Black carbon-inclusive modeling approaches for estimating the aquatic fate of dibenzo-p-dioxins and dibenzofurans.

A novel black carbon (BC) inclusive modeling tool is applied to estimate the distribution and long-term fate of dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in the Norwegian Grenland Fjords. Three versions of the model were developed in which sediment-water partitioning was described using (i) an amorphous organic carbon (AOC) partitioning sorption model without BC sorption, (ii) a combined AOC and BC sorption model based on the Freundlich isotherm, and (iii) a combined BC-AOC model based on the Langmuir isotherm. The predictive ability of the three different models was evaluated for 17 PCDD/Fs by comparison of model predictions with observed organic carbon normalized sediment-water partition coefficients (K(TOC)) and with measured concentrations. All three versions of the model were able to predict concentrations that were in reasonable agreement with measured particulate concentrations (i.e., within a factor of 4 of median values). Estimated particulate concentrations were less sensitive to the model choice because the majority of the mass of these hydrophobic chemicals is associated with particulates regardless. However, for estimation of K(TOC) or dissolved water concentrations, both versions of the combined AOC and BC sorption models provided greatly improved estimates compared to the AOC-only model.

Silica gel as a particulate carrier of poorly water-soluble substances in aquatic toxicity testing.

Aquatic toxicity tests were originally developed for water-soluble substances. However, many substances are hydrophobic and thus poorly water-soluble, resulting in at least two major implications. Firstly, toxicity may not be reached within the range of water solubility of the tested compound(s), which may result in the formation of solids or droplets of the tested substance and consequently an uneven exposure. Secondly, because of multi-phase distribution of the tested substance it may be complicated to keep exposure concentrations constant. To overcome such problems, we have introduced silica gel as a particulate carrier in a toxicity test with the benthic copepod Nitocra spinipes. The main objective of the current study was to evaluate whether a controlled exposure could be achieved with the help of silica gel for testing single poorly water-soluble substances. A secondary objective was to evaluate whether an equilibrium mass balance model could predict internal concentrations that were consistent with the toxicity data and measured internal concentrations of two model hydrophobic substances, i.e., the polybrominated diphenyl ethers BDE-47 and BDE-99. Larval N. spinipes were exposed for 6 days to BDE-47 and BDE-99, respectively, in the silica gel test system and, for comparative reasons, in a similar and more traditional semi-static water test system. Via single initial amounts of the model substances administered on the silica gel, effects on both larval development and mortality resulted in higher and more concentration-related toxicity than in the water test system. We conclude that the silica gel test system enables a more controlled exposure of poorly water-soluble substances than the traditional water test system since the concentration-response relationship becomes distinct and there is no carrier solvent present during testing. Also, the single amount of added substance given in the silica gel test system limits the artefacts (e.g., increased chemical load in test system) that a semi-static renewal may introduce when testing substances that partition to non-water phases. However, measured and modelled internal concentrations did not match toxicity, which may indicate that chemical equilibrium was not reached during the test. Further experiments are thus needed to explain the processes behind the observed positive effects of silica gel and a kinetic model would likely also be more appropriate to describe the concentrations and distributions in the two test systems.

Modelling the long-term fate of polychlorinated dibenzo-p-dioxins and furans (PCDD/Fs) in the Grenland Fjords, Norway.

The development and application of a predictive fate model (DIG--Dioxins in Grenland) for polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) in the marine environment of the Norwegian Grenland Fjords are described. The objective of the modelling study was to predict long-term future changes in PCDD/F concentrations in the fjord following the cessation of point source emissions. To assess the reliability of the model, the model performance was evaluated by comparing model results to field measurements collected between 1989 and 2001. Model bias (defined as the ratio of median predicted concentration and median observed concentration) for prediction of concentrations for three different PCDD/F congeners (2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 1,2,3,6,7,8-hexadibenzofuran (HxCDF) and octachlorodibenzofuran (OCDF)) in the fjord was between 0.53 and 24, which was deemed a satisfactory result for models of this type. The model was run to determine the dynamic change in concentrations between 1950 and 2050 and to examine the key fluxes of PCDD/Fs in the fjord. Between 1990 and 2050 sediment concentrations of TCDD, HxCDF and OCDF were predicted to fall at fairly constant but gradually slowing rates to concentrations 52, 98 and 88 times lower, respectively, of their 1990 values. Losses of PCDD/Fs from the bottom sediments in the Frierfjord were predicted to be a combination of sediment burial and net resuspension to the water column. Sediment burial was shown to be relatively more important in the fjord's deep-water sediments, whereas resuspension was relatively more important in the shallow sediments. For the shallower sediments, a net water-to-sediment flux was predicted for all three congeners up until the mid-1970s, when emission reductions were initiated, and thereafter a net sediment-to-water flux was predicted. The shallow sediments acted as net sources to the deeper sediments and to the fishing areas in the outer fjord.

