Thin layer capping of fjord sediments in Grenland. Chemical and biological monitoring 2009-2013

Summary

A field experiment on thin layer capping was initiated in the Grenland fjords in September 2009. A primary objective of the field experiment was to assess the capacity of the different cap designs to reduce bioavailability of dioxins as well as the disturbance and recovery expected of the benthic habitat and macrofauna communities. The test fields were investigated in samples collected in 2009, shortly after capping, and in 2010, one year after capping. The bioavailability was determined in box-cores transferred from the test fields to a mesocosm laboratory for ex situ measurements of uptake of dioxins in sediment-living organisms and passive samplers exposed in the overlying water. The results showed that caps containing activated carbon effectively reduced the bioavailability of dioxins, but adverse effects were found on benthic communities. In one of the two test fields treated with activated carbon, the community severely deteriorated during the first year after capping. In order to follow the further succession of the benthic community and the degree of sustainment of dioxin immobilization, extended monitoring was performed in 2012 and 2013, 3-4 years after cap placement. Here we report the results from this latter investigation and compare with the results obtained in the previous investigations. The new results confirmed maintenance of positive effects of activated carbon on the bioavailability of dioxins. The benthic habitat appeared to have improved since 2010, but adverse effects were still present in the macrofauna communities, in particular at one of the test fields treated with activated carbon at which recovery appeared to have stagnated at about two years after cap placement.