The record of paleo-environment in clayey aquitard pore water is much more effective relative to aquifer groundwater owing to the low permeability of clayey aquitard. Oxygen-18(18O), deuterium(D), and chemical patterns were determined in pore water samples extracted from two 500 m depth boreholes, G1 and G2, in western Bohai Bay, China. Shallow pore water samples(depth<102 m) are saline water, with the TDS(total dissolved solids) of 3.69–30.75 g/L, and deeper ones(depth=102–500 m) are fresh water, with the TDS<1 g/L. Content of major ions(i.e., Cl-, Na+, K+, Mg2+, SO2-4, Ca2+) is high in marine sediment pore water samples and gradually decrease towards to terrestrial sediment pore water, together with the Cl/Br and Sr/Ba ratios changing significantly in different sedimentary facies along the study profile, indicating that pore water may be paleo-sedimentary water and not replaced by modern water. δ18O profile and positive correlation between δ18O and Cl- of shallow saline pore water indicated diffusion as the main transport mechanism, and distinguished four transgressive layers since Late Quaternary(i.e., Holocene marine unit, two Late Pleistocene marine units and Middle Pleistocene marine unit), further supporting the finding that pore water retained the feature of paleo-sedimentary water. Climate was identified as the main influence on the isotopic signature of aquitard pore water and four climate periods were determined by δ18O profile.
The thickness of vadose zone plays a critical role in vertical groundwater recharge. The decline of water table since the past decades due to long-term groundwater over-exploitation has resulted in deep vadose zone in North China Plain. One-dimensional variably saturated flow models were established by Hydrus-1D software and simulations were run under steady and continuous declining water table respectively to estimate the impact of increase in thickness of vadose zone on recharge process, quantity and recharge time. Luancheng area was selected to estimate recharge quantity considering steady and continuous declining water table. The simulation results show that the increase in thickness of vadose zone delays recharge process to water table. The recharge quantity decreases first and then remains stable with the decline of water table. Under the condition of declining water table, the evaluation of recharge by the flux at water table overestimates the recharge quantity. The average annual recharge rate of Luancheng area is 134 mm/a.
