ABSTRACT

In this study we investigated the geological and biogeochemical processes that influence groundwater chemistry in the Boso Peninsula, Chiba, central Japan. We also discuss the factors that control groundwater chemistry within a sedimentary basin. Saline groundwater is found throughout the study area, while fresh groundwater is distributed unevenly in the central and southern parts of the study area. The distribution of Cl^- concentration indicates that groundwater in the study area is influenced by not only present-day seawater penetration but also presence of fossil brine water, although their extents remain uncertain. Since thermodynamic calculation of mineral saturation indices indicates that the groundwater is not in equilibrium with the host rock, it is considered that seawater has intruded to inland areas with and/or after progress of geochemical reactions. Consequently, changes in the concentration of various chemical species by geochemical processes (seawater-freshwater mixing is excluded) were calculated and used as a basis for the classification of samples into six reaction types according to the changing trends of the concentration values calculated by assuming that seasalt components of rainwater affect the groundwater chemistry. Chemical composition is not always correlated with a certain reaction type. The groundwater geochemistry of some reaction types is governed mainly by the dissolution of silicates (plagioclase, illite, chlorite and montmorillonite) and carbonates (calcite, dolomite, and siderite) and precipitation of carbonate minerals, as well as ion exchange of cations. The chemical properties of saline water are influenced by bacterial sulfate reduction, bromine, and the production of methane.