Radioactive dirt and rocks from Arizona uranium mines, such as this one in 1953, were used to build homes.

For 75 years, a factory in Columbus, Miss., churned out railroad ties treated with chemicals to resist weather and bugs. Even before Kerr-McGee Chemical closed the plant in 2003, the community in northeast Mississippi knew that its soil and water had been tainted with creosote, a toxic tar.

But today, cleanup has scarcely begun. No one is sure how bad the contamination is or how much it will cost to clean up the 90-acre facility and the houses around it.

Similar questions haunt many of the contamination sites covered by the reached last week between the federal government and Corp. , which bought some of Kerr-McGee's operations in 2006. In Nevada, for example, chemicals from making bombs and rocket fuel contaminated drinking water. And in Arizona, radioactive rock from a uranium mine was used to build homes.

The record environmental deal will divide upward of $4.5 billion among more than 2,700 sites across the country, many of which have been languishing during litigation over who would pay for cleanup. The rest of the settlement will pay claims by people who got sick from the pollution.

"We felt that remediation moneys will be sufficient, even with some cushion for the unknown," said John Hueston, a lawyer for the plaintiffs. "I don't think there's going to be a lot of money collectively left over."

It would cost up to $1.7 billion...