Heavy metals, light target

A floating plant shows Kolkata wetlands are turning toxic

By Biplab Das
Published: Monday 31 January 2005

researchers from the University of Calcutta and the National Bureau of Soil Survey and Land Use Planning have found high levels of toxic heavy metals in an aquatic plant species in Tiljala wetlands on the eastern fringes of Kolkata.

"A rise in the levels of heavy metals has caused genetic changes in a duckweed species scientifically known as Lemna minor," says Ashish Kumar Duttagupta, the lead researcher. The gene affected codes for the enzyme esterase, which is crucial to the detoxification process. The findings of the study were published recently in Environment International (Vol 30, No 6).

Spread over 250 square km, the Tiljala wetlands have become a virtual sink for urban and industrial wastes. The top layers of the waterbodies harbour large numbers of surface-active molecules that trap heavy metals. As duckweed is a floating plant, it is especially prone to contamination with heavy metals.

The study was based on about 50 samples of duckweed and water collected from the Tiljala wetlands. Nitric and hydrochloric acids were added to the samples to disintegrate and dissolve the plants. Each sample was then filtered and the concentration of heavy metals -- lead, cadmium, chromium, zinc, copper and mercury -- measured using a technique called Atomic Absorption Spectrophotometry. Same number of duckweed samples and water were collected from an unexposed domestic pond in a village near Batanagar, West Bengal and analysed in the same way. The team also exposed a duckweed population to heavy metals in laboratory conditions and compared it with an unexposed duckweed population.

In the samples collected from the wetlands, concentrations of lead and mercury were at least 20 times as high as the permissible limit -- lead was 1.14 mg per litre (allowed 0.05 mg/litre) and mercury was 0.02 mg/litre (allowed 0.001 mg/litre). The concentrations of other four heavy metals were within the limits prescribed by the World Health Organization. Lab studies also showed a similar profile of heavy metal concentration among exposed and unexposed duckweed.

"The study results are important because they stress the urgent need of biomonitoring of these wetlands," says Duttagupta. In the long run, heavy metals get more concentrated in a species, which through the food chain, can reach humans and exhibit carcinogenic effects.

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