Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:2:p8
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 2 (pt 8/15)
Character Range: 1271534–1274617

organisms. Soil organisms do vary greatly, with some microbes having generation times of hours, while earthworms have a generation time of approximately one year. A generic generation time of three months for soil organisms (microorganisms were not considered) was selected and the resulting categories of biodegradation rates can be found in Table 2 below.

Half-lives of contaminants depend on the soil physicochemical properties and therefore preference should be given on half-life values based on Australian soils. However, if this information is not available for Australian soils, then appropriate overseas studies can be used.
Table 2. Biodegradation rates, half-lives and the classification to be used in assessing the importance of the various exposure pathways for organic contaminants.
94% of contaminant degraded in (months)  t1/2 (days)  t1/2 Classification
<3                                       <22.5        Fast (F)
36                                      22.545      Moderately fast (M)
>6                                       >45          Slow (S)

2.3.1.2         Henry's law constant
Henry's law constant (H) is a measure of the volatility of the contaminant. The higher the volatility (or value of H) the more of the contaminant will volatilise and be found in the soil air and in the atmosphere. H is a temperature-dependent constant.

Together with the t1/2 of the contaminant, H is used to assess the transfer and persistence of the contaminant in the soil, as vapour transport for many contaminants may constitute an important pathway of loss and exposure to organisms.

Several researchers have used different cut-off values of H to class contaminants into volatile and non-volatile categories but, in most cases, for aquatic environments. Jury et al. (1983, 1984) categorised the behaviour of trace organic contaminants in soils using H (among other properties) and this is useful to assess the importance of the various exposure pathways for organic contaminants (see Table 3 below). Jury et al. (1983) used the Henry's law constant in dimensionless form as the ratio of concentration in the gas phase to concentration in the liquid phase, both in units of molar concentration, that is, H = (molar concentration in air)/(molar concentration in water)[4]. This is the most relevant form for estimation of the mass distribution of a chemical.

The dimensionless form of H based on concentrations (on a molar concentration basis) is the most commonly used of the dimensionless values (Staudinger & Roberts, 1996). The US EPA has published a calculator where Henry's law constant, H, can be estimated in different unit forms and at different temperatures. This can be accessed at www.epa.gov/athens/learn2model/part-two/onsite/esthenry.htm.
Table 3. Henry's law constant (H dimensionless) values to be used in assessing the importance of the various exposure pathways for organic contaminants
Henry's constant value  Classification
(cm3 air/cm3 solution)
 >2.5 x 10 -3           Highly volatile (H)
2.5 x 10-72.5 x 10 -5  Moderately volatile (M)
<2.5