Experimental results on effects of capping on fluxes of persistent organic pollutants (POPs) from historically contaminated sediments

One option proposed for the remediation of Oslo harbour, Norway, has been dredging of up to 700 000 m 3 of contaminated sediments and deposition at 60 m depth in a nearby intermittently anoxic fjord basin. Contaminant retention in the deposit area will be favoured by metal sulphide precipitation and the current absence of sediment dwelling organisms. In addition capping will be performed to protect against future fjord recovery and recolonisation with benthic animals. In order to assess the risk of contaminant leakage from the deposit area a long-term mesocosm experiment was performed to simulate the three expected successive phases: (1) anoxic basin water, (2) oxic water and (3) oxic water and presence of bioturbators. Fluxes of dissolved, bioavailable contaminants (PAHs, PCBs and DDTs) were measured by uptake in passive membrane samplers (SPMDs) located in the source seawater and downstream 0.25 m 2 boxes with capped and non-capped harbour sediments. The experiment showed that pyrene and fluoranthene was released from no-cap treatments at high rates (2545 and 830 pmol m − 2 day − 1 , respectively) during the initial, anoxic phase. The majority of components showed, however, maximum fluxes during the final, bioturbation phase: 243 pmol m − 2 day − 1 sumKPAH (six of the most cancerogenous PAHs), 19.6 pmol m − 2 day − 1 of sumPCB 7 and 13.6 pmol m − 2 day − 1 DDT. Cap layers reduced fluxes and bioaccumulation of PAH in the gastropode Hinia reticulata by 89–100%. Fluxes and bioaccumulation of PCBs was not reduced to the same extent suggesting that external sources may be relatively more important for PCB than for PAH levels in this organism. A minor release of PAH-components from one of the cap treatments indicated slight contamination from fossil fuel or explosives applied during production of the machined sand used as cap material for this treatment. Differences due to cap thickness variations between 10 and 50 cm were not found throughout the experimental period of 34 months.