Relative Contributions of Aqueous and Dietary Uptake of Hydrophobic Chemicals to the Body Burden in Juvenile Rainbow Trout

Abstract

This study assessed the relative contributions of aqueous versus dietary uptake of three hydrophobic chemicals, 1,2,4-trichlorobenzene (1,2,4-TCB), 1,2,3,4,5-pentachlorobenzene (PeCB), and 2,2′,4,4′,6,6′-hexachlorobiphenyl (HCBP). Juvenile rainbow trout (Oncorhynchus mykiss) were exposed separately to chemically spiked water and food for 4 days and 12 days, respectively. Chemical concentrations were measured in the food, water, and tissues, and this allowed calculation of uptake rate constants (k₁ from water exposure, k_d from food exposure). The k₁ values for the three test chemicals were approximately five orders of magnitude greater than the k_d values. Using these measured uptake rate constants, a simulation model was used to predict the relative aqueous versus dietary uptake when fish were exposed simultaneously to water and food contaminated with these hydrophobic chemicals. The model predicted for all three test chemicals that the two uptake routes would contribute equally to the chemical body burden in fish whenever the food:water chemical concentration ratio was near 10⁵. However, using food:water chemical concentration ratios that might be expected in nature, the model predicted that gill uptake could account for over 98% of fish body burden for both 1,2,4-TCB and PeCB uptake (log K_ow values of 3.98 and 5.03, respectively). For HCBP (log K_ow of 7.55), the model predicted that the dietary uptake could contribute over 85% of the body burden. Thus, depending on the actual food:water chemical concentration ratio, aqueous uptake via the gills can predominate even when the chemicals have a log K_ow value greater than 5.0. In addition, we confirmed that dietary uptake of hydrophobic xenobiotics increases with increasing log K_ow.