Sulfur isotope constraint on the provenance of salinity in a confined aquifer system of the southwestern Nobi Plain, central Japan

A combination of δ34S values and chemical compositions of brackish groundwater were used to examine the provenance of salinity in a confined aquifer system in the southwestern Nobi Plain, central Japan. A tongue of brackish confined groundwater (Cl->1000 mg/L), which extends from the shoreline of Ise Bay inland, has somewhat lower SO4/Cl ratios and higher δ34S values (approximately 79 per thousand) than those of the present seawater. Although the cation compositions of the brackish groundwater are consistent with values calculated for a mixture of fresh recharge water and seawater, the anion compositions cannot be adequately interpreted as derived from such a mixing process alone. Using a Rayleigh distillation model, we found that the groundwater chemistry could be explained by sulfate reduction in combination with the mixing of two types of seawaters, the present seawater and SO4-free seawater, with the fresh recharge water. The SO4-free seawater may consist mainly of fossil seawater that was trapped in an aquiclude for thousands of years. Model calculations based on Cl- and SO₄²⁻ contents and δ34S values showed that the present and fossil seawaters compose at most 10.7 and 9.4%, respectively, of the brackish groundwater volume, and the reduced SO₄²⁻ content is up to 353 mg/L in the confined aquifer. The model's discrimination of the two seawaters is also in agreement with the spatial distribution patterns of groundwater composition. From the west to the east in succession, the main influence on the groundwater chemistry is fresh recharge water, the present seawater, and the fossil seawater. Moreover, the area of intrusion of present seawater predicted by the model agrees well with the area of seawater intrusion determined from water quality data obtained a decade ago, indicating that the model calculations can be reasonably used to discriminate quantitatively between present and fossil seawaters.