World gives mapmakers a geochemical feast

January 5, 1996

An ambitious plan to map the globe's geochemistry could help pinpoint the causes of diseases, land desecration and minerals for exploitation.

The British Geological Survey is a key player in the mapping, which will involve dividing the world's land into 5,000 cells and taking about 125 samples - such as soil, water and stream sediments - from each.

Results from previous geochemical mappings have exposed parts of the world where vital nutrients are disappearing from the ground because of over-farming. Meanwhile in China, a 2,500 kilometre stretch that is low in selenium has been linked with two serious diseases, cardiomyopathy and osteoarthropathy. Such a link could not have been made with an ordinary geological map, which cannot highlight minor or trace elements. It takes a decrease of only two or three parts per million to define an area as deficient in selenium.

But the main purpose of the project is to provide a reference from which to measure environmental change, such as pollution. Scientists have gradually realised that in order to map anthropogenic pollution they must have a map of what chemicals are naturally in the background. If such a survey had existed before the Chernobyl disaster, for example, scientists would be able to give a much better account of the effects of the nuclear explosion.

Jane Plant, assistant director at the BGS, now co-chairs a group, the International Geosphere Biosphere Program, which recently published its final report calling for the baseline survey. The group has agreed a standard system for the world to follow, based on the BGS methods.

Earlier geochemical surveys cannot be correlated with each other. Each country or organisation has chosen its own standards and arbitrarily decided which elements to search for. An exception is studies of nuclear elements which have been done under the influence of the International Atomic Energy Authority.

A global map requires sensitive techniques to find tiny amounts of minor and trace elements in the soils. "We have established very robust sampling methods, very quantitative reproducable results," said Professor Plant.

Individual countries will do their own mapping. Funding is piecemeal at the moment. The report says it will cost $300-400 million.

Professor Plant says that the European part of the mapping could be finished by just after 2000. Other parts will take much longer.

In Britain, the BGS has mapped Scotland, Wales and half of England so far. Every sample is stored in a warehouse at its headquarters near Nottingham. Wales alone has been sampled in 15,000 places, with seven samples taken at each site, each tested for 100 different things. An element can be detected even if it makes up only a trillionth of the sample.

Douglas Miles, manager of the analytical geochemistry group, claims that the BGS has the largest geochemical X-ray fluorescence laboratory in Europe.

By blasting a sample with X-rays the scientists can make its constituent elements fluorescent: the different wavelengths of the fluorescence tell them which elements are there. A machine tests 60 samples an hour for 35 different elements.

The mass of information now stored on the BGS computer systems can be used for myriad planning decisions. In the case of arsenic for example, Martin Williams, geochemist, says: "We get frequent inquiries from people saying 'we have X parts per million of arsenic, which exceeds an EU trigger level'." He can tell how dangerous that arsenic is, based on other geological information about the area.

If you ask Dr Williams about the dangers of lead in an area, he will call up a computer map coloured according to the area's acidity, lead and sulphate levels: only in the parts exhibiting a particular blend of the three colours is there a risk that the lead may harm health.

One problem the BGS solved with its data was the case of the baffling cows in Quilquox, Caithness, where vets had to give cattle copper injections despite the fact that the soil contained plenty of copper. The survey showed that the land was high in molybedenum - which blocks the body's absorption of copper.

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