Environ. Sci. Technol. 1998, 32, 1754-1759
Occurrence, Sedimentation, and Spatial Variations of Organochlorine Contaminants in Settling Particulate Matter and Sediments in the Northern Part of the Baltic Sea B O S T R A N D B E R G , * ,† B E R T V A N B A V E L , † PER-ANDERS BERGQVIST,† D A G B R O M A N , ‡ R A S H A I S H A Q , ‡,§ C A R I N A N A¨ F , ‡ H A R A L D P E T T E R S E N , ‡,§ A N D CHRISTOFFER RAPPE† Institute of Environmental Chemistry, Umea˚ University, S-901 87 Umea˚, Sweden, and Aquatic Chemical Ecotoxicology, Department of Zoology and Department of Analytical Chemistry, Stockholm University, S-106 91 Stockholm, Sweden
Sediment trap sampling of settling particulate matter (SPM) was carried out in remote coastal and offshore stations in the Bothnian Bay (BB) and the Bothnian Sea (BS) located in the northern Baltic Sea. This was done to investigate occurrence, composition profiles, sedimentation, and spatial differences of PCBs, HCHs, hexachlorobenzene, DDTs, chlordane compounds, and dieldrin. The contamination load at the coastal stations was compared to that of surface bottom sediments. At one coastal station, sediment samples were collected in two different seasons (autumn and spring) in order to compare seasonal variations. All compound groups were found in the SPM and sediment samples analyzed. The contaminant concentrations in SPM at the coastal stations ranged, on dry weight basis, from 0.41 to 8.6 ng/g for chlordanes and HCHs and at the offshore stations from 3.2 to 31 ng/g for hexachlorobenzene and PCBs. The levels in SPM at the offshore stations were 10 times higher (on the average) than the coastal stations, likely because of an increased carbon content in the offshore SPM. The profiles of compounds in SPM reflect the corresponding sediment profile, and only small differences in concentrations and profile of compounds between autumn and spring surface sediments were observed. A comparison between the BS and the BB for offshore and coastal stations showed that the concentrations of compounds were similar although the sedimentation of contaminants, estimated by down fluxes collected in the sediment traps per unit of time, was 3-5 times higher in the BS than the BB. The total annual sedimentation volume of PCBs was approximately 1.1 ton/yr in the BS and 0.4 ton/yr in the BB. An extrapolation to total sedimentation of PCBs during 1 year in the whole Baltic Sea resulted in an approximate value of 7 ton/yr. * To whom correspondence should be addressed. Telephone: +46 90-786 5672; fax: +46 90-186155; e-mail: Bo.Strandberg@ chem.umu.se. † Umeå University. ‡ Department of Zoology, Stockholm University. § Department of Analytical Chemistry, Stockholm University. 1754
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 12, 1998
Introduction Organic environmental pollutants such as hexachlorocyclohexanes (HCHs), hexachlorobenzene (HCBz), DDT and its metabolites (DDTs), chlordane-related compounds (CHLs), dieldrin, and polychlorinated biphenyls (PCBs) are distributed globally by transport through air and water (1). The Baltic Sea is reported to contain high levels of halogenated organic compounds (2-4). Most literature is however focused on the levels of these compounds in organisms or in sediments. In the aquatic environment, these compounds can be dissolved in the water phase or associated with particles depending on their hydrophobicity and organic carbonwater partition coefficients (KOC) (5, 6). Particulate matter in the aquatic environment can be of natural origin like dead biological material (detritus) and living plankton organisms or have an antrophogenic origin (5). Sediment trap sampling of settling particulate matter (SPM) during a longer period of time reflects the contamination in the water column (7, 8). Since atmospheric input often dominates in remote water areas and since there is limited biotransformation on particles and in plankton (9), the levels in SPM give an indication of the situation in the air. Sediment trap fluxes are calculated as the total amount of matter or compounds obtained in the traps per unit of time. This includes contributions from resuspension of bottom sediments. An estimation of down fluxes of SPM associated compounds is of great interest for the general fate of organic pollutants, since sediments often serve as the final sink for these contaminants. Extrapolation of such flux estimations gives an indication of the contamination load in the region under study. Earlier reports using sediment traps in the Baltic Sea region have shown to be useful to estimate loads of polychlorinated dibenzodioxins and -furans (PCDD/ Fs) and polycyclic aromatic hydrocarbons (PAHs) in remote as well as antrophogenically influenced aquatic environments (5, 10, 11). Furthermore, SPM is of importance for the introduction of compounds to a variety of organisms, which can accumulate through ingestion or uptake over respiratory membranes (5). In the present study, the occurrence and profiles of organic contaminants in SPM and bottom surface sediments in remote coastal and offshore waters in the Bothnian Sea and Bothnian Bay are studied as well as spatial variations in terms of fluxes of these compounds in the sediment traps. An attempt is made to use the obtained fluxes to estimate the total sedimentation of organochlorine compounds in the Bothnian Bay, the Bothnian Sea, and the total Baltic Sea.