Testing common sediment-porewater distribution models for their ability to predict dissolved concentrations of POPs in The Grenlandsfjords, Norway.

This study compares in situ observed porewater concentration of persistent organic pollutants (POPs) with predictions by common solid-water phase distribution models. Bottom sediments were sampled in The Grenlandsfjords, Norway, and the interstitial porewater was isolated from the solids by centrifugation and filtration. Both phases were analysed for polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F), polycyclic aromatic hydrocarbons (PAH), and organic carbon. Based on the sediments' solid phase content of POPs, organic carbon, and soot carbon, we used the organic matter partitioning (OMP), and also the soot and organic matter partitioning (SOMP) model to estimate the porewater concentration. The OMP model gave better agreement to observations than the SOMP model for both PCDD/Fs and PAHs. The observed concentration of the PCDD/Fs in the sediments' porewater was much higher than in the deep water of the fjord. The logarithm of the organic matter-water partitioning coefficent (log K(OC)) in the porewater had positive linear regression on the logarithm of the octanol-water partitioning coefficient (log K(OW)). The slope of the regression model was indistinguishable from 1, except for the PAHs as a group which had a slope less than 1. This contrasts to previous studies undertaken in The Grenlandsfjords water column, where the slopes were higher than 1 for PCDD/Fs, and the K(OC) were much higher than the K(OW). One explanation may be that the influence of POPs adsorption to soots decrease because competitive sorption by other compounds in the sediment are higher than in the water column. This indicates that the sorption isotherms for these POPs need better understanding in order to be applicable in both the water column and the porewater.

Distribution of PCNs, PCBs, and other POPs together with soot and other organic matter in the marine environment of the Grenlandsfjords, Norway.

The apparently dissolved concentration of polychlorinated naphthalenes (PCNs) and three planar polychlorinated biphenyls (pPCBs) were sampled and analysed in the water column of a marine fjord system. We also measured how much of these persistent organic pollutants (POPs) were associated with suspended particles. The field observations showed that an unexpectedly high portion of the pollutants were particle-associated. The factor of deviation from model predictions had positive linear regression on the soot carbon:particulate organic carbon ratio of the particles, and on estimates of the soot-water distribution coefficient for the PCNs. The spatial distribution of surface sediment concentrations of PCNs and polycyclic aromatic hydrocarbons (PAH) were found to consistently follow the sediment content of soot (f(SC)) to a larger extent than the bulk organic matter (f(OC)). There were no systematic differences in the strength of correlation of sediments concentrations of other POPs, i.e. octachlorostyrene and organochlorine pesticides with sediment concentration to f(SC) and f(OC). Mechanisms possible of generating these types of observations, e.g. adsorption to soot carbon in the marine particle, are discussed.

Polychlorinated biphenyls in polysulfide sealants--occurrence and emission from a landfill station.

Approximately 80,000 kg polysulfide sealant containing 10,000-18,000 kg polychlorinated biphenyls (PCB) was deposited at a Swedish municipal landfill station during 1965-1973. Investigations during 1994 showed that soil layers underneath the landfill had concentration of PCB not alarmingly high. The concentration of PCB congeners in ground water samples was elevated 4-750 times compared to a reference sample. Based on samples of ground water, leachate water, and flux chambers measuring evaporation of PCB from the landfill surface, the emission of PCB was estimated to be 1 g sigmaPCB/yr. This very low rate was attributed to the high sorptive capacity of the sealant. Compared to a reference site, the evaporation flux was elevated for the most volatile congeners, but factors 20-1400 lower than from another landfill which was contaminated with PCB in paper-pulp fibres.