CHEMICAL AND ISOTOPIC CONSTRAINS ON THE EVOLUTION OF PORE WATER IN LOW PERMEABILITY CLAY AQUITARD OF BOHAI BAY, CHINA

Dill cores collected from an 200-m-thick Quaternary sedimentary sequence of North China Plain in western Bohai bay, China, were used to investigate the evolution of porewater and its residence time in low permeability aquitard. Porewater at all the depths are salty water with the TDS ranging from 2.2 g/L to 84.3g/L which decreased with depth. The higher Cl^- concentration (up to 39g/L), together with lower Cl/Br ratios (422-558) and enrich Sr²⁺ suggested that upper saline porewater (depth <23m) was formed by evaporation of residual seawater which invade to this place during Holocene transgression. While the depleted isotopic signature of saline porewater relative to seawater is attributed to the dilution of meteoric water and the limited depth of infiltration is 10m, in depth of 10m-23m porewater could be mixture of ancient seawater and original freshwater in terrestrial layer. The cause of deep brackish porewater (TDS, 2.2 g/L-6.3g/L) is dissolution of evaporite rather than seawater intrusion as the Br^- concentration is below the detection limit and the relationship between Cl^- and δ²H. In addition, high Na/Cl ratios of brackish porewater indicated considerable water-rock interactions, through which Na⁺is released from minerals into water, which also make a contribution to the salinity.

The shapes of Cl^- and δ²H profiles and associated 1-D transport modeling suggest diffusion is the dominate vertical transport mechanism across the aquitard. The upper saline porewater after dilution (<10m) has diffused at least 1ka to the underlying porewater forming the current shape and simulation results further supported that the residence time of saline porewater in depth10-23m is approximately 10ka BP, corresponding with the timing of seawater level rise during Holocene transgression. This study is significant for the prediction of saline porewater downward transport as the threat of higher salinity to deep drinkable groundwater although recently the influenced depth of seawater is about 50m.