Treated drinking water as a sentinel indicator for episodic solvent pulses affecting riverine and coastal ecosystems

Methylene chloride (dichloromethane, DCM) is a volatile industrial solvent widely used in manufacturing and surface-treatment processes. Its high volatility and episodic release patterns make it difficult to detect through conventional riverine and marine monitoring programmes, potentially obscuring short-duration contamination events with downstream ecological consequences. This study evaluates treated drinking water as a high-frequency sentinel matrix for identifying intermittent upstream solvent inputs within river catchments that ultimately influence estuarine and coastal environments. A nationwide dataset comprising 5,448 treated-water samples collected between January and December 2025 was analysed using gas chromatography–mass spectrometry (GC–MS) under routine regulatory surveillance. Although the overall mean DCM concentration was low (0.00405 ppm), the distribution exhibited extreme positive skewness (14.81) with rare but pronounced exceedances, including maximum concentrations of 0.338 ppm—substantially exceeding the World Health Organization drinking-water guideline of 0.020 ppm. This statistical pattern is consistent with episodic, pulse-type contamination rather than continuous background loading. Because drinking-water treatment processes do not generate DCM, these exceedances likely reflect intermittent upstream solvent releases entering river systems that simultaneously serve drinking-water intakes and discharge into downstream aquatic environments. Such transient solvent pulses represent an under-recognized chemical stressor with potential implications for estuarine ecosystems and aquaculture operations reliant on stable water quality. The findings demonstrate that treated-water volatile organic compound datasets can function as an effective sentinel monitoring framework, revealing otherwise hidden contamination events and supporting early environmental risk identification. Integrating sentinel chemical surveillance into broader water-quality monitoring strategies offers a practical pathway toward improved ecosystem protection and sustainable aquaculture management.