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GLERL 1999 Milestone Reports
GOAL: SUSTAIN HEALTHY COASTS
OBJECTIVE: PROMOTE CLEAN COASTAL WATERS TO SUSTAIN LIVING MARINE RESOURCES AND TO ENSURE SAFE RECREATION, HEALTHY SEAFOOD, AND ECONOMIC VITALITY.
PM: Number of Coastal and Great Lakes States
Provided with Improved Predictive Capabilities and Understanding
of Environmental Processes.
Milestone: Complete assessment of environmental factors that influence the bioaccumulation of PCBs by Diporeia spp. and develop a steady-state model.
Scientist: P.
Landrum
Contaminated sediments are recognized as a national problem of wide scope and extent (USEPA 1997). Methods to assess exposure and effects of contaminated sediments remain areas of active research. One of the major complications to evaluating and assessing the impact of sediment-associated contaminants is the ability to determine their bioavailability. Several approaches are used to evaluate the impact of sediment-associated contaminants, bulk sediment concentrations, normalized sediment concentrations to organic carbon for organic contaminants or acid volatile sulfide for heavy metals, or the concentration of contaminants in interstitial water. No one method has been completely successful in all cases.
This work examined the uptake and elimination kinetics for four polychlorinated biphenyl (PCB) congeners in the amphipod, Diporeia spp. to develop a model for improving the prediction of exposure of this important Great Lakes amphipod to sediment-associated PCB. This work was part of the multi-agency Lake Michigan Mass Balance Program designed to determine the mass flux of PCB and trans-chlordane in Lake Michigan and develop models to predict the concentration of these compounds in the food web, specifically in lake trout and salmon.
Results show that accumulation and loss kinetics were proportional to organism mass (size), environmental temperature and the octanol:water partition coefficient (Kow), which describes the relative lipophilicity of compounds. If organisms are at steady state with their environment, then the extent of accumulation can be described by the biota-sediment accumulation factor (BSAF). The BSAF for organisms exposed to organic contaminants in sediment is the ratio of the concentration in the organism normalized to the organism lipid concentration divided by the concentration in the sediment normalized to the organic carbon content of the sediment. The BSAF can also be determined as the ratio of the sediment uptake rate coefficient (ks) divided by the elimination rate constant (ke). The BSAF values determined for each set of conditions were then fit to incorporate the temperature and Kow variation to make the final model (Equation 1).
Incorporating variables that influence the kinetics into the simple model to generate BSAF values allowed us to compare the ability of a laboratory based model to predict the expected concentrations to actual measured values. The field-collected samples were taken from the same Lake Michigan site as the sediments for the laboratory studies. This comparison will answer two important questions: (1) Can a laboratory-based model come reasonably close to predicting the concentration of contaminants in field exposed organisms? (2) Are organisms in the field at or near steady state with respect to their exposure to sediment-associated contaminants?
The BSAF was determined from the model. The model assumed that Diporeia were in the medium to large size range, the size class used for the field samples, and that the exposure temperature was 60ºC which is an approximate temperature for the field exposure conditions. The model results (red squares) fit on the least squares fit to the field data (blue line and blue squares (Figure 1). The model was quite successful at predicting the field conditions, and the organisms in the field are at steady state. The accumulation is more complex than would be predicted by simple equilibrium partitioning theory, which would have produced values invariant with log Kow with a value of approximately 1.7. Further, changes in seasonal temperature will likely produce variation in the BSAF values and therefore changes in food chain exposure to predator fish. The laboratory BSAF model will provide improved prediction for food web exposure in the Lake Michigan Mass Balance Model.
Figure 1. Comparison of measured BSAF to model predicted BSAF values for PCB congeners in Diporeia spp. [click for eps file]
Reference
US Environmental Protection Agency. 1997. The Incidence and Severity of Sediment Contamination in Surface Water of the United States, Vol 103. EPA 823-R-97-006. Science and Technology Office, Washington DC.
Last updated: July 9, 2002 mbl