Equilibrium sampling of HOCs in sediments and suspended particulate matter of the Elbe River
Chemical quality of sediment and suspended particulate matter (SPM) is usually assessed by total chemical concentrations (Ctotal). However, the freely dissolved concentration (Cfree) is the ecologically more relevant parameter for bioavailability, diffusion and bioaccumulation. In recent studies, equilibrium sampling has been applied to determine Cfree of hydrophobic organic contaminants (HOCs) in the sediment pore water, whereas such data are missing for SPM. We applied solid-phase micro-extraction to measure and compare Cfree of PAHs and PCBs in pore water of sediments and SPM sampled along the German part of the river Elbe. Moreover, site-specific distribution ratios were evaluated and Cbio,lipid was predicted using Cfree.
Cfree of PAHs remained largely constant while Cfree of PCBs varied along the Elbe River. The highest Ctotal of PCBs and PAHs were found at Prossen (km 13) and Meißen (km 96). PCB Ctotal even exceeded the environmental quality standard for sediment and SPM in Prossen. Site-specific distribution ratios (KD) revealed a stronger sorption for PAHs compared to PCBs, indicating a higher availability of PCBs. Equilibrium partitioning concentrations in lipids (Clip↔sed) showed a high correlation with actually measured lipid-normalised concentrations (Cbio,lipid) in bream. This indicates that PCB bioaccumulation in this benthic fish species is closely linked to the sediment contamination.
In rivers, SPM functions as a transportation vehicle for HOCs along the stream until it eventually deposits to the sediment. This study demonstrates that due to weaker sorption of PAHs and PCBs to the SPM this matrix poses a higher risk to the aquatic environment compared to the sediment. The prediction of Cbio,lipid of PCBs was correct and shows that solid-phase micro-extraction is highly suited to predict lipid concentration, and thus a valuable tool for risk-assessment or sediment management.
KeywordsPassive sampling SPME Cfree HOCs SPM Sediment Elbe River
lipid-normalised concentration of PCBs in Abramis brama
freely dissolved concentration in the pore water of the sediment or suspended particulate matter
lipid concentration calculated from a potential equilibrium between Cfree of the sediment pore water and a generic lipid
concentration in the PDMS
total concentration in the sediment or suspended particulate matter
German Environment Specimen Bank
environmental quality standard
hydrophobic organic chemicals
site-specific distribution ratio (Ctotal/Cfree)
partitioning coefficient between a generic lipid and PDMS
partitioning coefficient between a generic lipid and SSP-M823
partitioning coefficient between PDMS and SSP-M823
partitioning coefficient between PDMS and water
polycyclic aromatic hydrocarbons
suspended particulate matter
total organic carbon
Water Framework Directive
Another parameter that completes the picture of bioavailability and accessibility is the site-specific distribution ratio (KD). KD is the ratio of Ctotal and Cfree and states the sorption strength of a contaminant to the sediment or SPM. To assess bioaccumulation, recent studies used equilibrium partitioning concentrations in lipids (Clip↔sed) for PCBs [3, 4]. These were obtained from equilibrium sampling in sediment and showed a high correlation with actual measured lipid-normalised concentrations in biota.
In the present study, the river Elbe was selected as study area. The Elbe River rises in the Czech Republic and flows through Germany where it enters the North Sea at Cuxhaven. In total, the length of the Elbe River is 1094 km. Although a majority of the Elbe flows through Germany, the main contribution to the polychlorinated biphenyl (PCBs) contamination of the Elbe originates from the Czech Republic . Another big issue is the Hamburg Harbour and the sediment management. Due to increasing draught of ships and sedimentation, Elbe sediments are regularly dredged to maintain the economic performance of the Hamburg Harbour. Moreover, as one of the largest rivers in Europe, the Elbe significantly contributes to the pollution load of the southern North Sea. There have been several monitoring programmes, however, only Ctotal was monitored. In several studies equilibrium sampling using SPME fibres has been successfully applied to determine Cfree of HOCs in the sediment pore water [2, 6], whereas such data are missing for SPM. As one of the important pathways for HOCs to enter the aquatic environment, SPM plays major role to understand the partition in the riverine system.
Provide the first dataset of Cfree of HOCs in SPM and compare these data with sediments.
Predict the internal PCB concentrations in biota using equilibrium sampling in sediment.
Show the downstream pattern of Ctotal and Cfree of the sediment/SPM pore water.
Identify site-specific distribution ratios (KD) and the influence of total organic carbon (TOC) and soot.
