Abstract

 The influence of upwelling and downwelling conditions on sediment toxicity in streams has been demonstrated in previous in situ studies. For example, vertical hydraulic gradients from -0.01 to 0.14 cm/cm reduced or increased, respectively, the bioavailability of contaminant stressors to in situ exposed benthic invertebrates. Laboratory studies were conducted with two model contaminants: the PAH fluoranthene (FLU) and the herbicide trifluralin (TF). Toxicokinetic parameters were determined for H. azteca and L. variegatus in water-only exposures to 0, 5, 20 and 50 µg/L of the test compounds and bioaccumulation was measured during exposures to 100 and 200 mg/kg of FLU and TF spiked into sediments. Mean uptake clearance rates (ku) ranged from 150-180 mL/g wet animal/h for FLU and 84-120 mL/g wet animal/h for TF. The elimination rates (ke) were 0.12-0.18 and 0.067-0.10/h for FLU and TF, respectively. Apparent mean (±SD) steady state tissue residues following exposures to sediments ranged from 0.15 (±0.033) to 0.33 (±0.062) µmol FLU/g wet wt and 0.061 (±0.016) to 0.17 (±0.055) µmol TF/g wet wt. Desorption of the bioavailable fraction of the chemicals Great Lakes sediments of differing organic contents (2.0 and 3.6% TOC) was also measured and ranged from 0.41-18.95 d. Data from these field and laboratory investigations were then combined with literature values of the feeding rates and chemical assimilation efficiencies of L. variegatus and H. azteca, and contaminant partitioning between sediments and pore water (i.e., Kp values) to parameterize a bioaccumulation model. The model predicted tissue concentrations at sites containing contaminated sediments and the estimates were compared to the results of two field studies. In general, the model predicted field observations within an order of magnitude and thus represents a useful tool that may reduce resource expenditures associated with site assessments and provide more accurate risk characterizations.