Botanical cleanup crews: using plants to tackle polluted water and soil - phytoremediation

Science News, July 20, 1996 by Tina Adler

In June, Terry and his colleagues completed construction of 10 experimental quarter-acre wetlands in Corcoran, Calif. They want to see if these wetlands reduce selenium concentrations in agricultural runoff to less than 2 ppb before it reaches the evaporation ponds. Grasses, such as cattails, bulrushes, and Spatina, grow in the wetlands.

The researchers are also investigating how much of the selenium in wetlands stays in sediments and plant tissues rather than being volatilized and dispersing into the atmosphere. Plants with large amounts of selenium could sicken birds and insects that eat them, the scientists fear. Studies from the late 1980s suggested that plants volatilize about 30 percent of the selenium entering a wetland. Terry's recent laboratory studies, however, show that volatilization rates differ from plant to plant and depend on the concentration of selenium in the soil. Terry expects to have more information on volatilization later this summer, after analyzing data on the selenium stored in plants in a 90-acre wetland in Point Richmond, Calif., that Chevron Corp. built in 1988. The company originally constructed the wetland, which features cattails and bulrushes, for its beauty. Now, the wetland is removing 70 to 75 percent of the selenium from the 10 million liters of wastewater that the company pumps through it every day, says Terry.

In central California, at least two farmers are using Indian mustard and tall fescue to extract selenium from irrigation water, reports Gary S. Banuelos of the U.S. Department of Agriculture's Agricultural Research Service in Fresno, Calif. He advises the farmers on the phytoremediation technology.

More farmers will probably become interested in this green technology, because a new law in California requires them to greatly reduce selenium concentrations in drainage water, Banuelos says.

To address the broader problem of oil contamination, Exxon and other companies (SN: 8/5/95, p. 84) find that plants stimulate bacteria that break down hydrocarbons. However, researchers must then contend with herbicides that companies sometimes use on contaminated spots to prevent fires, says Evelyn Drake of Exxon in Annandale, N.J.

This is just one of the many barriers to the widespread use of phytoremediation. Investigators must still find or develop plants that are superior hazardous waste handlers. For years, breeders have cultivated commercial plants primarily to produce a large yield.

Soil poisoned long ago will prove particularly difficult for plants to tackle because the chemicals have become firmly entrenched, says Cunningham.

"Technical and economic success will probably first come in treating water and airborne contaminants," he contends. "Soils are more difficult and more complex."

Unexpected costs continue to plague the phytoremediaton industry. For example, Cunningham says he needed legal advice just to determine how to meet environmental regulations for decontaminating a tractor used in a phytoremediation project.