SPM bulk samples were collected in September 2014 and treated according to Ricking et al. . Briefly, sedimentation boxes are permanently installed in federal monitoring stations with a flow rate of 8–10 L min−1. For sampling, supernatant water was removed and the remaining SPM was sieved (2 mm), homogenised and subsequently frozen on site. Frozen samples were stored in stainless steel containers and transported to the depot of the German Environment Specimen Bank (ESB) in a nitrogen vapour freezer. SPM samples were filled in amber glass bottles and stored at + 4° C until analysis. The geographical positions of the sampling stations are listed in the Additional file 1: Table S2.
Exhaustive extraction of sediments and SPM (C total)
Total concentrations of seven PCBs (PCB 28, 52, 101, 118, 138, 153, 180) and 16 PAHs (naphthalene, acenaphthylene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene, pyrene, benzo[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, indeno[cd-1,2,3]pyrene, dibenzo[a]anthracene and benzo[ghi]perylene) were determined in sediment samples. Freeze-dried sediments were sieved (< 2 mm) and ground. Approximately 5 g freeze-dried sediment of the original homogenised grab sample was extracted in a pressurised liquid extractor (ASE 200, Dionex, Idstein, Germany). Reduced extracts were cleaned with a copper–aluminium oxide column and subsequently fractionated with gel permeation chromatography using a ShodexTM CLNpakPAE 800 AC 8.0 × 300 mm column (Shodex Buisness, Showa Denko Europe GmbH, Munich, Germany) and acetone as eluent. The collected fractions were analysed by gas chromatography with triple quadrupole mass spectrometric detection (Chromtech Evolution Systems, Chromtech GmbH, Idstein, Germany).
Total concentrations of seven PCBs (PCB 28, 52, 101, 118, 138, 153, 180) and 16 PAHs (naphthalene, acenaphthylene, acenaphthene, fluoren, phenanthrene, anthracene, fluoranthene, pyrene, benzo[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, indeno[cd-1,2,3]pyrene, dibenzo[a]anthracene and benzo[ghi]perylene) were determined in SPM samples. For this, SPM samples were freeze-dried using a Christ freeze drier (Alpha 1–4 LD plus). The extraction was performed according to Witt et al.  with minor modifications. Briefly, 3–4 g of freeze-dried samples was extracted using an accelerated solvent extractor (ASE 2000, DIONEX). The samples were filled in ASE cartridges with an internal standard (PAH Mix-9 deuterated, Dr. Ehrenstorfer; PCB Mixture EC-4058, Cambridge Isotope Laboratories) and extracted with 60 mL acetone–hexane (v/v: 40/60) at 140 bar and 100 °C. The concentrated extracts were fractionated by HPLC with a silica gel column (MERCK, LiChrospher Si 100-5). Elemental sulphur was removed with 0.5 g activated copper. The copper was activated with HCl and then cleaned with methanol, acetone and n-hexane. The extracts were then concentrated to 500 µL and subsequently measured using a GC–MS.
Total organic carbon (TOC)
For the analysis of TOC, 100–700 mg of freeze-dried sediment/SPM was acidified with 1 mL hydrochloric acid (1 M) for 3–4 h and subsequently measured in a TOC analyzer (Eltra Helios, Eltra, Neuss, Germany). CNS content was measured with a CNS analyzer (Vario Macro, Elementar, Hanau, Germany) by weighing 20–120 mg of freeze-dried sample, depending on the TOC content, and adding 120% tungsten (VI) oxide for quantification of sulphur. In the CNS analyzer, the sediment was quantitatively decomposed into gaseous compounds by high temperatures. The sample was cleaned, separated in its components and elements were quantified by respective detectors.
Equilibrium sampling (C free)
Cfree of seven PCBs (PCB 28, 52, 101, 118, 138, 153, 180) and 12 PAHs (phenanthrene, anthracene, fluoranthene, pyrene, benzo[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, indeno[cd-1,2,3]pyrene, dibenzo[a]anthracene and benzo[ghi]perylene) were determined in pore water of sediment and SPM samples. Equilibrium sampling of HOCs in sediment and SPM was performed as described previously [2, 6] with minor modifications. Briefly, 7–10 g of thawed and homogenised sediments or SPM was filled in 12-mL round-bottom glass vials with polytetrafluoroethylene (PTFE) septum caps. Three 10-cm SPME fibres (Fiberguide Industries Inc.) with a 10-µm PDMS coating were placed in each vial. All vials were agitated on an overhead shaker in darkness at 20 °C for 14 days. SPME fibres were removed and immediately cleaned with Milli-Q water, dried with lint-free tissue and stored at − 20 °C in pre-cleaned aluminium foil. The HOCs in the fibres were analysed using thermal desorption in a GC–MS system. After the measurement, analyte concentrations in PDMS (CPDMS) were calculated based on the measured length and the specific volume of the fibre (0.0877 µL cm−1). Cfree was then calculated with Eq. 2 using PDMS to water partitioning coefficients (KPDMS:water) obtained from Witt et al. .
