The influence of multiple sewage sludge amendments on the PCB content of an agricultural soil over time
The polychlorinated biphenyl (PCB) content of soils from a long-term agricultural experiment that received 25 separate sewage sludge applications from 1942 to 1961 is presented, along with data from an untreated control plot. Archived plough layer (0 to 23 cm) soil samples were collected, stored, and processed in the same manner between 1942 and 1992 (i.e., before, during, and after sludge amendments) and samples of the applied sludges were available for analysis. Soil sigma PCB concentrations (defined as the sum of the 27 congeners quantified) on the control plot increased between 1942 (63 micrograms sigma PCB kg-1) and 1972 (560 micrograms sigma PCB kg-1) as a result of atmospheric deposition inputs; they subsequently declined to 13 micrograms sigma PCB kg-1 in 1992. A total of approximately 1 kg sigma PCBs/hectare was applied in sewage sludges (containing 0.14 to 4.33 mg sigma PCB kg-1) to the sludge-amended plot. Soil concentrations increased accordingly, to 640 micrograms sigma PCB kg-1 by 1960. However, because of the continued high atmospheric deposition inputs, concentrations on this plot also continued to increase until 1972. By 1992, the sludge-amended plot contained ca. 5 times the sigma PCB content of the control plot. By 1960, approximately 81% of the predicted sigma PCB added in sludge could be accounted for; this had decrease to < 50 and 21% by 1984 and 1992, respectively. Marked compound-specific differences in soil persistence were noted; generally lower chlorinated (3/4-Cl) congeners became less important contributors to the sigma PCB content of the soils over time, while 5-Cl homologues and above increased.
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