Thousands of homes, schoolyards and empty lots in 10 western states will have to be cleaned of radioactive uranium milling wastes under standards proposed yesterday by the Environmental Protection Agency. The seven-year project could cost $100 million, EPA estimated.

The sand-like wastes, called tailings, were left behind in massive piles when old uranium processing mills shut down in the 1960s. EPA estimated that 30 million tons of the material are heaped on 1,000 acres around 25 abandoned mills, blowing and washing freely over nearby communities.

Residents have treated the piles as free dirt over the years, putting the waste into landfills and house foundations and mixing it into cement. They did not know or did not care that the material contains 85 percent as much radioactivity as the uranium ore extracted from it.

Depending on the degree of contamination, tailings around and under buildings will be either dug out or sealed over, according to Donald Groelsema, remedial action program manager of the Department of Energy, which has responsibility for cleanup. Less severe cases may be handled by increasing ventilation in the buildings to get rid of the radioactive radon gases from the tailings.

Radon is a radioactive decay product of uranium. It emits alpha particles which have a very short range and can be stopped by a piece of paper, but they are dangerous when they adhere to dust particles and are inhaled. Radon levels below those found near mill tailings sites have been linked to increased lung cancer and leukemia rates.

Radon levels in some buildings surveyed in the 1970s, in fact, were found to exceed safety limits set for uranium mines. Many residents blamed apparently higher cancer rates on the tailings piles.

Groelsema said residents of most contaminated buildings probably would have to move out while the cleanup work is done. In a partial cleanup at Grand Junction, Colo., costs were roughly $15,000 per house and $75,000 per commercial building, including relocation expenses, he said.

Under the 1978 Uranium Mill Tailings Act, the federal government will pay 90 percent of the costs while the state government shoulders the other 10 percent. Groelsema said there was no way to know precisely how many sites will be eligible for cleanup under the law, which requires work at any location where radon levels are more than 0.05 of one "working level." The term refers to standards for uranium mines, which define 0.015 as safe for houses.

A 1971-73 EPA survey of towns around the abandoned mills found 22,795 "anomalies" of high radiation compared to normal background readings. However, the so-called Lucius Pitkin study, named for the firm that evaluated the findings, did not distinguish between natural and artificial sources of radiation, Groelsema said.

In Grand Junction, for example, the Pitkin study found 14,542 "anomalies" but DOE has found only about 800 sites for eligible for cleanup, Groelsema said.

Roger Williams, EPA's regional administrator in Denver, noted that yesterday's standards involve only the cleanup of contaminated areas away from the mills. Standards for disposal of the tailings piles themselves will be issued this summer and are expected to involve detailed specifications for burial.

The sites are massive, and disposal may cost as much as $600 million. One tailings pile in Salt Lake City covers more than 100 acres, EPA officials said.