[en] Full text: Davao City, one of the Philippines' major cities, is undergoing extensive urban and eco-tourism development. Generally, groundwater is the most important source of freshwater, supplying 97% of the city's water requirements. Davao City is generally mountainous, characterized by extensive mountain ranges, uneven distribution of plateaus and some lowlands. The city's urban and development areas are concentrated along a narrow coastal strip averaging 5 km in width and extending 56 km facing the Davao Gulf. It is in the coastal area where most of the production wells are located and where heavy abstraction is concentrated. Chemical and isotopic characterization of the water sources in Davao City were undertaken to provide an insight into the processes of groundwater occurrence, particularly on the origin and rate of recharge into the groundwater and evaluation of the aquifer's vulnerability to pollution. Field investigations were conducted from October 1998 to February 2002. The study area, 42km x 33km, is within the Talomo-Lipadas-Sibulan (TLSS) catchment basin. The groundwater aquifer in the TLSS is composed of reworked and redeposited overlapping flows of Quaternary pyroclastics. It has an upper unconfined aquifer composed of sand, gravel and occasional boulders which is tapped by shallow domestic wells. The deeper aquifer which is being tapped by wells of the Davao City Water District at depths ranging from 46 to about 140 meters below ground level is multi-layered aquifer separated by thin, relatively less permeable layers of clay. Three river systems, Lipadas River to the west, and Talomo and Davao Rivers to the east traverse the study area. These flow through the city and empty to the Davao Gulf, south of the city. Chemical composition of the groundwater shows that most of the waters in the Talomo-Lipadas-Sibulan catchment (TLSS), except for Well 1, are classified as Ca+Mg-HCO₃ waters. The fluid in Well 1 is a mixture of Ca+Mg-HCO₃ and Na+K-Cl waters. The high Cl, about 200 mg/L, is attributed to the presence of connate water in marine sediments at deeper levels. Composition diagrams of the water sources show three groups of water; one, mixed with river end members (Talomo River in the east and Lipadas River in the west); second, mixed with Well 1; and a third group of unmixed waters, found in the deeper aquifer. The mean isotopic composition of precipitation in Davao City has been established from data obtained for the period December 1999 to January 2002 from four stations located at different elevations in the watershed. δ¹⁸O values ranged from -13.51 per mille to -3.54 per mille and δ²H values ranged from -85.28 per mille to -16.13 per mille. A local meteoric line (LMWL) was established for the region with the equation δ²H = 8*δ¹⁸O +12. The isotopic composition of groundwater and surface waters in Davao City showed small variations, clustered along the LMWL. Groundwater from production wells with depths ranging from 90 m to 152 m, exhibited isotopic compositions ranging from -49.9 per mille to 39.90 per mille for δ²H and - 7.64 per mille to -6.38 per mille. This suggests a uniform groundwater recharge derived mainly from local precipitation. Differences in recharge altitude and mixing of different water origin could explain the slight variation in isotopic contents. The isotopic composition of precipitation exhibited an altitude effect. A gradient of -0.23 per mille/100m was obtained for ¹⁸O while a gradient of -1.6 per mille/100 m resulted for ²H. From these altitude effect lines, the elevations of recharge for the groundwater in the study area were estimated. The profile of per mille ¹⁸O in the groundwater from the Talomo-Lipadas catchment area reveals a distinct trend. The trend goes from less negative values inland to more negative values near the coast. As seen from water chemistry, three groups of water are revealed from the isotopic composition trend of the waters. The inland groundwater has isotopic composition similar to that of low altitude rainfall or river fed from low altitude. This has been estimated to be coming from elevations < 250 m. Groundwater with the most negative values is tapped along a 2-km wide region adjacent to the coast. This coastal water is derived from higher altitudes estimated to be at elevations of not greater than 500m. The coastal waters, on the other hand, exhibit isotopic enrichment approaching the Talomo River and Well 1 located on the eastern end of the study area. Isotopic enrichment and a corresponding increase in chloride concentration in adjacent wells indicate connection with Well 1. Isotopic values gradually decrease towards the coast. Mixing between the shallow low altitude groundwater and the deep high altitude groundwater produces this decreasing trend in isotopic composition. Tritium values from precipitation ranged from 1 TU to 11 TU while values from groundwater ranged from 0 TU to 6. Groundwater along the coast generally showed negligible or very low TU values. Representative samples from inland wells and from shallow wells had measurable tritium. Further investigation using CFCs showed representative wells from the coast yielded water with no detectable CFC indicating waters older than 50 years. Representative waters from inland had detectable CFCs with indication of mixture of old and recent waters, confirming the trend observed from stable isotopes. Recent recharge ages of 27 (55%), 22 (44%), 17(56%), and less than 10 years (8%) have been determined from the binary mixing model. (author)