TRANSPORT AND BIODEGRADATION OF TOLUENE IN UNSATURATED SOIL

Degradation of volatile organic chemicals during transport has receive d little attention in the past. In this study we report the results of a series of experiments on toluene movement through soil columns of d ifferent length in sterilized, pre-exposed and unexposed soil. Toluene was added to 25-cm-diameter soil columns through an inlet chamber tha t maintained a constant concentration throughout the experiment. The t oluene diffused through the soil to an outlet chamber at the top which was continuously swept with humidified air and samples were periodica lly analysed to determine toluene flux. The first experiment, conducte d under sterilized conditions, was used to measure the soil gas diffus ion coefficient, and subsequent experiments in which biodegradation oc curred were used to estimate the degradation rate coefficient by fitti ng the outflow to a mathematical model. The degradation rate was very rapid under both pre-exposed and unexposed soil conditions, correspond ing to a half-life of approximately 2 h when bacterial activity reache d high levels. Prior to this stage, the volatilization flux was very e rratic, implying that growth rates of the bacteria were out of phase w ith the transport process. Overall, the degradation process was not we ll described by a first-order model until the population stabilized. P re-exposure of the soil to the substrate prior to the transport experi ment greatly increased the rate of removal of toluene during transport . Under such conditions, a 30-cm cover could virtually stop volatiliza tion losses of the compound when the inlet concentration was well belo w saturation, and could decrease it by a factor of greater-than-or-equ al-to 30 even when the inlet concentration was saturated.