11 July 2005: Water pollution the natural way

US and Swedish environmentalists have confirmed high levels of toxic hexavalent chromium in the source of Santa Cruz County’s drinking water, echoing the plot of Hollywood blockbuster Erin Brockovich. 

Julia Roberts got the 2001 Best Actress Oscar for her depiction of real-life unemployed single mum, Erin Brockovich, who took on and won one of the biggest class action lawsuits against a major corporation. The Pacific Gas and Electric Company paid $333 million for poisoning a Californian community’s water supply with hexavalent chromium, which had made hundreds of people ill. 

But in this latest story - reality mimicking art mimicking reality - the source of contamination turns out to be natural, not man-made.

Environmental chromium comes in two forms, which differ in terms of geochemistry and effect on living organisms. Cr(III) is an essential trace element, while Cr(VI) is a potent carcinogen. Cr(VI) can also permanently damage eyes on direct contact, severely irritate and ulcerate the respiratory tract, and cause dermatitis and ulceration of the skin, leading to kidney damage. 

Cr(III) is the stable, relatively insoluble form of chromium at most environmental pH ranges. However, under certain oxidising conditions, it is the toxic Cr(VI) form of chromium that is more stable and also more soluble than Cr(III). These differences in solubility and toxicity led the California Department of Health in 2001 to request measurements of Cr(VI) as well as total chromium levels in water. 

The maximum limit of Cr(VI) in drinking water set by the state is 50 ?g/L. A preliminary 2001 study of Cr(VI) levels from the Aromas Red Sands aquifer serving Santa Cruz County showed high readings of between 6 and 36 ?g/L. 

Ana Gonzalez at the University of California, Santa Cruz, and colleagues report confirmatory data from the same aquifer. They sampled the groundwater at 11 sites in Santa Cruz County, 10 from the Aromas Sands aquifer and one from the adjacent Purisama aquifer to act as a control. The Purisama aquifer, unlike Aromas Sands, has virtually no detectable levels of Cr(VI) in its groundwater. 

The team measured Cr(VI), total chromium, iron and manganese levels in both aquifers. They collected and analysed sediment samples from both aquifers to determine qualitatively the particulate speciation of these three metals. 

Between 70-100 per cent of the total chromium (between 5 and 39 ?g/L) in the Aromas Sands aquifer is Cr(VI). How it got there involves a subtle interaction of naturally coupled redox reactions between various oxidation states of iron and manganese with chromium, which effectively rules out human origin. 

Fe(III) and Mn(IV), the oxidising states of iron and manganese respectively, are insoluble and found in the aquifer’s sediments. Sedimental Fe(III) and Mn(IV) particulates can oxidise Cr(III) to Cr(VI) at the right pH. Low concentrations of Fe(II) and Mn(II) species (the more soluble reducing forms of iron and manganese) in the water are mirrored by relatively higher concentrations of toxic Cr(VI). 

Conversely, when water concentrations of these reducing forms of the two metals are raised, Cr(VI) concentrations are reduced. 

This doesn’t necessarily mean that Brockovich got it wrong. ’Our findings do not discount the occurrence of industrial pollution in other instances,’ said Gonzalez cautiosly. ’But they do demonstrate that relatively high levels of hexavalent chromium may be due to natural processes, especially in areas where there are relatively high levels of naturally occurring Cr(III) in the sediments, and natural processes that can convert Cr(III) to Cr(VI).’ Lionel Milgrom