EFFECTS OF A FLUCTUATING WATER-TABLE - COLUMN STUDY ON REDOX DYNAMICSAND FATE OF SOME ORGANIC POLLUTANTS

The development of the redox conditions has been studied in an initial ly aerobic column filled with quartz sand coated with ferrihydrite and subjected to a fluctuating water table. The purpose of this study was to evaluate the effect of water table fluctuations on the redox dynam ics and the fate of selected organic pollutants. The column that was p ercolated continuously with electron accepters (O-2, NO3, SO4) and ele ctron donors (acetate and formate), was first operated under saturated conditions resulting in the classical redox zonation. After 4 months of operation, we started to fluctuate the water level and three draina ge-imbibition cycles were run each with a total cycle length of 1 mont h. The pulse of oxygen introduced by lowering the water table caused a partial and temporal oxidation of previously reduced species. To inve stigate the effect of the changing redox environment on the transport and transformation of organic pollutants, breakthrough experiments wer e performed with 4-nitrobenzoate and toluene as model pollutants repre sentative for nitro-substituted and volatile aromatics, respectively. The fate of 4-nitrobenzoate and toluene was studied under saturated co nditions in short pulse breakthrough experiments and evaluated using t he advection-dispersion model. 4-nitrobenzoate was transformed stoichi ometrically into 4-aminobenzoate caused by the reduction of the nitrog roup. The transformation rate varied with depth and with time, droppin g from 15.3 nmol g(-1) h(-1) after the first drainage-imbibition cycle to 1.5 nmol g(-1) h(-1) after 4 additional months of operation. Tolue ne was not degraded during the first breakthrough experiment and showe d a retardation factor of 2.06 which was ascribed to diffusion into en trapped air, originating from drainage-imbibition cycles, and to sorpt ion to biomass. After the 24-h pre-exposure to toluene, adaptation had occurred and in later experiments toluene was degraded within the fir st 6 cm. These data show that in an experiment that was well-described in terms of water flow, gas flow, and initial mineral phase compositi on, the microbial processes induced a chemical and physical heterogene ity. An additional heterogeneity in space and time was introduced by t he fluctuating water table. The 'history' of the column had consequenc es for the fate of organic pollutants and resulted in an unpredictable behaviour with respect to their transformation, transport and degrada tion. (C) 1998 Elsevier Science B.V. All rights reserved.