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Deterioration of the quality of groundwater under urban areas has become a major environmental concern worldwide. This paper presents a pilot case study (in the Taipei urban area) using multiple stable isotopes ( [Formula: see text] and [Formula: see text] , [Formula: see text] and [Formula: see text] , [Formula: see text] and [Formula: see text] , and ⁸⁷Sr/⁸⁶Sr) with the aim of understanding the subsurface nature and environmental status of such areas. A previous study has already characterized Taipei shallow groundwaters as sulfate-rich in comparison to other Southeast Asian metropolitan cities. Our chemical measurements clarified that the arsenic is the important environmental concerns in the aquifers. Dual isotope approaches for nitrate and sulfate suggested these ions were sourced mainly from municipal sewage leaking from sewer pipes in urbanized area, while chemical fertilizers applied in the local …