Groundwater Salinization and Flushing During Glacial-Interglacial Cycles: Insights From Aquitard Porewater Tracer Profiles in the North China Plain

Using natural tracer profiles to study migration of porewater salinity can help determine fluid fluxes and the timescales of salinization and freshening in coastal regions. Saline groundwater associated with evaporation and transgression events is widely distributed in the North China Plain (NCP). However, the fluid flux, time scales, and importance of different salinization processes are poorly constrained, particularly in areas beyond the limit of marine influence. Chloride (Cl) and delta O-18 profiles were obtained from two boreholes: one drilled at the edge of the piedmont alluvial fan (HS borehole) and one in the coastal region (G1 borehole). Numerical simulation of Cl and delta O-18 profiles confirmed diffusion-dominated solute transport but also indicated slow upward advective flow over geological timescales. The profile modeling at HS indicated that salinization related to evaporation has occurred since the Late Pleistocene. Assessment of alternative conceptual models indicates that intensive groundwater pumping in recent decades has also caused significant downward movement of saline porewater (e.g., movement of brackish water at similar to 0.6 m/yr), mixing modern or Holocene water with Pleistocene water. The coastal Cl profile modeling in contrast shows that porewater salinity can be primarily attributed to salt diffusion during four transgression/regression cycles since the late Middle Pleistocene. The Cl transport appears never to have reached an equilibrium state during the glacial-interglacial cycles. This suggests not only that trapped Holocene seawater is still present and leaching into adjacent sediments but also that Pleistocene water has never been entirely flushed from the deep part of the coastal sediments.