High-salinity paleowater from low-permeability aquitards in coastal areas can be a major threat to groundwater resources; however, such water has rarely been studied. The chemical and isotopic compositions of porewater extracted from a 200-m-thick Quaternary sedimentary sequence in the western coastal plain of Bohai Bay, China, were analyzed to investigate the salinity origin and chemical evolution of porewater in aquitards. Porewater samples derived at depths shallower than 32 m are characterized by Cl-Na type saline water (total dissolved solids [TDS], 10.9-84.3 g/L), whereas those at depths greater than 32 m comprise Cl·SO4 -Na type brackish water (TDS, 2.2-6.3 g/L). Saline porewater is interpreted as evaporated seawater prior to halite saturation, as evidenced by Cl-Br relationships. Although substantial dilution of saline porewater with meteoric water is supported by a wider Cl- range and δ2 H-δ18 O covariance, the original marine waters were not completely flushed out. The deeper brackish porewater is determined to be a mixture of fresher porewater and brine groundwater and had a component of old brine of less than 10%, as indicated by a mixing model defined using δ2 H and Cl- tracers. Porewater δ2 H-δ18 O relationships and negative deuterium excess ranging from -25.9‰ to -2.9‰ indicate the existence of an arid climate since Late Pleistocene in Tianjin Plain. The aquitard porewaters were chemically modified through water-rock interactions due to the long residence time.
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