A Risk Assessment for Hydrocarbon-Contaminated Soil at a Well Site in Alberta, Canada

Abstract

In Alberta, Canada, there are lots of former oil and gas well sites which have pollutants. Thus, there are a growing number of oil and gas well sites requiring remediation to remove pollutants such as benzene. Management of these sites, as dictated by provincial regulation, can be achieved through Tier 1, Tier 2 or exposure controls. Tier 1 guidelines are generic and are designed to be protective under a range of conditions. The Tier 2 option allows for site-specific factors to be incorporated into guideline adjustments. However, it is not well known how much Tier 2 reduces risks and cost expenditures. An oil and gas well in Alberta provided a study site to evaluate whether remediating to Tier 2 guidelines was effective in reducing the human health risks and project expenditures. Therefore, I compared the human health risks through screening post-remediation soil-exposure concentrations to update the exposure, toxicity/effects assessments and risk characterizations. I completed this by using a three-phase partition model to convert soil concentrations into soil vapour concentrations. Soil vapour concentrations were then used to calculate indoor air concentrations and finally to a incremental lifetime cancer risk (ILCR). I found that remediation to Tier 2 guidelines eliminated 75% of contaminants of potential concern (COPCs). I also found there was a reduction in the remaining COPC benzene to below the in surface and subsurface soils. I used soil borehole and confirmatory lab analysis to estimate soil volumes to remediate to Tier 1. This estimated Tier 1 volume was compared to the known volumes excavated during the 2023 remedial excavation. Volumes were translated into project costs by using the unit rate in Canadian dollars (CAD). I found there was a 44% reduction costs related to earthworks through the remediation to Tier 2 guidelines. Overall, this means the Tier 2 pathway was effective at reducing human health risk and while reducing project costs.