Snails increase metal toxicity of soils

Contaminated soils may contain greater fractions of toxic heavy metals than is commonly estimated, according to a new report. Metals typically classified as inaccessible to organisms because they are tightly bound to soil may be absorbed by snails, thus increasing the fraction of metal that enters the food chain.

Contaminated soils are typically rated according to their ‘bioavailable’ metal content – the amount of metal dissolved or suspended in water percolating through the soil that could be absorbed by living organisms. The proportion of pollutants that bind tightly to soil particles is presumed to be inaccessible to organisms feeding on soil.

But according to a paper to be published in Environmental Science and Technology, Renaud Scheifler and colleagues at the University of Franche-Comté in France have found that snails are able to absorb cadmium bound to soil particles. In a two week experiment studying snails living in cadmium-contaminated soil from a disused lead and zinc smelter, the team found that the snails had adsorbed 16% of the soil-bound cadmium.

The findings, reported in New Scientist, suggest other heavy metals could also be more bioavailable than was previously assumed. But more work is needed to establish whether risk assessment procedures for contaminated soils need rethinking, says Scheifler. His team will now investigate other metals such as zinc, copper, lead and mercury using a range of organisms. “We have to work with other invertebrates to find whether the snail is a particular case,” he says.

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