Standardizing Risk Assessment Approaches base on Residual Mass vs. Numerical Endpoints

Conventional approaches for managing salts in soil at Alberta Sites, such as Tier 1 guidelines and the subsoil salinity tool (SST) are based on remediating to numerical guideline values. While convenient at simple sites, remediating to numerical guideline values will always be unnecessarily conservative, due to simplistic models assuming blocks of soil with uniform concentration.

In contrast, the residual mass approach takes the actual current distribution of contaminant, and models the predicted future movement of the plume to make predictions of maximum future concentrations at receptor locations. If maximum predicted future receptor concentrations exceed allowable limits, then contaminant hotspots are sequentially removed from the model until the allowable limits are no longer exceeded. The hotspots removed from the model to achieve this goal define the soil to be remediated. The residual mass approach was developed for salt contamination, but can equally well be applied to other contaminants.

Residual mass approaches to risk-based remediation are relatively new in Alberta and there will be value in standardizing approaches and clarifying expectations for implementation to facilitate regulatory review and acceptance.

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