Site-specific distribution ratios (K D)
PCB concentrations in muscle tissue from Abramis brama (Cbio) in ng kg−1 wet weight (ww) and fat contents (%) were obtained from the ESB. These fish samples were collected in 2014 during a monitoring and at the same sampling stations of this study (Additional file 1: Table S1). Since PCBs mainly accumulate in lipid, Cbio was normalised to the lipid content (Cbio,lipid) in µg kg−1 lipid.
Partitioning ratios between lipid and the silicone SSP-M823 (Speciality Silicone Products Inc.) (Klip:SSP) were obtained from Jahnke et al.  and ratios between PDMS fibres and SSP (KPDMS:SSP) were published by Gilbert et al. .
Total concentrations (C total) in sediment and SPM
In both matrices, the highest concentrations of PCBs and PAHs were found at Prossen (km 13) and Meißen (km 96) with a decreasing trend downstream the River Elbe. Regarding the sums of PAHs and PCBs, there was no substantial difference in contaminant levels between sediment and SPM samples, except for Prossen which had elevated PCB concentrations in SPM of 169 μg kg−1 which is twice the Ctotal of the sediment with 79 µg kg−1. At this station, the environmental quality standard of 20 μg kg−1 (for single PCBs in sediment and SPM, German Surface Water Regulation ) is exceeded for PCB 138 and 153 in the sediment and for PCB 138, 153 and 180 in the SPM. Single concentrations of Ctotal of sediment and SPM samples are available in the Additional file 1: Tables S3–S6.
Freely dissolved concentrations (C free) in sediment and SPM
Sorption of HOCs
Influence of TOC and soot
Linking C free and C biota
Several studies have shown a close link between sediment contamination and bioaccumulation of PCBs in different benthic organisms [3, 4, 13, 14]. The authors applied coated jars to measure the concentration in the PDMS (CPDMS) with sediment samples und used this to predict the concentration in the lipid of fish (Clip) using partitioning coefficients between silicone and lipid.
Total concentrations (C total) in sediment and SPM
The exceedance of the environmental quality standard in sediment and SPM at Prossen indicates a recent contamination near the German–Czech border. These findings agree with a monitoring programme by the RBC Elbe that revealed an exceedance of the EQS (20 µg kg−1 in sediment and SPM, WFD) since 2012 with an increasing trend. This PCB contamination mainly consists of higher chlorinated PCBs, which are presumed to originate from a Škoda plant near Prossen.
Freely dissolved concentrations (C free) in sediment and SPM
Cfree of PCBs and PAHs determined in the present study in sediments from the Elbe was in the same order of magnitude compared to sediment data from the Elbe [2, 4]. For PCBs and PAHs, the highest Cfree in sediment and SPM was found in Meißen, while the highest Ctotal was found in Prossen. This demonstrates that a high Ctotal does not necessarily result in high Cfree values. This indicates a stronger sorption of HOCs to the sediment in Prossen which is supported by high site-specific distribution ratios determined in the present study (Fig. 3). Cfree of PCBs is further comparable to levels in sediments from the Baltic Sea . Cfree of PAHs was further comparable to sediment data from the Oslo Harbor , whereas they were six times higher compared to data from the Baltic Sea . Consequently, the PCB and PAH contamination level in the river Elbe can be characterised as moderately elevated.
