BERKELEY -- To the astonishment of Chevron engineers, cattails and wetland vegetation planted to beautify barren land near a Bay Area oil refinery have put themselves to work as star pollution busters.

The wetland is pulling 89 percent of the toxic chemical selenium from millions of gallons of wastewater flowing daily from the refinery and removing it before the dangerous chemical can reach the San Francisco Bay, according to a new UC Berkeley study to be published this week in "Environmental Science and Technology." The researchers say this daunting task has defeated manmade filtration systems but seems to come naturally to the humble wetland rushes and their ecosystem.

Such wetlands could be a breakthrough environmental advance throughout the western U.S., where soils are high in selenium. In the West, agricultural runoff tainted with selenium has left thousands of evaporation ponds so badly contaminated they are essentially toxic dump sites. Some of these ponds cover thousands of acres.

Should the wetlands technique work for agricultural land, "this approach will revolutionize water treatment in the western U.S.," said Professor Norman Terry, a UC Berkeley plant biologist in the College of Natural Resources and principal investigator on the new study. "There will be thousands of acres of constructed wetlands put in."

Selenium is one of the major toxic elements found in industrial waste and agricultural runoff throughout the western United States. It caught the world's attention at the Kesterson national wildlife refuge in California's Central Valley in 1983 when selenium leached from farmland polluted the waters and birds hatched with distorted beaks or feet and damaged brains protruding through eye sockets.

Selenium is stripped from crude oil during the refining process and released in the wastewater. Since the San Francisco Bay drains contaminated water from six major oil refineries, it is a dumping ground for thousands of pounds of selenium each year. Shell Oil Co. was fined $12.5 million in 1995 for excessive selenium dumping, and discharge levels from several other refineries substantially exceed permitted limits. But company officials claim no method exists to remove selenium from refinery wastewater without spending millions of dollars and creating hazardous waste.

Chevron is reaping the reward of its good works. Its wetland was intended not as a filtration system but as an environmental beautification project.

"You can have a natural wetlands-a marsh-or you can have one tailor-made for wastewater cleanup," said Terry. Since Chevron had 90 acres of land sitting around not growing much, they decided to beautify and restore the area into a wetland, which it originally was.

Planted near the mouth of San Pablo Bay in Richmond in the late '80s, the restored wetland grows typical California marsh plants including saltmarsh bulrush, cattail and rabbitfoot grass. The wetland traps nearly 90 percent of the wastewater selenium. Of this, 10 to 30 percent is released harmlessly to the air in a process referred to as volatilization, and the rest is stored in roots, shoots and sediments.

The hope that volatilization might prove effective for pollution cleanup was the main reason for investigating the Chevron wetlands. In volatilization, the marsh plants accumulate selenium in their roots, convert it into a non-toxic gas and release it to the atmosphere.

"The beauty of volatilization is it leaves no hazardous waste at all, the whole thing works beautifully," said Terry. Recent work in the Terry laboratory indicates wetland plants can be genetically engineered to greatly increase the amount of selenium they send into the atmosphere.

The Chevron study findings have led to a new experiment to treat industrial wastewater in 10 quarter-acre wetland cells in Corcoran, California near Bakersfield, sponsored by the University of California Salinity Drainage Taskforce, the Tulare Lake Drainage District and other collaborators. Initial data looks promising, said Terry.

He pointed out that heating, flooding and drying, and other manipulations could also raise the volatilization rate, as could altering the mix of plants. "There's clearly a lot of room to maximize volatilization," said Terry. The gas the marsh plants produce is dimethylselenide and huge amounts exist in the atmosphere from volcanoes, soil and plants. The amount produced from constructed wetlands would be negligible.

The bottom line is selenium, in this and other forms, "is cycled just like carbon and water," said Terry. "There are 100,000 metric tons floating around the northern hemisphere alone, and the small amount we're putting in compared to what's up there is not a problem."

Two other potential fates for selenium entering the wetland are chemical immobilization in sediments at the bottom of the marsh or absorption into plant tissues. To remove selenium accumulated in plant leaves and stems, the researchers suggest periodic wetland harvesting by drying and mowing. This would prevent selenium from endangering birds and animals. Composted cuttings could be used as soil additives elsewhere and mowing might increase the rate of volatilization considerably, said Terry.

The San Pablo Bay, which houses the Chevron wetland, is one of five basins of the San Francisco Bay estuary system. Water passes through the wetland in about seven to 10 days. After building it, Chevron officials were amazed at the excellent habitat it made for wildlife and birds.

Despite how affordable constructed wetlands are, "engineers feel some hesitation about working this way," said Terry. "They don't always want to get involved with biological systems. We don't know much about them and they seem too complicated and too unreliable."

Other authors on the study are UC Berkeley graduate student Drew Hansen and postdoctoral researcher Adel Zayed, and Peter J. Duda of Chevron Products Company. "Duda deserves a lot of credit," said Terry. "He set up the wetland."

The UC Berkeley study was funded by Chevron, the National Water Research Institute, the University of California Water Resources Center and the Electric Power Research Institute.