Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:4:p7
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 4 (pt 7/24)
Character Range: 959713–962677

(US EPA 2000).
To assess exposure to volatiles, it is necessary to estimate the concentration of the vapour in the air that the exposed population breathes. The most direct approach to the quantification of these exposures is to use direct measurements of indoor or ambient air. Vapours in ambient air are relatively easy to sample; however, the collection and interpretation of this data can be difficult.

An indoor air sampling program may be expensive if many samples over a reasonably long period are needed to get representative results. In homes and workplaces, gaining access can be difficult and may lead to unnecessary concern on the part of the occupants.

Depending on the volatile compounds considered, ambient air results may be difficult to interpret since many other sources in addition to the site soil and groundwater can be present. For these reasons, ambient air measurements are not generally available. When they are available they may be unsuitable as a means to assess risks associated with soil and groundwater source alone, because of the inability to distinguish whether a soil- or groundwater-derived component is present or not. Where affected by background sources, the collection of indoor or ambient air measurements may not be considered the most appropriate approach.

Where direct measurements are not available, indoor and outdoor ambient air contaminant concentrations can also be predicted by modelling from measured soil vapour concentrations. Soil vapour measurement is the preferred route in most situations where a vapour issue (from a subsurface source) is considered likely to exist. However where soil vapour measurements are used in risk assessments, the relevance of the data with respect to the CSM and the data quality need to be evaluated prior to use. Guidance on the development and use of vapour CSMs is provided in Schedule B2 and references there-in.

In the absence of measured soil vapour concentrations, it is also possible to model the generation of vapour from soil, groundwater and non-aqueous phase liquids. This procedure adds another level of uncertainty to the process, and may lead to inaccurate results.

The uncertainties associated with the use of a model for these purposes should be well understood and discussed in relation to the nature of the volatile contaminants assessed.

Where unresolved uncertainties or unacceptable risks are predicted by modelling vapour concentrations, direct measurement of soil vapour and/or exposure concentrations indoors and outdoors should be obtained.

Additional information on developing a multiple-lines-of-evidence approach to assessing vapour risk can be found in Schedule B2.

    4.4.2          Indoor air concentrations:
The direct measurement of indoor air concentrations may be appropriate where practical and where the data can be adequately interpreted. When assessing impacts from contaminated land it is important that background influences on