EFFECT OF THE INPUT PATHWAY ON THE DISTRIBUTION OF 1,2,3,4,7-PENTACHLORODIBENZO-P-DIOXIN IN AN AQUATIC MESOCOSM

The effect of the input pathway on the distribution of 1,2,3,4,7-penta chlorodibenzo-p-dioxin (P5CDD) in small-pond mesocosms was determined by introducing C-14-labeled dioxin (468 mu g) into the water sorbed to sediment particles and by a surface sprayover in a water-miscible org anic solvent. Rates of clearance from the water column were rapid and unrelated to the input method in either case, with pseudo-first-order half-lives (t(1/2)) of 0.38 +/- 0.19 d (r(2) = 0.81) and 0.57 +/- 0.08 d (r(2) = 0.98) for the particle-sorbed and sprayover applications, r espectively, over the first 24 h after input. However, redistribution path ways were strongly influenced by the route of entry of P5CDD into the ecosystem. Ponds receiving a sprayover of P5CDD showed higher con centrations of the dioxin in surface microlayers with larger amounts a lso collected in air above these ponds. Concentrations of P5CDD in the bottom sediments of these ponds were significantly lower (0.6 ng/g dr y weight) compared to concentrations in sediments of ponds receiving a sediment slurry input (1.2 ng/g dry weight). Evaluation of the result s using several fugacity-based aquatic-fate models indicated that when P5CDD entered the aquatic ecosystem in the sorbed state, sedimentatio n dominated its redistribution. However, with a sprayover input, film- to-water transfer, partitioning and deposition to bottom sediment, vol atilization, and photolytic degradation in surface microlayers were im portant processes affecting the fate of P5CDD in the mesocosm.