From sensor challenges to environmental insights: Nutrients, metals, and community tools

Freshwater ecosystems worldwide are increasingly exposed to multiple pressures arising from land-use change, urbanisation, climate variability, and extractive activities such as artisanal and small-scale mining. Among the most persistent threats are heavy metals and nutrient enrichment, both of which can compromise ecosystem health and human water security. Despite advances in analytical chemistry and environmental monitoring technologies, large regions of the world still lack adequate surveillance systems capable of detecting water quality degradation in a timely and spatially representative manner. Addressing this challenge requires both technological innovation and new monitoring paradigms capable of operating in resource-limited contexts. This thesis explores how accessible sensing technologies, participatory monitoring, and advanced analytical frameworks can be combined to improve the understanding and monitoring of freshwater quality across diverse environmental contexts. The work is structured around three complementary research themes. The first examines the potential of bio-based colourimetric sensors for detecting heavy metals in freshwater environments, critically evaluating their analytical performance, operational constraints, and suitability for decentralised monitoring. The second investigates how citizen science monitoring can be used to detect basin-scale water-quality patterns by analysing long-term observations of nitrate, phosphate, and turbidity across seven Italian river basins. The third and fourth chapters extend this approach to investigate the dynamics of trace metals in river systems, first by examining interactions between metals and fluorescent dissolved organic matter (fDOM) and then by applying an integrated monitoring and modelling framework to assess metal variability in mining-affected river basins in Sierra Leone. Across these studies, I demonstrate that combining low-cost sensing concepts, citizen science observations, and advanced chemometric approaches can reveal spatial and temporal patterns in freshwater quality that are difficult to detect using traditional monitoring alone. Land use emerged as a dominant driver of nutrient variability in Mediterranean river systems, while climatic conditions modulated basin-specific responses. In mining-affected environments, metal concentrations were influenced by both regional geological context and anthropogenic disturbance, with Bayesian modelling revealing systematic increases in several metals associated with mining activity and seasonal hydrological variability. Taken together, the results highlight the opportunity that integrated monitoring frameworks present to expand water quality surveillance in data-limited regions. By bridging analytical chemistry, environmental science, and participatory monitoring, this thesis contributes to the development of scalable strategies for improving freshwater quality assessment and supporting global water governance efforts, including the monitoring targets of Sustainable Development Goal 6.