style lysimeter tests of pesticide field dissipation, but has no policy. She believes that lysimeters could be useful for testing a new generation of pesticides that are applied at very low rates, ounces per acre rather than pounds. "In these cases, more controlled lysimeter studies might be a good approach," said Behl. —JANET BYRON
Toluene boosts bioremediation of TCE Native bacteria successfully biodegraded trichloroethene (TCE) in the groundwater at an Edwards Air Force Base, Calif., demonstration project completed in March. Researchers believe the in situ bioremediation technique can be applied to anaerobic and aerobic processes involving a wide variety of chlorinated solvents. "The test has shown that this method can successfully reduce the levels of TCE in groundwater by 96% to 98%," said project head Perry McCarty of Stanford University. "While we were specifically applying this to the cometabolism of TCE, there are many other applications where one needs to bring chemicals and contaminants and bacteria together." McCarty reported results of the full-scale demonstration at the American Chemical Society's meeting in April. McCarty's team built two dual-screen groundwater circulation wells into the contaminated aquifers and added oxygen and toluene to break down the TCE into carbon dioxide, water, and chloride. The toluene was degraded by 99.98% over the course of the project to a final concentration of 1.21.3 pg/L, a level well below EPA's threshold of 1000 pg/L and the California state maximum of 100 pg/L. To modulate bacterial growth and prevent clogging the pores of the aquifer with excess bacterial growth, the researchers injected hydrogen peroxide into the wells. In fact, the hydrogen peroxide represents the "major cost of concern" to McCarty. "Clogging wasn't as big a problem as we feared." McCarty's team used a hydrological model to evaluate the effects of factors such as adsorption and transport. "The results and the modeling came out very close together," he said, stressing that the technique requires advance modeling with sitespecific data. —KELLYN S. BETTS
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VOL. 31, NO. 7, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS • 3 0 5 A
