<p>The origin and evolution of underground brines are controversial in coastal plains with complex geological structures and multiple transgression and regression events. Hydrochemical and isotope analyses were conducted on porewater extracted from the coastal aquitard of Laizhou Bay, China, to explore the evolution of pore brines. High Cl⁻ concentrations (up to 95&#xa0;g/L), variations in Cl/Br and ⁸⁷Sr/⁸⁶Sr ratios, and depleted δ³⁷Cl values (–0.37—0‰), indicated that porewater present in Late Pleistocene strata originated from paleo-seawater associated with transgression and regression periods. Two types of aquitard brine were observed: (1) high Cl⁻ and normal Cl/Br (62.84–348.09) in an area characterized by paleochannels; and (2) high Cl⁻ and low Cl/Br (30.16–90.71) located in a topographic depression with low-permeability clayey soil. Both of these brines mixed with fresh groundwater. Significant cation exchange reactions resulted in Na⁺ and K⁺ losses and enrichment of Ca²⁺ and Mg²⁺ in pore brines. Silicate dissolution resulted in high ⁸⁷Sr/⁸⁶Sr ratios (0.7099–0.7109), while carbonate precipitation caused Sr losses. Porewater with heavy δ³⁷Cl in the Holocene and Late Pleistocene strata was ascribed to recharge from atmospheric precipitation and halite dissolution, respectively. Variations in δ³⁷Cl revealed the mixing of salty porewater (low Cl⁻ and heavy δ³⁷Cl) with two types of aquitard brine (light and heavy δ³⁷Cl) along with its migration. Br enrichment was associated with the reduction of nitrate and sulfate during organic matter degradation. This study can provide a reference for research on groundwater quality evolution in low-lying coastal plains with offshore extension.</p>