International Symposium on Isotope Hydrology: Revisiting Foundations and Exploring Frontiers - CN225

Contribution ID: 114 Type: Poster

DISTRIBUTION OF STABLE ISOTOPES OF LIQUID WATER IN FRESHWATER RESOURCES WITHIN WATER RESOURCES REGION X, SOUTHERN

Stable isotope abundance of hydrogen (δ2H) and oxygen (δ18O) in liquid water from different freshwater resources in the Water Resources Region X, Southern Philippines were used to gain insight about the hydro- logical processes in the area. Water Resources Region X is divided into Cagayan-Tagaloan Basin and Agusan Basin. Cagayan-Tagaloan Basin is subdivided into , East and West and MisOr. Agusan Basin is subdivided into Agusan, Surigao and Dinagat. Data presented here were the results of the reconnaissance water sampling campaign from September 2012 to August 2013. Precipitation samples were collected between October 2012 to September 2013 from areas of City (646 masl) and Manolo Fortich (571 masl), Bukidnon and San Francisco, Agusan del Sur (223 masl) to establish the local meteoric water line (LMWL) and to identify the role of precipitation as an input in groundwater and river systems. Precipitation samples were found to have δ18O values ranging between -3.62‰ to -11.94‰ and δ2H values of -17.30‰ to -86.56‰. LMWL for Agusan Basin was determined to be δ2H = 7.84 δ18O + 9.96 ‰ (r2 = 0.998) using rain collected from San Francisco, Agusan del Sur. LMWL for Cagayan-Tagaloan Basin was determined to be δ2H = 8.11 δ18O + 10.19 ‰ (r2 = 0.985) using rain collected from Malaybalay City and Manolo Fortich, Bukidnon. The weighted annual average for δ18O was determined to be -6.87‰ and -9.00‰ for Agusanand Cagayan-Tagaloan Basins, respectively. Weighted average for Cagayan-Tagaloan Basins was compared with the long-term weighted average of the nearest GNIP station of almost the same elevation at Ilomavis, Mt. Apo, Davao City (740 masl) which is -8.06‰ for δ18O and was adapted as the reference index. The estimated average (-6.87‰ and -8.06‰ for δ18O) was used as an index to demarcate possible interconnection between different freshwater resources such as rivers, springs, shallow and deep groundwater based on corresponding δ18O values obtained for each water type. The δ18O abundance of waters from rivers in Cagayan-Tagaloan Basin collected at high elevation areas, such as in Bukidnon, which are tributaries to the Tagaloan river, showed more depleted values (data range: -8.1‰ to -10.32‰; mean: -9.25‰ and s.d.: 0.78‰; median: -9.27‰; n=6) as compared to the estimated index. The difference may be attributed to the depletion of oxygen-18 containing water during downpour caused by altitude effects. Groundwater driven from deepwells and springs from these areas have more depleted average δ18O values (-8.30‰ (s.d.=0.4‰; n=12) and -8.37 (s.d.=0.52; n=6) for deepwells and springs, respectively) as compared to index value which may indicate local recharge process. On the other hand, δ18O values for both deepwell and spring groundwater obtained at lower elevation areas particularly in Cagayan de Oro, showed values ranging from -7.47‰ to -8.4‰ and exhibited more enriched mean δ18O values (7.95‰ (s.d.:0.25; n=11) and -7.87 ‰ (s.d.: 0.28; n=6) for deepwells and springs, respectively) as compared to the estimated index of -8.06‰ which may be recharged at lower elevations. The δ18O abun- dance of waters from rivers in Agusan Basin (i.e. Agusan and Surigao) showed more enriched values (data range: -4.4‰ to -6.28‰; mean: -5.66‰ and s.d.: 0.45‰; median: -5.78‰; n=18) as compared to the estimated index which may indicate evaporative enrichment. On the other hand, δ18O abundance for both deepwell and spring groundwater generally exhibited more enriched mean δ18O values (6.17‰ (s.d.: 0.50; n=7) and -5.73‰ (s.d.=0.57; n=27) for deepwells and springs, respectively) as compared to the isotopic index of -6.87‰ which may be recharged at lower elevations. Isotopic data for freshwater resources in Water Resources Region X is reported for the first time. Primary author: Mr RACADIO, Charles Darwin (Philippine Nuclear Research Institute) Co-author: Mr MENDOZA, Norman (Philippine Nuclear Research Institute) Presenter: Mr RACADIO, Charles Darwin (Philippine Nuclear Research Institute)

Track Classification: Surface Water Isotopic Studies