BIODEGRADATION OF TRICHLOROETHYLENE AND TOLUENE BY INDIGENOUS MICROBIAL-POPULATIONS IN VADOSE SEDIMENTS

The unsaturated subsurface (vadose zone) receives significant amounts of hazardous chemicals, yet little is known about its microbial commun ities and their capacity to biodegrade pollutants. Trichloroethylene ( TCE) biodegradation occurs readily in surface soils; however, the proc ess usually requires enzyme induction by aromatic compounds, methane, or other cosubstrates. The aerobic biodegradation of toluene and TCE b y indigenous microbial populations was measured in samples collected f rom the vadose zone at unpolluted and gasoline-contaminated sites. Inc ubation at field moisture levels showed little activity on either TCE or toluene, so samples were tested in soil suspensions. No degradation occurred in samples suspended in water or phosphate buffer solution; however, both toluene and TCE were degraded in samples suspended in mi neral salts medium. TCE degradation depended on toluene degradation, a nd little loss occurred under sterile conditions. Studies with specifi c nutrients showed that addition of ammonium sulfate was essential for degradation, and addition of other mineral nutrients further enhanced the rate. Additional studies with vadose sediments amended with nutri ents showed similar trends to those observed in sediment suspensions. Initial rates of biodegradation in suspensions were faster in uncontam inated samples than in gasoline-contaminated samples, but the same per centages of chemicals were degraded. Biodegradation was slower and les s extensive in shallower samples than deeper samples from the uncontam inated site. Two toluene-degrading organisms isolated from a gasoline- contaminated sample were identified as Corynebacterium variabilis SVB7 4 and Acinetobacter radioresistens SVB65. Inoculation with 10(6) cells of C. variabilis ml(-1) of soil solution did not enhance the rate of degradation above that of the indigenous population. These results ind icate that mineral nutrients limited the rate of TCE and toluene degra dation by indigenous populations and that no additional benefit was de rived from inoculation with a toluene-degrading bacterial strain.