Effects of a fluctuating water table: column study on redox dynamics and fate of some organic pollutants

Abstract

The development of the redox conditions has been studied in an initially 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 dynamics and the fate of selected organic pollutants. The column that was percolated continuously with electron acceptors (O 2, NO 3, SO 4) and electron 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 drainage–imbibition cycles were run each with a total cycle length of 1 month. The pulse of oxygen introduced by lowering the water table caused a partial and temporal oxidation of previously reduced species. To investigate the effect of the changing redox environment on the transport and transformation of organic pollutants, breakthrough experiments were performed with 4-nitrobenzoate and toluene as model pollutants representative for nitro-substituted and volatile aromatics, respectively. The fate of 4-nitrobenzoate and toluene was studied under saturated conditions in short pulse breakthrough experiments and evaluated using the advection–dispersion model. 4-nitrobenzoate was transformed stoichiometrically into 4-aminobenzoate caused by the reduction of the nitrogroup. The transformation rate varied with depth and with time, dropping 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. Toluene was not degraded during the first breakthrough experiment and showed a retardation factor of 2.06 which was ascribed to diffusion into entrapped air, originating from drainage–imbibition cycles, and to sorption to biomass. After the 24-h pre-exposure to toluene, adaptation had occurred and in later experiments toluene was degraded within the first 6 cm. These data show that in an experiment that was well-described in terms of water flow, gas flow, and initial mineral phase composition, the microbial processes induced a chemical and physical heterogeneity. An additional heterogeneity in space and time was introduced by the fluctuating water table. The `history' of the column had consequences for the fate of organic pollutants and resulted in an unpredictable behaviour with respect to their transformation, transport and degradation.