Findings: Bark reveals pollution level

April 5, 2002

The Darley Dale lead smelter has been recycling metal for years. It sits in the bottom of a glacial valley, yet the contour map published in the Journal of Environmental Monitoring shows the plant at the summit of a lofty peak, writes Steve Farrar.

This is bad news for nearby villagers as the study is testing a new way to measure pollution with great accuracy. The contour lines mark out concentrations of lead.

The technique is based on the fact that airborne particles accumulate in the bark of trees. They become incorporated into the trunk and provide a time-sensitive record of pollution.

The information can be extracted from a small sample of bark through a spectroscopic analysis in the laboratory.

Research by Cameron McLeod, professor of analytical science at Sheffield University, and colleagues, has turned this method into a practical way to assess pollution.

Levels of a particular contaminant can vary greatly from tree to tree. The team standardise them by measuring the ratio of the pollutant to another airborne substance ambient in the environment.

As both chemicals should be affected by local conditions to the same degree, a high ratio will reliably reveal heightened levels of the target substance.

Any chemicals can be studied - from uranium to dioxin - but in the proof-of-principle study around the Derbyshire smelter, the pollutant was lead and the comparison chemical aluminium from wind-blown soil.

The scientists collected samples of bark from ten-year-old oaks and ashes over an area of 4 km2 around the smelter. Analysis revealed lead concentrations ranging up to 2.5 per cent of the bark's weight.

The contour map plotted from this data - the first of its kind - has surprised the scientists by revealing particularly high levels of pollution.

Their analysis of tree bark from sites around the world, revealed at a recent conference, found the least contaminated site - the Atlantic island of Tristan da Cunha - had 10,000 times less lead than the area immediately around the smelter.

Professor McLeod said the inhabitants of two nearby villages were outside the worst affected zone but he was still concerned for their health.

Lead can damage neural tissues and impair brain development. Professor McLeod said a follow-up study should measure the lead content of local children's blood.

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