VOLUME 19 (Supplement)

Measurement of pre-monsoon and post-monsoon tritium activity concentrations without electrolytic enrichment in selected coastal waters of Sta. Ana, Cagayan, Philippines

Tritium (³H) is an important environmental radionuclide because it occurs primarily as tritiated water and can be transported through hydrological and oceanographic systems. However, published measurements of tritium in Philippine coastal seawater remain limited. This study determined tritium activity concentrations in selected coastal waters of Sta. Ana, Cagayan using liquid scintillation counting (LSC), compared pre-monsoon and post-monsoon measurements, and established an initial reference dataset for future monitoring. Surface coastal water samples were collected along a 0–15 km coastal transect during five campaigns from March to December 2025.

During the pre-monsoon period, eight of twelve samples yielded concentrations above Lc, although only one exceeded the applicable MDA, with a maximum of 12.5 ± 3.6 TU. During the post-monsoon period, all eight samples were above Lc, with six exceeding MDA. The measured signals ranged from 6.2–12.5 TU pre-monsoon and 4.9–19.4 TU post-monsoon. The greater post-monsoon detection frequency and broader concentration range suggest temporal variability potentially associated with seasonal hydrological inputs and coastal mixing, although specific processes cannot be established from the present dataset. Comparison with Philippine and international measurements placed the Sta. Ana observations within the broader range reported for environmental and coastal marine waters. The study provides an initial reference dataset for northern Luzon coastal seawater and a basis for continued monitoring.