[en] The International Atomic Energy Agency (IAEA) recognizes the importance of water in the international agenda as water shortages and competition for water becomes greater the world (IAEA, 2012). Hence, the IAEA Water Availability Enhancement Project (IWAVE) was initiated, aimed at enabling Member States to enhance the availability of freshwater through science based, comprehensive assessments of national water-resources. The IWAVE Project was undertaken to build capacity within Member States and assist them in gathering and using scientific information to fully assess the availability and quality of water resources. It provides assistance to improve national capacity for collecting, managing, and interpreting water-resources data, and to use advance techniques to simulate hydrologic systems for resource management. The IWAVE Project focuses on the fundamental hydrological data and understanding that are essential for conducting a comprehensive national water-resources assessment. The Philippine, although endowed with abundant water resources, faces the threat of the resource scarcity, particularly with respect to groundwater for which demand has been perceived to substantially exceed supply. The Philippine Development Plan 2011-2016 recognizes the problem concerning the water resources in the country which include, among others, the lack of investment in water, inefficient water resource management, and the threat to water security and sustainability of the impact of climate change. Rapid and uncontrolled urban development has reduced aquifer recharge and has in some cases resulted in the decline of groundwater level and saltwater intrusion. In view of the situation, the National Water Resources Board of Department of Environment and Natural Resources (DENR-NWRB) and the Philippine Nuclear Research Institute of the Department of Science and Technology (DOST-PNRI) have joined with IAEA in leading the IAEA Water Availability Enhancement Project (IWAVE) in the Philippines, which has been underway since 2010. A specific remedy identified to address the data and information gap particularly on groundwater resources is the formulation and implementation of the optimum methodology for application of nuclear and isotope techniques are currently underutilized in the Philippines for the assessment of groundwater resources. To demonstrate effectiveness and efficiency of these approaches to groundwater resource, the DENR-MGB which has launched its Groundwater Resource and Vulnerability Assessment Project (GRVAP), collaborated with the DENR-NWRB and DOST-PNRI, under the Philippines with data analysis and interpretation to Water Resources Regions (WRR) 2 and 10. The isotope hydrology approaches that are developed and tested will be replicated by the MGB into the assessment of the remaining WRRs in the future. The application of isotope techniques is expected to accelerate and enhance the assessment of groundwater resources in the country. Field investigation were conducted with successful engagement of the Water Districts, the regional offices of DOST, DENR-MGB, DPWH, NIA and the LGUs through awareness seminar and training on the application of isotopes techniques in water resources assessment. The training particularly covered proper sample collection of groundwater, surface water and rainwater, and sample handling and storage .The study areas covered Region 2 in North Luzon (which consists of the provinces of Cagayan, Isabel, Quirino, Nueva Viscaya, Ifugao and Kalinga Apayao), Region 10, Northern Mindanao (Bukidnon, Misamis Occidental, Misamis Oriental) and Caraga Region (Agusan Del Norte, Agusan Del Sur, Surigao del Norte and Surigao Del Sur) Stable isotopic composition (oxygen and hydrogen) and chemical composition (major ions) of groundwater and surface water were determined before and after the rainy season whenever possible, to assess seasonal variation. Rains stations for isotopes measurements in precipitation were installed at selected areas. Samples of rainwater collected represented the integrated precipitation for a one-month period and analyzed for stable isotope and tritium composition. Mean residence time and age of selected groundwater samples were determined by tritium, tritium-helium, and C-14 dating techniques. Isotopes ratio of hydrogen and oxygen isotopes in water was performed using laser stable isotopes spectrometer and continuous flow isotope ratio mass spectrometer. Tritium analysis was performed b electrolytic enrichment prior to determination of tritium activity by low level liquid scintillation counting. Carbon 14 activity was measured by accelerator mass spectrometry (AMS). 