ARTICLE pubs.acs.org/est
Remediation of Contaminated Marine Sediment Using Thin-Layer Capping with Activated Carbon—A Field Experiment in Trondheim Harbor, Norway † Gerard Cornelissen,*,†,‡,§ Marie Elmquist Krusa, Gijs D. Breedveld,†,|| Espen Eek,† Amy M.P. Oen,† † ^ Hans Peter H. Arp, Caroline Raymond, G€oran Samuelsson,^ Jenny E. Hedman,^,# Øystein Stokland,3 and Jonas S. Gunnarsson^ †
)
Norwegian Geotechnical Institute (NGI), P.O. Box 3930 Ulleval Stadium, N-0806 Oslo, Norway ‡ Applied Environmental Sciences (ITM), Stockholm University, 10691 Stockholm, Sweden § Department of Plant and Environmental Sciences, University of Life Sciences, Ås, Norway Department of Geosciences, Oslo University, Oslo, Norway ^ Department of Systems Ecology, Stockholm University, 106 91 Stockholm, Sweden # Department of Contaminant Research, Swedish Museum of Natural History, 104 05 Stockholm, Sweden 3 Marine Bunndyr AS, Trondheim, Norway
bS Supporting Information ABSTRACT: In situ amendment of contaminated sediments using activated carbon (AC) is a recent remediation technique, where the strong sorption of contaminants to added AC reduces their release from sediments and uptake into organisms. The current study describes a marine underwater field pilot study in Trondheim harbor, Norway, in which powdered AC alone or in combination with sand or clay was tested as a thin-layer capping material for polycyclic aromatic hydrocarbon (PAH)-contaminated sediment. Several novel elements were included, such as measuring PAH fluxes, no active mixing of AC into the sediment, and the testing of new manners of placing a thin AC cap on sediment, such as AC+clay and AC+sand combinations. Innovative chemical and biological monitoring methods were deployed to test capping effectiveness. In situ sediment-to-water PAH fluxes were measured using recently developed benthic flux chambers. Compared to the reference field, AC capping reduced fluxes by a factor of 210. Pore water PAH concentration profiles were measured in situ using a new passive sampler technique, and yielded a reduction factor of 23 compared to the reference field. The benthic macrofauna composition and biodiversity were affected by the AC amendments, AC + clay having a lower impact on the benthic taxa than AC-only or AC + sand. In addition, AC + clay gave the highest AC recoveries (60% vs 30% for AC-only and AC + sand) and strongest reductions in sediment-to-water PAH fluxes and porewater concentrations. Thus, application of an AC-clay mixture is recommended as the optimal choice of the currently tested thin-layer capping methods for PAHs, and more research on optimizing its implementation is needed.
’ INTRODUCTION An innovative in situ technique to reduce the risk of hydrophobic organic chemicals (HOCs) in sediments is activated carbon (AC) amendment.1 Strong HOC sorption to AC particles leads to reductions in porewater concentrations, CPW, and consequently to lower bioavailability and uptake in benthic organisms.1,2 Several laboratory studies have demonstrated both strong reductions in release fluxes and CPW.1 Two earlier pilot studies on AC amendment in the field have been established: one at Hunters Point, in San Francisco Bay, CA,2,3 and the other at Grasse River, NY.1 The first field test aimed at remediating polychlorinated biphenyl (PCB)-contaminated mud flats in the San Francisco Bay, and the r 2011 American Chemical Society
second field study was carried out on a permanently inundated freshwater river bed also contaminated with PCBs. In both of these studies heavy equipment was used to mix the AC into the sediment (with the exception of one plot at Grasse River where no mixing was done1), experimental plots were in the order of 5 5 m, and granulated AC was used without additions of sand or clay. Both studies demonstrated a decreased bioaccumulation Received: April 6, 2011 Accepted: June 14, 2011 Revised: June 10, 2011 Published: June 14, 2011 6110
dx.doi.org/10.1021/es2011397 | Environ. Sci. Technol. 2011, 45, 6110–6116
