The Trinacional Commission for the Trifinio Plan is managed by the respective national deputy presidents and works under the slogan ‘Water without borders’, recognizing the importance of water resources for these three countries. This study includes homogenization of existing geological descriptions of different geological units in the Trifinio region. Physical and chemical parameters were measured in the field for about 140 samples. Geoelectrical campaigns were carried out in selected areas to reveal information about the location and hydraulical properties of the principal aquifers. Thus, the first hydrogeological map of the Trifinio region was generated with a scale of 1:100 000. The preliminary results have shown the chemical and isotopic similarities between most surface waters and groundwaters, suggesting both a fast dynamic for most systems and the relevance of local sources of recharge. The precipitation presents a high temporal variation in deuterium (-105.60 ‰ ? ?2H 21.66 ‰) and oxygen-18 (-14.76 ‰ ? ?18O ? 1.37 ‰), due to the combined effects of amount and altitude. Also, a strong seasonally effect can be found. Tritium was determined in 29 samples from wells, springs, rivers and lakes from El Salvador. Observed maximum activity was of 2.09 UT and a minimum of 0.00 UT. Tritium concentration in precipitation shows a tendency to decrease their activity to background noise. The results indicate that at least four samples showed no Tritium activity, which could be interpreted as water corresponding to intermediate and regional flows where the Tritium in the system has disappeared. 20 samples showed an activity greater than 1 TU reaching even 2.09 UT. Taking into account that the natural concentration of tritium when it enters the hydrogeological system is of approximately 4 UT, the trend observed at the Ilopango Station (outside the Trifinio region) and the half-life of tritium (12.3 years), there is a significant component of young water (less than 12 years) in the majority of the sites.