Elsevier

Applied Geochemistry

Volume 18, Issue 2, February 2003, Pages 327-338
Applied Geochemistry

An investigation of geochemical factors controlling the distribution of PCBs in intertidal sediments at a contamination hot spot, the Clyde Estuary, UK

https://doi.org/10.1016/S0883-2927(02)00128-2Get rights and content

Abstract

The concept that total organic C (TOC) is the main factor dominating the sorption of PCBs to sediment is over simplified. Numerous discrepancies are found when trying to compare concentrations predicted from laboratory observations to field concentrations. Some studies show a lack of correlation between PCB and TOC or particle size, but state that it is the origin of the organic matter or the clay swelling that is most important in determining the partitioning to sediment. It may also be argued that the discrepancies are merely a reflection of localised inputs of PCBs. An evaluation of the influence of these factors was undertaken at an intertidal site in the Clyde Estuary, previously highlighted as being highly contaminated by PCBs. Analysis of a series of sediment samples failed to show a strong correlation of PCB content with TOC or particle size. Separation into grain-size fractions and subsequent analysis suggested that both variation in organic matter source and mineralogical composition exerts an influence on congener distribution with implications for the mobility of PCBs within intertidal sediments.

Introduction

Organic compounds of anthropogenic origin such as Polychlorinated Biphenyls (PCBs) have entered the marine and estuarine systems by atmospheric transport and deposition, direct and indirect discharges, and via riverine inputs. Upon entering the aquatic system, PCBs partition between water, air, sediment, particulate matter and biota (Mackay, 1989). PCBs can be referred to as “particle-reactive” chemicals due to their low vapour pressures, low solubilities and corresponding high octanol–water partition coefficients (Shui and Mackay, 1986). In the aquatic system, a two phase Freundlich model has conventionally been used to predict or describe the equilibrium abundance of an organic pollutant in the sediment column from concentrations in associated water and vice versa (Burgess et al., 1996a). The ratio of the concentration of a chemical (e.g. PCB congener) in sediment to the concentration of the same chemical in water is referred to as the partition coefficient (Kp). It has been demonstrated that the sorption of non-polar compounds in laboratory experiments is correlated to the organic C content of the sorbent (Karichkoff et al., 1979, Means et al., 1980, Schwarzenbach and Westall, 1981). As a consequence, the equilibrium partition concept for hydrophobic organic contaminants is normalised to the percentage of organic C, giving the equation:Kp=Koc.focwhere Koc is the normalised C partition coefficient and foc is the weight fraction of organic matter (% organic C/100) (Kolemans et al., 1997). This is based on the assumption that organic C from different sources has similar affinities for PCB congeners (Ferraco et al., 1990).

Several authors have discussed the reliability of using the partition coefficient in the marine environment to predict the fractionation of the organic pollutant between sediment and water. Coakley et al. (1993) investigated the vertical and spatial trends of PCBs in sediment and showed no correlation to TOC but directly related the concentrations to changes in anthropogenic sources. Achman et al. (1996) found no correlations between total PCBs and TOC but a difference was noted for the varying types of TOC found, suggesting that the origin of the TOC must be taken into account. Tyler et al. (1994) investigated PCDD/Fs in the Clyde and suggests that concentrations were better correlated to lipids in the Clyde area. More recent studies in the Clyde (Balls et al., 1997, Hess, 1998, Edgar et al., 1999) could not identify a strong correlation between PCBs and TOC or particle size.

There is further complexity in understanding PCB behaviour from observations that within the sediment column, redistribution of PCBs can be influenced by the concentration of colloidal organic matter (Burgess et al., 1996b, Butcher et al., 1998, Pedersen et al., 1999) which limits the usefulness of the partition coefficient model. In addition, evidence from laboratory and field studies, suggests that these effects are related to the degree of chlorination of PCB congeners and estuarine mixing conditions (Baker et al., 1986).

From earlier work (Edgar et al., 1999) a site was identified with variable total PCB concentrations, comparable with the most contaminated systems world-wide and with typically low proportions of TOC and fine grained sediments. The objective of this study was to further investigate this site and assess the significance of TOC and particle size factors on the distribution PCBs within the sediments. Some previous work in the UK coastal waters (Thompson et al., 1996) and the Mediterranean Sea (Peirard et al., 1996) identified the variable role of different sediment grain size fractions in releasing and sorbing PCBs. Thus the effect of variation in sediment composition on PCB mobility has important implications for predicted “bioaccumulation factors” and would have serious implications for the management of the aquatic ecosystem (Maund and MacKay, 1998). The advantage of selecting a contaminated site was that physical fractionation of samples would be possible; leaving quantifiable PCB concentration levels in the sub samples.