Sorption of HOCs
Up to now, this is the first study that used SPME to measure Cfree of HOCs in SPM. Although it is rather unlikely, that SPM and the surrounding water will establish an equilibrium under realistic conditions, the ratio of Ctotal and Cfree (KD) helps to understand the partitioning of HOCs between SPM and water and thus the sorptive behaviour to SPM. For SPM, Fig. 4 shows a weaker sorption and generally more scattered KD values compared to the sediment. For the sediment, Fig. 4 demonstrates stronger sorption of PAHs compared to PCBs. These different sorptive behaviours of PAHs and PCBs to the sediment are in agreement with other studies Bucheli and Gustafsson  and Koelmans et al. . Jonker and Koelmans  suggested that this results from different chemical structures of PAHs and PCBs. They concluded that pore sorption in soot is the main factor for PCBs and PAHs. Additionally, PAHs are able to from non-covalent interactions because of their planar structure. This indicates that when there are more possibilities for PAHs to develop a bond, the sorption of PAHs to the sediment is stronger. We suggest that these different sorption capacities in both matrices result from ageing processes during and after sedimentation of SPM to the sediment.
Influence of TOC and soot
The findings of the present study agree with those of Koelmans et al. , who concluded a saturation of the soot content. When the soot content is saturated, HOCs would then bind to the TOC content. Finally, we demonstrated that PCBs and PAHs prefer binding to soot over TOC and thus that soot-like material is highly suited for remediation applications in aquatic systems.
Linking C free and C biota
For all sampling sites, we found a high correlation of Cbio,lipid and Clip↔sed indicating that bioaccumulation of PCBs in this benthic fish species is linked to the sediment contamination and moreover to the bioavailable fraction (Cfree) rather than Ctotal. This was previously demonstrated by Schäfer et al.  for Elbe sediments. However, in the present study, Clip↔sed in Neuenfelde was slightly higher compared to the lipid-normalised PCB concentrations in bream. The equilibrium partitioning theory assumes that the equilibrium cannot be exceeded but if so, it could indicate biomagnification . However, since Clip↔sed is normalised to a generic lipid reflecting an ideal organism results might slightly differ. Previous studies showed that the majority of predictions of Cbio,lipid were below or at equilibrium and did not markedly exceed the equilibrium [4, 10, 3, 13]. Moreover, considering that the Cfree and Cbiota were investigated independently, these findings demonstrate that passive sampling is in general suitable to predict the internal contamination of organisms living close to or in the sediment.
In this study, we applied equilibrium sampling to compare PCB and PAH contamination of SPM and sediment from the German part of the River Elbe. We demonstrate that Ctotal of PCBs and PAHs clearly showed a decreasing trend downstream the River Elbe in SPM and sediment whereby the decrease was stronger in the SPM matrix. Cfree remained largely unchanged for PAHs and varied for PCBs. This again underlines that Ctotal does not reflect the actual exposure. The highest Ctotal was measured in Prossen and exceeded the environmental quality standard (EQS of the German Surface Water Regulation) for Ctotal of PCBs in sediments and SPM. We further observed similar Ctotal in sediment and SPM (except for Prossen). However, in SPM HOCs were better available due to a weaker sorption, which indicates that SPM poses a higher risk to the aquatic environment compared to the sediment. The correlation of Ctotal to the TOC and soot content revealed a strong preference for soot and we further demonstrated that the soot content in Elbe sediments was saturated with PAHs. Moreover, the present study confirms the close link between sediment contamination and bioaccumulation, which further demonstrates that equilibrium sampling is highly suited to predict lipid concentration, and thus a valuable tool for risk-assessment or sediment management. In summary, parameters derived from equilibrium sampling (Cfree/chemical activity, KD, Clip↔sed) are valuable for today’s sediment assessment and should be a part of the sediment management.
Phenanthrene, anthracene, fluoranthene, pyrene, benzo[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, indeno[c,d-1,2,3]pyrene and benzo[ghi]perylene.
GW, SS and NN developed the basic idea. NN performed the passive sampling experiments and the total extraction of SPM samples. JB and CM performed the total sediment extraction. NN did the data analysis and wrote the draft of the manuscript. All authors contributed on specific aspects. All authors read and approved the final manuscript.
We thank Julia Bachtin (BfG) for technical support. Sediment samples from Prossen were collected and analysed within the AnPassa project funded by the German Environment Agency (UBA; 713 22 230). We thank Jan Koschorreck (UBA) for the cooperation and providing the fish data and Kendra Majewski for proof reading of the manuscript. Last of all, we would like to express our gratitude to the reviewers for the valuable comments on the manuscript.
The content of this manuscript has not been submitted or published before.
The authors declare that they have no competing interests. All authors approved the submission of this manuscript.
Availability of data and materials
Measured concentrations (Ctotal/Cfree) are available in the Additional file 1.
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Ethics approval and consent to participate
This study was funded by the German Federal Ministry of Education and Research (BMBF; 03F0669D).
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