Isotopic Assessment of Shallow Groundwater Resources in the Cagayan Valley Region, Philippines

ASM Sc. J., 21(2), 2026

Published on July 10, 2026

https://doi.org/10.32802/asmscj.2026.0133

Author: Jayzelle S. Ventura, Lanie A. Alejo, Orlando F. Balderama, Alvin John B. Felipe, Englebert O. Manmano, Charles Darwin T. Racadio

Abstract

Effective groundwater management is of high importance, particularly in the light of the changing climate and increasing demand due to the increasing population. This study found that the main source of water for both surface and groundwater from the selec ted water samples over the Cagayan Valley Region is of meteoric origin. A local meteoric water line (LMWL) of δ2H = 8.22 δ18O + 12.31 was developed in this study which closely resembles the Philippine Meteoric Water Line (δ2H = 8δ18O + 12). Stable isotope analysis also revealed that surface water and groundwater can have considerable connection. Supported by tritium analysis, the groundwater age implies that the groundwater is recharged with new water making shallow well water relatively you ng where the selected wells record a young groundwater age from 2 years to more than 22 years. The estimated average recharge rate is 197.96 mm/year, accounting for 9.89% of the region's average annual rainfall. Further analysis also implies that the altit ude effect also prevails over the region where higher altitude provinces have a more depleted isotopic composition. The result of the study supports the applicability of isotope hydrology in a more comprehensive groundwater evaluation.

Keywords: groundwater age, isotope signature, recharge rate, shallow well, stable isotope, tritium analysis