3He was determined at Isotope Hydrology Laboratory of International Atomic Energy Agency in Vienna, Austria. Benchmark isotopic composition of the water sources in the regions were obtained and mapped. The isotopic indices of freshwater sources were established. In Region 10, water samples from springs, deep wells, hand pumps and dug wells, and river water were collected within the study area from September 2012 to June 2014. Monthly integrated precipitation samples were also collected at different points within the study area from October 2012 to March 2015. A total of 85 boreholes, 29 handpumps/dugwell, 59 spring and 36 river samples were collected. The local water meteoric water line (LMWL) was established for Region 10 and this was according to equation, δ2H = 8 δ 18 O + 10. Weighted means of - 6.94 %0 for δ 18 O and -44.66 %0 for δ2H where obtained for the Agusan Basin and weighted means of -8.58%0 for δ18O and -59.4 %0 for δ2H were obtained for Cagayan-Tagaloan Basin. These were used as the isotopic indices for precipitation in the two basins within the study area. Based on the isotopic signatures of the water sources in WRR 10, the region can be divided into two major basins, namely tge Cagayan-Tagaloan Basin, covering the provinces of Misamis Oriental abd Bukidnon, and the Agusan Basin, covering the provinces of Agusan del Sur and del Norte, and Surigao del Norte. The two watersheds exhibit significantly distinct groundwater isotopic signatures. Local meteoric water lines of δ2H = 7.86 δ18O + 9.93 was established for Agusan Basiin and δ2H = 8.08 δ18O + 10.06 for the Cagayan-Tagaloan Basin. Agusan Basin groundwater are relatively enriched in the heavy isotopes compared to the groundwater in Cagayan-Tagaloan Basin. This is attribute to different moisture source of precipitation on the groundwater recharge is evidenced by the signature of precipitation during the high rainfall months reflected in most groundwater and springs. Deep groundwater in the Agusan Basin reflects the isotopic signature of rainfall in the rainy months of November to May, corresponding to the Northeast Monsoon or ''Amihan''. On the other hand groundwater in the Cagayan-Tagaloan Basin, carries the isotopic signature of the rainfall in the rainy months in the basin, from May to July. There are no apparent seasonal changes in the isotopic signatures of deep groundwater in this area. C-14 dating of the deep groundwater in Cagayan-Tagaloan Basin appear to be relatively young, from 80 pMC to more than 99 pMC. Groundwater dating in Cagayan de Oro City using Tritium-helium resulted in water ages between 18 to 72 years. Recharge rates of 422 to 625 mm/year were calculated for Cagayan de Oro City. Chemical analysis also suggest that the deep groundwater of the area can classified under Ca-Mg-HCO-3 type of water which is also characteristic of dynamic groundwater systems.In the Cagayan-Tagaloan Basin, groundwater flows from the uplands of Bukidnon down to coastal areas of the Misamis Oriental, probably originating from the Kalatagan-Kalatungan mountain ranges. In the Agusan Basin, water flows from the eastern mountain ranges down to the plains of Agusan provinces. An isotopic lapse rate of -0.06 %0 / 100 m was obtained for 18O and -0.53 %0 / 100 m for 2H was calculated using the long-term weighted average of the isotopic signatures of the precipitation samples versus elevation. However, these values are significantly small compared with the global average and observed data in the Philippines. Isotopic lapse rates were calculated using springs, assuming their elevations as the points of recharge. Using the springs as the proxies, isotopic lapse rates of -0.17 %0 δ18O and -1.06 %0 δ2H / 100 m were calculated for Agusan Basin, for Cagayan-Tagaloan Basin, isotopic lapse rate -0.24 %0 δ18O and -1.4 % δ2H / 100 m were calculated using springs from Bukidnon. These values are in agreement with the global average and observed values in some areas in the Philippines. Shallow groundwater in both basins appear to be recharged by local precipitation, being more enriched enriched in heavy isotopes compared to deep groundwater. Shallow groundwater in Cagayan-Tagaloan Basin fall also under the Ca-Mg-HCO3 type of water. Spring samples for both watershed also appear to be recharged by local precipitation with a few exceptions which may be recharged at high elevations. CDO West groundwwater is different from CDO East groundwater. CDO East water may be influenced by the water coming from Bukidnon area. While