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

Executive Summary

The criteria and standards in Ontario for the utilization of sewage biosolids on agricultural land are outlined in the publication entitled Guidelines for the Utilization of Biosolids and Other Wastes on Agricultural Land (OMEE and OMAFRA, 1996). They have been developed to facilitate the recycling and use of sewage biosolids on agricultural land, while protecting the quality of food, the health of consumers and livestock, and the quality of the environment.

In 1995, the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRARA) conducted a field survey to study pH, phosphorus and heavy metal levels in soils which had received two or more applications of sewage biosolids. The analytical results and findings of that survey are outlined in the report entitled Analytical Results, Findings and Recommendations of the 1995 OMAFRA Sewage Biosolids Field Survey (OMAFRA, 1995).

In 1996, further monitoring efforts were carried out in regard to the sewage biosolids land application program in Ontario. The Biosolids Utilization Committee (BUC) endorsed a proposal to carry out a limited field survey to study the concentrations of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) in: 1) agricultural soils which had received long-term, multiple applications of sewage biosolids; and 2) sewage biosolids utilized on agricultural land. This report presents the analytical results, findings and conclusions of that field survey.

PCDDs and PCDFs (PCDD/Fs) are organic chemical compounds which occur naturally as by-products of combustion of organic materials (i.e. during forest fires). Therefore, they can never be totally eliminated from the environment. PCDD/Fs are also toxicants which, if they occur above acceptable levels in environmental media, may pose a risk to human and animal health. PCCD/Fs can occur in sewage biosolids, and the public has expressed concern that the concentrations will increase in agricultural soils which receive sewage biosolids, and pose significant risk to human and animal health.

Since PCDD/Fs are toxicants, it is important that the concentrations of PCDD/Fs in sewage biosolids utilized on agricultural soils, and the concentrations in the receiving soils, not exceed concentration limits which are: 1) specified in relevant Ontario guidelines; or 2) used by the Ontario Ministry of the Environment (OME) in the waste management approval process set out under Part V of the Environmental Protection Act (R.S.O., 1990). They are as follows:

1. the interim maximum concentration of PCDD/Fs allowed in waste materials applied on agricultural land is 100 ng/kg I-TEQ (International Toxicity Equivalents) (Pers. Comm., 1997) (Note: 100 ng/kg I-TEQ = 100 ppt I-TEQ);
2. the generic soil remediation PCDD/F concentration for agricultural land use is 10 ng/kg I-TEQ, as specified in the Guideline for Use at Contaminated Sites in Ontario (OMEE, 1997). This criterion, however, can not be used for sites defined as sensitive in the guideline, without a site specific risk assessment that demonstrates its acceptability for that site; and
3. the background soil remediation PCDD/F concentration for all land uses is 7 ng/kg I-TEQ, as specified in the Guideline for Use at Contaminated Sites in Ontario (OMEE, 1997). This value can be used as a remediation criterion for any site.

Further to 2) and 3), the 10 ng/kg I-TEQ value is an effects based value and has most often been used by the OME as the maximum concentration of PCDD/Fs allowed in agricultural soils which receive waste materials containing PCDD/Fs. The background 7 ng/kg I-TEQ value, however, could also be used if deemed necessary (i.e. sensitive site). Therefore, it was included in this study for comparison purposes.

Purpose and Objectives

The main purpose of the 1996 field survey was to study the concentrations of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) in a limited number of agricultural soils which had received long-term, multiple applications of sewage biosolids. A secondary purpose was to study these parameters in selected sewage biosolids utilized on Ontario agricultural land. Numbers of soil and sewage biosolid samples were limited by the high cost of PCDD/F analysis, estimated at $1,000.00 per sample. Specific objectives of the study were:

1. to determine the concentrations of PCDDs and PCDFs (PCDD/Fs) in Ontario agricultural soils which had received three applications of sewage biosolids;
2. to determine the concentrations of PCDD/Fs in sewage biosolids utilized on Ontario agricultural land, specifically those produced by water pollution control plants which had supplied sewage biosolids to the treated soils; and
3. to compare the measured concentrations of PCDD/Fs in the treated soils, and in the sewage biosolids, to: a) the concentration limits specified in relevant Ontario guidelines, or used by the OME; and b) the concentrations reported in other studies.

