In situ remediation of contaminated sediments using thin-layer capping : effectiveness in contaminant retention and ecological implications

Abstract: Hydrophobic organic contaminants (HOCs) often reside in sediment sorbed to particles, most tightly to particles with high content of organic carbon. If persistent, such pollutants can accumulate in the sediment for many years and constitute a contamination risk for sediment-living organisms and organisms at higher trophic levels, including humans. Awareness of the problem has brought many countries to start remediation initiatives to clean up sediments. Since traditional remediation techniques are associated with complications (e.g. release of contaminants, disruptive to benthic communities), or constraints (handling of large amounts of contaminated material, expensive, size and depth limitations), there is a need for new effective alternatives. In situ remediation through thin-layer capping (a few centimeter cover) with a sorbing material such as activated carbon (AC) has been proposed as an alternative remediation method. Compared to traditional remediation techniques, AC amendment in a thin layer means less material handling and lower costs and is assumed to be less disruptive to benthic communities. The objectives of this thesis are to investigate the ecological effects from thin layer capping (TLC) as well as the efficiency in contaminant retention. In Paper I we studied the ecological effects from nine different materials, proposed for TLC remediation, on benthic structural (Bacteria, Macro- and meiofauna) and functional responses (e.g. O2 oxygen respiration and nutrient fluxes). In Paper II we investigated the potential of thin layer capping with activated carbon in reducing aqueous contaminant concentrations and bioaccumulation of HOCs in two different benthic organisms exposed to intact cores from an in situ pilot remediation project. The results in Paper I show that all the tested materials can disturb the benthic system, but to different degrees. Activated carbon showed minor or intermediate disturbances on functional endpoints, however,  the species abundance was reduced by ca 50 %, which is worrying. Nevertheless, the bioaccumulation study in Paper II shows that sediment capped with a thin layer of clay amended with activated carbon led to great reductions in the uptake of PAHs and PCBs by the benthic fauna as well as reduced aqueous concentrations in pore water . In conclusion, this thesis shows positive results concerning the contaminant retention potential of TLC with AC, but short term and long term ecological effects on benthic fauna have to be investigated before this technique can be recommended in large scale capping projects.