water in CDO West appears to be influenced by local recharge as seen in comparability of the isotopic signature of shallow and deep wells. This may be due to the interaction between the unconfined and semi-confined aquifers. In Region 2, field investigations were conducted from 2012 to 2013 covering the provinces of Cagayan, Isabela, Nueva Viscaya and Kalinga Apayao. A total of 49 samples from shallow wells, 107 samples from boreholes or deep wells 3 samples from springs and 21 river water samples were collected. In August 2013, selected deep groundwater in Cagayan, Isabela and Nueva Viscaya were resampled for tritium-helium groundwater age dating technique (3H-3He). Rain stations for isotopes measurements in precipitation were installed at Calamagui (23 m.a.s.l.) and integrated samples of monthly precipitation were collected from June to December 2012. The WRR2 LMWL was established to follow the best fit line, δ2H = (8.6 +- 0.2) δ18O + (13.3 +- 1.8) with a correlation of coefficient (r2) of about 0.984. This coincides both with the Philippine Meteoric Water Line (PMWL). The annual rainfall index for WRR2 was estimated to be aboout -7.3%0 δ18O and -50.5%0 δ2H. Stable isotope values of deep and shallow groundwater from Cagayan, Isabela, Nueva Viscaya and Kalinga provinces generally plotted along the Philippine Meteoric Water Line (PMWL) reflecting the effect of altitude and the proximity to the ocean on the isotopic signatures. Shallow groundwater in municipalities near the coast are most enriched in the heavy isotope while shallow groundwater in municipalities near mountain slopes in Nueva Viscaya abd Aurora are most depleted in the heavy isotope. The shallow groundwater in Isabela exhibit intermediate O-18 values. This trend indicates high contribution of local recharge from percolation of the rain or influence water from the river or irrigation water. For Cagayan deep wells, the δ18O values ranged from -5.4 %o to -8.4 %o, most lying along the LMWL. The good permeability of soil favors local recharge. The enrichment in heavy isotope in the coastal groundwater that was also observed in the shallow groundwater further reflects the influence of local precipitation on the recharge. Meanwhile, the wide δ18O range could be attributed to the characteristics of well design (tapping multiple aquifers) or the heterogeneity of geologic overlay throughout the Cayagan plains. In Isabela Province, the deep groundwater δ18O values ranged from -5.4 %o to -9.0 %0. The more enriched values are believed to be influenced by evaporation possibly from the contribution of river water. The deep groundwater in Kalinga Apayao Province exhibit of δ18O values between -6.4 %0 to -8.0 %0. The relatively depleted isotope values are attributed to fractionation of light to heavy isotopes due to altitude effect. Isotope values of samples collected from various segments of the Cagayan River, traversing the provinces of Isabela and Cagayan, and the rivers in Nueva Viscaya and Kalinga Apayao, mostly lie along the PMWL. The relatively depleted isotopes values in rivers from the higher elevation, particularly in Kalinga Apayao and Mountain Province are consistent with attitude effects. Observed isotopes values lying above the LMWL may be an indication of contamination from anthropogenic sources. Only a few springs were sampled from the elevated areas of Kalinga Apayao and Nueva Viscaya. The δ18O values ranged from -8.8 %0 to -9.8%0.Groundwater samples collected from boreholes in Tuguegarao, Ilagan and Nueva Vizcaya were determined to have detectable tritium and groundwater ages estimated using 3H-3He dating technique ranged from 3 to 38 years old. Groundwater ages obtained reveal mixed groundwater ages of about 9 to 28 years in Isabela, 33 to 38 years in Cagayan and about 3 years in Nueva Vizcaya. This trend is consistend with the conceptualized direction of downstream to Cagayan.The results of investigations in Region 2 provided insight on the general hydrological features of groundwater and surface water alike. Findings based on the isotope technique were found to be consistent with previous hydrogeological investigations. Both shallow and deep groundwater were found to be recharged locally by rain. The wide variability in stable isotope range in shallow and deep wells can be attributed to differences I geological overlay along the plains of Cagayan and Isable province. Furthermore, factors such as boreholes design could have influenced the wide range in isotope values in deep wells as different aquifers may had been tapped for freshwater productions. (author)