Comparing total and accessible concentrations of hydrophobic organic contaminants in sediments and suspended particulate matter in the Danube River

Contamination of aquatic ecosystems by hydrophobic organic contaminants (HOCs) is often assessed based on their concentrations in riverbed sediment and suspended particulate matter (SPM). However, total HOC concentration (C-TOT) in sediment or SPM is of limited value for evaluating the exposure of benthic or pelagic organisms. The accessible HOC concentration (C-AS) presents a useful parameter quantifying the overall pool of HOC in sediment or SPM available for fast partitioning to the water phase or biota. We applied a novel approach of ex situ sequential equilibrium partitioning with silicone elastomer sampler at a high sampler/SPM phase ratio to measure C-AS of HOC in SPM from the Danube River. We compared C-TOT and C-AS in SPM and surface layer sediment collected at the same sites to evaluate whether HOC monitoring in the two matrices provides equivalent information on environmental quality. At most sites, there was a good agreement and correlation of organic carbon (OC)-normalised C-TOT in SPM and sediment for polychlorinated biphenyls (PCBs) and the majority of organochlorine pesticides (OCPs). In contrast, C-TOT of polycyclic aromatic hydrocarbons (PAHs) in SPM were up to a factor 10 lower in SPM than in sediment. Site-specific differences of OC-normalised C-AS concentrations in SPM and sediments were observed for PCBs and OCPs, with accessibility mostly lower in SPM than in sediment. The highest accessibility in SPM was observed for PCBs, ranging between 15 and 30%. The accessibility of OCPs varied from 0 to 23%. SPM and riverbed sediment samples provide complementary but not mutually interchangeable information on HOC contamination.

Automated summary

The contamination of aquatic ecosystems by hydrophobic organic contaminants (HOCs) is often evaluated based on their concentrations in riverbed sediment and suspended particulate matter (SPM). However, the total HOC concentration (C-TOT) in sediment or SPM has limited value in assessing the exposure of benthic or pelagic organisms. The accessible HOC concentration (C-AS) provides a useful parameter for quantifying the overall pool of HOC available for partitioning to the water phase or biota. This study compared C-TOT and C-AS in SPM and sediment samples collected from the Danube River and found that they provide complementary but not interchangeable information on HOC contamination.