 1982; de Miguel et al., 1997) and water (e.g. Flores-Rodriguez et al., 1994). Finally the metals accumulate in soils and, since cities often are located in connection to water, also in sediments. The sediments can therefore provide time integrated fingerprints of aquatic environments, at least for metals that have a reasonably strong affinity to natural particles. The sedimentary record can be used to investigate how emissions of metals vary both geographically and chronologically. A number of studies on dated sediment cores from different geographical areas demonstrate that the concentrations of several metals, e.g. Cd, Cu, Pb and Zn, have decreased during the last ca yr (e.g. Bollhöfer et al., 1994; Callender and Van Metre, 1997; von Gunten et al., 1997). This decrease is generally preceded by a period of increasing metal concentrations, starting during the 19th century and in some cases much earlier (e.g. Renberg et al., 1994). Water, Air, and Soil Pollution: Focus 1: , Kluwer Academic Publishers. Printed in the Netherlands.

2 152 J. STERNBECK AND P. ÖSTLUND A large scale anthropogenic impact on the sedimentary metal concentrations in the Baltic proper started after the second world war, with the atmosphere as a major pathway for metal pollution (Hallberg, 1991). Since the seventies, however, the atmospheric deposition of most metals has decreased in the Nordic countries (Rühling et al., 1996). Both Cd and Cu display a declining trend in the water column of the Baltic Sea during the period (HELCOM, 1996). In general, the declining metal levels are a response to several measures that have been taken to reduce emission of metals to the ecosystems: (i) the use of many metals (e.g. Cd, Pb and Hg) has become regulated by the authorities; (ii) incineration and combustion plants are equipped with more efficient flue gas cleaning techniques; and (iii) the establishment of efficient sewage plants. Whether metal concentrations in sediments of a large city such as Stockholm respond to these measures on a time scale of one or a few decades is, however, not obvious. For instance, (i) the road traffic intensity has increased and (ii) large amounts of metals are stored in buildings and constructions and may be released by corrosion. The aim of this study is to use the sedimentary record to give a comprehensive view of metal pollution in a large city (Stockholm, ca one million inhabitants). Nine metals and arsenic have been analysed in a large number of sediment samples from 117 stations, located in central Stockholm and its surroundings. Furthermore, the pollution is put into a time frame by the use of dated sediments cores. The possible sources of metals in these sediments are only briefly discussed in this paper, since other papers in this volume address this subject. 2. Study Area The city of Stockholm is located at the threshold between a large freshwater lake (Mälaren) in the west, and the Baltic Sea in the east (Figure 1). Lake Mälaren flows eastward, mixes with slightly brackish water in central Stockholm and flows through a wide archipelago before reaching the open Baltic proper, which is the largest subbasin in the Baltic Sea. HELCOM (1996) has classified Stockholm as a main problem area regarding metals. A large number of small lakes are located in the surroundings and these are generally not hydrologically connected to central Stockholm. Therefore, we have divided the investigated stations in two groups: (1) central Stockholm including Lake Mälaren and (2) the other lakes. Sediments were sampled at 117 stations (the location of these are presented further on), 81 of which were situated in central Stockholm or immediately west or east of this area. The remaining 36 stations are located in 12 lakes, mainly situated west and south of central Stockholm.

