Iodine-129 for determining the origin of salinity in groundwater in Pampanga, Philippines

Abstract

Assessing groundwater vulnerability from salinity contamination is vital and relevant to meet the increasing demand for freshwater. Iodine-129 (129I, half-life = 15.7 million years), a radioisotope of iodine, was used as an environmental tracer for the possible origin of salinization in groundwater (e.g., natural rock weathering, evaporated water, seawater, brine fossil water, contamination). In July 2017 (wet season), thirty-two (32) water samples were taken from production wells of different localities in Pampanga, a province in the Philippines that relies heavily on groundwater for freshwater sources. Hydrogeochemical (mainly Cl) and stable water isotopes (δ2H and δ18O) were able to identify seven samples potentially affected by seawater intrusion. The salinity origin of these samples was investigated using iodine-129 and iodine-127 isotopes by generating two graphs: 129I vs. chloride and 129I/127I ratio vs. 1/127I. 129I vs. Cl graph was capable of showing a clear distinction between different salinity origins. Five out of the seven samples were being affected by evaporated water, one sample from possible wastewater, and one sample from brine fossil water. A conceptual model was produced to summarize the results. Compiled end-members (e.g., natural brine, seawater, modern rain) were plotted in the 129I/127I ratio vs. 1/127I graph to show the interaction between two recharge sources. The results of this study will be helpful to the government, civil society, and other organizations for monitoring, policymaking, and management of the groundwater and the subsurface formations that will be crucial to continuously supply the freshwater needs of the present and future generation.