Field Survey of Polychlorinated Dibenzo-p-dioxins and Polychlorinated Dibenzofurans (PCDD/Fs) in Sewage Biosolids Treated Agricultural Soils

5.0 Conclusions

The objectives of the study were attained with one minor exception. The sewage biosolids targeted for inclusion in the study were those produced by water pollution control plants (WPCPs) which had previously supplied all of the sewage biosolids applied on the treated soils. It was thought that this would provide a good indication of the total amount of PCDD/Fs which had been applied to the treated soils. It was found, however, that five of the treated soils included in the study had not received sewage biosolids from a single-source WPCP or regional storage facility. Rather, they had received sewage biosolids from two or more sources.

Measured and mean concentrations of PCDD/Fs were determined for: 1) ten Ontario agricultural soils which had received three applications of sewage biosolids; 2) ten adjacent or nearby agricultural soils which had not received sewage biosolids; and 3) seven sewage biosolids utilized on Ontario agricultural soils.

The concentrations of PCDD/Fs determined for the treated soils were compared to: 1) the generic soil remediation concentration of 10 ng/kg I-TEQ for agricultural land use (OMEE, 1997); 2) the background soil remediation concentration of 7 ng/kg I-TEQ for all land uses (OMEE, 1997); and 3) the concentrations reported in other studies. It was found that the ten I-TEQ concentrations for the treated soils were all well below the 7 ng/kg I-TEQ background concentration. They also fell within the normal range of background concentrations for soils comprising uncontaminated rural parkland sites (OMEE, 1994).

Moreover, it was found that there was no significant difference in the concentrations of PCDD/Fs between the sewage biosolids treated and non-treated soils. Therefore, there was no evidence from this limited study that three applications of sewage biosolids in accordance with the sewage biosolids guidelines (OMEE and OMAFRA, 1996), increased the concentrations of PCDD/Fs in the receiving soils.

The concentrations of PCDD/Fs in the sewage biosolids were also compared to the interim maximum concentration of 100 ng/kg I-TEQ currently allowed in waste materials utilized on agricultural land (Pers. Comm., 1997). It was found that all of the sewage biosolids I-TEQ concentrations were well below the 100 ng/kg I-TEQ criterion.

In regard to individual congeners, the 2,3,7,8-TCDD congener is of greatest concern from a toxicity perspective. It was not detected in any of the twenty soils (treated and non-treated), or seven sewage biosolids sampled as part of the study.

Thus, based on the findings of this limited study, it is concluded that the application of sewage biosolids to agricultural soils, in accordance with the sewage biosolids guidelines (OMEE and OMAFRA, 1996), has not resulted in increased PCDD/F risk to human and animal health.