Dispersion of sediment DDTs in the coastal ocean off southern California

Abstract The masses of DDT compounds (DDTs) in surface sediments of the Palos Verdes Shelf (PVS) and Santa Monica Bay (SMB) have declined over the last two decades, following the ban on DDT production in 1970. This mass reduction could result from a number of biological and physical processes, including biodegradation and/or dispersal away from the sites. We integrated existing data with our new data of DDTs from different compartments in the coastal zones off southern California to assess the importance of the dispersal mechanism. The synthesis of the data indicated that: (1) historically deposited DDTs have been remobilized upward in the sediment column; (2) DDTs have been resuspended into the water column; and (3) sewage-derived DDTs have been redeposited into distant areas. Resuspension of DDTs from contaminated sediments was evident from the close correlation between the DDT concentrations in the water column and surface sediment at three locations with different DDT levels. The current distribution patterns for linear alkylbenzenes and DDTs in surface sediments at SMB were also suggestive of dispersal of DDTs. The distribution of DDTs in the surface sediments exhibiting a gradient from the outfalls to offshore and the general spatial distribution pattern in the basins precluded the possibility of either aerial fallout or surface runoff as being an important source of DDTs. These results are consistent with the hypothesis that resuspended DDTs in the discharge zones are being dispersed to distant areas. The percent of DDEs in total DDTs was uniformly high (∼90%) in the PVS and SMB sediments, but varied widely in sediments of the Santa Monica and San Pedro Basins. The percent DDEs were particularly low (as low as 10%) in certain subsurface sections of the sediments near two dumpsites containing DDT wastes (from prior to 1970) comprising of low proportions of DDE. However, the top-layers of the basin sediments contained DDT residues with high %DDEs similar to that of sediments on the PVS, suggesting a common source from the historic DDTs in the wastewater discharges. The available data is insufficient to confirm the possibility of anaerobic degradation of DDEs in the sediment cores investigated which could also result in the mass reduction of DDTs in the post-1970 sediments.