Spatial and temporal controls on biogeochemical indicators at the small-scale interface between a contaminated aquifer and wetland surface water

Abstract

This high-resolution biogeochemical study investigated spatial and temporal variability in the mixing interface zones within a wetland-aquifer system near a municipal landfill in the city of Norman, Oklahoma. Steep biogeochemical gradients indicating zones of enhanced microbial activity (e.g. iron/sulfate reduction and fermentation) were found at centimeter-scale hydrological and lithological interfaces. The small resolution study was achieved by combining passive diffusion samplers with capillary electrophoresis for chemical analysis. The spatial and temporal variability of biogeochemical processes found at the interfaces was evaluated in a depth profile over a period of three years. Correlations between geochemical parameters were determined using Principal Component Analysis (PCA) and the principal factors obtained were interpreted as a dominant biogeochemical process. Factors scores were mapped by date and depth to determine the spatial-temporal associations of the dominant processes. Fermentation was the process controlling the greatest variability in the dataset followed by iron/sulfate reduction, and methanogenesis. The effect of seasonal and hydrologic changes on biogeochemistry was evaluated from samples collected in a wet/dry period from three locations exhibiting upward, downward, and negligent hydrologic flow between aquifer and wetland. PCA was used to identify the principal biogeochemical processes and to obtain factor scores for evaluating significant seasonal and hydrological differences via analysis of variance. Iron and sulfate reduction were dominated by changes in water table levels and water flow paths, whereas methanogenesis and bacterial barite utilization were dominated by season and associated with a site with negligible flow. A preliminary study on microbial response to changes in geochemical nutrients (e.g. electron acceptors and electron donors) was conducted using in situ microcosms with the purpose of quantifying iron and sulfate reduction rates. Problems encountered in the experiment such as leaks in the microcosms did not allow the determination of respiration rates, therefore the experiments will be repeated in the future. The results suggest that iron and sulfate reduction were stimulated with the addition of sulfate and ferrihydrite (electron acceptors) and acetate and lactate (electron donors). This research demonstrates the importance of assessing biogeochemical processes at interface zones at appropriate scales and reveals the seasonal and hydrological controls on system processes.