Tier I Look Up Table criteria were calculated based on slab on grade construction assuming one meter of clean surface soils below the slab overlying the impacted sub-surface soils.
The RBCA model developers assumed that a source would not be located beneath a building (i.e., tank installation) and that contaminant migration from the source tank would migrate below the building in a manner which results in a clean 0 to 1.0 meter soil zone below the existing slab on grade structure. The Atlantic RBCA model does not consider advection when calculating indoor air impacts.
Tier I criteria may be used for residential furnace oil contamination (#2 fuel oil) in typical cases. The Atlantic PIRI Committee considered several furnace oil tank release scenarios using Tier II assessment where the product is near the basement wall and floor (next to approximately 30% of the wall/floor area). The Tier II scenario calculations assumed impacted soil and groundwater within 10 cm of the basement floor and walls and produced higher remedial criteria than the Tier I Look Up Table.
As such, use of the Tier 1 criteria at sites is reasonable for many sites with basements, however, the Site Professional is responsible for comparing the site to the other Atlantic PIRI default conditions before using the Look Up Table. This specifically includes the condition of the floor and walls for cracks and dirt floor sections, including sumps with open bottoms.
How are basements considered at Tier II?
If a basement exists, the site-specific depth to contaminated soil or groundwater should be calculated from the basement floor, not the ground surface.
If you conduct Tier II calculations on a building with a basement you should consider the buried sections of walls that can transmit vapours to the interior. This can be done by considering the sum of the basement wall and floor area in contact with soil as the modified floor area and recalculate a new Building Volume / Area Ratio (Screen 8.4). When calculating the building volume, only the basement height should be used since complete mixing of basement air and first floor air is unlikely. Ensure that the Building Volume / Area figure is placed in the correct exposure box for your site (Residential or Commercial).