GLEN AVON -- Buried under a box canyon in the boulder-studded Jurupa Mountains, a slow-moving plume of polluted groundwater traveling from a toxic dump here is gradually succumbing to a high-tech attack from state scientists and engineers.

It's a battle that is only just beginning; experts concede that it may take more than 400 years and hundreds of millions of dollars to fully eliminate the toxic stew here at the Stringfellow Acid Pits -- one of the nation's most notorious toxic dumps.

But for state officials and long-suffering residents, any progress is a monumental achievement. And more gains, they believe, are on the way, now that state officials are in the final stage of taking control of the massive cleanup effort from a consortium of private companies.

In assuming that responsibility, the state has been implementing new -- and at times controversial -- techniques to better identify and treat the solvent-laced water flowing beneath the three-acre pits that were dug into Pyrite Canyon. The residents say that the state's involvement has stepped up the pace of the lengthy project.

"As jaded as I am, I must say [state officials have] improved" the cleanup efforts, says Penny Newman, a retired local schoolteacher and environmental activist in this dusty, largely Latino working-class town just south of the San Bernardino-Riverside county line.

Between 1956 and 1972, Stringfellow's customers -- including such industrial behemoths as General Electric Co., Alcan Aluminum Corp., Weyerhaeuser Co. and two aerospace concerns since acquired by Boeing Corp., Rockwell International and McDonnell-Douglas -- legally poured the bulk of some 35 million gallons of waste into the leaking pits. But in 1972, state water-quality officials ordered a halt to the dumping when they noticed pollutants from the pits were leaking into the groundwater. Then in 1978, a huge amount of rain forced the state to release more than a million gallons of polluted water into Pyrite Canyon, which drained into a creek bed that flowed through the community.

After much outrage from activists such as Ms. Newman, the dump's customers started the pits' cleanup in 1980, with the state overseeing their efforts. Then in 1982, Stringfellow was declared a Superfund site. Sixteen years later, after decades of litigation, a California court declared that the state was responsible for the cleanup. Costs have already exceeded $150 million and are continuing to accrue at a rate of about $10 million to $12 million a year, state officials say. A 1989 study estimated the final tab at $740 million.

"We've been very skeptical [of the state officials]," says Ms. Newman. "In the past, we always said that they just talked a good game."

But she and others have been impressed with the state's use of the latest high-tech weapons. The most promising: three-dimensional seismic imaging, which employs sound waves, digital technology and computers to pinpoint subterranean fractures and fissures where toxic liquids may lurk. Geologists can use 3D imaging as one of several tools to look underground, just as physicians can choose among magnetic-resonance imaging, CAT scans or old-fashioned X-rays to see into a patient's body.

"We've found it's useful," says Robert Wood, chief of environmental restoration at Edwards Air Force Base. Still, Mr. Wood adds, "it's much more expensive [than the less sophisticated 2D imaging] and it should be used judiciously."

Moreover, a number of leading academic geophysicists caution that 3D imaging isn't the magic bullet for finding and remediating toxic messes. They believe that Brian Herridge, president of Resolution Resources Inc., a small Minneapolis geophysical consulting firm that developed and has been pushing the 3D technology in California, has oversold it, particularly by saying it can see underground pockets of super-concentrated organic chemicals that separate out from surrounding groundwater. These chemicals are called DNAPLs, a scientific acronym for Dense Non-Aqueous Phase Liquids.

Mr. Herridge is "out there claiming a lot of things in the data that there's no way of confirming," says Ernie Majer, a geophysicist at the University of California's Lawrence Berkeley National Laboratory.

The use of seismic imaging at shallow depths is an adaptation of a deep-earth technique that is standard procedure in exploring for oil. The 42-year-old Mr. Herridge invented the 3D digital technology for finding toxic underground pools after working for 18 years as a geologist in private industry. He says the marketplace provides all the confirmation he needs.

"My clients aren't dumb. They're some of the smartest people in the world," says Mr. Herridge, who holds a bachelor's degree in geology from the College of St. Thomas in St. Paul, Minn. "They don't hire me over and over again" just because of a slick sales presentation. At North Island, he says, the Navy looked at the images and said, "You can see the DNAPLs."

Despite the criticism, state and federal environmental officials contend that 3D imaging gives them a powerful tool by providing detailed underground pictures. Those shots, if properly interpreted, can be used to sink wells directly into large volumes of contaminated ground water, which are then pumped to the surface and treated to remove pollutants. State officials say the 3D imaging has allowed them to fine-tune the placement of a series of extraction wells located just north of Glen Avon. Those wells capture and treat ground water before it flows under Highway 60 into residential neighborhoods. The technique saves money, officials say, because fewer "dry wells" are dug. At Stringfellow, wells can cost as much as $10,000 apiece.

Now "we know where to drill, expecting to get results, instead of doing random drilling, throwing darts at a dart board," says Allen R. Winans, a senior engineering geologist with the state Department of Toxic Substances Control at Stringfellow.

Though Toxic Substances Control officials haven't quantified any cost savings yet, they say the $500,000 they've paid Resolution for seismic work has already proved worthwhile. The data they've collected so far has resulted in a library of geological information. The enhanced pumping and treatment program, they say, has reduced levels of the most prevalent solvent, trichloroethylene, or TCE, to a maximum of 30 parts per billion today from 400 parts per billion in 1987. The contamination in town south of the freeway is now "nondetectable," they say. What's more, they say the added precision of the 3D seismic imaging as a siting tool has doubled extraction rates in some areas.

Perhaps just as important, the new imaging is helping geologists avoid making serious mistakes that could endanger public health and safety, says Allen Wolfenden, Toxic Substances Control's top administrator at Stringfellow. "It could take a long time to figure out that the plume [of ground water] is continuing to migrate," he says. "Our cost of error could be very high" without 3D imaging.

Indeed, Resolution Resources' Mr. Herridge touts Stringfellow as one of his company's biggest successes. California is "way out front of everyone" in terms of getting results with the technology, he says. "If there were a lot more projects like this, we'd get a lot more [cleanups] done and waste a lot less money."

The projects' results haven't won over critics, though. Seismic studies like those Mr. Herridge promotes undoubtedly have yielded some "beautiful data" at some sites, says James Rector, an associate professor of applied seismology at UC-Berkeley, but that doesn't mean the imaging will work everywhere. The seismic technology is "not a panacea" for quickly cleaning up all underground pollution, he says.

Another academic critic, geophysics professor Don Steeples of the University of Kansas in Lawrence, adds that transferring seismic techniques used in deep-depth oil exploration for use in shallow toxic-pool searches isn't as simple as Mr. Herridge claims. Seismic images bounce back much faster at shallow depths and produce less accurate readings than those for deep searches, he says.