Fate of 2,5,4'-trichlorobiphenyl in outdoor ponds and its uptake via the food chain compared with direct uptake via the gills in grass carp and rainbow trout.

A field experiment was carried out to investigate the fate and the extent to which food chain transfer of a PCB congener (2,5,4'-trichlorobiphenyl; 3-CB) could affect its potential for bioaccumulation. Three 35-m3 ponds were each stocked with 25 rainbow trout (a carnivore) and 20 grass carp (a herbivore). 3-CB was supplied to each pond at a nominal concentration of 14 micrograms liter-1. Samples of water, sediment, grass carp, rainbow trout, aquatic plants, and invertebrates were removed at intervals from 0 to 28 days after treatment and residues of 3-CB were determined using gas-liquid chromatography with electron-capture detection. The fate of 3-CB in the ponds was determined by transport rather than degradation processes. Evaporation accounted for 86-87% loss and sorption onto sediment and biota for 11-12% loss of 3-CB from the pond water after 28 days. The kinetics of transport between water, air, sediment, and biota were studied after fitting the data to a three-compartment model. This model was used to calculate rates of evaporation (ke) and sorption. The calculated value for ke was in good agreement with predictions based on fundamental relationships between Henry's constant, windspeed, and ke. Residues of 3-CB in rainbow trout accumulated to a significantly greater extent (P less than 0.05) than in grass carp. This difference could not be explained by differences in growth rates, distribution of lipids, or by a difference in rates of metabolism. The most plausible explanation is that there was a difference in accumulation of 3-CB residues via the food chain. This experimental work supports the conclusion, suggested by a modeling approach of other workers, that food chain accumulation of PCBs can be an important route for uptake when environmental concentrations are very low.