Tidal Variation on the Groundwater Quality of the Coastal Aquifer of Basilan Province, Bangsamoro Autonomous Region in Muslim Mindanao, Philippines

Abstract

This study analyzed the physicochemical properties of coastal groundwater, determined the Groundwater Quality Index (GWQI), and assessed saltwater intrusion across the mainland of Basilan Province, Philippines, from May to October 2022. Groundwater samples were collected from 20 wells across nine municipalities and one city during both high and low tides. Parameters such as electrical conductivity (EC), total dissolved solids (TDS), pH, chloride, sulfate, calcium, magnesium, sodium, potassium, carbonate, and bicarbonate were measured using standard analytical techniques. The GWQI was computed using weighted indices based on established guidelines, while saltwater intrusion patterns were evaluated based on dominant ion compositions.

The results revealed significant spatial and tidal variability in water quality. EC ranged from 12.7 to 1790 mg/L at high tide and 15.7 to 1486.8 mg/L at low tide, while TDS increased notably during low tide, peaking at 1840 mg/L. pH values were generally stable (5.3–8.3). Chloride and sulfate concentrations were higher at low tide, while calcium peaked during high tide and magnesium during low tide. Sodium and potassium levels showed minimal tidal variation. GWQI analysis indicated that groundwater in Tuburan, Akbar, Tipo-Tipo, Ungkaya Pukan, and Sumisip was consistently of "Excellent" quality, while Maluso declined from "Excellent" to "Good" during low tide. Lantawan, Albarka, and Lamitan showed mixed water quality, and Moh. Ajul consistently exhibited "Poor" water quality. Saltwater intrusion patterns were categorized as either "Intrusion" or "Freshening," with Tuburan and Moh. Ajul is exhibiting stable intrusion conditions, and other areas are showing varying transitions due to tidal and hydrogeological influences.

The findings underscore the vulnerability of Basilan’s coastal aquifers to seawater intrusion, especially during low tide when dilution is reduced. The consistently poor water quality in Moh. Ajul demands urgent remediation and monitoring. It is recommended that a comprehensive groundwater management plan be implemented, including controlled groundwater extraction, regular water quality monitoring, and community awareness programs to mitigate the long-term impacts of saltwater intrusion and ensure sustainable use of groundwater resources.