The long-term fate of four polychlorinated dibenzo-p-dioxin (PCDD) congeners in sediments and their bioavailability to benthic invertebrates were studied in littoral enclosures located in a small lake at the Experimental Lakes Area in northwestern Ontario. The PCDDs sorbed on sediment particles were added to the 5-m-diameter enclosures on June 12, 1985, and concentrations In sediment cores (0 to 6 cm) were determined over 1,800-d (-5 years). Concentrations of 1,3,6,8-TCDD, 1,3,7,9-TCDD, HpCDD, and OCDD in the sediment declined, with half-lives of 4.4rt 1.7,4.6* 1.9, 6.2rt3.1, and 6.0rt2.8 year, respectively. Evaluation of the data for 1,3,6,8-TCDD and OCDD using a sediment-water exchange model suggested that the decline in PCDD concentrations was due to losses to the water column via diffusion of DOC-associated PCDDs in pore water and via particle resuspension. After 5 years the PCDDs remained bioavailable to freshwater mussel (Anodonfu grundis) and crayfish (Orconecfes viribs) exposed to sediments for up to 78 d. The crayfish had higher mean biota-sediment accumulation factors (BSAFs) for 1,3,6,8-TCDD and 1,3,7,9-TCDD (0.31-0.53) than did mussels (0.07-0.13), whereas BSAFs for HpCDD and OCDD in the two species were similar (0.03-0.09).
