Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:10:p2
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 10 (pt 2/2)
Character Range: 1227220–1228928

concentration addition method results in higher estimates of toxicity than the response addition method, it is not necessary to use the more complicated two-step mixed model method.

Given the above, it is appropriate to use the concentration addition model to estimate the toxicity of mixtures irrespective of the type of joint action, unless there is specific information in the literature about a mixture that shows that this model is inappropriate.

The hazard quotient (HQ) method described below is a modification of the concentration addition model that takes into account the use of EILs in the ERA framework. The HQ method requires the ratio of existing soil contaminant concentrations and the EIL for each individual chemical to be calculated.

 HQ = X/E

where X is the concentration of a contaminant in soil, and E is the EILSOIL for that contaminant.

The sum of the HQ for each contaminant is calculated. The total toxicity of the contaminants present at a site, assuming they conform to concentration addition, is calculated by summing the HQs for each contaminant. The resulting value is called the Hazard Index (HI).

 HI = HQA + HQB + HQC

where HQA  is the HQ for contaminant A (that is, XA/EA), HQB  is the HQ for contaminant B (that is, XB/EB), and HQC  is the HQ for contaminant C (that is, XC/EC).

Where HI is equal to or less than 1, ecological values are assumed to be protected.

Where HI is greater than 1, there is potential for adverse impacts to ecological values.

That is, the sum of effects of simultaneous sub-threshold exposures to several contaminants may induce an effect equivalent to greater than the maximum tolerable dose for a single contaminant given in isolation.