Experts say hefty, additional carbon filters -- a considerable expense for hundreds of wells across the Valley -- could be used to clean up wells tainted with TCP.

The chemical has been connected with several kinds of cancer in animal tests, including stomach, kidney, liver and pancreatic.

The chemical also is known as an industrial solvent, as well as a cleaning and degreasing agent.

In the 1940s, TCP became part of Shell's fumigant, called D-D. In the 1950s, it became part of Dow's Telone. TCP and several other chemicals in the fumigants were byproducts or impurities from the process of making allyl chloride, which is used in manufacturing plastics.

Dow and Shell were following common business practices by processing such waste into another product for sale, says a retired chemical engineer who worked and researched for 40 years in the field.

There are few options for disposal of the waste stream -- burn it, bury it or illegally dump it somewhere, like a river.

To avoid disposal expense, chemical companies regularly look for legitimate ways to incorporate the waste in other products, said Robert H. Schwaar, an expert witness in the cases. Schwaar is a retired chemical engineer and researcher from the nonprofit SRI International, which originated as Stanford Research Institute in the 1940s.

"The disposing of waste is not inexpensive," he said. "But if you can recover some costs by using the waste in another product, then it's all to the good."

The fumigant worked well because of the ingredient 1,3-dichloropropene. Chemical companies found a market in Western states.

A Shell memo in 1983 said D-D sales averaged 3.5 million gallons annually. Most farmers were using it to eliminate pests for potatoes, sugar beets, trees, vines and vegetables.

In 1981, Shell made about $6.3 million from D-D sales, the memo said. The company also avoided a $3.2 million cost for disposal of the leftover chemicals from its allyl chloride operation, the memo said.

The "garbage" memo from Dow was a brief analysis of the most effective ingredient in Telone -- 1,3-dichloropropene. It hints that the company's experts did not consider TCP and the other ineffective waste chemicals dangerous.

Instead, they were weighing the costs of leaving small amounts of these chemicals in the fumigant and fully explaining them to EPA.

"It's not that we are particularly concerned about their safety, but rather we can't justify the costs of their toxicological studies," the memo said.

The companies could not support their early claims that all the ingredients of the fumigant were effective in killing nematodes. One memo suggests that lowering the amount of unproven chemicals might protect the ground water.

Shell took D-D off the market in 1984. By the 1990s, Dow changed the formula for Telone II to take most of the garbage chemicals out. It is still in use but not considered a threat.

The TCP already in the Valley's ground water, however, probably will not degrade much over time, Schwaar said.

Should Dow and Shell have known that TCP could have posed a threat to human health? Schwaar said Dow and Shell should have at least suspected it because there are health hazards in products created with chlorine, which was used in making TCP.

Said Schwaar: "It's hard to think of very many chlorinated organic compounds that are really safe."