—Results of study of natural waters of the Baidar valley (southwestern Crimean Peninsula) obtained during the 2018–2019 field works are presented. Major groundwater resources of the study area are confined to the Upper Jurassic aquifer complex, which serves as a recharge source for the aquifer systems of the Plain Crimean and the Azov–Kuban’ artesian basins and hydrogeologic folded region of the Crimean Mountains mega-anticlinorium. The regional waters are fresh and ultrafresh, predominantly of calcium bicarbonate composition, with TDS varying from 208 to 1269 mg/dm3. The study enabled their classification into eight geochemical groups: (1) waters of a regional fracture zone in carbonate-terrigenous rocks affected by continental salinization; (2) waters of a regional fracture zone affected by leaching of aluminosilicates and sulfide oxidation; (3) waters of a regional fracture zone dominated by sodium aluminosilicates in the fracture filling (long-term interaction in the water–rock system), affected by continental salinization; (4) regional fracture zones dominated by sodium aluminosilicates affected by anthropogenic continental salinization; (5) groundwaters in fracture–vein aquifers affected by leaching of aluminosilicates and sulfide oxidation; (6) fracture–vein aquifers affected by leaching of sodium aluminosilicates (long-term interaction in the water–rock system); (7) waters in fractured karst aquifers; and (8) surface waters subjected to continental salinization. Fracture karst waters, which were found to be most protected against human impact and continental salinization processes, are slightly alkaline (pH = 7.7), fresh (with average TDS = 444 mg/dm3), with low silicon concentrations (2.23 mg/dm3), and of calcium bicarbonate composition. Waters residing in regional fracture and fracture–vein zones are affected by continental salinization and anthropogenic load and are neutral to alkaline (pH = 7.1–8.6), predominantly fresh (TDS = 269–1269 mg/dm3), with average silicon concentrations of 4.61–4.70 mg/dm3, of calcium bicarbonate composition, with high concentrations of sulfate ion, magnesium, and sodium. The waters of the Chernaya River, Chernorechensk reservoir, and lakes, which are also affected by continental salinization, are slightly alkaline (pH = 8.3), brackish (TDS = 207–364 mg/dm3), with an average silicon concentration of 1.18 mg/dm3, of calcium bicarbonate composition, with high concentrations of chlorine ion, magnesium, and sodium. The calculated intensity of chemical-element migration in waters of the background composition follows the descending order: very strong, I17.7 > Br14.4; strong, Se2.83 > B2.22 > Sr1.46 > Sb1.12 > Be1.07 > Hg1.06; moderately strong, Zn0.74 > Mo0.50 > Li0.46 > Sc0.41 > Ag0.18 > As0.16 > Si0.123 > Ba0.122; weak, Cr0.10 > Cu0.096 > Bi0.080 > Sn0.068 > Tl0.067 > P0.062 > Ni0.043 > Ta0.040 > Ge0.034 > Cd0.028 > Fe0.026 > Rb0.024 > Co0.023 > Pb0.020 > W0.017 > V0.012; very weak (inert), Nb0.008 > Hf0.0033 > Mn0.0031 > La0.0029 > Cs0.0022 > Ti0.0018 > Ga0.0016 > Y0.0013 > Al0.0008 > Zr0.0008. All the studied waters are found to be of atmospheric origin and located along the global (GMWL) and local (LMWL) meteoric water lines. Their δ18O value varies from –9.9 to –3.3‰, and δD value, from –64.2 to –32.5‰. Sedimentary carbonate rocks, atmospheric carbon dioxide, organic compounds, and hydrolysis of aluminosilicate minerals serve as the source of δ13C bicarbonate ion in natural waters of the Baidar valley. Surface waters have a heavier carbon isotope composition (δ13C = –9.2 to –6.2‰), which is due to atmospheric CO2, plant growth, and associated microbial activity. Fracture karst waters are characterized by a lighter carbon isotope composition (δ13C = –12.8 to –11.0‰) because of their interaction with dispersed organic matter. Waters of the regional fracture and fracture–vein zones display the widest variation in δ13C (–15.5 to –6.9‰), which is associated with a mixed type of “isotope supply” to the waters. A complex hydrogeochemical field that has formed in the Baidar valley tends to be increasingly affected by the anthropogenic factor.
