Comparative study of grab, DGT, and bryophyte sampling as monitoring program for quality management: case of arsenic in freshwater

Selecting the most appropriate method for monitoring arsenic in freshwaters is crucial due to the diversity of available approaches and their inherent objectives, advantages, and limitations. This study addresses this challenge by conducting a comparative analysis of three well-established monitoring methodologies that have been among the most used in the last 30 years. Grab sampling, passive sampling using diffusive gradients in thin films (DGT), and bryophyte sampling were selected to monitor arsenic in a main river and its tributaries over a full hydrological year. On the one hand, grab and passive samplings both indicate stable concentrations of dissolved arsenic throughout the monitoring period and across the whole studied area. They also highlight a stable fractionation and redox speciation of arsenic, predominantly present as labile As(V). On the other hand, bryophyte monitoring exhibits significant spatiotemporal variations of arsenic content, with differences reaching up to tenfold. These contents are not solely determined by arsenic occurrence in water but result from arsenic bioavailability and its bioaccumulation in the organisms, which are both influenced by environmental factors and uptake mechanisms. Thus, the results of this study highlight the importance of clearly stating the goals of a monitoring programme in order to identify the most suitable method and implementation, ensuring relevant environmental management decisions.