A set of 10 surface sediment samples were collected by hand from the Battery Park, on the southern shore of the inner Clyde estuary, Greenock, Central Scotland (Fig. 1). The samples were collected at low tide on the 15th May 1998 and stored at −20 °C until freeze-dried. They were subsequently analysed to determine concentrations of PCB congeners, TOC and the % of sediment <63 μm. The sediment at the site is well mixed and a fresh bulk sediment sample was collected in June 1998 at the hot spot shown in Fig. 1. This bulk sample was separated into 8 size fractions (4000–2000; 2000–1000; 1000–500; 500–250; 250–180; 180–95; 95–64;<64 μm) by sieving the freeze-dried sediment (Thompson et al., 1996) and the new samples were analysed for PCB concentrations, % TOC and the mineralogy determined by powder XRD.

For particle size analysis a subsample was removed and analysed using a Malvern Mastersizer E particle size machine. The remaining sediment was homogenised by mortar and pestle. A subsample of 10–20 mg was used for total organic C (TOC), determined using a Perkin-Elmer CHN Elemental Analyser (model 2400) after acidification with 14% HCl to remove carbonate. Previous work on sediment-PCB distribution has used a variety of markers for organic material influence and the limitations of TOC are acknowledged in this study. However, TOC, supported by C/N ratios does offer a method to probe source characteristics (Muller and Mathesius, 1999, Muller and Voss, 1999) and was considered appropriate for this initial investigation, following the observations made in earlier work at this site (Edgar et al., 1999).

Details of the analytical procedures used for PCBs are given elsewhere (Edgar et al., 1999). To summarise, freeze dried sediment samples for PCB analysis (1–10 g) were placed in pre-extracted thimbles along with CB 209 (Promochem, Welwyn Garden City, Herts) used as an internal standard and soxhlet extracted overnight with methyl tertiary butyl ether. Activated Cu powder (Allrich, 40 Mesh, 99.5% Kupler, pulver) was also present during the extraction to remove any S present. After extraction, the samples underwent clean up procedures by alumina and silica columns (Wells et al., 1985). The extracts were then reconstituted in 2,2,4 trimethylpentane and two internal standards, 2,4-dichlorobenzyl alkyl ethers with alkyl chain C6 and C16, were added. Determination of 22 chlorobiphenyls congeners; CB 31, 28, 52, 49, 44, 74, 70, 101, 110, 149, 153, 105, 138, 158, 187, 128, 156, 157, 180, 170, 189, and 209, was carried out by GC-ECD with a HP5 60 m column. Calibration of the GC was by an internal standard method using point calibration. Data were collected and analysed by the VG Minichrom system (Duinker et al., 1991, Wells, 1992).

The quality of PCB data was monitored as part of the routine laboratory quality control scheme (Kelly and Campbell, 1995). The within batch coefficient of variation, derived from the analysis of replicate samples was 10% for PCB congeners and between batch variation control was 21%. No problems of contamination were found in any method blanks. In addition regular analyses of a certified reference material (National Research Council of Canada) was performed to check accuracy, and results were within acceptable limits. A laboratory reference material was also used, which was a bulk sediment sample from the Garroch Head dump site (Wells and Kelly, 1991). Sample detection limits based on the lowest standard are shown in the tables of data and are generally <0.06–0.09 μg/kg dry sample weight. Variability is due to sample intake weight, and is suitably low in this study.

The 8 size fractions from the bulk sediment sample were analysed by powder XRD, with a preliminary assessment undertaken at the University of Wales, Cardiff, and a follow up, high resolution study at the Natural History Museum, London. The initial screening analysed the sediment samples on a Nonius powder diffractometer with an INEL PSD, using CuKα radiation against a quartz standard. It showed the mineralogy to be significantly different in the sediment <500 μm compared to that >500 μm and led to further studies at the Natural History Museum, London using identical equipment. Quantification of the mineral content in each sample was by a rapid phase quantification method, developed at the Natural History Museum, London (Batchelder and Cressey, 1998, Rodgers and Cressey, 2001), and demonstrated to overcome many of the sample preparation difficulties encountered in the quantification of mineral phases in environmental samples by XPD. A sensitive and reproducible analysis allowed quantification to a few % level (see Table 4).

Section snippets

PCB distribution in bulk sediment samples

Individual CB concentrations from the 10 bulk samples taken from the Battery Park are given in Table 1 and show the area to be heterogeneous with values ranging from 0.10 to 1670 μg kg−1. The distribution of PCB concentration is variable over a few metres, suggesting that the characteristics of the sediment within the Battery Park may have accounted for the differences in concentrations (Fig. 1). Table 2 shows the % TOC values for the 10 samples, with the lowest % TOC value being 0.5% and the

Conclusion

For the Clyde Estuary area, no significant correlation has been observed between total PCB and TOC or particle size. Whether this is due to the influence of multiple input sources or variations in sediment characteristics is unclear and requires further investigation. However initial work at one site (the Battery Park, Greenock), highlights the role of sediment geochemistry, which may be more important than is presently thought. It suggests that the differences in organic matter composition as

Acknowledgements

The authors are grateful to the University of Paisley and FRS Marine Laboratory for financial support to P.J.E. through a PhD studentship. They also wish to acknowledge the assistance of the Crystallography Group, University of Wales, Cardiff (now at the University of Southampton) and the Natural History Museum, London for provision of powder XRD analysis and useful discussions.

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