Four months after the supertanker Amoco Cadiz went aground on the rocks off this Breton fishing village, French soldiers are still scratching away the polluted surface sand around the little harbor with any triangular gardener's trowels.

"We'll never get it clean. The sea will have to do the rest," said a bare-chested young soldier who, along with the rest of his 100-man tank unit, had been desultorily troweling the sand for two weeks.

A couple hundred yards off the coast, the prow of the ship that spilled more oil into the sea than any previous tanker still juts into the air in the direction of the beach. The superstructure points out toward the open sea.

When the tanker broke up on the rocks, an estimated 1.5 million barrels of crude oil flowed out, polluting the shores and waters for miles in all directions.

U.S. scientists subsequently estimated that about one-third of the oil was washed ashore, while the rest was lost to evaporation and the sea. Nevertheless, the spill caused havoc among wildlife and brought severe financial losses to the shell-fishing, seaweed-harvesting and tourist industries. One estimate put the cleanup costs and business losses at more than $100 million.

Most sports along the 250 miles of heavily indented coastline that were hit are cleaner looking than Portsall. Appearances, however, are often deceiving. Sixty miles of road from the site of the wreck, the bay of Morlaix, where half of Brittany's oyster production in located, scientists estimate that there are 40,000 to 50,000 tons of oil brought in by the tidal action and trapped in the fine sandy sediment. At the time of the disaster, little surface oil was visible on the huge bay.

Scientists say that long narrow inlets like the bays at Morlaix and Lannion face the most trouble future. Bactaria that "eat" and destroy petroleum deposits require huge amounts of oxygen, and the waters of the narrow bays do not get stirred up enough to supply it.

So, the petroleum is expected to stagnate there, making some of the most beautiful maritime vistas in Brittany ecological wastelands, with unpredictable consequences for the food fish chain. The bays serve as nurseries for flat fish such as Dover sole.

In the Aber Benoit, a fjord-like estuary near the wreck, the bottom looks clean at low tide. But every footprint in the oozy mudflats is black and filled with strands of telltale petrolem iridescence.

Alain Madec, 57, a third-generation oysterman and the biggest producer in the Aber Benoit, is still destroying his oysters so, he says, "there will be no suspiciou placed on the rest of Brittany's oysters."

He spoke optimistically of resuming production in one to two years.

But Breton scientists who know the Abers well do not share his optimism. They think it will be more like a decade before edible oysters can again be growing in the mud flats of the Aber Benoit.

Except in the deep bays, the ecological effects are severe, but not as bad as at first feared. Most of the northern coast of Brittany is not going to be a desert. Some species may be replaced by others, but life will continue.

It has taken 11 years for Britain's Cornwall coast, hit by the Torrey Canyon oil spill to recover completely. Those parts of the Breton coast exposed to sea and wind action may require less time. A very important lesson was learned in the Torrey Canyon disaster and applied in Brittany at the insistance of Breton's mayors and scientists: almost no detergent was used.

The Torrey Canyon oil war cleaned up very fast with detergents but they doubled the toxic effects on marine life. What the oil did not kill, the detergents did.

Nevertheless, scientists express fear for the future of a number of species of marine animal and plant life. Lucien Laubier, head of the Oceanological Center of Brittany, said it would take at least a year to judge the full effects.

The petroleum hit Brittany at the beginning of the spring just as the eggs and plant spores were being put into the sea.

"We could see the cadavers of adult animals," Laubier said, "but the eggs and larvae are tiny, and they disintergrate fast. They are 100 to 1,000 times more sensitive to pollution than grown animals. We can suppose that a very large proportion of the eggs and the young animals were killed in a band extending three or four miles out to sea."

Yet, some species have had a population explosion, like certain shrimp that fed on the microorganisms that eat petroleum proteins. In many places, larger animals have been replaced by teeming populations of almost microscopic ones that do much the same work on the beach.

Aside from the oystermen, the hardest hit are probably the 1,000 Bretons who live off harvesting seaweed, whose extracts are used in a large variety of chemical processes involving production of glue, animal fodder and gels for toothpaste, beauty products and ice cream.

Eighty percent of France's commercial seaweed production was in the area hit by the oil of the Amoco Cadiz. This year's harvest was practically wiped out and scientists express fear that so many plant spores were destroyed that seaweed may be radically reduced for several years. That could have disastrous effects on the food chain of marine life as well as on the seaweed-extraction industry.

Fishing was perhaps the least hard hit of Brittany's maritime industries. About 10,000 fish were found dead, the evuivalent of one fishing boat's catch. The fish fled the area immediately when the oil came, and fishing was halted in the region for only a month to six weeks.

Now, say scientists, fish taken from right under the wreck of the Amoco Cadiz seem perfectly good to eat, but they look underfed.

There are those, including some local marine biologists, who note that the fishermen were quick to return to sea because they are notorious for cheating on their taxes. Since they were being indemnified by the government for spill damages on the basis of the low incomes they report on their tax returns, they hurried back to work to maintain their real income levels.

In any case, court cases involving the Amoco Cadiz will probably go on for years. The suits over the Torrey Canyon are only now being finally settled. A recent French senate investigation committee report estimated that cleanup costs and damages would total more than $100 million.

A University of Brest economist said the government was putting heavy pressure on experts to confine their estimates of losses to damage, directly attributable to the oil, like the destroyed oyster beds, and not to count businesses that are suffering because there are so few visitors to Brittany this year.

It is not only the beach towns, whose shores were polluted, where tourism is down this year. Only a quarter of Brittany's coast was actually hit. Even towns in the interior and on the south coast of Brittany, where there was no oil at oil, are reporting dramatic declines in business. In the Finistere district, which covers parts of both the north and south coasts, the chamber of commerce reports that business was down 40 percent in the hotel, and that there were 60 percent fewer foreigners. West German tourist, the richest and freest spending of all, were reported to be almost completely absent.

Merchants along the main business street of Roscoff estimated business was down 25 to 50 percent.

There are a lot of incalculable costs as well. Col. Philippe Millon, head of the French League for the protection of birds that runs the great bird sanctuary of Seven Isles, asked. "How do you place a value on a dead puffin?"

Scientists believe about 20,000 birds must have been lost. The hardest hit were the puffins, guillemots and auks - three related birds that fish at sea.

Seven Isles was established in 1913 as a puffin sanctuary. There were then 15,000 to 20,000. It is the only puffin colony on the European continent and before the Amoco Cadiz, there were 500 puffin pairs nesting there. Now, there are only 300.

The puffins have been declining everywhere because of the increasing pollution of the seas. "If pollution just continues at its steady rate of 6 million tons of petroleum products into the sea a year, the puffins, the guillemots and the auks are condemned to extinction," said Col. Millon.

Claude Chasse, the research director at the University of Brest's marine biology laboratory, spoke of the contamination of oysters. He told of how oysters from Alain Medec's beds in the Aber Benoit had been found with a dramatically high level of 300 parts per million of petroleum, a level at which they give off a very noticeable odor and are inedible.

They were put in sea water in an unaffected zone. Within 25 days, they had flushed themselves out to 66 parts per million, almost as low as the local oysters, which have 56 parts per million.

The story had two almost contradictory points. The first was that nature is good at righting itself. The second was that even at 56 parts per million, oysters are approaching the limits of what is tolerable.

"We are very close," said Chasse, "to having nothing but inedible products in the sea. The Amoco Cadiz was only the visible part of the pollution. Hydrocarbons from the rivers near our cities and from the constant offshore washing out of oil tanker holds is gradually raising the general pollution level of shellfish."