Environmental Science & Technology by benthic biota, a decreased CPW, and no significant negative effects on benthic organisms.1,2 The present study was performed in a marine underwater environment in Trondheim Harbor, Norway, and describes a field experiment where thin-layer in situ capping with AC was tested. Thin capping would be applicable for systems with low hydrodynamic energy and even surface. To supplement the previous U.S. pilots, the current study added several elements that have not been studied in situ before that relate to making the process more feasible; (1) no direct sedimentAC mixing (bioturbation was relied on to mix the AC into the sediment) in order to reduce stress to the benthic community and to reduce handling costs during placement; (2) a comparison of three different amendment mixtures: (a) AC-only, (b) AC mixed with clay (AC+clay), in order to facilitate AC placement underwater, and (c) AC covered by a thin layer of sand (AC+sand), to test if sand or clay would prevent erosion of the AC; (3) powdered AC was used since it gave superior sorption effects in laboratory trials compared to granulated AC, probably because of faster kinetics and/or reduced fouling;4 (4) the experiment was carried out in a marine system (46 m depth, with plot sizes about 15 15 m), making this the largest field study published up until now; (5) in situ sediment-to-water PAH fluxes were measured using recently developed benthic flux chambers;5 (6) the studied compounds were polycyclic aromatic hydrocarbons (PAHs); (7) in situ pore water concentration (CPW) profiles were measured using a new passive sampler technique,6 where a 17 μm thin polyoxymethylene (POM) sampler is directly inserted into the sediment, and (8) effects of the AC were also studied on the benthic macrofauna composition and biodiversity (this has only been described for mudflats before2). PAH bioaccumulation in benthic organisms will be described in a subsequent paper. Results from the present study demonstrates the feasibility of thin-layer capping with AC in shallow marine areas, as a means to reduce PAH release fluxes, while maintaining benthic macrofauna composition and biodiversity, providing an attractive remediation alternative to more intrusive methods like dredging or conventional capping.
’ EXPERIMENTAL SECTION Field Establishment. The field experiment was carried out in a channel in the outer part of the Trondheim harbor, Norway (the “Canal”, SI Figure S1). Due to the proximity of the Trondheim fjord (200 m away) the tidal amplitude in the Canal is 12 m. Water depth is 46 m depending on the tide. Sediments in the Canal were moderately contaminated with PAHs (sum of 16 EPA-PAHs were 16 ( 6 mg/kg dry weight (n = 3); SI Table S1). This 40% standard deviation indicates that unfortunately some contamination heterogeneity existed at the field site. The Canal is a net deposition area, with deposition rates in the order of 510 mm per year. Five experimental field sites were established in April 2008: (1) reference site, no capping, (2) site capped with AC-only, (3) site capped with AC mixed with bentonite clay (AC+clay), (4) site capped with AC and then covered by 5 mm of sand (AC+sand), and (5) capping with sand only (5 mm), to discern the effect of AC from that of the sand. The purpose of the clay was 3-fold: (i) to create a viscous slurry to facilitate placement, (ii) to maintain the AC placement, that is, protect it from lateral advection and increase the longevity of the capping treatment (tidal current up to 20 cm s1) and (iii) to add a more natural and viable soft-bottom
ARTICLE
substrate to the indigenous benthic fauna. The purpose of the sand capping over the AC was also to protect the AC from erosion. The size of the experimental fields was ca. 225 m2 (15 15 m). The amount of AC used per field was 1000 kg, aiming at approximately 5 kg AC per m2. Powdered AC (Silcarbon TH90 Extra; 0.02 mm average particle size; 80% < 0.045 mm; TOC 76 ( 3%) was used since it had proven effective for lowering pore water concentrations in pilot laboratory trials with several sediments.7 AC was mixed with a 10% w/w NaCl (Industrial grade, Industrial Salt A/S, Trondheim, Norway; > 95%) solution in a 100-L cement blender (AC:water 1:3 v/v; 10 min) on a pontoon in the harbor. The purpose of the salt was to saturate the AC pore system with water that was slightly heavier than surrounding water, to facilitate AC particle settling. For the AC-only and AC+sand fields, this slurry was pumped out with a flexible manually operated hose (inner diameter 5 cm) that was moved evenly above the area of the seafloor to be covered by the particular amount of AC in the blender. The slurry was released approximately one meter above the sediment bed at a rate of 20 L min1. For the AC+clay field, AC and bentonite clay (dry powdered white sodium montmorillonite clay, Borgestad AS, Porsgrunn, Norway, 1000 kg; mean particle size 3 μm; pH 8.5; density 2.6 kg/m4) were mixed 1:1:6 with 10%-NaCl solution and pumped out as described above. For the AC+sand and sand-only fields, a 5 mm thick sand cap (particle size 01 mm construction sand) was placed, 24 h after placing the AC, by spraying the dry sand under the water surface using standard sand blasting equipment. Monitoring and Sampling. In May 2007, eleven months before the capping treatments, six initial stations in the reference field were sampled with grab samples (0.1 m2, van Veen), for an initial benthic community assessment. Amendment with the capping materials was carried out April 24, 2008 (see SI Figure S1 for the positions of the amendment fields). Following amendment, the benthic community was surveyed five and eleven months after capping, respectively (Benthic grab samples). An overview of the sampling schedule is presented in SI Table S2. One day after capping and five months after capping (September 2008), the fields were visually documented using an underwater Remotely Operated Vehicle equipped with a video camera. Twelve months after capping, sediment tube cores (Kayak Perspex cores, 8 cm in diameter) were collected and sliced in 1 cm sediment sections down to 5 cm depth, and then in 2 cm sections between 5 and 11 cm depth. TOC and AC Quantification. Total organic carbon (TOC) was determined with catalytic combustion elemental analysis at 1030 C after microacidification (1 M HCl). AC quantification was carried out in the sliced core sediments taken 12 months after amendment (see above), as well as in pooled 03 cm samples from each field. The AC amount was determined by measuring the difference between the TOC content in the capping fields and in the reference field, and correcting for the TOC content of the AC (76%). Since TOC content was 510 times smaller than AC content in most AC-amended layers, this approach yields results of sufficient accuracy, as earlier shown by Cho et al.2 Sediment-to-Water PAH Fluxes. Sediment-to-Water PAH Fluxes were measured in situ using benthic diffusion chambers.5 Briefly, a closed stainless steel chamber (inner diameter 250 mm, area 0.049 m2) containing an infinite sink material for HOCs (a 1 mm thick sheet of polydimethylsiloxane, PDMS) held 40 mm above the sediment bed, was placed on the seafloor at the field 6111
dx.doi.org/10.1021/es2011397 |Environ. Sci. Technol. 2011, 45, 6110–6116
Environmental Science & Technology sites during three months. PAH flux measurements were carried out at three occasions at 03, 47, and 912 after amendment, using duplicate (AC+sand, AC-only, reference, sand) or triplicate (AC+clay) chambers per field. After exposure, the chambers were retrieved from the sediment floor and the PDMS sheets were cleaned with tissue, solvent-extracted with heptane (20 mL; glass-distilled purity, Burdick and Jackson, Muskegon, MI), cleaned up with dimethylformamide and silica gel and the PAHs were quantified using GC-MS, as described in Eek et al.5 and SI Table S1. Sediment-to-water PAH fluxes (ng m2 d1) were then calculated from the amounts of PAHs in the PDMS, the surface area of the chamber, and the exposure time.5 Fluxes were corrected for the PAH amount present in the water inside the chamber (20% of measured values for five of the PAHs (naphthalene, acenaphthene, acenaphthylene, phenanthrene, and dibenz[a,h]anthracene) and therefore these data were considered too unreliable and omitted from the analysis. Relative in Situ Porewater (CPW) and Water (CW) Concentrations were determined in profiles from 5 cm above the seabed to 15 cm below it, with a 5 cm resolution. We developed a novel method, where we inserted the passive sampler material polyoxymethylene (17 μm; POM-17), mounted on a rod, directly into the sediment, as described by Oen et al.6 and depicted in SI Figure S2. POM-17 equilibration under such static condition was first tested in the laboratory, by exposing POMs to sediments spiked with performance reference compounds (PRCs, SI Figure S3) and leaving them in sediments up to 80 days. These measurements revealed that most PAHs were >80% equilibrated within 80 days (SI Figure S3). This observation agrees with findings that PCBs, which are more hydrophobic than the currently studied PAHs, come close to equilibrium at around 100 days in in situ exposed POMs.6 It also agrees with similar findings that sediment-embedded 26 μm thick polyethylene, which is more strongly sorbing than POM-17, requires less than a month for PAH equilibration under static conditions.8 Note, however, that as here we are mainly comparing relative concentrations (based on uptake in POM), it is not necessary that the POM comes into complete equilibrium, provided that deployment and retrieval were done consistently, and uptake rates were consistent, for all samples. In the field, the POM-17 rods were exposed for 60154 days, between 9 and 12 months after capping. After exposure, POM strips were analyzed as described in Cornelissen et al.,9 with similar procedures, internal standards and detection limits. Briefly, POM was cleaned, extracted with heptane, after which the extract was cleaned-up with a silica column and analyzed by GC-MS using deuterated internal standards.9 