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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 enclosures (34 m3) in a small oligotrophic lake in the Experimental Lakes Area. Tetrachlorodibenzofuran was added as asingle dose (10.4 g/34 m3) or as five small multiple additions over a 5-d period in a sediment slurry. Tetrachlorodibenzofuranwas rapidly redistributed, mainly to bottom sediments (water column t1/2 ~1.8 d), reflecting removal on settling particles. Between 0 and 120 d, 80 to 90% of TCDF in the water column was associated with particles (>1 mum). The highest concentration of TCDF in suspended particles was consistently observed in the smallest size fraction (0.22-1 mum) at 326 to 464 d posttreatment. Mode of addition had no effect on TCDF concentrations in water (unfiltered) or surficial sediments (0-2 cm) or pore waters throughout the experiment. Mean TCDF concentrations in surficial sediment (0-120 d) were 1,830 1 1,180 pg/g (organic carbon [OC] basis), whereas they averaged 1,260 1 596 pg/gOC from 318 to 851 d posttreatment. Increasing concentrations of TCDF in lower sediment layers at 852 d suggests that TCDF was either diffusing into the sediment or undergoing burial. Fugacity (f) calculations indicated that the TCDF in enclosures shifted from disequilibriumfavoring the water column (days 0-9) to a disequilibrium with respect to bottom sediments (fwater/fsediment = 0.06-0.003) from day 21 onward. Initially, TCDF was more bioavailable to filtering and deposit feeders (mussels, Chironomidae, Hexagenia sp., and zooplankton) in enclosures receiving multiple additions; however, differences were rarely statistically significant. Concentrations of TCDF in all organisms were initially high, and they decreased in later sampling periods. Biota-sediment-accumulation factors (BSAFs) in mussels and crayfish were >1 in the early sampling periods, which reflected [ABSTRACT FROM AUTHOR]