Influence of the sedimentary environment on the salt migration ability of ancient brine in the coastal plain of Laizhou Bay

Objective The coastal sedimentary environment is fundamental to the comprehending the evolution and movement of groundwater. To investigate the impact of the distribution and mobility of brackish groundwater along the southern coast of Laizhou Bay, Methods this study collected samples from two archetypal Quaternary boreholes (LZ01 and LZ02), extracted pore water using the mechanical squeezing technique, and measured the grain size composition of the sediments and the Cl- content of the pore water. Results Both boreholes are dominated by fine-grained sediments with an average particle size of 5.5ϕ. Based on the sediment lithology and grain size parameters, it was found that the LZ01 and LZ02 boreholes contained 10 and 9 alternating sedimentary facies, respectively, revealing three sets of marine stratigraphic units since the late Pleistocene. The porewater was found to be predominantly composed of brine (saline) water, with the Cl- profile displaying three peaks corresponding to the three sets of marine stratigraphic units. These results demonstrate the presence of residual ancient marine brine water in the sediment and its permeation into neighboring sediments. In this study, it was observed that the hydraulic index of sedimentary water exhibited a clear zonal pattern, with the vertical change rate of Cl- serving as a crucial indicator. Notably, under moderate hydraulic conditions, Cl- displayed a robust diffusion and migration capability in pore water, as confirmed by detailed particle size analysis. The dominant particle size range for pore water migration was 45.61-111.47 μm. Conclusion The paleo-marine transgression events in the study area controlled the distribution of brine, while low permeability and lithological combinations played a significant role in the preservation of ancient brine. The results of this study provide a theoretical basis for research on the origin of brine, groundwater salinization, and migration in the Laizhou Bay coastal plain.