Lunch and Learn: Ontario’s Evidence Informed Approach for Source Allocation Factors: Moving Beyond the 20% Default

Ontario’s Evidence-Informed Approach for Source Allocation Factors (SAFs): Moving Beyond the 20% Default

Presented by Marco Pagliarulo:

This lunch‑and‑learn introduces the Ontario Ministry of the Environment, Conservation and Parks’ (MECP) draft revised approach to selecting source allocation factors (SAFs) for the Brownfields program. The presentation highlights a shift from default assumptions to an evidence‑informed framework that more transparently integrates background exposures into the selection of SAFs.

SAFs play a central role in deriving human health-based, media-specific criteria by ensuring that total exposure to contaminants across environment media and background sources remains within acceptable limits defined by threshold toxicity reference values (TRVs). In Ontario’s Brownfields program, SAFs are applied to develop criteria for soil and groundwater.

Historically, default SAFs (commonly 20%) have been applied in Ontario’s Brownfields program, reflecting a simple but often overly conservative approach.

This presentation introduces Ontario’s draft revised approach to SAF selection, based on a modified subtraction method. The method retains the conceptual foundation of subtracting background exposure from the TRV but incorporates two key refinements:

(1) comparison of the TRV to a reasonable upper estimate of total background exposure; and

(2) selection of one of three informed default SAFs (20%, 50%, or 80%) based on the proportion of the TRV remaining after accounting for the proportion consumed by background exposure.

The approach integrates multi-media background exposures, including diet, house dust, treated tap water, indoor air, and consumer products, with a focus on toddler exposures as they are in a vulnerable life stage. A structured process is used to identify dominant exposure sources, construct a reasonable upper estimate of total background intake, and assign one of three possible SAFs using transparent decision criteria. This framework explicitly acknowledges the uncertainty and variability in exposure data while avoiding both excessive conservatism and over-allocation of the TRV.

Key assumptions, practical simplifications, and limitations will be highlighted, along with applications within Ontario’s Brownfields program. Overall, the draft revised approach represents a shift toward a more transparent, consistent, and evidence-informed framework for SAF selection.