Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:4:p7
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 4 (pt 7/8)
Character Range: 1048548–1051666

speciation will normally have to be made using available understanding of the site conditions and site history. It is recommended that where analytical information is not available, risk assessments considering metals should account for metal species using a reasonable worst-case assumption regarding the presence of toxic forms of the metal. TRVs should then be selected for relevance to the assumption presented.

The reasonable worst-case assumption should be derived from the following:
    * knowledge of the soil type, organic carbon content and moisture content
    * the pH and redox conditions
    * the form that the metal was in when it was released to the environment.
Other indicators such as iron content and the presence of other contaminants that may influence metal speciation may also be useful.
Table 7. Elemental species that may be present and influence the toxicity of elements in soil (WHO 2006)
Element   Aerobic soil                   Anaerobic soil
Arsenic   Ca3(AsO4)2, Mg3(AsO4)2, As2O5  As2S3
Cadmium   Cd(OH)2, CdCO3                 CdS
Chromium  Cr(OH)3 (low to neutral pH)    Cr(OH)3
Lead      PbO, PbCO3, Pb3(CO3)(OH)2      PbS
Mercury   HgCl2, HgO, Hg(OH)2            HgS
Nickel    NiO, NiCO3, Ni(OH)2            NiS

The reasonable worst-case assumption should be to assume that the metal in soil is the most toxic form that could be present given the understanding of the conditions. Forms that are not stable under environmental conditions, or that would require implausible soil processes to produce, should be discounted.

An alternative means of determining elemental speciation of aqueous solutions is through geochemical equilibrium speciation modelling. Examples of such models include MINTEQA2 (Allison et al. 1991) and PHREEQC (Parkhurst & Appelo 1999). Where reliable analytical methods cannot be found to conduct relevant elemental speciation analyses, geochemical modelling methods can be used to predict species in solution and phases that are likely to precipitate from solution. The accuracy of such methods is largely dependent on the accuracy of the input data. Such geochemical modelling methods are applicable to both soil solutions and aqueous environments.

Typical input data required to run geochemical models includes basic geochemical field parameters (pH, electrical conductivity, redox potential, dissolved oxygen and temperature), major ion chemistry (sodium, potassium, calcium, magnesium, ammonium, chloride, carbonate, bicarbonate, sulfate, nitrate, nitrite, phosphate), iron, manganese and the trace metal chemistry of the soil solution. The essential data needed to perform a speciation calculation is the temperature, pH, and the concentration of elements and element valency states.

Where there is reasonable evidence to suggest that a metal contaminant may be present as a species that is less toxic than the toxicity assumed by the HIL, analytical or modelling methods may be useful in characterising the geochemistry of the particular environment. In this way, a more accurate prediction of the toxicity of the metal contaminant can be made.