Greenpeace may have won the political battle to stop Shell sinking its oil platform, Brent Spar, at sea but it has not won the scientific debate argues Tony Rice.

Brent Spar is safely at anchor in Erfjord, Norway, awaiting Shell's next move. Why do I believe that the best environmental option would be to sink it in the deep ocean, as originally planned, far from our already stressed, overfished and polluted inshore waters?

First, because in relation to the size and resilience of the deep sea Brent Spar is small beer. Its effect would be negligible. Second, because disposed of in this way there is no chance of the Spar's limited toxic materials re-entering man's environment. Third, because the alternative of land-based disposal itself involves an environmental cost and carries a significant risk of accidental damage to the shallow North Sea.

Why do I say that Brent Spar is relatively insignificant? Is it not a vast structure packed with toxic chemicals that will wreak havoc in the deep ocean environment, as Greenpeace claims? Brent Spar is not an oil drilling rig, but a floating oil storage buoy and one of only a handful of similar structures in the whole of the North Sea. Although designed to stand vertically in the water, like a huge fishing float, the Spar's structure is very much like that of a ship. It is 29 metres in maximum diameter and some 140 metres high, 30 metres standing clear of the water and housing crew accommodation, a helipad and machinery, and the remainder submerged. The underwater section is basically a large cylinder made up of storage tanks, air-filled buoyancy tanks and a heavy ballast section to keep the structure upright. It weighs a total of about 14,500 tonnes, made up of some 7,700 tonnes of steel and 6,800 tonnes of haematite ballast (iron ore, chemically similar to rust) embedded in concrete.

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So Brent Spar is big, but not enormous. It weighs rather less than a modern cross-channel ferry. Tragically, the wrecks of hundreds of ships of this size litter the ocean floor all over the world. No one has suggested that these have caused appalling environmental damage.

But isn't Brent Spar full of nasty chemicals? Well, not really. Greenpeace questioned the figures released by Shell for the amounts of "contamination" in the Spar because these were based on few samples and a good deal of guesswork. It would be futile to become embroiled in this argument. No one will know the true contents until we see the results of the survey currently being conducted by Det Norske Veritas (the Norwegian equivalent of Lloyds). So to get round this particular red herring let us assume that all of Shell's figures are underestimated by a factor of four or five; my arguments are not affected.

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Shell says that the Spar's equipment, paint and sludges in the tanks contain a variety of heavy metals including about 14 tonnes each of copper and zinc, and much smaller quantities (grams/kilograms) of cadmium, lead, mercury, nickel, chromium and arsenic. At least some of these metals can harm living organisms in very low concentrations. But again let us put Brent Spar into perspective. Each year the river Rhine carries a cocktail of heavy metals into the southern North Sea which includes 25 times as much copper as Brent Spar, 150 times as much zinc, 500 times as much cadmium, 10,000 times as much mercury, 20,000 times as much lead, 30,000 times as much nickel and arsenic, and 200,000 times as much chromium.

According to Shell the tanks contain about 100 tonnes of sludge, consisting mostly of sand but with about 10 tonnes of heavy bituminous residues, a bit like road tar. In addition, Shell says there are about 40 tonnes of oil and wax lining the tank walls. It was this figure for hydrocarbons that Greenpeace particularly questioned. But whether the total figure for hydrocarbons on Brent Spar is 50, or 5000 tonnes, it pales beside the 84,000 tonnes disgorged off Shetland by the Braer in January 1993 - and where is that now?

Brent Spar also contains, horror of horrors, radioactivity! But the activity is from naturally occurring chemicals and not man-made nasties brought aboard by Shell. The chemicals found their way into Brent Spar from the seabed in the oils and associated sands and were concentrated and deposited on the insides of the tanks much like the scale on the insides of kettles, but in this case in barium sulphate. This is a phenomenon well-known to the oil industry and one that they deal with worldwide all the time. An independent ICI assessment concluded that the levels of radioactivity were so low that they would probably be exempted from the relevant legislation and could be disposed of like non-radioactive waste on land. The material in the scale would be classified as low level radioactive waste, like contaminated glassware and clothing from medical radiological labs. However, since in removing the scale there is a significant risk of the workmen inhaling the radioactivity, the material should be treated with great care on land. It would also have to be disposed of at a properly licensed and supervised site such as Drigg, near Sellafield. But the scale would not be considered a serious risk except to the workers actually handling it.

Finally, Shell estimates that about 20ml (a couple of spoonfuls) of polychlorinated biphenols (PCBs) may remain in the Spar's two large transformers. PCBs are nasty synthetic molecules similar in structure and action to DDT. They are very stable and therefore accumulate in animals and plants with serious physiological consequences. They have many commercial applications, but their production and use are now strictly controlled. Unfortunately, they are still hanging around in many shallow-water areas. The muds in the mouth of the River Oder in the southern Baltic, for example, contain a Brent Spar's worth of PCBs in every 500 tonnes.

So what do I believe would be the effect of its disposal in the deep sea? Greenpeace says that we do not know enough about the deep ocean to answer this question. I believe that after more than a century of study we do.

