Predicting bioavailability of sediment-associated organic contaminants for Diporeia spp. and oligochaetes.

Biota-sediment accumulation factors (BSAF) were calculated for Diporeia spp. and oligochaete worms exposed to polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) from field-collected sediment. These data were compared to the contaminant fraction extracted from sediment with Tenax resin using a 24 h extraction. A previous laboratory study suggested a linear relationship between log BSAF and the contaminant fraction rapidly desorbed from sediment. However, the BSAF data in our study did not fit this relationship. Better predictive regressions for both PCBs and PAHs were found when the log of the lipid-normalized organism contaminant concentrations were plotted against the log of the Tenax-extracted organic carbon-normalized sediment contaminant concentration. Regression lines for the two species had the same slope, but the Diporeia intercept was 2.3 times larger. When adjusted for a 6 h Tenax extraction, based on a regression between 6 and 24 h Tenax extractions, data from this study and two other studies that included multiple oligochaete species fit a single predictive regression. The exception included some PAHs that fell below the regression line. Thus, a single relationship generally predicted bioaccumulation across sediments, compound classes, oligochaete species, and among laboratories.

Influence of long contact times on sediment sorption kinetics of spiked chlorinated compounds.

The desorption kinetics of hexachlorobenzene (HCB) and 2,4,4'-trichlororbiphenyl (PCB 28) spiked to a field sediment were studied using a gas-purge technique. A contact time of up to 1,461 d was used to assess long-term changes in desorption kinetics. Purge-induced desorption experiments lasted from 300 to more than 4,000 h. Fast-, slow-, and very-slow-desorbing fractions could be distinguished. The desorption patterns changed with contact time from mainly fast- and slow-desorbing fractions toward the domination of slow- and very-slow-desorbing fractions. The desorption pattern for HCB after a contact time of 1,461 d became comparable to previously reported desorption patterns observed for in situ contaminants. An additional spike of HCB, PCB 28, 2,4,6-trichlorobiphenyl, and 2,2',4,5'-tetrachlorobiphenyl applied to the sediment purged for more than 4,000 h showed that very slow sites were accessible for these compounds within a few hours. Only very small fast-desorbing fractions could be detected after a contact time of just 48 h. These results indicate that domination of very slow desorption is caused not only by long contact times but, perhaps, also by the accessibility of specific sites within the sediment matrix.

Tenax extraction mimics benthic and terrestrial bioavailability of organic compounds.

Biota to sediment accumulation factors (BSAFs) are widely used to describe the potential accumulation of organic contaminants in organisms. From field studies it is known that these BSAFs can vary dramatically between sediments of different origin, which is possibly explained by the variation in bioavailability of organic contaminants in sediments. In the present study it is shown that the variability in BSAF values for different sediment samples obtained at two Dutch freshwater sites could largely be explained by the variation in Tenax-extractable concentrations in these sediments. Variations of a factor of about 50 could be explained. The ratio between concentrations in biota and Tenax-extractable concentrations in sediment varied slightly between sediments and contaminant class, but was close to the theoretically expected value of 2. This is a strong indication that Tenax-extractable concentrations of contaminants in sediments are an excellent indicator of available concentrations.

Slow and very slow desorption of organic compounds from sediment: influence of sorbate planarity.

The kinetics of desorption of in situ chlorobenzenes, PAHs, and PCBs from four different sediments was studied employing Tenax beads as an infinite sink for sorbates. Rate constants for slow desorption were 2.9+/-0.4 x 10(-2) x h(-1), irrespective of the extent of sorbate planarity. Rate constants for very slow desorption were 2.1+/-0.5 x 10(-4) and 6.7+/-1.4 x 10(-4) x h(-1) for planar and non-planar compounds, respectively. Comparison with literature data suggests a priori estimates for rate constants for slow desorption to be 3 x 10(-2) x h(-1), and to be 2 x 10(-4) and 7 x 10(-4) x h(-1) for very slow desorption of planar and non-planar compounds, respectively. The ratio between the fractions in the very slowly desorbing domain and the rapidly desorbing domain was 15-38 for planar compounds which is higher than for non-planar compounds for which the ratio was 2.8-5.2. The ratio between the fractions in the slowly desorbing domain and the rapidly desorbing domain was 1.3-1.8 and independent of the sorbate planarity. The difference in influence of sorbate planarity on the very slowly desorbing domain as compared to the slowly desorbing domain points to different environments for the slowly and the very slowly desorbing fractions.

Influence of sorbate planarity on the magnitude of rapidly desorbing fractions of organic compounds in sediment.

Organic compounds in sediments are known to distribute between rapid, slowly, and very slowly desorbing sites. This distribution is relevant to bioavailability and risk assessment of organic compounds in sediment. In this study, the fraction desorbing to Tenax in 6 h was measured for a range of organic compounds in sediment differing in their extent of planarity. The aim was to determine the influence of the extent of planarity on the distribution over the rapidly desorbing sites on the one hand and the slowly and very slowly desorbing sites on the other. The magnitude of rapidly desorbing fractions, calculated from the fractions desorbed to Tenax in 6 h, decreased with increasing extent of planarity, expressed as sorbate thickness. For a thickness of less than 5.5 to 6 A, rapidly desorbing fractions are approximately equal to those for fully planar compounds, such as polycyclic aromatic hydrocarbons, which have a thickness of about 3 A. This is in agreement with previously reported differences in sediment-water distribution coefficients between planar polycyclic aromatic hydrocarbons (PAHs) and nonplanar PCBs. The present findings suggest that simple molecular modeling of the thickness of nonplanar organic compounds enables the estimation of the affinity for rapidly desorbing sites relative to planar compounds.