Influence of seawater intrusion on the hot springs in a coastal area: The case of the Anak-Sinchon Uplift, Korean Peninsula

•TDS in saline hot springs show decreasing trends away from the coast.•Mg2+ is used to determine the time order between heating and seawater intrusion.•Dolomitization and anhydrite precipitation are the dominant water-rock interactions. Temperatures and chemical compositions of hot springs are decisive of their roles in heating, power-generation, therapeutic applications and recreational activities. However, the influence of seawater intrusion on hot springs are still not fully understood. Typical ten hot springs in and around the Anak-Sinchon Uplift in a coastal area of the northern Korean Peninsula were investigated. All the hot springs were near-neutral to weakly alkaline. High TDS value of 25300 mg/L was found at the hot spring closest to the coastline. TDS values, and concentrations of Na+, K+, Ca2+, Cl− and SO42− in the hot spring waters near the coast showed decreasing trends with the distance from the coastline. According to the calculation of seawater fraction, three hot springs near the coast were contaminated by seawater. Temperatures of the geothermal reservoir were estimated to range from 103 to 170 °C, which were interpolated by thin plate spline to predict the local trend of the reservoir temperature distribution. Mg2+ concentration was used to determine the time order between the heating process and seawater intrusion. Among the three seawater-contaminated hot springs, very low Mg2+ concentrations in two hot spring waters probably indicate that they were mixed with seawater before being heated in the geothermal reservoir and not affected by seawater intrusion again afterwards; relatively high Mg2+ concentration in the hot spring water closest to the coastline likely indicates that it was mixed with seawater again during the ascent from the geothermal reservoir. The geochemical modeling performed with PHREEQC indicates that dolomitization of calcite and precipitation of anhydrite were the dominant water-rock interactions for the seawater-contaminated hot spring waters.