Spa tourism as a source of pharmaceutically active compounds in surface waters: a multi-evidence assessment of occurrence and environmental risk

Tourism-related activities are increasingly recognized as sources of pharmaceutically active compounds (PhACs) in aquatic environments; however, the contribution of spa tourism and associated pathways remains poorly quantified. This study investigated the occurrence, spatiotemporal variability, and environmental risk of caffeine (CAF), carbamazepine (CBZ), and diclofenac (DCF) in the Zala River catchment (Hungary), which is strongly influenced by spa and medical tourism. A multi-evidence approach was applied, combining multi-year monitoring data (2018, 2024, and 2025), questionnaire-based information on pharmaceutical usage, and a multiparametric mass-balance model. The results revealed pronounced spatial variability linked to spa-impacted river sections, with DCF consistently dominating ecological risk based on both PNEC- and EQS-based assessments. DCF concentrations frequently exceeded risk thresholds and showed significant correlations with the number of medical treatments at selected sites. Questionnaire data indicated that approximately 30% of visitors used DCF-containing medications, supporting the interpretation of bathing-related release pathways. Mass-balance modelling further indicated that observed DCF concentrations in thermal waters can be reasonably explained by direct inputs from bathers, particularly via topical application. Overall, the findings suggest that spa tourism can act as a plausible and locally relevant contributor to pharmaceutical contamination in surface waters, although its role should be interpreted alongside other sources such as municipal wastewater and healthcare facilities. The integration of monitoring data, behavioural information, and mechanistic modelling provides a transferable framework for identifying and characterizing tourism-related sources and pathways in aquatic environments.