Experimental Section Sampling Area Characteristics, Sampling Sites, and Sampling Techniques. The Baltic Sea is a shallow (mean depth 55 m) basin covering 412 560 km2. It consists of a series of basins, most of them separated by shallow areas in combination with sills. It is linked with the Atlantic Ocean through the Danish Straits (Figure 1). The northernmost of these basins are the Bothnian Bay (BB) (mean depth of 41 m), the Bothnian Sea (BS) (mean depth of 62 m), followed by the Baltic Proper. The BB and BS are enclosed by Sweden and Finland, and the catchment area is relatively large (270 000 km2 for BB and 230 000 km2 for BS). A number of rivers with approximately the same water volume run into the BB and BS. The region is sparsely populated (∼2 million), but both the BB and the BS are surrounded by several industrialized cities on both the Swedish and Finnish side. S0013-936X(97)00789-X CCC: $15.00
1998 American Chemical Society Published on Web 05/06/1998
1991 and in spring 1992. The coastal stations are remote and probably they have a good water exchange with the open sea. The HF station has however a relatively high influence of freshwater due to the outflow of several rivers in that area. Characteristics of sampling procedures including sampling time, geographical coordinates and depths, and placement of the traps are given in Table 1. A detailed description of the sediment traps and sampling techniques were presented elsewhere (12, 13). In short, the sediment traps were self-buoyant and bottom anchored with cylindrical glass collection vessels mounted in PVC cylinders. Chloroform was added to the glass vessels for preservation of the trapped settling matter. The samples of surface bottom sediment (0-1 cm) were taken with a modified Ponar grab sampler, which allows free water passage through the sampler during descent and sediment penetration.
FIGURE 1. Sampling stations in the Baltic Sea and a section bottom topography from the Bothnian Bay in the northern Baltic Sea through the Danish Straits to Skagerrak. The length of the section is approximately 2300 km. Due to its location in high latitudes, relatively poor water mixing, and low salinity, the BB and sometimes also the BS can be ice-covered for up to 6 months a year. Therefore, the biological productive season in this northern part of the Baltic Sea is only 4-5 months as compared to 8-9 months in the southern Baltic Proper. The primary phytoplankton production is only 25 g of C m-2 yr-1 in the BB and 110 g of C m-2 yr-1 in the BS as compared to 160 g of C m-2 yr-1 in the Baltic Proper (9). Sediment traps were positioned in coastal and offshore stations in the BB and the BS (Figure 1). Surface bottom sediments were sampled in the coastal stations, at Harufja¨rden (HF) in autumn 1991, and at Simpna¨s (SN) in autumn
Cleanup procedures and chemical analysis. The sample cleanup procedure used was a multiresidue method aimed at simultaneously determining numerous organochlorine and polyaromatic hydrocarbon compounds (14). The analytical steps in this sample cleanup as well as the parameters for the analytical instrumentation are thoroughly described by van Bavel et al. (15). Briefly, the samples with additional blanks were placed in a cellulose timbles and wet extracted in a Soxhlet (Dean Stark) apparatus with toluene (24 h) followed by a mixture of hexane and acetone (59/41, 24 h). Prior to extraction, the chloroform was evaporated from the SPM samples, and an internal standard mixture containing 13C-labeled compounds was added. The internal standards used for quantitation of native compounds in this study were 13C-labeled lindane (γ-HCH), HCBz, p,p′-DDT, dieldrin, and PCB IUPAC Nos. 80 and 153. After evaporation of the solvents used for extraction, the residue was weighed to determine the amount of extractable organic matter. The dry weight of the samples was determined, as the weight of the sample left in the Soxhlet timbles after extraction. The total carbon content (TC) was determined in a subsample using an element analyzer (Carlo Erba EA 1108) according to standard procedures. The extracted organic phase was enriched using a polyethylene film dialysis technique (16). A total of 10% of the dialysis permeate was used for analysis of organochlorine pesticides and bulk (Cl3-Cl10) PCBs. The 90% fraction was analyzed for mono- and non-ortho-PCBs, PCDD/ Fs, PCNs, and PAHs, which will be presented elsewhere. The 10% fraction was further cleaned on a Florisil column (17). Prior to analysis, tetradecane as keeper and a 13C-labeled recovery standard (PCB 101) was added. The solvent was evaporated to the final volume of 30 µL of tetradecane. Analysis and detection were accomplished using highresolution gas chromatography/low-resolution mass spectrometry (HRGC/LRMS). The MS instrument was a Fisons MD800 operating in electron impact (EI) mode using selected
TABLE 1. Characteristics of Sediment Trap and Bottom Surface Sediment Sampling station
SR5 (BS) sediment trap (n ) 1)a
F9 (BB) sediment trap (n ) 1)
SN (BS) sediment trap (n ) 1)
HF (BB) sediment trap (n ) 2)
SN (BS) sediment (n ) 4)
SN (BS) sediment (n ) 4)
HF (BB) sediment (n ) 3)
sampling time (from) (to) latitude (N)
Jan 17, 1991 Mar 7, 1992 61°05′03′′
May 13, 1991 Nov 18, 1991 64°42′50′′
Jun 16, 1992 Jun 12, 1993 59°54′90′′
Aug 1991
Apr 1992
Aug 1991
59°54′90′′
59°54′92′′
65°26′90′′
longitude (E)
19°32′94′′
22°04′32′′
19°10′92′′
19°10′92′′
19°10′90′′
22°55′70′′
water depth (m)
115
116
41
41
41
25
placement of trap from bottom (m)
15
15
10
Aug 17, 1991 Aug 26, 1993 65°34′72′′ 65°34′86′′ 22°45′70′′ 22°45′95′′ 24 26.5 10
a
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TABLE 2. Concentrations (ng/g dw) of Organochlorines in Settling Particulate Matter (SPM) and Sediments (SED)a offshore stations station sample
SR5 (BS) SPM (n ) 1)c
F9 (BB) SPM (n ) 1)
R-HCH β-HCH γ-HCH HCHs
11 4.3 7.1 22
16 2.1 4.1 23
HCBz
3.4
3.2
coastal stations SN (BS) SPM (n ) 1)
HF (BB) SPM mean (n ) 2)
SN (BS) SED (spr 1992) mean (n ) 3)
HF (BB) SED (au 1991) mean (n ) 3)
2.5 1.2 1.3 5.0
3.5
