DDT and related compounds in pore water of shallow sediments on the Palos Verdes Shelf, California, USA

摘要

For nearly two and a half decades following World War II, production wastes from the world's largest manufacturer of technical DDT (1-chloro-4-[2,2,2-trichloro-1-(4-chlorophenyl)ethyl]benzene) were discharged into sewers of Los Angeles County. Following treatment, the wastes were released via a submarine outfall system to nearshore coastal waters where a portion accumulated in shallow sediments of the Palos Verdes Shelf (PVS). An investigation of the pore-water geochemistry of DDT-related compounds (DDX) was undertaken in an effort to understand factors controlling the rate of reductive dechlorination (RDC) of the major DDT degradate, 4,4'-DDE (1-chloro-4[2,2-dichloro-1-(4-chlorophenyflethenyl]benzene). Equilibrium matrix-solid phase microextraction (matrix-SPMEeq) combined with automated thermal desorption-gas chromatography/mass spectrometry (TD-GC/MS) was used to determine freely dissolved concentrations of ten DDX analytes in sediment cores collected from three locations on the PVS (stations 3C, 6C, 8C, which are 7 km, 2 km, and 0 km, respectively, downcurrent from the outfall system). Pore-water concentrations (pM) of the principal DDX compounds involved in RDC were: 3C-DDE: 6.0-24, DDMU (1-chloro-4-[2-chloro-1-(4-chlorophenyl)ethenyl]benzene): 11-160, DDNU (1-chloro-4-[1-(4-chlorophenyl)ethenyl]benzene): 1.8-68; 6C-DDE: 5.6-170, DDMU: 5.6-177, DDNU: 1.7-87; 8C-DDE: 27-212, DDMU: 31-403, DDNU: 5.5-89. Variations in the spatial distribution of DDX analytes in pore water reflect several factors including proximity to the outfalls, RDC reaction rates, and natural variability in sedimentation and post-depositional transport processes. A comparison of pore-water data produced using matrix-SPMEeq/TD-GC/MS and whole-core squeezing/solvent extraction/liquid injection-GC/MS indicates that the majority of the DDE in the upper sediment column (= about 10 cm) is associated with dissolved/colloidal organic matter. Below that depth, freely-dissolved DDE predominates. The principal organic geochemical phase controlling sorption of DDE in PVS sediments are residual hydrocarbons, the vast majority of which originated from petroleum refinery wastes. Organic carbon-normalized sediment-water distribution coefficients (K-OC) were calculated from solid-phase and pore-water concentrations of DDX and organic carbon. Log K-OC values (L/kg) were relatively invariant across the shelf and with depth in the sediment column. Shelf-wide compound-specific coefficients (log K-OC) were: DDE: 7.5 +/- 0.11, DDMU: 6.92 +/- 0.13, DDNU: 6.37 +/- 0.19. The spatial uniformity of K-OC means that biological exposure and availability of the DDX compounds can, in principle, be estimated from solid-phase chemical measurements.