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The most dramatic effect would be physical. As the Spar reached the seafloor it would topple on to its side and possibly break up. Parts of it would penetrate a metre or so into the soft chalky sediments and a great plume of mud would be thrown up into the water column. More or less all of the animals living on and within the sediments in an area of 5,000 to 10,000 square metres (about the size of two football pitches) would be killed. Since life is ten to one hundred times less abundant at this depth than in shallow seas, the numbers involved would be correspondingly less than if the same thing happened, for instance, in the North Sea. Nevertheless, many millions of animals would die. Some would be relatively large animals similar to ones we see in shallow seas - sponges, sea-anemones, shrimp and crab relatives, snails, starfishes, brittlestars, sea cucumbers and fishes, though none that we have ever seen on a fishmonger's slab. But the vast majority would be tiny creatures measuring up to a millimetre or so and living within the mud. Most would be nematodes (or thread worms), probably the most abundant multi-celled animal group on earth and found in all environments, including soil. Nematodes are killed in their millions whenever we build a motorway - or dig a landfill site!

The plume of mud would be carried away from the Spar in the rather slow, near-bottom, water currents, gradually settling to the bottom once more. Though the finest particles might be carried several kilometres, most material would settle within a few hundreds of metres of the Spar, and any significant smothering effect would be even more limited. In any case, the deep sea communities in many parts of the ocean are quite used to these calamities, with mud being whipped up and moved around at a variety of space and time scales.

If the bulky materials, such as the sludge, oil and ballast, were spilled catastrophically as the structure disintegrated they would kill more or less everything in a very local area. But as with the physical disturbance, the deep sea as a whole is quite prepared to deal with hydrocarbons. Oil and gas seep through the ocean floor in many areas, attracting specialist bacterial and animal communities. Brent Spar would probably do the same, so that after a few months or years the seabed in its vicinity might have been changed, but by no means necessarily for the worse.

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Finally, the heavy metals, radioactive materials and PCBs are in such small quantities in the Spar, or would leach so slowly from it, that they would never reach toxic concentrations except, perhaps, in the immediate neighbourhood of the source. Again, the deep sea is used to dealing with seemingly toxic cocktails. Each year, hydrothermal vents on the mid-oceanic ridges pump out thousands of tonnes of heavy metals (though not PCBs). Yet vents are quite difficult to find, and they support incredibly rich communities of organisms specialised to exploit them.

What would certainly not happen is that Brent Spar's "toxic" materials would come back to man's environmental ambit. Even at the relatively modest 2,300 metres depth of the proposed disposal site there are no significant means, physical or biological, of bringing material back to the surface. The near-bottom water currents are part of the general deep oceanic circulation with, in the Atlantic, a mean time for return to the surface of 200-300 years. Similarly, although a small number of animals on the deep-sea floor send their young by stages up into the overlying water, and some midwater animals swim up and down in the water column, these upward transport mechanisms are very minor relative to the general tendency for sinking particles to scavenge material from the water and carry it down to the sediments. And fish, like all the other animal life at 2,000 metres down, are so much less abundant than in shallow water that there would be absolutely no chance of commercial fishing in the vicinity of Brent Spar for human consumption.

Brent Spar could be towed once more to the deep disposal site in its current, relatively safe, vertical orientation. In contrast, to take it to a land disposal facility it will have to be turned horizontally and towed through the shallow waters of the North Sea. There is a significant risk that the structure will disintegrate in the process and spew its contents into the already polluted inshore waters from which we obtain most of our fish and shellfish catch. Even if it can be brought safely to shore, its disposal will have environmental costs. It will consume fuel, it will cause pollution like any other environmental process, and if the Spar were simply buried it would cause the deaths of lots of little organisms just as in my deep-sea scenario. The whole process would also involve a significant risk of death or injury to the workforce that gets the job.

What about the other 50 or so structures in the North Sea for which deep-sea disposal is considered to be a possibility? Here I am less sure. First, I would like to see Brent Spar disposed of and monitored properly. Armed with this direct information from Brent Spar we could then take a much more rational look at the others. I am in total agreement with Greenpeace in advocating a reduction, indeed cessation, of pollution in the shallow inshore seas. In fact, I would rather there were no oil rigs or storage platforms in the ocean at all, even if it meant dearer petrol and a simpler lifestyle. But I suspect that few would agree with me. Consequently, whether we like it or not, we are saddled with the problem of how to deal with the Brent Spars and all the other "waste" problems that our technological world increasingly presents us with. The world's oceans cover 361 million square kilometres, twice as much as the emergent land. Shallow inshore waters, the continental shelves including the North Sea, represent only about 6 per cent of this total. The rest, with an average depth of more than four kilometres, contains 1.3 billion cubic kilometres of water, more than 300 million tonnes each for every man, woman and child on earth today. The hands-off policy of Greenpeace may seem very attractive, but I believe that to ignore the oceans in trying to solve our survival equation would be quite irresponsible.

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Tony Rice is senior benthic biologist at the Institute of Oceanographic Sciences, Surrey.