Treated drinking water as a sentinel indicator for episodic solvent pulses in riverine–coastal systems

Methylene chloride (dichloromethane, DCM) is a volatile chlorinated solvent widely used in industrial manufacturing and surface-treatment applications. Its high vapor pressure, rapid dispersion, and episodic release characteristics complicate detection in conventional riverine and coastal monitoring programmes, potentially obscuring short-duration contamination events. Such transient inputs may remain under-recognized despite their environmental significance. This study evaluates treated drinking water as a high-frequency sentinel matrix for identifying intermittent upstream solvent releases within river catchments that ultimately discharge into 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), statistical analysis revealed extreme positive skewness (14.81), characterized by rare but substantial exceedances. Maximum concentrations reached 0.338 ppm, markedly exceeding the World Health Organization drinking-water guideline of 0.020 ppm. The highly skewed distribution pattern is consistent with episodic pulse contamination rather than continuous background loading. As drinking-water treatment processes do not generate DCM, these exceedances likely reflect intermittent upstream solvent discharges entering river systems that supply potable water and influence downstream aquatic environments. The findings highlight the value of leveraging existing regulatory monitoring datasets as an environmental chemical surveillance tool capable of revealing otherwise hidden contamination dynamics. Integrating sentinel chemical analysis into broader water-quality monitoring frameworks offers a practical and resource-efficient approach to strengthening early detection, improving environmental risk assessment, and advancing sustainable water management strategies within riverine–coastal systems.