Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:4:p6
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 4 (pt 6/24)
Character Range: 956936–959952

vapour intrusion modelling used to estimate vapour concentrations in ambient air from measured soil vapour, soil, groundwater and non-aqueous phase product data
    * particulate modelling used to estimate the concentrations of dust generated from surface soil
    * groundwater fate and transport modelling, used to estimate groundwater concentrations at an exposed population location where direct measurement is not possible, or to predict future groundwater concentrations at an exposed population location
    * plant uptake modelling, used to estimate concentrations of contaminants in crops and vegetables from soil and groundwater data
    * animal uptake modelling, used to estimate concentrations of contaminants in eggs, fish and meat.
These methods are described herein, with the exception of groundwater fate and transport modelling for which guidance is presented in Schedule B2. It should be noted that the level of uncertainty associated with the use of any model for the purpose of estimating exposure concentrations should be carefully considered and discussed.

4.4              Exposure point concentrations  volatiles

    4.4.1          Introduction
Volatile substances are those that are capable of changing from liquid to vapour phase (that is, volatilising) under ambient conditions. While there are a few definitions of what may be considered of significance with respect to volatilisation, a volatile substance can be defined as having a Henry's law constant of greater than or equal to 10-5 atm/m3/mol and its vapour pressure greater than 1 mm Hg at room temperature (NJDEP 2005). In addition to these measures, a substance should be assessed as volatile if its saturated vapour concentration results in exposure concentrations that are a risk to the exposed population. Some chemicals with low Henry's law constants, or low vapour pressures, are so toxic that even a small amount that moves into the vapour phase could be enough to contribute to a risk. Hence both measures of volatility and toxicity need to be considered.

Vapours may arise primarily from three processes (refer to US EPA (2004a) for additional detail and equations):
    * by desorption from soil organic matter, described by a function of soil concentration, Henry's law partition coefficient, soil-water partition coefficient (a function of organic carbon content and soil organic carbon partition coefficient) and soil properties (bulk density, volumetric air content and volumetric water content)
    * from groundwater plumes, described by the Henry's law partition coefficient between water and air
    * from non-aqueous phase liquids, depending on the vapour pressure of the volatile compounds in the non-aqueous phase and the mole fraction of the compound of interest (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