Magnet turns detector

Measure of the soil’s remanence can quantify pollution  

By Sugandh Juneja
Published: Saturday 15 May 2010

soil and rock layer composition is governed by human activities such as disposal of factory and domestic wastes, construction and vehicular pollution. These change the magnetic properties of the soil particles. A recent study by geologists from the University of Pune measured the soil’s magnetism in the Pune metropolitan region to quantify pollution.

The team collected 118 soil samples over an area of 900 hectares during pre- and post-monsoon seasons. The samples were subjected to Isothermal Remanent Magnetisation (irm), a technique of exposing materials to electromagnetic fields of known intensities and directions at a constant temperature. Once the electromagnetic field is removed, no magnetic property should remain in the material. Some magnetization is, however, left and can be measured. It is termed remanence. That is how iron nails acquire temporary magnetic properties in presence of a magnet.

When the soil samples were put through irm, degrees of remanences pointed towards the presence of heavy metal pollutants. S J Sangode, the lead author, said: “This technique is very useful if the source of the signal can be identified.”

The currently used geochemical methods for soil analysis are pollutant-specific. “irm is rapid and apt for an initial and basic analysis,” said Aranya Bhattacharjee, physicist at Delhi University.

The study can suitably be applied only to certain settings. Sangode said it is ideal for Delhi where the bedrock is weakly magnetic and the pollutants are highly so, producing a good contrast. It is too complicated for Mumbai’s highly magnetic bedrock. “Systematic sampling and a database is urgent for each metropolitan area to benchmark the levels of pollution for any given year so that it can be compared with succeeding years,” he added.

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