Tier 2 Modification Models, Exposure Monitoring and Domestic Use Aquifers in Alberta

Tier 2 offers a way to move beyond generic Tier 1 soil and groundwater remediation guidelines by excluding exposure pathways or adjusting stable, readily measured site parameters. This course explains which model parameters can be adjusted, such as source dimensions, depth to groundwater, hydraulic gradient, hydraulic conductivity, soil organic carbon, bulk density, moisture content and soil vapour permeability, along with their allowable ranges and the linked parameters that must be measured together. It then works through the fate and transport models used for human, ecological and groundwater exposure pathways, including direct contact, indoor vapour inhalation, food ingestion and the protection of potable water, aquatic life, livestock watering and irrigation, and the sub-fraction approach used for petroleum hydrocarbon (PHC) fractions.

The course also sets out the adjustments that are not supported at Tier 2, including receptor characteristics, exposure scenarios, receptor locations, building properties and recharge rate, and the site conditions that call for a site-specific risk assessment (SSRA) instead, such as fractured bedrock and other preferential flow paths, contamination close to a building foundation, discharge to stagnant water bodies and sensitive ecological habitats. Data requirements for contaminant characterization and physical site conditions are covered, followed by the use of soil vapour and groundwater measurements at or near the point of exposure. It concludes with how a Domestic Use Aquifer (DUA) is defined by hydraulic conductivity, thickness and sustained yield, and when the DUA drinking water pathway can be excluded, including interbedded units, large-diameter bored wells, municipal by-laws, peat and muskeg, and shallow saline groundwater.

What Will Be Covered:

• Model parameters that can be adjusted at Tier 2 and their allowable ranges for Alberta sites

• Inter-dependent parameter sets, including soil porosity and moisture, source dimensions, and hydraulic conductivity and gradient

• Calculation of guidelines for petroleum hydrocarbon fractions from aliphatic and aromatic sub-fractions

• Human exposure pathway models for direct contact, indoor vapour inhalation and ingestion of produce, meat and milk

• Ecological and groundwater pathway models, including leaching, dilution and lateral transport to surface water

• Site-specific adjustments not supported at Tier 2 and conditions requiring a site-specific risk assessment

• Contaminant characterization and physical site data needed to support Tier 2 modification

• Point-of-exposure and exposure pathway measurements for soil vapour and groundwater, and the monitoring they require

• Criteria for defining a Domestic Use Aquifer based on hydraulic conductivity, unit thickness and sustained yield

• Exclusion of the DUA drinking water pathway and special cases such as bored wells, by-laws, peat and saline groundwater