Geochemical Characterization of Natural Groundwater on the Southern Slopes of the Caucasus Mountains on the Russian Black Sea Coast

The Mzymta and Sochi watersheds, located on the southern slopes of the Caucasus Mountains on Russia's Black Sea coast, are characterized by a remarkably complex geological setting. The predominant rocks are mudstones, siltstones, and shales, with smaller amounts of carbonate and clay-carbonate rocks, and occasionally igneous rocks. The area is also characterized by the occurrence of polymetallic, gold, sulfide, rare metal, and rare-earth mineralizations in addition to hosting a thermal hydrogen sulfide groundwater aquifer. The objective of this study was to determine groundwater genesis patterns in an area with such contrasting geological and hydrogeological structures. Based on the chemical analysis of 33 natural groundwater springs, five chemical types of water were identified dictated by the composition of the host rock as well as the occurrence of mineralizations, seepage of mineralized water, and local geochemical features of the host rock. The high sodium concentration and mineralization of groundwater near the thermal hydrogen sulfide groundwater aquifer are due to the infiltration of seawater. The consistent presence of excessive amounts of rare-earth elements (REEs) in all groundwaters in the area is a reflection of the regional geochemical specialization of the rocks. Groundwater, like the terrestrial components of the natural environment, inherits rare-earth fractionation patterns from the rocks that make up the area. Thus, rare-earth totals and fractionation patterns can be used as a reliable criterion to interpret the regional specificity of groundwater.