Laguna de Bay-Philippines. Etude de la protection et de l'utilisation des ressources en eaux

Laguna de Bay water resources development (Philippines Islands)

Bureau des Experts - SOGREAH

Abstract

Laguna de Bay is a shallow brackish lake of 900 km² (Fig. 1) discharging to Manila bay through the Napindan and Pasig Rivers. These rivers flow through Greater Manila (5 million inhabitants) and transport most of its domestic and industrial waste to the sea. Mostly because of periodic salinity and large pollution, the lake water resources are not presently efficiently used. However, within one or two decades, Laguna de Bay water will be needed for the supply of the expanding population (14 to 16 million in year 2000). The study, carried out on 1973-1974, has covered at the prefeasibility stage all technical and economical aspects, including environmental and sanitary conditions and institutional and financial aspects. The study covered the following points : 1. Flood control and regulation of Laguna de Bay discharges The hydraulic regime of the lake and of the Marikina, Napindan and Pasig rivers was reviewed with the help of a mathematical model reproducing the rivers, the lake and the tidal elevation. Main recommendations were : The construction of the Mangahan floodway (Fig. 2 and 3) to protect Manila against overflow of the Pasig caused by flash floods from the Marikina by diverting excess flows to Laguna de Bay. The rate of internal return based on flood damage reduction is 14 %. The construction of a control dam across the Napindan (Figs. 2 and 4) to control Laguna de Bay discharges and protect the lake against the penetration of salt and polluted water when, at the end of the dry season, the lake level may become lower than tide elevation in Manila Bay reversing the flow of the Pasig-Napindan rivers. Mostly based on irrigation and fishery benefits the I.R.R. is nearly 30 %. 2. Present water quality and pollution - Future evolution A fully instrumented laboratory was built and operated, thousands of samples taken. The study included chemical analysis of more than fïfty products, biologic studies of plankton, fauna and flora with special attention to algae and fishes. Nitrogen and phosphorous cycles were also studied. This water quality study is continued by the Philippine administration as the results of one year of observation must be continuously updated. Main results are : Apart from chloride, magnesium, calcium and SO₄² , entering the lake from the Napindan-Pasig, a problem to be solved by the Napindan control dam, the chemical water content is suitable for water supply. Pesticides, heavy metals and other poisonous substances have not yet reach a dangerous level. However urban and industrial development of the area renders regulation and control mandatory. - Presently the most prejudicial pollution is caused by the excess of nitrogen and phosphorus compounds causing a proliferation of blue green algae (anacystis) which in summer may exceed one million anacystis per ml. (Fig. 5). These algae blooms have in 1973 already reduced by 15 % the value of fishery production and will make water treatment for municipal use very costly if not impossible. As illustrated on figure 6 the main causes of this pollution, the nitrogen has proven to be the main villain, are human population, livestock and inflow from the Pasig-Napindan. Without control the pollution will triple by year 2000 destroying fisheries, making the water unfit for water supply and causing an ecological disaster with severe consequences to the environmental and sanitary conditions. 3. Possible utilisation of Laguna de Bay water resources Benefits from three main utilizations were evaluated : Irrigation for which the present quality of the water will be acceptable as soon as the Napindan control dam will have stopped saline inflows. Fishery, mostly through the raising of bangus (milk fïsh) in fish pens. The bangus which reproduces only at sea, feeds on the phytoplankton and algae abundant in the lake and unused by the original fish population. Water supply for Greater Manila when around 1990-95, other sources of water supply will have been exhausted. The study at feasibility level of the treatment plant with the help of a 8 m³/h pilot plant was included. 4. Preservation of water quality : objectives, pollution control program, economic justification, institutional aspects, financial feasibility The 1973 value of the nitrogen pollution has been taken as a limit not to be exceeded : it already causes sizeable damage to fishery and will periodically make water treatment costly and near the limit of actual possibilities. As illustrated on figure 7 the main points of the pollution control program up to year 2000 are : stopping the pollution from the Pasig by the Napindan control dam, reducing livestock pollution by regulations and levies, controlling industrial pollution by regulations, levies and construction of 2 main sewers, reducing urban pollution and improving sanitary conditions by progressive implementation of sewerage system including the use of sewerage after lagooning for irrigation. The corresponding internal rate of return based only on direct benefits, mostly fishery as the discounted value of water supply starting in 1995 is small, exceeds 20 %. It has been suggested to entrust the water management of Laguna de Bay basin to an administration centralizing all aspects, pollution control included. The possibility of financing the costs by reasonable levies has been established.