Identifying Sources and Load Reduction Scenarios in the Nutrient Loading Problem of the Manila Bay Watershed (Philippines)

Manila Bay is an important port in Southeast Asia near the densely populated city of Metro Manila, Philippines. The bay drains an area of 19,720 km² populated by 31 million people or roughly 34% of the whole population of the country. Evidence of hypoxia and eutrophication has been observed in the bay. The study used a scaled down version of the global nutrient export from watersheds (Global NEWS) model along with statistical data, coefficients, and land use maps to calculate the nutrient loads from the Manila Bay watershed. The scenario building feature of the model looked into four major aspects of the nutrient loading problem: sewage connections and treatment, decentralizing urban population, population growth rates, and phosphorus content regulation in detergents. Of the total nitrogen (TN) and total phosphorus (TP) loads, 66% and 59% respectively, came from point sources (domestic). Agricultural activities account for 22% of TN and 30% of TP. Majority of the domestic waste load comes from densely populated provinces like Metro Manila which is adjacent to the coast of Manila Bay. Rural provinces associated with agricultural activities situated further north of the bay, contributed most to the nutrient loads from agriculture. Smaller but more densely populated sub-watersheds like the Pasig and Cavite River sub-watersheds each contributed almost as much as the biggest sub-watershed (Pampanga River Basin) to the total nutrient load. Unfortunately, with the continued high population growth driven principally by migration to Metro Manila, nutrient loading from the domestic sector will continue to increase. Sewage treatment will help decrease the loads but tertiary treatment is required for a significant decrease. A ban on phosphorus content in detergents could help decrease total P loads by 7% which is half as effective as tertiary treatment which decreases loads by 15%, albeit at a lesser cost. Decongesting the densely populated urban centers by providing urban amenities and jobs in rural areas and provinces in other parts of the country is a way to help address the nutrient problem of Manila Bay. The tool used in this study provided a relatively rapid way to assess the contributions of nutrient sources, build scenarios for different solutions, and consider options that may be useful for policy makers and management bodies.