Isotopic Data for Inferring Groundwater Dynamics in Cagayan de Oro City, Philippines

Abstract

A groundwater study was conducted in Cagayan de Oro City (CDO) located in the north-central part of Mindanao, the Philippines using isotopic techniques. The study identifies the recharge sources of the groundwater in the city and estimates the groundwater age and groundwater recharge rate. Monthly integrated samplings of rainfall were conducted in three locations of varying altitudes from October 2012 to March 2015. Groundwater samples from production wells and shallow wells were also collected within the same period at least twice, during the dry and the wet seasons. The samples were analyzed for their stable isotopic compositions and groundwater in selected deep wells was dated using tritium-helium dating. A meteoric water line of δD = (8.26 ± 0.21) δ18O + (11.56 ± 1.88) was calculated using the precipitation-weighted reduced major axis (PWRMA) method. Isotopic compositions of groundwater show that local precipitation recharges the shallow aquifer. An interaction exists between the shallow and deep aquifer possibly due to the absence of a well-defined multilayer aquifer based on available lithological profiles. Coastal shallow wells appear to be recharged from 35–80 meters above sea level (masl), while deep wells appear to be recharged from 200–300 masl. A recharge rate of 380 mm/yr was estimated, which is more than 20% of the average annual rainfall and twice the estimated recharge using the precipitation-based method.