Buried Contaminants Research Articles

Toolset for assessment of natural recovery from legacy contaminated sediment: Case study of Pallanza Bay, Lake Maggiore, Italy

The aim of this study was to develop a toolset that can be used by site managers to assess and monitor natural attenuation processes in sediments contaminated with legacy hydrophobic organic contaminants. The toolset is composed of sediment traps to measure quality and deposition rate of incoming sediment under different hydrodynamic conditions, sediment cores to show trends in sediment bed concentrations over time, and passive samplers attached to a porewater probe frame to assess the mobility of buried contaminants and possible contaminant flux from sediment. These three tools were used together for the first time to assess the mobility of dichlorodiphenyltrichloroethane (DDT) contaminants in sediment in Pallanza Bay, Lake Maggiore, Italy. Depositing sediment and sediment cores were consistent in showing that DDT-contaminated sediment is undergoing burial by cleaner sediment. Elevated DDT concentrations from historical contamination seemed to be effectively buried and immobilized by ongoing deposition by cleaner sediment, because the positive flux from the elevated DDT concentration in the sediment porewater should not advance towards the sediment surface. The monitoring toolset introduced in this study enabled us to more effectively assess ongoing natural attenuation processes and provide more risk relevant data than traditional methods used in monitored natural recovery projects, such as bulk sediment concentrations from sediment cores. Our field assessment results suggest that incoming sediment from the Toce River have reduced DDT concentrations in the sediment compared to historic levels, and will continue to do so in locations where higher DDT concentrations are found within the bioactive layer.

Linking Cellulose Fiber Sediment Methyl Mercury Levels to Organic Matter Decay and Major Element Composition

Methylation of mercury (Hg) to highly toxic methyl Hg (MeHg), a process known to occur when organic matter (OM) decomposition leads to anoxia, is considered a worldwide threat to aquatic ecosystems and human health. We measured temporal and spatial variations in sediment MeHg, total Hg (THg), and major elements in a freshwater lagoon in Sweden polluted with Hg-laden cellulose fibers. Fiber decomposition, confined to a narrow surface layer, resulted in loss of carbon (C), uptake of nitrogen (N), phosphorus (P), and sulfur (S), and increased MeHg levels. Notably, fiber decomposition and subsequent erosion of fiber residues will cause buried contaminants to gradually come closer to the sediment-water interface. At an adjacent site where decomposed fiber accumulated, there was a gain in C and a loss of S when MeHg increased. As evidenced by correlation patterns and vertical chemical profiles, reduced S may have fueled C-fixation and Hg methylation at this site.

Influence of contaminant burial depth on the bioaccumulation of PCBs and PBDEs by two benthic invertebrates ( Monoporeia affinis and Marenzelleria spp.)

The bioaccumulation of buried polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs) added to specific depths in sediment (2.0–2.5, 5.0–5.5 and 10.0–10.5 cm) was studied in two infaunal species with similar feeding habits (surface deposit-feeders) but different bioturbation modes. The deep-burrowing polychaetes Marenzelleria spp. ( Mz) displayed up to 36 times higher tissue concentrations of buried (spiked) contaminants than the surface-dwelling biodiffusing amphipod Monoporeia affinis. The differences in bioaccumulation were most pronounced for less hydrophobic contaminants due to the bioirrigating activity of Mz. Contaminants buried at shallow depths displayed higher accumulation than more deeply buried contaminants. In contrast, the bioaccumulation of unspiked (native) contaminants with a uniform vertical distribution in the sediment was similar between the species. For Mz, the BSAFs increased with increased K OW for the uniformly distributed contaminants, but decreased for the buried contaminants, which indicates that the dominant uptake routes of the buried contaminants can differ from the uniformly distributed contaminants. The surface sediment concentration of buried contaminants increased in Mz treatments, showing that Mz bioturbation can remobilize historically buried contaminants to the biologically active surface layer and increase the exposure for surface-dwelling species.

Bioturbation-Driven Release of Buried PCBs and PBDEs from Different Depths in Contaminated Sediments

Bioturbation can remobilize previously buried contaminants, leading to an increased exposure of aquatic biota. The remobilization of buried polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs) from three different sediment depth layers (2.0-2.5 cm, 5.0-5.5 cm, and 10.0-10.5 cm) was studied in a laboratory experiment with two benthic macrofauna species, the amphipod Monoporeia affinis and the polychaete Marenzelleria spp. Remobilization of PCBs and PBDEs was significantly higher in the presence of Marenzelleria spp. than in M. affinis treatments and controls (without macrofauna). The highest remobilization occurred from the most shallow layers (2.0-2.5 cm > 5.0-5.5 cm > 10.0-10.5 cm), but contaminants were remobilized due to bioturbation from layers down to at least 10 cm. Congeners with lower hydrophobicity were remobilized to a higher extent than more hydrophobic congeners. The contaminant distribution between the particulate and the dissolved phase in the water column depended on hydrophobicity and burial depth of the contaminant, with congeners from deeper layers displaying an increased distribution to the particulate phase. Release fluxes and sediment-to-water mass transfer coefficients (MTCs) show that bioturbation by the polychaete Marenzelleria spp. can lead to a significant remobilization of buried contaminants from Baltic Sea sediments.

