Abstract

Hydrogeochemical investigations of the Yilgarn Craton and its margins have been dominantly in areas of deep (20–100 m) regolith, within unconfined aquifers where water tables are commonly 10–60 m below surface. These groundwaters contacting weathered Archaean rocks have a surprising (on the basis of its normal aqueous chemistry) lack of correlation between Cr content and acidity. Trivalent Cr has a soluble chemistry similar to that of Al, being extremely insoluble under neutral conditions, with appreciable solubilities only below pH 4. In contrast, dissolved Cr concentrations in the studied Yilgarn groundwaters can be very high, with no pH relationship, and are strongly correlated with the presence of ultramafic rocks. Groundwaters in contact with fresh and weathered ultramafic rocks contain consistently high (0.01–0.43 mg l−1) dissolved Cr concentrations, whereas waters in contact with other lithologies have Cr concentrations below detection (< 0.005 mg l−1). This effect is highly robust at the Yilgarn sites tested and offers a potential method for recognizing the presence of ultramafic rocks, even where they are intensely weathered.

If the dissolved Cr were present as Cr3+, those groundwaters with Cr concentration greater than 1 μg l−1 and pH above 6 would be strongly over-saturated with respect to secondary Cr oxides. Comparison of ICP-AES and spectrophotometric analyses indicate that Cr in these groundwaters is present as Cr6+ (i.e. CrO42−), which has a much higher solubility than Cr3+. A high oxidation state of dissolved Cr is also suggested by its highly anti-pathetic relationship with dissolved Fe, possibly due to the capacity of dissolved Fe2+ to reduce CrO42− to the less soluble Cr3+ ion. A similar anti-pathetic relationship is observed between dissolved Cr and Mn. In non-reducing (i.e. Fe– and Mn–poor) groundwaters, CrO42− will be relatively stable and potentially mobile. However, the mechanism by which CrO42− is released into groundwater is not known.

These naturally occurring concentrations of dissolved Cr6+ are, in many cases, well above the WHO maximum concentration allowed in drinking water (0.05 mg l−1). In at least one area (Lawlers mining district, Western Australia), otherwise potable groundwaters cannot be used for human consumption due to concentrations of Cr6+ up to six times greater than that allowable.

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