Occurrence and enrichment mechanism of arsenic-rich groundwater from eastern coastal China

Groundwater arsenic (As) contamination represents a noteworthy occurrence within coastal aquifers. This work employs an approach of chemical analysis and geochemical modelling to study the occurrence and enrichment mechanism of As-rich groundwater within coastal aquifers in East China. Recent hydrogeological survey demonstrates that the As contaminated samples are clustered in Rudong Bay and As contamination occurs within both reducing and oxidizing coastal aquifers. The Piper, Gibbs, and Rock-Weathering diagrams signify shallow and deep groundwater are Na–HCO₃ and mixed Na·Ca·Mg–HCO₃ facies respectively, and the hydrochemistry of contaminated groundwater is governed by weathering and dissolution. Moreover, high salinity in shallow groundwater can be associated with seawater intrusion. The PHREEQC modeling predicts that As (V) (HAsO₄²⁻) is the dominant As speciation in shallow groundwater, while As (Ⅲ) (H₃AsO₃) is the counterpart in deep groundwater. Statistical analysis of chemicals and modeling calculations suggest that alkaline desorption is responsible for As enrichment in shallow groundwater, while reductive dissolution is the primary processes governing As enrichment in deep groundwater. Redox and pH vibrations play an important role in generating the differences of As enrichment mechanism between shallow and deep aquifers in the coastal plain. The findings of this study may enhance understanding the occurrence and enrichment mechanism about As-rich groundwater within coastal area aquifers.