POM-water equilibrium distribution coefficients9 were used to calculate porewater concentrations from POM contents. Description of analytical procedures for PAH determination and selected PAH congeners can all be found in Cornelissen et al.9 Benthic Community Analyses. Taxonomy determination was done according to the Norwegian standard procedure (NS 942310). Briefly, benthic macrofauna species over 1 mm were preserved in a neutralized 8% formalin solution and stained with 0.01% Rose Bengal. The animals were sorted out, counted and identified to species or the lowest taxon possible under a stereomicroscope. The following species were omitted from the analyses according to current practice: nematodes, the copepod Cyclorhiza eteonicola, juvenile blue mussels Mytilus edulis, and
ARTICLE
juvenile echinoderms (Asteroidea, Ophiuroidea, Spatangoidea) and species which live on hard bottom substrates (i.e., Balanus balanus, Verruca stroemia, Ascidiacea). Species abundance (total number of individuals) and richness (number of taxa) were determined for each field site from two to three grab samples per field and sampling event. Species diversity was calculated using the ShannonWiener index (H0 ). A benthic quality index (BQI) based on a combined assessment of species tolerance, abundance and diversity was calculated and used to evaluate the ecological status of the benthic community.11 The ecological status of the benthic community at each field site was then compared to the reference site and classified into one of five categories: high, good, moderate, poor, and bad according to the guidelines within the European Water Framework Directive (WFD).11 Both the classification method adopted by the Norwegian Environmental Authorities10 and by the Swedish Environmental Protection Agency12 were used, based on the H0 and BQI values, respectively. Statistical Testing. The species composition at the field sites was compared to the reference site using multivariate statistics with the software PRIMER (Plymouth Laboratories, England). Both dendrograms and MDS diagrams were plotted and the distance between groups was tested using the BrayCurtis index of similarity. Species richness, abundance, diversity index (H0 and BQI) were compared with univariate statistics using two-way ANOVA with treatment and time as fixed factors. Treatments were compared using SNK (StudentNewmanKeuls) posthoc comparisons. Homogeneity of variances was tested using the Cochran’s C test. PAH fluxes and pore water concentrations were compared using student t tests and 95% confidence intervals.
’ RESULTS AND DISCUSSION Field Establishment. Underwater video footage (SI Figures S4S6) revealed the following information for the different fields: (1) AC-only: the AC was clearly visible on the seafloor on day one as a black layer on top of the sediment covering most of the field except for a few small patches of bare sediment (SI Figure S4). After 5 months, the sediment surface was not black anymore, but more of a natural gray-brownish color. (2) AC+clay: the AC+clay suspension was spread unevenly in sausage-like structures on the sediment surface on day one (SI Figure S5). Five months later, however, the images revealed a more homogeneous and smooth sediment surface. The visual observations also revealed the presence of epibenthic macrofaunal species, for example, a few asteroid seastars (Asteroidea) and many highly mobile brittle stars Ophiura sarsi. (3) AC+sand: Video recordings during sand placement showed that the covering of AC by sand was successful and did not lead to massive AC resuspension, since the AC remained on the sediment surface while the sand was deposited on top of it (SI Figure S6). The sand formed a surface layer above the AC. Five months later the sand was still on the sediment and no AC was visible at the surface. Cap disturbance showed that the AC was still in place as a dark layer under the sand (SI Figure S6). AC Contents. AC was initially applied as an approximately 25 mm thick surface layer. After 12 months the AC was mixed into the sediment down to a depth of 34 cm (Figure 1), which is a normal bioturbation depth in marine coastal systems.13 The 6112
dx.doi.org/10.1021/es2011397 |Environ. Sci. Technol. 2011, 45, 6110–6116
Environmental Science & Technology
ARTICLE
Figure 1. TOC and AC contents as a function of depth. Resolution of 1 cm down to 5 cm depth, and of 2 cm below 5 cm depth. Each dot represents duplicate measurements from either one core (AC-only and AC+ sand), two cores (AC + clay) or three cores (REF, the untreated reference field). Numerical values plus standard deviations are shown in SI Table S3. Because AC was measured as (TOCsampleTOCREF), the low AC contents (