3 METALS IN SEDIMENTS FROM THE STOCKHOLM REGION Materials and Methods Sampling was performed during October 1996-May Positions were determined using differential GPS (DGPS). Sediment cores were retrieved with a gravity corer (inner diameter 5 cm) and sectioned in the field into 2 cm slices. Three cores were retrieved at each station and the samples from each level were mixed. The mixed samples were stored in PP bottles at 4 C in the dark until analysis. The sediment cores covered on the average the upper 40 cm (range 4 72 cm). Most sediment cores were analysed at three depths, the surface, the middle and the lower section of each core and 394 samples were analysed in total. Sediments from 13 stations in central Stockholm were analysed at six depths and these sediments were also dated, together with sediments from 7 lake stations, see below. All samples were freeze-dried and analysed for As, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb and Zn following digestion with HNO 3 (1:1) at 2 atm (ca 120 C) according to the Swedish standard method. Certified reference materials was used (Table I). Mercury was analysed with CV-AAS, As with flame-aas using hydride generation, and the other metals using either flame- or graphite furnace AAS. The laboratory is accredited for these analyses. Sediment dating was performed using the 210 Pb method. This procedure also allowed the sediment accumulation rates to be determined. Freeze-dried samples were analysed for 210 Pb, 214 Pb and 137 Cs using γ -spectrometry. Supported 210 Pb was determined by assuming radioactive equilibrium with 214 Pb. The CRS method (e.g. Oldfield and Appleby, 1984) was used at 14 stations because it appeared that sediment accumulation rates showed significant variations with sediment age. For the other stations the CIC method (e.g. Oldfield and Appleby, 1984) was used. Porosity variations was accounted for in the calculations. 137 Cs was dispersed during the Chernobyl accident in 1986 and is thus frequently used as a marker for checking the precision of 210 Pb-datings. In addition to 137 Cs, PCB was used in the older sediments to verify the 210 Pb-derived ages. PCB was not produced before 1930 and the content of PCB in older sediments should thus be low, controlled by either bioturbative mixing at the time of sediment deposition, or possibly by downward diffusion. Analytical details of PCB are given in Östlund et al. (1998) and the data will be published elsewhere. 4. Results and Discussion 4.1. GENERAL CHARACTERSTICS In the surface sediments, loss on ignition ( organic matter) averaged 30 ± 10% in the lakes and 13 ± 5% in the central Stockholm sediments. Also the water content was generally higher in the lakes than in central Stockholm. A majority of the lakes showed Mn enrichment in the surface sediments, indicating oxic sediment

4 154 J. STERNBECK AND P. ÖSTLUND Figure 1. The Stockholm region. Freshwater is shown in light grey and brackish water in dark grey. The approximate area of central Stockholm is indicated by the circle. TABLE I Recovery of standard reference materials As Cd Cr Cu Hg Ni Pb Zn QC Loam Soil cert. value, µgg A, VKI recovery % n QC Municipal cert. value, µgg Sludge A, VKI recovery % n River sediment cert. value, µgg CRM 320, BCR recovery % n 2 3

5 METALS IN SEDIMENTS FROM THE STOCKHOLM REGION 155 surfaces. Only a few stations in central Stockholm showed this feature. However, anoxia may prevail also in the lakes during the summer season HEAVY METAL CONCENTRATIONS AND GEOGRAPHICAL DISTRIBUTION Metal concentrations are mainly presented and discussed in an aggregated form rather than as single values. All data can be found elsewhere (Östlund et al., 1998) and are presented as µgg 1 dry weight. In general, the concentrations of especially Hg but also Cu, Pb, Zn and to some extent Cd and Cr are strongly elevated as compared to, e.g., surface sediments in the Baltic proper (Table II). Except for Cr, this elevation is even more apparent when comparing the concentrations with preindustrial levels. In contrast, the concentrations of As, Co and Ni are close to preindustrial levels. Those metals that are most significantly affected by anthropogenic activities also show the largest difference between central Stockholm and the lakes. This is illustrated in Figure 2 with median values, 25- and 75-percentiles as a measure of the general level of contamination. Median values, and not mean values, were chosen for comparison of the groups because concentrations generally does not follow normal distribution but rather log-normal distribution. Mercury display a sevenfold elevation in central Stockholm relative to the lakes. This is followed by Pb and Cu (two-fold), Cd, Zn and Cr. There are no pronounced differences between the median concentrations of As, Co or Ni, which is in agreement with the fact that these metals show no evidence of significant anthropogenic contamination. The 90- and 95-percentiles occasionally contrast to this general pattern of contamination. These values reflect point sources and individual lakes rather than the overall level of contamination. Metals in surface sediments pose a certain ecological risk because these metals may be assimilated by zoobenthos or may be mobilised following improving redox conditions. The latter process is most important for Cd and Zn (e.g., Sternbeck et al., 1999). Therefore, the mean and median concentrations of Cd, Cu, Hg, Pb and Zn in the uppermost 4 cm are also presented in Table II. It appears that the difference between all samples and the surface sediments is negligible for the median concentrations. The mean values of Cd, Hg and Zn are, however, significantly lower for the surface sediments. The explanation for these observations is that extremely high concentrations of Cd, Hg and Zn are less frequent in surface sediments compared to deeper layers, but that the overall contamination level still is high in the surface sediments. The time trends in metal accumulation rates will be discussed in chapter 4.4. To examplify the scale of contamination, metal concentrations can be classified into five groups, from very low to very high as suggested by the Swedish Environmental Protection Agency (Swedish EPA, 1999). The criteria concentration for high (including very high) was chosen to represent the highest 5% of samples from a large nation-wide survey of lake sediments. This classification does not give an

