We evaluate the hydrogeology of the area around the Soğuksu spring in the Aydıncık district, Mersin, Turkey, with a focus on the groundwater and aquifer systems. An increasing population and the expansion of agricultural land have resulted in increasing demand for water in the Aydıncık area. The Soğuksu spring, discharging from the Aydıncık aquifer, is the major source of water for Aydıncık and the surrounding area. To establish a sustainable water management plan for the region it is essential to determine the characteristics of the hydrogeological structure of the area. Water samples taken from the Soğuksu spring and other small springs in the Aydıncık area were analysed and the spring recession curve, hydrochemistry, stable isotopes (δ18O, δ2H), and tritium isotopes (3H) were used to characterize the aquifer. The master recession coefficient obtained from analysis of the spring discharge data for 1999–2012 indicates one exponential term. Analysis of the groundwater composition identified two groups, the Ca–HCO3 and Ca–Mg–HCO3 groups, based on the major cation and major anion in the sampled water, as expected from karstic carbonate aquifers. The stable isotopes demonstrate the predominance of low-elevation precipitation. High tritium concentrations between 2.32 and 4.25 TU suggest recent recharge of groundwater from rainfall.