Science Diliman, Vol 12, No 2 (2000)

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Tidal Effects on Groundwater in a Very Small Tropical Island: A Study on the Groundwater Resources of Pag-asa Island, Kalayaan Island Group

John Ong, Nancy Aguda, Caroline Jaraula, Zenon Mateo, Chelo Pascua, Joseph Foronda


The Pag-asa Island, with its very small land area and low relief, has a very limited fresh water supply occurring as a thin freshwater lens. Climate, topography, vegetation, lithology, human abstractions, and tides affect the volume of the freshwater lens. Topographic and hydrogeologic surveys, coupled with a 72-hour groundwater-monitoring program were done to assess the effects of tides on the freshwater lens.

Groundwater parameters measured in wells during the monitoring program include variations in water table depths, specific electrical conductivity (SEC), and temperature. Changes in these parameters were then correlated with the observed variations of the tides.

The groundwater levels oscillate with the tides at varying amplitudes. The hydraulic properties of the lithologies making up the island's aquifer influence the amplitude of the oscillations. Groundwater level oscillations are least in the reef materials and greatest in the sandy materials where it is nearly simultaneous with the tidal variations. High electrical conductivity values are marked in wells built near the coasts and in sandy materials.

The average annual precipitation is approximately 2,020 mm. Based on empirical studies, the estimated sustainable yield for small tropical islands is 6% of the lowest annual rainfall or about 20,300 m3/yr for Pag-asa Island.

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