Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:10:p1
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 10 (pt 1/4)
Character Range: 1744149–1746987

10             Trivalent chromium

10.1          Chromium (III) compounds considered
Chromium occurs in a number of oxidation states: II, III, IV, V and VI. The two dominant states in soils are trivalent (III) and hexavalent (VI) Cr. The only forms of Cr (III) for which there was toxicity data were chromium chloride, chromium nitrate and chromium sulphate.

10.2          Exposure pathway assessment
Chromium is the seventh most abundant element (McGrath & Smith 1990). It is also an essential element for humans and for some groups of organisms (Crommentuijn et al. 2000), yet the hexavalent form is generally considered to be highly toxic and a carcinogen.

The two key considerations in determining the most important exposure pathways for inorganic contaminants, such as Cr (III), are whether they biomagnify and whether they have the potential to leach to groundwater. A surrogate measure of the potential for a contaminant to leach is its watersoil partition coefficient (Kd). If the logarithm of the Kd (log Kd) of an inorganic contaminant is less than 3 then it is considered to have the potential to leach to groundwater (Schedule B5b). The log Kd reported by Commentuijn et al. (2000) for Cr (with the oxidation state not identified) was 2.04 L/kg; therefore, Cr has the potential in some soils to leach to groundwater. However, the ability of Cr to migrate from soil to either groundwater or surface water depends greatly on its oxidation state. Hexavalent Cr is highly water-soluble whereas trivalent Cr is almost insoluble in water and immobile in soil (Bartlett & James 1988; Cervantes et al. 2001). Therefore, Cr (III) is unlikely to pose an environmental risk by leaching. In addition, Cr (III) cannot cross most cells (Cervantes et al. 2001). In contrast, Cr (VI) is actively transported across cell membranes (Dreyfuss, 1964; Wiegand et al. 1985). Chromium (III) is not known to biomagnify (Scott-Fordsmand & Pedersen 1995; Heemsbergen et al. [2008]) and therefore only direct toxicity routes of exposure were considered in deriving the SQGs for Cr (III).

10.3          Toxicity data
Unlike the preceding elements, there is a lack of ecotoxicity data for Cr (III). This is reflected by the fact that the US EPA (US EPA 2008) could not derive Eco-SSL values (which require toxicity data for species belonging to three different types of organisms) for Cr (either as III or VI) for soil invertebrates and plants. Also, neither the Canadians (CCME 1999h,) nor the Dutch (Crommentuijn et al. 2000) have SQGs for Cr (III) but simply total Cr.

Extensive searches of the available scientific literature were conducted on ISI web of knowledge, the US EPA ECOTOX database (http://cfpub.epa.gov/ecotox), the Dutch RIVM e-toxbase database (http://www.e-toxbase.com – this is not publicly available), the database of the French