6 156 J. STERNBECK AND P. ÖSTLUND Figure 2. The median metal concentrations for the small lakes (n = 108) and central Stockholm including lake Mälaren (n = 286). Samples represent all depths. The bars represent median values and 25- and 75-percentiles, the whiskers represent 10- and 90-percentiles, and the triangles represent 5- and 95-percentiles. absolute measure of the scale of pollution, but rather compares the concentrations with the general state of pollution in Swedish, mainly non-urban, lakes. An element that shows large-scale pollution in the entire Sweden will thus, in absolute terms, be given a higher acceptance than elements whose concentrations are not large-scale anthropogenically affected. According to this classification, 79% of the samples in central Stockholm have Hg concentrations that are high (> 1 µg g 1 ) or very high and many of these samples exceed the criteria for high Hg concentrations by a factor 10 or more. In the lakes, 26% of the samples are high or very high in Hg. Percentages for Cd, Cu, Pb, and Zn are given in Table III. Mercury and Cu deviate most strongly from the general pollution state in Swedish lakes. This does not necessarily imply that Hg and Cu are the metals most strongly elevated relative to natural concentrations, but they are the most typical urban metals.

7 METALS IN SEDIMENTS FROM THE STOCKHOLM REGION 157 TABLE II Mean and median content (µg g 1 d.w.) of metals in this study and regional reference values for comparison. For those metals that are significantly elevated, mean and median concentrations of the surface sediments (0 2 or 2 4 cm) are also given As Cd Co Cr Cu Hg Ni Pb Zn Present study, mean of all samples (n = 394) and of surface sediments (n = 118) Present study, median of all samples (n = 394) and of surface sediments (n = 118) Baltic proper, surface sediments a Baltic proper, preindustrial a Lakes in southern Sweden, regional background b Lakes in southern Sweden, preindustrial b a mean values, Borg & Jonsson (1996); b Swedish EPA (1999). TABLE III The percentage of samples with metal concentrations being classified as high or very high according to Swedish EPA (1999) Cd Cu Hg Pb Zn Central Stockholm 16% 82% 79% 26% 28% Lakes 11% 53% 26% 17% 23% Criteria concentration, µg g 1 dw > 7 > 100 > 1.0 > 400 > 1000 The geographical distribution of Cd, Pb and Hg in surface sediments (0 2 or 2 4 cm, corresponding to ca. 1995) are shown in Figure 3, with the concentrations divided into three intervals. It is apparent that central Stockholm still is a pronounced source of Hg to the aquatic environments, which is also supported by the very high Hg accumulation rates (see chapter 4.3). As already indicated, the lakes surrounding Stockholm display a consistently lower impact regarding Hg whereas sediments east of Stockholm (downstream) display higher Hg concentrations (Figure 3). This pattern indicates that the Hg emitted in Stockholm rapidly reaches the aquatic environment and mainly is transported via the water route. Since Hg

8 158 J. STERNBECK AND P. ÖSTLUND Figure 3. The concentrations of Hg, Cd and Pb in surface sediments (0 2 or 2 4 cm). The approximate area of central Stockholm is indicated by the circle. The smallest lakes are not drawn on the map but are indicated by the concentration symbols. The size of these lakes are roughly represented by the lateral distribution of the symbols. is strongly bound to particles in natural waters, a major transport process may be resuspension. This geographical pattern is also found in the distribution of Cd and Pb (Figure 3) and Cu and Zn (data not shown) in surface sediments, although not as consistent as in the case of Hg. Certain lakes do display strong elevations of Cd, Pb, Cu and Zn concentrations. Because these higher concentrations are restricted to certain lakes it is suggested that these lakes are affected by local sources rather than by large-scale emission from central Stockholm METAL FLUXES An example of the 210 Pb profiles is given in Figure 4. Minor irregularities in certain 210 Pb profiles indicate non-constant sediment accumulation rates. In those cases, this was accounted for by using the constant rate of supply (CRS) model (Oldfield and Appleby, 1984). The calculated sediment accumulation rates were generally higher in central Stockholm (mean values g cm 2 y 1 ; n = 13) than in the lakes (mean values g cm 2 y 1 ; n = 6; one outlier at 0.29 g cm 2 y 1 ).

