Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:1850:p51
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 1850 (pt 51/117)
Character Range: 489268–492366

degradation products as appropriate.

    9.3.3          Undeveloped land
Assessing the potential for vapour intrusion to a future building on vacant land poses unique challenges. Some of the investigative tools of the vapour intrusion pathway (for example, indoor air and sub-slab sampling) are not possible when there is no slab or structure present, though others (soil, soil vapour and groundwater sampling) may be able to be used with appropriate precautions or adjustments.

As for existing buildings, a multiplication factor (x10 or x100 as appropriate) due to biodegradation may be able to be applied to relevant HSLs if the proposed maximum building size can be determined with a high degree of certainty, and the exclusion/inclusion criteria listed in Davis et al. (2009c) can be fulfilled (refer Schedule B4).

    9.3.4          Preferential migration pathways
If there is significant vapour migration via preferential pathways that connect a contaminant source to a building, then the measurement of contaminant concentrations in soil vapour may not be representative of vapour concentrations that would migrate into the indoor environment. Other investigative techniques (for example, vapour measurements in utilities or indoor air measurements) may provide more representative data for the evaluation of the inhalation exposure pathway in these circumstances (API, 2005).

    9.3.5          Environmental factors
VOC concentrations in the environment are highly variable, and collecting sufficient data to thoroughly understand and predict their temporal and spatial distribution can be time-consuming and costly.

VOC levels in ambient air can vary greatly over time. Diurnal fluctuations occur due to changes in vehicle traffic (for example, rush-hour effects), commercial activity, and as a result of atmospheric heating and cooling cycles, air pressure changes and wind speed. These fluctuations and their impact on the data analysis can be reduced by collecting time-integrated samples. Additional information on environmental factors and their effects can be found in Davis et al. (2009a).

To compensate for these inherent uncertainties, consideration should be given to identifying and characterising the main factors that may lead to a reasonable worst-case exposure scenario. The time period selected for sample collection should be appropriate to characterise the site-specific exposure scenario.
Rates of vapour intrusion are affected by both short-term and seasonal changes in weather conditions. Changes in barometric pressure associated with the arrival of weather fronts can move gases into or out of the vadose zone. This phenomenon, known as barometric pumping, increases the rate of vapour emission as low pressure systems arrive and decreases rates when transiting to higher pressure. This effect is only of importance for soil vapour where sampling is shallow (less than 12 m). Wind can also enhance vapour intrusion rates by depressurising a building relative to the underlying soil, causing more vapours to enter the building from the sub-surface. Similarly,