86 research outputs found
Large-Scale Field Study on Thin-Layer Capping of Marine PCDD/F-Contaminated Sediments in Grenlandfjords, Norway: Physicochemical Effects
A large-scale field experiment on in situ thin-layer
capping was
carried out in the polychlorinated dibenzodioxin and dibenzofuran
(PCDD/F) contaminated Grenlandsfjords, Norway. The main focus of the
trial was to test the effectiveness of active caps (targeted thickness
of 2.5 cm) consisting of powdered activated carbon (AC) mixed into
locally dredged clean clay. Nonactive caps (targed thickness of 5
cm) consisting of clay without AC as well as crushed limestone were
also tested. Fields with areas of 10 000 to 40 000 m<sup>2</sup> were established at 30 to 100 m water depth. Auxiliary shaken
laboratory batch experiments showed that 2% of the applied powdered
AC substantially reduced PCDD/F porewater concentrations, by >90%
for tetra-, penta- and hexa-clorinated congeners to 60–70%
for octachlorinated ones. In-situ AC profiles revealed that the AC
was mixed into the sediment to 3 to 5 cm depth in 20 months. Only
around 25% of the AC was found inside the pilot fields. Sediment-to-water
PCDD/F fluxes measured by in situ diffusion chambers were significantly
lower at the capped fields than at reference fields in the same fjord,
reductions being largest for the limestone (50–90%) followed
by clay (50–70%), and the AC + clay (60%). Also reductions
in overlying aqueous PCDD/F concentrations measured by passive samplers
were significant in most cases (20–40% reduction), probably
because of the large size of the trial fields.The AC was less
effective in the field than in the laboratory,
probably due to prolonged sediment-to-AC mass transfer times for PCDD/Fs
and field factors such as integrity of the cap, new deposition of
contaminated sediment particles, and bioturbation. The present field
data indicate that slightly thicker layers of limestone and dredged
clay can show as good physicochemical effectiveness as thin caps of
AC mixed with clay, at least for PCDD/Fs during the first two years
after cap placement
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