9 METALS IN SEDIMENTS FROM THE STOCKHOLM REGION 159 Figure 3. Continued.

10 160 J. STERNBECK AND P. ÖSTLUND Figure 4. Example of a 210 Pb profile vs. sediment depth in a sediment core from central Stockholm. The ages are calculated with the CRS method. The right figure shows 137 Cs and accumulated dry mass vs. 210 Pb derived ages. The use of sediment traps in central Stockholm showed annual average fluxes of particulate matter to be g cm 2 y 1 (Broman et al., 1994), in excellent agreement with the sediment accumulation rates calculated here. Sedimentary metal accumulation rates is a better measure of metal fluxes in the environment than mere concentrations, since concentrations of metals depend on the dilution by detrital and organic matter. The accumulation rates of metals was calculated by combining sediment accumulation rates with metal concentrations. This procedure was performed for 13 stations in central Stockholm and Lake Mälaren and for 7 lake stations, and show the variation of fluxes with sediment depth/age. The range and the median values are presented in Table IV together with literature values for comparison. The range of metal accumulation rates in central Stockholm are very similar to settling rates measured by sediment traps (Broman et al., 1994; Table IV). This indicates that sedimentary accumulation rates of these metals reflect fluxes in the water column, and are not significantly affected by diagenetic processes. The geographical pattern that emerged when comparing concentrations between central Stockholm and the lakes (Figure 3) is repeated when comparing fluxes. Actually, when comparing the metal accumulation rates, central Stockholm is even more clearly distinguished from the lakes than when comparing concentrations. The atmospheric wet deposition of metals at a station ca. 40 km north of Stockholm (during 1993) is given (Table IV) in order to show the magnitude of long-range metal transport to Stockholm. It is apparent that this external input of metals is negligible in central Stockholm, even if the gathering effect of the drainage area is accounted for. Wet deposition at one station in central Stockholm is also shown

11 METALS IN SEDIMENTS FROM THE STOCKHOLM REGION 161 TABLE IV Accumulation rates of metals in different sedimentary environments during the 20th century, and deposition rates in the Stockholm region. Results of the present study are given in the first three rows. Since the values in the lakes are far from being normally distributed, median values are given rather than mean values Cd Cu Hg Pb Zn (mg m 2 y 1 ) 13 stations in central Stockholm at c. 1995; ( ) ( ) ( ) (90 870) ( ) median (range) 7 lakes at c. 1995; median (range; outlier a ) ( ; (24 420; ( ) (44 990) ( ) 67) 3500) Total range in central Stockholm during the period Sedimenttraps, annual mean at 2 stations in central Stockholm, b Atmospheric wet deposition in central Stockholm 1996 c Atmospheric wet deposition north of Stockholm 1993 d Lakes in N. Sweden e Kiel Bay, S.Baltic Sea f Oderhaff, east German coast g Finnish forest lakes, h Lake Constance, Germany i City region, U.S.A. j Rural region, U.S.A j 130 a Except for one outlier; b Broman et al. (1994); c SLB-analys (1998); d Munthe (1994); e Renberg (1986); f Lapp & Balzer (1993); g Leipe et al. (1995); h Kähkönen et al. (1998); i Bollhöfer et al. (1994); j Callender and Van Metre (1997); & For Cd and Cu there is one pronounced outlier among the lakes. in Table IV and is apparently neither sufficient for explaining the sedimentary accumulation rates. The present accumulation rates of Cd, Hg and Pb in the lakes are much higher than in more remote lakes in northern Sweden, although Cd and Hg are in good agreement with the rates in certain Finnish forest lakes, some of which were not

