Geostatistics-based regional characterization of groundwater chemistry in a sedimentary rock area with faulted setting

Sophisticated modeling of a hydrogeological structure and the environment are crucial for underground construction. This study characterizes hydrochemical properties of groundwater in a sedimentary rock area, clarifying their spatial distribution and correlation with geologic structure, and interpreting the groundwater source and chemical evolution. Water samples, geological columns, and well logs to 1-km depth were taken at 10 sites in Horonobe of northern Japan. Toward the objectives, a 3D model of Cl⁻ concentration was produced in conjunction with resistivity logging data through kriging estimation and sequential Gaussian cosimulation. Variography shows that the dip of the main Omagari Fault is a control on the spatial correlation structure of Cl⁻ concentration. The 3D model shows that this fault and its auxiliary constitute a clear boundary between high and low saline waters, and that Cl⁻ concentrations tend to change in accord with sedimentary layer structures. The integration of stable isotope analysis suggests that deep saline water with heavy δD and δ¹⁸O originated from fossilized seawater, whereas shallow freshwater with light values is of meteoric water origin. Dilute saline water in the deep part of study area is partially attributable to dehydration of silica minerals. Vertical and lateral groundwater flows are estimated to prevail near the Omagari Fault and be general in other zones, respectively. Difference in the depth of transition zones may be caused by the dominant flow among downward, ascending, and lateral. Consequently, geostatistical techniques and data integration are useful to depict regional groundwater systems with a data set of water investigation limited by quantity and location.