Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:2:p7
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 2 (pt 7/15)
Character Range: 1268644–1271807

potential in the food web.
    * A site-specific pathway for sloping land is pathway 5 and this should be assessed for contamination situated on slopes where down-slope migration of the contamination is possible. It will not be considered for general EIL derivation.
    * Pathway 9 requires harmonisation of air quality guidelines with the soil quality guidelines but will not be used in the current process. Inhalation is more a human health issue and therefore the health investigation levels (HILs) using human toxicology assessment of inhalation is a much more accurate measurement of potential risk.
Box 2. Soil compartments, routes of environmental exposure and the key physicochemical properties that govern the distribution of a contaminant
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Properties controlling the environmental fate and exposure routes of chemicals:
  (1)     soil porosity, water holding capacity (WHC), soilwater partition coefficient (Kd), precipitation
  (2)     octanolwater partition coefficient (Kow), soil pH, pMn+ (free ion), ionic activity, electrical conductivity, and dissolved organic carbon (DOC)
  (3)     soil pH, cation exchange capacity (CEC), Kd, organic matter (OM), clay, DOC
  (4)     diet, metabolism, octanolwater partition coefficient (Kow)
  (5)     ingestion rate (diet), metabolism, absorption through skin, soil pH, CEC, Kd, OM, clay, DOC, Kow
  (6)     sublimation constant (Ks)
  (7)     amount soil ingested, Kd, metabolism
  (8)     boiling point, Kow, Henry's gas law constant (KH)
  (9)     boiling point, Kow, surface area, turbulence, wind speed
  (10)  erosion, plant coverage, WHC, % moisture
  (11)  sublimation constant (dust to air), Kd (air to dust), density of dust
  (12)  lung type, Kd, Kow, breathing rate x volume
  (13)  wind speed, vicinity of water body.

2.3.1         Exposure pathway assessment for organic contaminants
The environmental fate of organic contaminants is largely controlled by three physicochemical properties:
    1. half-life (t1/2)
    2. Henry's law constant (H)
    3. octanolwater partition coefficient (Kow) which, in general, determines a contaminant's potential to cause secondary poisoning.

2.3.1.1         Half-life
The half-life (t½) of a contaminant is a measure of persistence of the contaminant in the environment. It represents the time taken for 50% of the contaminant to be lost from the environment. The loss may occur through biodegradation (microbially mediated degradation) or abiotic pathways (hydrolysis, oxidation, reduction, etc.). The more persistent a contaminant in the environment (that is, larger t½), the longer is the potential exposure time of species to the contaminant and the more deleterious the effects that could occur[3].

In order to classify contaminants in terms of their half-lives, the most relevant comparison is their persistence (based on half-life) to the generation time of soil 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