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

Abstract

AbstractThe 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 a single dose (10.4 g/34 m3) or as five small multiple additions over a 5‐d period in a sediment slurry. Tetrachlorodibenzofuran was rapidly redistributed, mainly to bottom sediments (water column t½ ∼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 μm). The highest concentration of TCDF in suspended particles was consistently observed in the smallest size fraction (0.22–1 μm) 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 ± 1,180 pg/g (organic carbon [OC] basis), whereas they averaged 1,260 ± 596 pg/g OC 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 disequilibrium favoring the water column (days 0–9) to a disequilibrium with respect to bottom sediments (fwate/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 greater bioavailability of the added particle‐borne 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.