Sample collection and processing. Surface water samples (1L) were collected every four weeks, whenever physically possible, from nine different locations in Luxembourg from April to November 2019 (Figure ) and eight different locations from April to August in 2020 in accordance with the tri-annual sampling strategy employed at AGE. In this strategy, four locations monitored in compliance with the WFD are consistently sampled every four weeks (locations 1-4, Figure ) while the other locations throughout Luxembourg are divided into 3 regions and are alternately sampled during a 3-year cycle.