The groundwater quality of an aquifer for the different uses to which the water is destined depends on natural and anthropic processes, which must be understood to perform a sustainable use of groundwater. With the support of the International Atomic Energy Agency, hydrochemical and isotopic techniques have recently been applied in the aquifer of the Azua Plain to assess the origin of the chemical composition of groundwater, whether it is the result of natural processes or is affected by contamination, and in case of contamination what is the origin and its extension. The study focused on samples of rainwater falling on the aquifer surface (Sisal Station) and also on a higher area (Peralta Station), of rivers flowing across the Azua plain (Jura and Tábara rivers), and of groundwater. The major chemical components (Ca2+, Mg2+, Na+, K+, HCO3-, Cl-, SO42- and NO3-), stable isotopes (18OH2O, 2HH2O, 13CDIC, 15NNO3 and 18ONO3) and radioactive isotopes (3H, 14C) were considered. The main results show that in the municipality of Azua the waters are contemporary, while in the municipalities of Estebanía and Las Charcas the waters are also young but somewhat less. The chemical composition of part of the groundwater samples studied is of natural origin, and is mainly due to dissolution of carbonates and to cation exchange. But part of the aquifer is affected by pollution of mainly agricultural origin. A conceptual model of aquifer contamination, the identification of pollution sources and natural attenuation processes has been established through the interpretation of isotope nitrate data.