[en] This study investigates surface water quality in Luxembourg with the help of citizen scientists. The fundamental question explored relates to uncertainty and judgements on what constitutes adequate data sets, comparing official data and citizen science. The case study evaluates how gaps and uncertainties in official data for the United Nations Sustainable Development Goal 6 (UN SDG 6), Indicator 6.3.2 on water quality, and the EU Water Framework Directive (WFD), can be served with citizen science. In two Water Blitz sampling events organised in collaboration with the NGO Earthwatch, participants sampled water bodies at locations of their choice, using field kits to estimate nitrate and phosphate concentrations. Samples were collected (428 in total) over two weekend events, providing snapshots in time with a good geographic coverage of the water bodies across the country: 35% of nitrate and 29% of phosphate values were found to exceed thresholds used by the European Environment Agency to classify the nutrient content in water as good. Our study puts forward recommendations on how citizen science data can complement official monitoring by national agencies with a focus on how such data can be represented to serve the understanding and discussion of uncertainties associated with such ordinal data sets. The main challenge addressed is high levels of natural variation in nutrient levels with both natural and anthropogenic multi-factorial causes. In discussing the merits and limitations of citizen science data sets, the results of this study demonstrate that a particular strength of citizen science is the identification of pollution hotspots in small water bodies, which despite being critical for ecosystem wellbeing are often overlooked in official monitoring. In addition, citizen science increases public awareness and experiential learning about factors affecting surface water quality and policies concerning it.