Estimates of suspended sediment entering San Francisco Bay from the Sacramento and San Joaquin Delta, San Francisco Bay, California

This study demonstrates the use of suspended-sediment concentration (SSC) data collected at Mallard Island as a means of determining suspended-sediment load entering San Francisco Bay from the Sacramento and San Joaquin River watersheds. Optical backscatter (OBS) data were collected every 15 min during water years (WYs) 1995–2003 and converted to SSC. Daily fluvial advective sediment load was estimated by combining estimated Delta outflow with daily averaged SSC. On days when no data were available, SSC was estimated using linear interpolation. A model was developed to estimate the landward dispersive load using velocity and SSC data collected during WYs 1994 and 1996. The advective and dispersive loads were summed to estimate the total load. Annual suspended-sediment load at Mallard Island averaged 1.2±0.4 Mt (million metric tonnes). Given that the average water discharge for the 1995–2003 period was greater than the long -term average discharge, it seems likely that the average suspended-sediment load may be less than 1.2±0.4 Mt. Average landward dispersive load was 0.24 Mt/yr, 20% of the total. On average during the wet season, 88% of the annual suspended-sediment load was discharged through the Delta and 43% occurred during the wettest 30-day period. The January 1997 flood transported 1.2 Mt of suspended sediment or about 11% of the total 9-year load (10.9 Mt). Previous estimates of sediment load at Mallard Island are about a factor of 3 greater because they lacked data downstream from riverine gages and sediment load has decreased. Decreasing suspended-sediment loads may increase erosion in the Bay, help to cause remobilization of buried contaminants, and reduce the supply of sediment for restoration projects.

The use of tracers to evaluate the importance of bioturbation in remobilising contaminants in Baltic sediments

Large areas of the bottom sediments of the Baltic Sea are temporarily or permanently anoxic. These sediments are also an important sink for a variety of contaminants. Reoxygenation of bottom waters allows recolonisation by benthic infauna, which may have important implications for the fate of buried contaminants. This study used tracers to experimentally examine the role of bioturbation by benthic infauna in transporting sediment-associated contaminants in the Baltic Sea. Three different tracer methods were used in two experiments, using three key Baltic macrofaunal species: the amphipod crustacean Monoporeia affinis; the Baltic clam Macoma baltica; and the priapulid worm Halicryptus spinulosus. In the first experiment, a reoxygenation–recolonisation scenario was recreated in the laboratory, using hypoxic sediment cores collected in the field, to determine if there was remobilisation of buried 137Cs from the Chernobyl nuclear accident in 1986. The potential for the infauna to bury newly settled surface contamination was also investigated, using a fluorescent particle tracer. In the second experiment, artificially-created radiolabelled tracer layers ( 14C and 51Cr) were used to quantify both upward and downward movements of organic matter and sediment-associated contaminants by bioturbation. In both experiments there were clear visual differences between the sediment effects of the three species. Halicryptus spinulosus buried deepest into the sediment, creating a network of burrows, Monoporeia affinis burrowed actively in the upper few centimeters of the sediment, and Macoma baltica was quite stationary, but appeared to filter- and deposit feed at the sediment surface. Mixing depths in the hypoxic sediment varied from 4.0 ± 3.5 cm for M. baltica to 7.8 ± 2.1 cm for H. spinulosus. Biodiffusion rates ( D b) were similar for all treatments but biotransport rates ( r) were significantly different between treatments, mainly due to a high r value for H. spinulosus. In the experiment with radiolabelled tracer layers, 51Cr was transported more than 14C, and tracer originally at the surface transported more than tracer buried 4 cm below the surface. There was also transport of all tracers in treatments without added macrofauna. The most likely explanation is bioturbation by the meiofauna that were undoubtedly present in both experiments. Bioturbation by macrofauna both buries surface contaminants and remobilises those that are buried, but the effects are small and on a similar scale to transport caused by meiofauna. In addition, 137Cs profiles at the hypoxic site indicated that resuspension and redeposition of sediment by physical processes had occurred, and also showed that contaminants from the last 40 years were still present in the top 5–10 cm of the sediment, well within active mixing depths. At this site, as at many others in the Baltic, physical processes are likely to be far more important than biological processes in the redistribution of contaminants on a decadal timescale.

Depositional history of organic contaminants in Narragansett Bay, Rhode Island, USA

Sediment cores were taken at three locations in Narragansett Bay, Rhode Island, USA in 1997 and analyzed for a variety of organic contaminants including polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), chlordanes, linear alkyl benzenes (LABs), benzotriazoles (BZTs) and dichlorodiphenyltrichloroethanes (DDTs) and metabolites. The distributions of these chemicals at Apponaug Cove and in the Seekonk River indicate that there was a disturbance in the depositional environment relative to cores collected at these locations in 1986 demonstrating the potential for buried contaminants to be remobilized in the environment even after a period of burial. Sharp breaks in the concentration of several organic markers with known dates of introduction were successfully used to determine the sedimentation rate at Quonset Point. Both the Quonset Point and Seekonk River cores had subsurface maximums for DDTs, PCBs, PAHs and BZTs, which are consistent with expected inputs to the environment. The Apponaug Cove core showed an increase of most contaminants at the surface indicating a recent event in which more contaminated sediments were deposited at that location.