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<100m)are saline water,with the TDS(Total dissolved solids)of 3.69~30.75g/L,and deeper ones(depth=100~500m)are fresh water,with the TDS <1g/L.Content of major ions(i.e.,Cl-,Na+,K+,Mg2+,SO42-,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.