Large-Scale Field Study on Thin-Layer Capping of Marine PCDD/F-Contaminated Sediments in Grenlandfjords, Norway: Physicochemical Effects

Abstract

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(2) 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.

Response of marine benthic fauna to thin-layer capping with activated carbon in a large-scale field experiment in the Grenland fjords, Norway

“…Species with activities at the sediment-water interface would be the ones most exposed to the fine particles of AC used in this study and accordingly, it is primarily the suspension and surface deposit feeding species that were the most affected in our study. Mixing of the AC in clay together with loss of AC particles and a further mixing and dilution in the bottom sediment resulted in a final 2% AC dry weight concentration in the AC+clay treatments (Cornelissen et al 2012). Thus, the small particle size of the AC, rather than the concentration, may explain the severity of the deleterious effects on benthic communities in this study.…”

“…Clay was suction-dredged at 10 m depth in the inner part of the Ormerfjord, where the top 10 cm of the sediment was discarded to obtain a clean clay sediment (TOC 1.8%; <1 ng kg −1 TEQ) (Cornelissen et al 2012). For the AC+clay treatment, powdered AC (Jacobi Carbons, BP2 fine powder; average particle size of 20 μm; 80% smaller than 45 μm; TOC 76%) was mixed in a 1:10 dw/dw ratio with clay (the same as in the Clay-30) in a large tank on the ship.…”

“…The achieved thickness of the caps were after 1 month measured to 11 ± 6 and 12 ± 3 mm for AC+clay-30 and AC+clay-95 respectively, 37 ± 11 mm for Clay-30, and 21 ± 12 mm for Lime-30 (Eek et al 2011). After 9 months, the final AC concentration was 2% by dry weight of sediment measured in the AC-treated fields (Cornelissen et al 2012). A more detailed description of how the capping materials were applied and their effects on reducing contaminant fluxes are presented in Cornelissen et al (2012, 2015).…”

“…The moderate amount of material needed in thin-layer capping makes the method particularly suitable for remediation of large and deep areas. Activated carbon has a strong sorption capacity for hydrophobic organic contaminants (HOCs) (Cornelissen et al 2005; Grathwohl and Kleineidam 2000; Luthy et al 1997), and thin-layer capping with AC has proven to be an efficient method to decrease the bioavailability and the sediment-to-water fluxes of HOCs such as PCBs, PAHs, dioxins, and furans (Beckingham and Ghosh 2011; Cho et al 2009, 2007; Cornelissen et al 2012, 2011; Josefsson et al 2012; Kupryianchyk et al 2013b; Lin et al 2014; McLeod et al 2007; Millward et al 2005; Samuelsson et al 2015; Zimmerman et al 2004, 2005). …”

“…The ecological effects on the benthic macrofauna were measured 1 and 14 months after capping. The capping efficiency on sediment-to-water fluxes have been presented in Cornelissen et al (2012, 2015). Compared to other studies with thin-layer capping on marine sediment, this field experiment was carried out on a much larger scale and at greater depth.…”