Anoxic sediments as a source of persistent organic pollutants: the role of organic carbon and iron reduction
Mineralogy, Petrology and Geochemistry
Final Report Abstract
Pollution of the Baltic Sea by persistent organic pollutants (POPs) is of great environmental concern and there is a need to understand processes of transport and transformation of these compounds in this environment. Sediments constitute a significant reservoir for POPs but can also act as a source of these chemicals to the water column. Changing redox conditions can alter sorption characteristics and may therefore influence the partitioning of organic compounds between sediment and water phase. The major Baltic inflow events of oxic water in 2014-2016 oxidized the long-term anoxic Gotland deep and offered a good opportunity to study the effect of changing redox conditions on sediment POPs. During the time of the project, several Baltic inflow events occurred and changed the conditions under which the Gotland sediments were planned to be investigated. During the wintertime 2015/2016 two additional Major Baltic Inflows of moderate intensity followed the first one in December 2014. The development of the inflow situation allowed now to collect sediments from different cruises with different redox regime and different history of ventilation and hence to assess the effect of different redox conditions on the distribution of POPs. We sampled sediments for POP analysis before the inflow situation, during the first inflow and at the end of the third inflow at different locations within the Gotland basin but also in the Farö deep which stayed anoxic during the investigations. The samples are still stored to be analyzed as the results from the field study are thought to complement the laboratory incubation results. In laboratory sediment slurry incubations we investigated the effect of oxygen (anoxic vs. oxic) on the partitioning between sediment and water phase of polychlorinated biphenyls (PCBs) and Hexachlorocyclohexanes (HCHs). In anoxic incubations, more of the PCB congeners were found in the water phase than in oxic incubations, hence, the anoxic sediment was less sorptive for the compounds than oxic sediment. The main cause was the higher pH in anoxic incubations, likely due to microbial reduction activity. Partitioning of HCHs was less affected by the different treatments. However, in anoxic incubations, biodegradation of compounds was considerably stronger than in oxic incubations, especially for γ-HCH. Surprisingly, the sediment-water partitioning factors for the investigated PCBs were not linearly related to the octanol-water partitioning factors. Especially for the higher chlorinated PCBs, no increase in sediment-water partitioning factors with increasing chlorination state was observed. We could show that changing redox conditions have an influence on the partitioning of PCBs between sediment and water phase. The outcome of the field study will show if these processes are also relevant for field conditions and the situation of the Baltic inflows with its ventilation of anoxic basins in the Baltic.
