ALBONS, Spain Enric Navarro was dumbfounded when the letter arrived from the testing lab of the Spanish organic farmers association in late February, informing him that his organic crop contained 12 percent genetically modified corn. Hearing that his plants had been modified by biotechnology was almost as traumatic for Navarro as finding they contained nuclear waste.

For four years, he has lovingly planted hundreds of varieties of trees, shrubs, flowers and herbs to attract just the right mix of insects so that he would not need fertilizers or weed- killers on his precious seven hectares. "If I could not farm organic, I would not farm," Navarro said. "I could not sleep at night if I sold that crop."

He burned the corn in the field to rid his farm of what he calls a "contaminant." But he does not know how the genetically modified seed blew in. He cannot claim compensation for his losses. Also, since pollen lingers, he is not sure when, if ever, it will be safe to use the field to farm organic corn again.

As the European Union cracks open the door to genetically modified crops, Navarro's tale serves as a caution about the risks, scientific uncertainties and the hazy policies now in place to deal with problems that will almost certainly arise.

For eight years, Spain was the only EU member state to allow commercial cultivation of genetically modified crops. In the last 18 months, the European Commission has approved 11 genetically modified seeds for planting in the bloc. In 2005, France, Germany, Portugal and the Czech Republic began planting small commercial plots.

The cornerstone of the EU's policy is the political conviction that genetically altered crops and conventional crops can coexist as long as proper safeguards are in place - such as keeping a distance between the two types of fields and imposing a liability scheme for accidents.

But scientifically, there are strong disagreements about whether coexistence is possible, and at what cost.

"Coexistence is feasible in the vast majority of places, so long as farmers talk to each other and cooperate," agreeing, for example, not to place GM and conventional seeds of the same crop in adjacent fields, said Simon Barber of EuropaBio, an industry group in Brussels. Ordeals like Navarro's, he said, should be rare.

But many scientists - not just those with Green credentials - believe that the small, closely spaced farms of Europe make such coexistence difficult if not prohibitively expensive.

"My experts all agreed that coexistence often just doesn't work, it isn't possible," said Chantal Line Carpentier, an agricultural economist who assembled an independent panel of international experts to study the issue in North America.

The study was requested by Mexico in 2002, after GM corn was discovered contaminating fields of native crops in Oaxaca, hundreds of miles south of the United States. Mexico had not permitted GM cultivation, for fear that the heartier, but more uniform, genetically modified variants would edge out its dozens of unique strains of maize.

That report, "Maize and Biodiversity," prepared by the North American Council on Environmental Cooperation, concluded that the GM corn - which came from the United States - might have a long-term effect on Mexico's ecology and biodiversity and should be more thoroughly studied and monitored.

The United States and Canada attacked its conclusions. "We are deeply disappointed that the CEC secretariat has produced a report that ignores key science about biotechnology," reads a letter of protest from the U.S. Environmental Protection Agency.

But some farmers believe the report did not go far enough. "Saying that GM and non-GM farming can coexist is nonsensical," said Julian Rose, an organic farmer from England who has helped organize Polish farmers against modified crops. "It's like saying that noise and silence can coexist in a room."

The biotech industry considers that "coexistence" has been achieved if mixing is below 0.9 percent. It argues that foods in the EU could be labeled GM- free if they contained less than that amount. The concept infuriates advocates of organic foods, who liken it to allowing a bit of meat in vegetarian products. But even industry analysts admit that 100 percent GM free foods are not practical once GM farming begins on a large scale. "Coexistence has become a problem in Europe because some people want zero percent tolerance," Barber said. "And that is, quite frankly, unobtainable."

There are simply too many ways that mixing occurs: Mills grind crops from different farms, a cookie contains oil made from imported GM soy. The GM corn in Oaxaca was most likely the progeny of GM ears that had been legally imported for animal feed, whose kernels had been illegally used for planting.

With so many routes, environmental groups say it inevitably spreads past the 0.9 percent limit and to areas where it is unwelcome.

"When the government of Catalonia says there's no evidence of genetic pollution, what they mean is they didn't look," said Anna Rosa Martinez of Greenpeace in Barcelona. Last year, Greenpeace tested 40 organic farms, and nearly 20 percent had some level of contamination, from 0.7 to 12 percent.

Suzette Jackson, a spokeswoman for Greenpeace International, said: "We would like to keep Europe as a supplier of non-GM food, and when you look at countries with a lot of GM crops, it eventually becomes really hard, or hugely expensive, to maintain regular farming."

Spain allowed GM cultivation in 1998. Twelve percent of corn is now GM - 50,000 to 60,000 hectares - about half of that in Catalonia.

Farmers are free to choose what to plant, but representatives of GM seed companies now regularly hold dinners touting the benefits of modified seeds, which are patented. Some variants produce pesticides, others have stalks that resist wind or need less water.

While some farmers signed on, others - like Navarro - said no, preferring the independence and quality they see with traditional seeds.

Traditional farmers in Mexico, and many in Europe, save seed from one season's crop to plant the next. It is cheaper and allows selection of unique varieties. Such replanting is forbidden under the agreement for GM corn seed that farmers sign with companies like Monsanto and Syngenta.

In 2004, knowing that GM corn was growing in his area, Navarro planted just a small patch of land to see if he could grow without contamination. Successful, he later planted two large corn fields. "But it was very windy here last fall," he said, "so perhaps it blew in some stalks from another field, and contaminated me. I don't know, I will never know."

His two fields are 70 and 100 meters, 230 and 330 feet, from his neighbors' farms, a distance often deemed adequate to prevent mixing. But the GM seed could have come on the wind or on a truck tire, from anywhere.

He would like an investigation to prevent a recurrence. But there is no reliable log of which farmers plant GM seeds in the area, and farmers are not likely to confess, for fear of being sued.

In Denmark, to prepare for GM farming, the government is creating a liability pool that all GM farmers will have to pay in to.

The EU agriculture commissioner, Mariann Fischer Boel, has told EU states to try to guarantee coexistence, but it is unclear how, or at what cost. Can farmers afford to maintain buffer zones of 100 meters between fields? Would it work to create zones specifically designated for GM crops? Will the GM crops harm the environment?