GEOLOGICAL SOCIETY OF THE PHILIPPINES

Fresh groundwater protection strategies for Pagasa Island, Philippines

 

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

and John Ong

National Institute of Geological Sciences, University of the Philippines, Diliman, 1101 Quezon City

 

A b s t r a c t

 

Pagasa Island, with an area of 31.4 ha, is underlain by an unconfined aquifer of limestone and beach sand.  The central part of the circular island has a surface elevation of 1.5 to 2.0 masl.  The static water level in the center of the island was 26.9-cm above mean sea level on 2 May 1999 following a week of rainfall, and declined to 20.6-cm asl three days later.  The island is covered with mangrove vegetation except in the central and southern portions of the island.  As of May 1999, there are 17 wells on the island distributed along an east-west zone passing through the center of the island.  Sixteen of these wells are open wells 1.43 to 2.48 m deep that are dug 0.38 to 0.73 m below water table.  Five wells are abandoned due to saline taste or foul odor.

The greatest threat to ground water contamination on the island comes from entry of saline water and sewage.  Other threats are posed by abandoned open wells and unprotected storage of petrochemicals.

Some latrines on the island are constructed within the recharge zone (in the central part of the island), possibly contaminating nearby downslope wells.  It is good practice to construct latrines near the coastline within the discharge zone.  The separation distance between latrines and some wells down the groundwater flow direction are as close as 15 to 25 m.  Based on the soilís poor filtering capacity for bacteria and microorganisms in limestone aquifers, the minimum separation distance from a latrine to a well down the flow direction should be at least 50 m.

The dug wells have to be properly designed to minimize contamination.  Some wells are not lined in the upper part making the entry by surface runoff and debris possible.  Only half of the wells are designed with an extended well lining above the ground for protection from surface pollution.  Some wells do not have a concrete slab constructed on the ground surface.  It is desirable for the top of all dug wells to be covered, and a handpump fitted, to further prevent contaminants from entering the well.  Abandoned wells have to be plugged with reinforced concrete slab or with pressure treated lumber.

Five wells have average specific electrical conductivity (SEC) values >1000 mS/cm.  Wells constructed within 50 m from the coastline were found to be salty during dry months.  Two wells within a high permeability zone in the central part of the island exhibit relatively high SEC values.  Wells with tidal efficiencies >0.10 generally exhibit high SEC values.  The only well with a motorized pump, with SEC value of 825 ĶS/cm, is already tapping the brackish aquifer and is prone to further degradation by upconing and saltwater intrusion.  Due to the very shallow boundary between the freshwater-brackish zones, the use of motorized pumps is discouraged.  Wells affected by brackish water can still be used for sanitary flushing.  Water resources on the island can be supplemented by rainwater harvesting systems.

Efforts must be made to avoid contamination from petrochemicals (fuels) since these are not filtered effectively by the soil and can reach substantially greater distances from the source.  These fuels are stored in separate sites in steel barrels that are placed directly on barren ground.  These fuels are better stored in the coastal area in aboveground petrochemical storage tanks (ASTs).

 
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