Field Survey

Ten of the paired sites included in the 1995 OMAFRA field survey were selected and sampled again in 1996. Each site consisted of a "treated" plot located in an agricultural field which had received three applications of sewage biosolids, and a "non-treated" plot located in an adjacent or nearby agricultural field comprising soils with similar physical characteristics, which had not received sewage biosolids. There were a total of 14 farmer co-operators.

Soil samples (0-15 cm depth) were taken at varying intervals along three transects located in each of the treated and non-treated plots. At least 25 samples were taken along the transects, and combined to obtain a composite sample for each plot. The composite sample was then thoroughly mixed, and a portion taken for analysis. A total of 10 treated plots and the same number of non-treated plots were sampled.

Sewage biosolid samples were taken at 7 water pollution control plants (WPCPs) or regional storage facilities which had supplied all, or a portion of, the sewage biosolids applied to the treated soils. Five samples taken at different times were combined into a composite sample for 6 of the WPCPs or storage facilities. Each composite sample was thoroughly mixed, and a portion was taken for analysis. Five samples also taken at different times from the Southampton WPCP were analyzed individually for PCDD/Fs.

Findings and Conclusions

Two different conventions were used to derive overall toxicity values for the soils and sewage biosolids included in the study. The present international convention (U.S. EPA, 1989) was used to calculate values expressed as International Toxicity Equivalents (I-TEQs), and an earlier international convention was used to calculate values expressed as Homologue Toxicity Equivalents (H-TEQs). The latter values were required for comparison with Ontario background concentrations in soils at uncontaminated rural parkland sites (OMEE, 1994).

The mean concentration of PCDD/Fs in the treated soils was 1.9 ng/kg I-TEQ, and the maximum concentration for these soils was 3.5 ng/kg I-TEQ. These concentrations are well below the generic soil remediation concentration of 10 ng/kg I-TEQ for agricultural land use (OMEE, 1997). They are also well below the background soil concentration of 7 ng/kg I-TEQ (OMEE, 1997), which is the upper limit of Ontario background concentrations in soils at uncontaminated rural parkland sites (OMEE, 1994).

Statistical analysis of the analytical results indicated that there were no significant differences in PCDD/F concentrations between the treated and non-treated soils. Thus, there is no evidence from this study that three applications of sewage biosolids in accordance with the Ontario guidelines (OMEE and OMAFRA, 1996), increased the concentrations of PCDD/Fs in the receiving soils.

The 2,3,7,8-tetrachlorodibenzo-p-dioxin (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), nor in the seven sewage biosolids included in the study.

Using the H-TEQ convention for deriving overall toxicity values, the mean concentration of PCDD/Fs for the treated soils was 1.2 ng/kg H-TEQ, and for the non-treated soils it was 1.3 ng/kg H-TEQ. Both values are slightly lower than the mean value of 1.7 ng/kg H-TEQ reported in the Ontario background database (OMEE, 1994). Moreover, the highest value of 4.1 ng/kg H-TEQ determined for the non-treated soils at the Oakville #2 site, is slightly lower than the 98th percentile value of 4.8 ng/kg H-TEQ also reported in the background database. Thus, the concentrations of PCDD/Fs in both the treated and non-treated soils are within the normal range of Ontario background soil concentrations (OMEE, 1994).

The mean concentration of PCDD/Fs in the sewage biosolids was 10.8 ng/kg I-TEQ. Concentrations ranged from 5.9 ng/kg I-TEQ in the Whitby sewage biosolids, to 17.8 ng/kg I-TEQ in the Halton sewage biosolids. These values are well below the interim maximum concentration of 100 ng/kg I-TEQ currently allowed in waste materials utilized on agricultural land (Pers. Comm., 1997). The mean value of 10.8 ng/kg I-TEQ for the sewage biosolids also compares favourably with values reported by Webber and Nichols (1995), and Webber and Bedford (1996), who examined concentrations of organic chemical compounds in 16 Canadian sewage biosolids, including 7 from Ontario. They found that the mean concentrations of PCDD/Fs in the sewage biosolids varied from 4 ng/kg I-TEQ to 120 ng/kg I-TEQ. The mean concentration for the 16 sewage biosolids examined was 30 ng/kg I-TEQ, and the mean for the 7 Ontario sewage biosolids was 10 ng/kg I-TEQ.

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.

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