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

Numerous studies show that the quality of groundwater, directly discharged as submarine groundwater discharge (SGD) to the coastal zone, around the world has declined throughout the past decades with increasing human activities. High-nutrient SGD might lead to coastal eutrophication, hypoxia and an increase of algal blooms. This phenomenon is critical for many South East Asia regions, highly SGD-prone due to climatic and geological settings, where anthropogenic induced groundwater pollution is an increasing concern for a population mostly stretched along coastlines. However, very few studies on SGD have been conducted so far in this part of the world despite the potential of this issue. Here, we present results from a modular variable-density groundwater flow model (MOCDENS3D), applied to the south-eastern coastal zone of Bataan, Philippines, on 12 km of coastline where preliminary SGD flux measurements were made earlier. Different scenarios were run to evaluate the sensitivity of SGD, mainly to presence or absence of clay layers in the subsurface, and to changes in rainfall. Model results are consistent with previously measured SGD rates using seepage meters and geochemical tracers. SGD rates peak during the rainy season. The presence of confining layers greatly affects the offshore extent of SGD, and therefore its potential impact on the regional environment. INTRODUCTION Submarine groundwater discharge has been recognized as a potential direct pathway for water and dissolved nutrients from land to sea (Johannes, 1980; Charette, 2001; Slomp and van Cappellen, 2007; Dürr et al., 2008; Spiteri et al., 2008). Nutrients of anthropogenic origin, e.g. from septic tanks, agricultural fertilizers or factory wastes, find their way to the ocean via SGD. SGD is a widespread phenomenon and might be of ecological significance, especially in areas where its magnitude rivals surface runoff. Occurrences can result in coastal eutrophication, bacterial development, hypoxia, increase of algal blooms, and fish/shellfish mortality. Over the years, increasing efforts have been made to study, quantify and qualify SGD around the world to understand its behavior, potential influence and related risks. Despite several initiatives to generate a well defined database for SGD, South East Asia remains in the shadow of SGD research. Yet, this area presents typical characteristics of SGDprone areas such as high relief, high rainfall and karstic or highly permeable sediments, or volcanic terrains (Bokuniewicz et al., 2003). SGD might be critical to understand in many SE Asian regions where anthropogenic induced groundwater pollution is an increasing concern for a population mostly stretched along coastlines. The Philippines is a region where coastal zones (Total land area=300,000km; Total Coastline=36,289 km) represent nearly half of the country surface area, and are thus a key SGD study site. 75% of the population live in coastal areas and the population growth is the highest of SE-Asia (National Statistics Office Philippines), which means that anthropogenic activities potentially have a huge impact on groundwater resources in terms of quantity, and quality and thus on freshwater SGD and related nutrients. The economy of this country is largely dependent on coastal resources (e.g. fish, corals, sea grass, etc.), and its marine habitat is one of the most diverse in the world (Carpenter & Springer, 2004). It appears as a major issue to fully understand the mechanism, variability and impact of SGD in this region, as well as the factors affecting its characteristics. The results of this study also represent a significant contribution to the SGD world database. This study focuses on the Southeastern coastal zone of the Bataan Peninsula, in the North eastern section of Manila Bay (fig. 1). Manila Bay is one of the areas heavily affected by harmful algal blooms (HAB) in the Philippines (Taniguchi et al., 2008). Results of the study made by Taniguchi et al. (2008) confirmed the occurrence of SGD in this area, together with elevated nutrient levels. SGD flux from the surficial aquifer was estimated to be about 12.4 m m day (8.3x10 my) with magnitude range of DIN (dissolved inorganic nitrogen) input at 42-96% of river input. Thus, SGD, rather than surface loading of nutrients by rivers alone, may be triggering the observed HAB outbreaks. METHODS A hydrogeological model has been created using MOCDENS3D (Oude Essink, 1998), a modular variable-density groundwater flow and solute transport code based on MODFLOW and MOC3D, to understand the behavior and sensitivity of SGD. The code was adapted for density variability to assess the hydrogeologic factors influencing the magnitude of SGD at a regional scale. A 30m digital elevation model (ASTER GDEM), precipitation data (21yr record Lamao Nursery, Limay and 50yr record Port Area and Science Garden, Manila), SGD field measurements, sediment core/bore logs and well information (70 wells) were used. Different scenarios were run to evaluate the sensitivity of SGD to seasonal climate variability, topography, geology, and land-use changes. The model area is 9.6 x 15.5 km and created at 100m resolution. It is run using 600 stress periods, each equivalent to 30.5 days or 1 month. Two scenarios were run with the model in steady state: 1) confining clay layers, identified from the core/bore logs, are present in the subsurface; 2) the confining clay layers are removed (a homogenous sand aquifer is assumed for the entire low-lying part). Figure1: Index map. Study area highlighted with the rec tangular box. Major topographic features and Municipalities are also shown. Study area bounds 14 o 24’0.6” – 14 o 32’49.06” N and 120 o 29’36.5” – 120 o 39’48.6”