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

1.0 Introduction

1.1 Background

Sewage biosolids are the nutrient rich black residues remaining after municipal wastewater treatment at a water pollution control plant. They are a valuable agricultural fertilizer because they contain nitrogen, phosphorus, some trace minerals, and organic matter, which are all beneficial to the soil. The utilization of sewage biosolids on agricultural land can benefit farmers, municipalities and society when carried out in accordance with sound utilization guidelines.

The criteria and standards in Ontario, which must be adhered to in order to utilize 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). The guidelines supplement the General - Waste Management Regulation (Regulation 347), made under Part V of the Environmental Protection Act (R.S.O., 1990). 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. Hereafter, they are referred to as the "sewage biosolids guidelines".

Members of the public have expressed concerns in regard to the utilization of sewage biosolids on agricultural land. The concern relevant to this study is whether the application of sewage biosolids increases soil levels of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs), and whether the resulting levels pose an increased risk to human and animal health.

PCDDs and PCDFs (PCDD/Fs) are chlorinated tricyclic aromatic hydrocarbons with similar molecular structures (Figure 1) and physical and chemical properties. They occur in a wide variety of environmental media and have caused great concern in the general public as well as intense interest in the scientific community. Much of the public concern revolves around characterization of these compounds as among the most potent "man-made" toxicants. While this characterization is warranted, it must be balanced by the knowledge that PCDD/Fs occur naturally in soils 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 can also occur in sewage biosolids. However, the concentrations of PCDD/Fs in sewage biosolids are roughly the same as those found in vacuum cleaner dust, and are considerably lower than in house furnace filter dusts (Berry et al., 1993). Nevertheless, PCDD/Fs are toxicants. Therefore, it is important that the concentrations of PCDD/Fs in sewage biosolids applied on agricultural land, and the concentrations in the receiving soils, not exceed acceptable levels.

The most widely studied of the PCDD/Fs is the 2,3,7,8,-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) congener. It often is called simply "dioxin", and represents the reference compound for this class of compounds. Dioxin-like compounds are the subset of this class of compounds which are generally agreed to produce dioxin-like toxicity. They include all of the compounds with chlorines substituted in the 2,3,7,8- positions. In order to determine the overall toxicity of these compounds, each is assigned an individual International Toxicity Equivalence Factor (I-TEF) value as defined by international convention (U.S. EPA, 1989). Each I-TEF value is an estimate of the toxicity of that compound relative to the toxicity of the 2,3,7,8-TCDD congener, which is assigned a value of 1.0.

The sewage biosolids guidelines (OMEE and OMAFRA, 1996) do not specify the maximum concentrations of PCDD/Fs allowed in: 1) sewage biosolids utilized on agricultural land; and 2) agricultural soils which receive applications of sewage biosolids. While this has lead to public concern and anxiety, concentration limits are either specified in relevant Ontario guidelines, or are used by the Ontario Ministry of 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 PCDD/F concentration 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.

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In 1995, the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRARA) conducted a sewage biosolids field survey to study pH, phosphorus and heavy metal concentrations in 36 agricultural soils which had received two or more applications of sewage biosolids. The report for that study is 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 field survey of PCDD/F concentrations in: 1) agricultural soils which had received long-term, multiple applications of sewage biosolids; and 2) sewage biosolids utilized on agricultural land. The analytical results, findings and conclusions of that field survey are presented in this report.

1.2 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. 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.

Further to 1) above, most agricultural soils in Ontario which have received sewage biosolids, have not received more than three applications under three separate OME Part V approvals. Treated soils which had received three applications were therefore targeted for inclusion in the study.

1.3 Constraints and Limitations

The study was constrained by the high cost of PCDD/F analysis, estimated at $1,000.00 per sample. This limited the numbers of soil and sewage biosolids samples. Only the minimum number of paired sites needed for statistical analysis of the data were studied. Further, only a single sample, rather than replicate samples, was taken for: 1) each of the treated and non-treated soils comprising the paired sites (20 samples); and 2) each of the sewage biosolids (6 samples), with the exception of the Southampton sewage biosolids (5 samples). The five samples taken at the Southampton water pollution control plant were analyzed individually for PCDD/F concentrations.

The paired sites were good candidates for the study. However, replicate sampling of a greater number of paired sites would have been preferred to allow more comprehensive statistical analysis, and to improve the confidence level of the findings.

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