Multiple isotope (H, O, N, S and Sr) approach elucidates complex pollution causes in the shallow groundwaters of the Taipei urban area

SummaryDeterioration 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 (δDH2OδDH2O and δ18OH2Oδ18OH2O, δ15NNO3δ15NNO3 and δ18ONO3δ18ONO3, δ34SSO4δ34SSO4 and δ18OSO4δ18OSO4, 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 agricultural fields partly contribute in the suburb area. Significant decreases in nitrate and sulfate concentrations due to denitrification and sulfate reduction were observed in some groundwaters, which induced increases in isotope ratios up to 24.9‰ for δ15NNO3δ15NNO3 and 27.2‰ for δ34SSO4δ34SSO4. Nitrate groundwater pollution in the Taipei Basin is not a subject of serious concern (<44 mg/L limit for NO3- for all analyzed samples) at the present time due to attenuation by denitrification. However, arsenic is preferentially released in groundwater (>100 μg/L) in anoxic environment under which sulfate reduction occurs. The Sr isotope data suggested that the arsenic is derived from alluvial deposits of Oligocene–Miocene sedimentary rock origin. This study disclosed the complex causes of environmental changes in Taipei urban shallow groundwater. It is also important as an excellent case study demonstrating the efficient use of multiple isotope diagnosis in the urban groundwater environmental field.

Research highlights
► The status of urban groundwater environment was investigated in the Taipei Basin.
► The multiple stable isotope ratios (δDH2OδDH2O and δ18OH2Oδ18OH2O, δ15NNO3δ15NNO3 and δ18ONO3δ18ONO3, δ34SSO4δ34SSO4 and δ18OSO4δ18OSO4, and 87Sr/86Sr) were used.
► Complex causes of environmental changes were disclosed.
► This study demonstrates the efficient use of multiple isotope diagnosis in the urban subsurface environmental field.