Geochemical signature of pore water from core samples and its implications on the origin of saline pore water in Cangzhou, North China Plain

• High-depth-resolution profiling of aquitards in the North China Plain is first done.
• Evaporite dissolution is the main cause of the saline pore water in Cangzhou.
• Sediments of low permeability possibly allow prolonged water–rock interactions.

Groundwater is reported to have TDS over 5 g/L at depths down to 160 m in Cangzhou, which is about 70 km away from the coast, in the North China Plain. Ongoing is the debate on the origin of this saline water. Two commonly suggested causes are (1) intensive evaporation in the Late Pleistocene and (2) transgression in the Quaternary. Core samples from a borehole drilled down to 450 m were collected for pore water extraction, allowing the first high-depth-resolution geochemical and isotopic profiling of aquitards in the region. The high Ca2 + and SO42 − concentrations, together with high Cl−/Br− mass ratios, indicate the evaporite dissolution origin of the saline pore water at 30 to 125 m. Relatively enriched, yet negative stable isotope ratios (δ2H and δ18O) of pore water show that the saline water originates from fresh water and there is no evidence for seawater involvement. Also, the exceptionally high Na+/Cl− mass ratio and the negative base exchange indices point to considerable water–rock interactions, through which Na+ and K+ are released from minerals into water. It is likely that the prolonged water–rock reactions give rise to high F− concentration in the relatively stagnant pore water.