Evaluating salinity variation and origin in coastal aquifer systems with integrated geophysical and hydrochemical approaches

Public concerns have been dramatically increased over potential saltwater intrusion resulting in freshwater resources shortage in coastal aquifers in the past decades. Investigating the distribution and origin of saline water is a key factor to understand coastal groundwater evolution and further assist its management. Here we evaluate the horizontal and vertical spatial variability of the coastal groundwater salinity and its potential key influencing factors based on integrated hydrochemical and geoelectrical approaches in the Pearl River Estuary (PRE), South China. The electrical resistivity tomography and geochemical data show a decrease of salinity from the coast to the inland with a water type varying from Cl–Na to HCO3–Ca at the regional scale. Points with higher/lower salinity values than those in the surrounding environment occasionally exist in the study region. In the cross section, the zone 2–90 m below the surface has low resistivity values, which correspond to high-salinity values in the subsurface. The moderate resistivity values at 0–2 m depth illustrate an infiltration of freshwater. The complex salinity distribution pattern is mainly controlled by the heterogeneity of formation and distribution of primary flow paths, while the coastal groundwater salinity evolution is shaped by the joint influence of paleo-seawater intrusion, the ion mixing processes, and the water–rock interaction.