12 162 J. STERNBECK AND P. ÖSTLUND TABLE V Present anthropogenic accumulation of metals in central Stockholm and proposed preindustrial concentrations Cd Hg Cu Pb Zn Burial flux 28 ± 8 25 ± ± ± ± 1500 (kg yr 1 ) Preindustrial concentration (µgg 1 ) affected by local point sources (Kähkönen et al., 1998; Table IV). The rate of Hg accumulation in central Stockholm is, however, about one order of magnitude higher. The fluxes of Cu, Pb and Zn are significantly higher in the lakes than in both the Finnish lakes and the coastal stations in the Baltic Sea (Table IV). Lead fluxes in central Stockholm are very similar to values from an urban area in USA (Table IV). In spite of the Swedish measures undertaken to reduce metal emissions, especially for Cd and Hg, large amounts of Cd, Hg, Cu, Pb and Zn are apparently still mobile in the aquatic environments of central Stockholm. In attempt to estimate the annual anthropogenic accumulation of metals in Stockholm sediments, we summarized calculations for each subbasin (data not given in this paper). The percentage of accumulation bottoms in each basin was taken or estimated from Lindstöm et al. (2000). Concentrations were corrected for assumed natural concentrations (Table V), which are generally less than 10% of the present concentrations (Zn: ca 20%). The present annual accumulation of metals in the sediments of central Stockholm (ca. 18 km 2 sediment area) is presented in Table V. Conclusively, the metal accumulation rates of Cd, Cu, Hg, Pb and Zn are generally much higher in central Stockholm than in the surrounding lakes. The anthropogenic load of Cd and Hg to the lakes appears slightly higher than in certain more remote areas, whereas the load of Cu, Pb and Zn is significantly higher. This may be interpreted as that the emissions of Cu, Pb, and Zn are more strongly related to local point sources and densely populated areas than especially Cd is TIME TRENDS IN METAL ACCUMULATION RATES In line with the introductory discussion, we have investigated time trends in metal accumulation rates in the 13 sediment cores from central Stockholm and lake Mälaren. This is accomplished by comparing metal accumulation rates at each sediment depth (samples from six different depths analysed in each core) with the corresponding 210 Pb-age of each depth. We restrict the discussion to the trends of Cd, Cu, Hg and Pb. The dated sediment cores cover the period The results are presented in an aggregated form with mean values calculated for every tenth sample, in

13 METALS IN SEDIMENTS FROM THE STOCKHOLM REGION 163 chronological order (Figure 5). Standard deviations are shown in order to make it possible to evaluate whether the time trends are significant. Since the sediment cores represent different subbasins it would not be expected that the accumulation rates are of a similar magnitude. Therefore, the standard deviations are given with only a 95% confidence interval. Both Cd, Cu, Hg and Pb show generally increasing accumulation rates from 1900 to ca After that period, the same metals show declining trends, although the trend for the whole dataset is statistically significant only for Cd. However if the change in accumulation rates are evaluated separately for each station (data not shown), the following conclusions can be drawn. In 11 of the 13 stations Cd accumulation rates have decreased more than 25% during the period ca to For Cu, Hg and Pb 10 stations show this behaviour. Actually, these four metals have declined much more than 25% at many of these stations. Of the stations not showing a decline, two are common for the four metals. At this moment we can only speculate about the reasons for Cd levels declining more strongly than the other metals. The use of Cd in pigments, stabilizers and as a protective for corrosion was banned during the eighties in Sweden, whereas the use of Hg was regulated 1991 and Pb additives in petrol was banned Although these regulations may contribute to the observed trends, the fluxes generally start to decline earlier (Figure 5). The development of the sewage plants may have had a relatively larger impact on the emissions of Cd. Copper is to a large degree, and Pb to some extent, used in constructions that are exposed to the corrosive atmosphere. Geochemically, Cd is generally considered as a more mobile and less particle associated element than Cu, Hg and Pb. Cadmium may thus be flushed out of the system more rapidly than the other metals. Therefore, erosion of contaminated soils and resuspension of contaminated sediments may contribute with Pb, Hg and Cu for a longer period of time. This explanation is analogous to de Miguel et al. (1997), who showed that following the elimination of petrol-pb, contaminated soil particles were a major source of Pb in urban air. 5. Conclusions We have shown that large amounts of Cd, Cu, Hg, Pb and Zn are emitted in the Stockholm city area, whereas the concentrations of As, Co and Ni are close to preindustrial levels. Copper and Hg are the most urban metals, as compared to results from a nationwide survey of metals in lake sediments. The present anthropogenic accumulation of metals in central Stockholm sediments is estimated to: Cd 28 ± 8kgy 1 ;Cu2.4± 0.7 ton y 1 ;Hg25± 7kgy 1 ;Pb2.4± 0.7 ton y 1 ;and Zn 5.1 ± 1.5 ton y 1. Dispersion of these pollutants appears mainly to occur by transport in the aquatic environment in Stockholm. In lakes upstream or lakes that are not hydrologically connected to central Stockholm, the pollution appears not to be related to central Stockholm. Actually, the anthropogenic fluxes of Cd and Hg

