Analysis of Submarine Groundwater Discharge to Manila Bay: 3D Density Dependent Hydrogeological Modeling of the South-eastern coastal zone of Bataan, Philippines

Abstract

Submarine groundwater discharge (SGD) has been recognized as a potential direct pathway for water and dissolved nutrients from land to sea. SGD is a widespread phenomenon and might be of ecological significance, since it can play an important role in the marine geochemical cycles of elements and can lead to environmental deterioration of coastal zones especially in areas where its magnitude rivals surface runoff. SGD occurrences can result in coastal eutrophication, bacterial development, viruses, hypoxia, increase of algal blooms and harmful algal blooms (red tide), marine biota, fish/shellfish mortality and even human casualties. The effects associated with nutrients via SGD are biological, economical and social, thus SGD is a phenomenon which cannot be taken for granted. Understanding the mechanism behind its occurrence and the factors influencing it are all important especially to coastal management. This thesis examined the different hydrogeologic processes and factors which influence the magnitude of SGD flux, also the effects of changing climate conditions, sea level rise and how anthropogenic induced factors influence the SGD. The selected study area is located on the Southeastern coast of the Bataan Peninsula in the Philippines. It is situated on the SE slopes of Mt. Mariveles, a potentially active stratovolcano. The area stretches on 12 km of coastline where preliminary SGD flux measurements were made earlier. A regional scale modular variable-density groundwater flow model (MOCDENS3D) has been used to estimate the magnitude of submarine groundwater discharge to Manila Bay. Key features of the groundwater system include high relief in the upper slopes and narrow low relief coastal plain. The model domain incorporates both the terrestrial recharge and re-circulated water in the coastal sediments. Different scenarios were run to evaluate the sensitivity of SGD to different hydrogeologic parameters, e.g. changes in geology, topography, also the changes in water sources and sinks such as rainfall, rivers, and groundwater extraction wells. Model SGD rates peak during the rainy season and there’s a month delay to the peak rainfall. SGD shows major influence of rainfall (recharge), geology and topography. Model calculations suggest that there is substantial contribution of freshwater from terrestrial origin which discharges to the bay. The presence of confining layers greatly affects the offshore extent of SGD, and therefore its potential impact on the regional environment. Model results are consistent with previously measured SGD rates using seepage meters and geochemical tracers by Taniguchi et al. (2008).