Long-term fate and bioavailability of sediment-associated 2,3,7,8-tetrachlorodibenzofuran in littoral enclosures

The fate and bioavailability of 2,3,7,8-tetrachlorodibenzofuran (TCDF; 4,6- tritium labeled) was examined over a 3-year (852-d) period in littoral encl osures (34 m(3)) in a small oligotrophic lake in the Experimental Lakes Are a. Tetrachlorodibenzofuran was added as a single dose (10.4 g/34 m(3)) or a s five small multiple additions over a 5-d period in a sediment slurry. Tet rachlorodibenzofuran was rapidly redistributed, mainly to bottom sediments (water column t(1/2) similar to 1.8 d), reflecting removal on settling part icles. Between 0 and 120 d, 80 to 90% of TCDF in the water column was assoc iated with particles (>1 mu m). The highest concentration of TCDF in suspen ded particles was consistently observed in the smallest size fraction (0.22 -1 mu m) at 326 to 464 d posttreatment Mode of addition had no effect on TC DF concentrations in water (unfiltered) or surficial sediments (0-2 cm) or pore waters throughout the experiment. Mean TCDF concentrations in surficia l sediment (0-120 d) were 1,830 +/- 1,180 pg/g (organic carbon [OC] basis), whereas they averaged 1,260 +/- 596 pg/g OC from 318 to 851 d posttreatmen t. increasing concentrations of TCDF in lower sediment layers at 852 d sugg ests that TCDF was either diffusing into the sediment or undergoing burial. Fugacity (f) calculations indicated that the TCDF in enclosures shifted fr om disequilibrium favoring the water column (days 0-9) to;a disequilibrium with respect to bottom sediments (f(water)/f(sediment) = 0.06-0.003) from d ay 21 onward. Initially, TCDF was more bioavailable to filtering and deposi t feeders (mussels, Chironomidae, Hexagenia sp., and zooplankton) in enclos ures receiving multiple additions; however, differences were rarely statist ically significant. Concentrations of TCDF in all organisms were initially high, and they decreased in later sampling periods. Biota-sediment-accumula tion factors (BSAFs) in mussels and crayfish were >1 in the early sampling periods, which reflected greater bioavailability of the added particle-born e TCDF By 851 d, BSAFs had declined to 0.02 to 0.04, reflecting removal of TCDF from the water column and from surficial sediments.