14 164 J. STERNBECK AND P. ÖSTLUND Figure 5. Time trends in the accumulation rates of Cd, Hg, Pb and Cu in the 13 stations located in central Stockholm. The filled symbols represent mean values and standard deviations for consecutive groups of ten samples. One high outlier for Cu is omitted.

15 METALS IN SEDIMENTS FROM THE STOCKHOLM REGION 165 to the lakes appears only slightly higher than in certain more remote areas, whereas the fluxes of Cu, Pb and Zn are significantly higher. The results highlight the importance of avoiding processes that lead to sediment resuspension, e.g. dredging. The accumulation rates of especially Cd show a declining trend during the last 20 yr. Also Hg, Pb and Cu show this trend though not quite as strongly as Cd. Acknowledgements We wish to thank Britta Dusan and Kerstin Hommerberg who carried out the metal analyses. We also thank Tomas Hjorth who assisted during sampling. Financial support was given by the Swedish Environmental Protection Agency and Environment and Health Protection Administration in Stockholm. References Bollhöfer, A. Mangini, A. Lenhard, A. Wessels, M. Giovanoli, F. and Schwarz, B.: 1994, Environ. Geol. 24, 267. Borg, H. and Jonsson, P.: 1996, Mar. Poll. Bull. 32, 8. Broman, D., Lundbergh, I. and Näf C.: 1994, Environ. Poll. 85, 243. Callender, E. and Van Metre, P. C.: 1997: Environ. Sci. Technol. 31, 424A. de Miguel, E. Llamas, J. F. Chacón, E. Berg, T. Larssen, S. Royset, O. and Vadset, M.: 1997, Atmos. Environ. 31, Flores-Rodriguez, J. Bussy, A.-L. and Thévenot, D. R.: 1994, Wat. Sci. Tech. 29, 83. Hallberg, R.: 1991, Ambio 20, 309. HELCOM: 1996, Baltic Sea Envir. Proc. No 64B. Kowalczyk, G. S. Gordon, G. E. and Rheingrover, S. W.: 1982, Environ. Sci. Technol. 16, 79. Kähkönen, M. A. Suominen, K. P. Manninen, P. K. G. and Salkinoja-Salonen, M. S.: 1998, Environ. Sci. Technol. 32, Lapp, B. and Balzer, W.: 1993, Geochim. Cosmochim. Acta 57, Leipe, T. Neumann, T. and Emeis, K.-C.: 1995, Geowissenschaften 13, 470. Lindström, M., Jonsson, A., Brolin, A. and Håkansson, L.: 2000, Water, Air, and Soil Pollution: Focus 1(3 4), Munthe, J.: 1994, Report to the County Administrative in Stockholm, in Swedish. Oldfield, F. and Appleby, P. G.: 1984, Lake Sediments and Environmental History. Leicester University Press. p. 93. Östlund, P. Sternbeck, J. and Brorström-Lundén, E.: 1998, IVL Report B1297 (in Swedish). Renberg, I.: 1986, Hydrobiol. 143, 379. Renberg, I. Wik Persson, M. and Emteryd, O.: 1994, Nature 368, 323. Rühling, Å. Steinnes, E. and Berg, T.: 1996, Nord 1996:37, Nordic Council of Ministers. SLB-analys: 1998, Report Nr 1:98 (in Swedish). Environment and Health Protection Administration in Stockholm. Sternbeck, J., Skei, J. Verta, M. and Östlund P.: 1999, TemaNord 1999:594. Nordic Council of Ministers. Swedish Environmental Protection Agency: 1999, Report Von Gunten, H. R. Sturm, M. and Moser, R. N.: 1997, Environ. Sci. Technol. 31, 2193.