Dispersion of sediment DDTs in the coastal ocean off southern California

The masses of DDT compounds (DDTs) in surface sediments of the Pales Verdes Shelf (PVS) and Santa Monica Bay (SMB) have declined over the last two dec ades, following the ban on DDT production in 1970. This mass reduction coul d result from a number of biological and physical processes, including biod egradation and/or dispersal away from the sites. We integrated existing dat a with our new data of DDTs from different compartments in the coastal zone s off southern California to assess the importance of the dispersal mechani sm. The synthesis of the data indicated that: (1) historically deposited DD Ts have been remobilized upward in the sediment column; (2) DDTs have been resuspended into the water column; and (3) sewage-derived DDTs have been re deposited into distant areas. Resuspension of DDTs from contaminated sedime nts was evident from the close correlation between the DDT concentrations i n the water column and surface sediment at three locations with different D DT levels. The current distribution patterns for linear alkylbenzenes and D DTs in surface sediments at SME were also suggestive of dispersal of DDTs. The distribution of DDTs in the surface sediments exhibiting a gradient fro m the outfalls to offshore and the general spatial distribution pattern in the basins precluded the possibility of either aerial fallout or surface ru noff as being an important source of DDTs. These results are consistent wit h the hypothesis that resuspended DDTs in the discharge zones are being dis persed to distant areas. The percent of DDEs in total DDTs was uniformly hi gh (similar to 90%) in the PVS and SMB sediments, but varied widely in sedi ments of the Santa. Monica and San Pedro Basins. The percent DDEs were part icularly low las low as 10%) in certain subsurface sections of the sediment s near two dumpsites containing DDT wastes (from prior to 1970) comprising of low proportions of DDE. However, the top-layers of the basin sediments c ontained DDT residues with high %DDEs similar to that of sediments on the P VS, suggesting a common source from the historic DDTs in the wastewater dis charges. The available data is insufficient to confirm the possibility of a naerobic degradation of DDEs in the sediment cores investigated which could also result in the mass reduction of DDTs in the post-1970 sediments, (C) 1999 Elsevier Science B.V. All rights reserved.