Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:6:p3
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 6 (pt 3/4)
Character Range: 3024844–3027861

by evaluating the pathways driving the HIL values (that is, percentage contributed by each) and assessing the likely reality of the proportions of exposure from each pathway. In the case of both the vegetable uptake and airborne dust pathway a number of contaminants showed unrealistic proportions of exposure. Further consideration of the model assumptions and algorithms in both cases led to the conclusion that sensitivity to input values, rather than problems with the algorithms, was the cause. The subsequent sensitivity analysis is described below.

6.5              HIL sensitivity analysis
Site-specific exposure scenarios provide the most reliable information for assessing potential human health risks. In order to allow the HIL values to be applied across a variety of Australian sites, however, generic scenarios were applied to estimate the magnitude of potential exposure.

In sensitivity analyses, the values of parameters suspected to drive exposure risks are varied and the degree to which changes in the input variables result in changes to the risk estimates are summarised and compared (US EPA 1989). Throughout the process of deriving the HILs, sensitivity analyses were performed to provide a 'reality check' for the data adopted and to identify the key parameters influencing the resultant HIL values.

The HIL values for all of the contaminants of concern are sensitive to both the toxicity criteria and background exposure allocation applied in the risk characterisation model. Similarly, human behavioural factors such as body weight, exposure frequency and duration have a significant effect on the HIL value derived. Those assumptions derived from enHealth (2012a) were not varied, since these were considered policy decisions. Other exposure parameters identified as having a significant influence on the derived HIL values differ according to the physicochemical characteristics of the contaminants and, in particular, the volatility of the individual chemicals. The approach taken for the key sensitive parameters is broadly summarised below.

    6.5.1         Soil fraction of organic carbon
The vegetable ingestion pathway is highly sensitive to Foc. Friebel and Nadebaum (2011) selected a value of 0.3% organic carbon based on data for subsurface soils from ANRA (2001) in the derivation of petroleum HSLs since many contaminated sites, particularly development sites, have been stripped of their more organic-rich topsoils. However, this value proved unsuitable for use with the vegetable ingestion pathway, resulting in unrealistic exposure percentages deriving from vegetables. While the underlying reason for this effect is in the model formulation, increasing the Foc produced much more realistic results. It was also considered unlikely that vegetables and fruit would actually be cultivated in soils with an organic carbon content as low as 0.3% and a value of Foc= 2% was selected to apply to the vegetable pathway. In areas where lower levels of organic carbon are present