SEATTLE, FEB. 12 -- A Washington state family that operates a sand-mining company has acquired a swath of the National Dunes Recreation Area on the Oregon coast for post-Civil War-era prices, and environmentalists and coastal residents are blaming Ulysses S. Grant.

Or at least they blame the Mining Law of 1872 that Grant, as president, signed into law to spur settlement of the West.

Using the 119-year-old law and three decades of perseverance, the Duval family of Bellevue, Wash., a Seattle suburb, has acquired a patent to mine sand -- and the silica within it -- from a part of the coast at the mouth of the Umpqua River.

The Duvals have agreed to place ownership of the land in escrow until April 10 to give federal agents time to engineer an exchange. Federal representatives realize getting back the part of the recreation area will cost 1991 prices.

"The law was passed back in a time when the exploration and exploitation of the West was encouraged," said Tony Vander Heide, a U.S. Forest Service officer. "I don't foresee a mining operation ever occurring there, because we have some strong land-use laws, but it will cost taxpayers money."

The 780 acres of rolling sand hills were sold, in accordance with the law, for $1,950. That breaks down to the required $2.50 an acre, the fair market value for western grazing and farm land in 1872.

The land is valued at $350,000 to $750,000, but the family believes the site's potential mineral value of about $12 million -- as estimated by the General Accounting Office -- should be considered in any exchange or sale.

The Duval family was able to prove that the silica, used in glassmaking, is marketable and "uncommon" in the dune area because of the absence of iron and other contaminants.

The site is amid a 40-mile recreation area that attracted more than 2 million visitors in 1989. It lies in a roadless area maintained for its scenic beauty and used by migrating birds.

The battle is not as unusual as it might seem. The GAO, the investigative arm of Congress, estimates that $48 million worth of federal land has been sold to individuals and companies for only $4,500 since 1970 under terms of the law.

The measure has been widely abused by speculators, especially in California, Nevada and Colorado, the GAO says.

However, the Duvals are not speculators but victims of 32 years of governmental delays and heavy-handedness, according to their Portland attorney, Jerry Fish, who said Maurice Duval of Bellevue first filed the claim to mine silica in 1959.