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Vertical flow filters and vertical flow constructed wetlands are established wastewater treatment systems and have also been proposed for the treatment of contaminated groundwater. This study investigates the removal processes of volatile organic compounds in a pilot-scale vertical flow filter. The filter is intermittently irrigated with contaminated groundwater containing benzene, MTBE and ammonium as the main contaminants. The system is characterized by unsaturated conditions and high contaminant removal efficiency. The aim of the present study is to evaluate the contribution of biodegradation and volatilization to the overall removal of benzene and MTBE. Tracer tests and flow rate measurements showed a highly transient flow and heterogeneous transport regime. Radon-222, naturally occurring in the treated groundwater, was used as a gas tracer and indicated a high volatilization potential. Radon-222 behavior was reproduced by numerical simulations and extrapolated for benzene and MTBE, and indicated these compounds also have a high volatilization potential. In contrast, passive sampler measurements on top of the filter detected only low benzene and MTBE concentrations. Biodegradation potential was evaluated by the analysis of catabolic genes involved in organic compound degradation and a quantitative estimation of biodegradation was derived from stable isotope fractionation analysis. Results suggest that despite the high volatilization potential, biodegradation is the predominant mass removal process in the filter system, which indicates that the volatilized fraction of the contaminants is still subject to subsequent biodegradation. In particular, the upper filter layer located between the injection tubes and the surface of the system might also contribute to biodegradation, and might play a crucial role in avoiding the emission of volatilized contaminants into the atmosphere.
Solute concentration variability is of fundamental importance for the chemical and ecological state of streams. It is often closely related to discharge variability and can be characterized in terms of a solute export regime. Previous studies, especially in lowland catchments, report that nitrate is often exported with an accretion pattern of increasing concentrations with increasing discharge. Several modeling approaches exist to predict the export regime of solutes from the spatial relationship of discharge generating zones with solute availability in the catchment. For a small agriculturally managed lowland catchment in central Germany, we show that this relationship is controlled by the depth to groundwater table and its temporal dynamics. Principal component analysis of groundwater level time series from wells distributed throughout the catchment allowed derivation of a representative groundwater level time series that explained most of the discharge variability. Groundwater sampling revealed consistently decreasing nitrate concentrations with an increasing thickness of the unsaturated zone. The relationships of depth to groundwater table to discharge and to nitrate concentration were parameterized and integrated to successfully model catchment discharge and nitrate export on the basis of groundwater level variations alone. This study shows that intensive and uniform agricultural land use likely results in a clear and consistent concentration-depth relationship of nitrate, which can be utilized in simple approaches to predict stream nitrate export dynamics at the catchment scale. (C) 2016 Elsevier Ltd. All rights reserved.