Groundwater sulfate in the Pearl River Delta driven by urbanization: Spatial distribution, sources and factors

Groundwater sulfate contamination becomes a big concern in urbanized areas where urbanization expansion continues. This study investigates the spatial distribution of sulfate in shallow groundwater of the Pearl River Delta (PRD) where urbanization has lasted for more than 40 years, and discusses sources and factors for groundwater sulfate contamination in various aquifers and areas with different land-use types by using hydrochemistry and principal component analysis. A total of 330 groundwater samples, 19 river samples, and 9 landfill leachate samples were collected from the PRD. The results show that groundwater SO42- concentration in the PRD is up to 857.57 mg/L, and the SO42--rich (>150 mg/L) groundwater accounts for 7.27%. SO42--rich groundwater in the coastal alluvial aquifers is 3.8 times that in the alluvial-proluvial aquifers, while fissured and karst aquifers are absence of SO42--rich groundwater. In coastal alluvial aquifers, SO42--rich groundwater in urbanized areas is 8.5 times that in peri-urban areas. By contrast, in alluvial-proluvial aquifers, SO42--rich groundwater just occurs in urbanized areas rather than other areas. Exogenous input driven by urbanization is the primary factor for SO42- rich groundwater in the PRD. The occurrence of SO42--rich groundwater in urbanized and peri-urban areas of the PRD is mainly attributed to the industrial wastewater infiltration and acid precipitation. Lateral flow from rivers is another prominent source for SO42--rich groundwater in the PRD. As a consequence, in order to reduce the emergence of SO42--rich groundwater in areas of accelerated urban expansion such as the PRD, strengthening the treatment and supervision of wastewater from township enterprises and waste gas from industrial enterprises is recommended.