Groundwater-derived nutrient inputs to the Upper Gulf of Thailand
Introduction
Groundwater is an important source of nutrients and other dissolved constituents to the coastal ocean. In some areas groundwater inputs may rival those of nearby rivers although few comparisons are available. Estimates of the location and magnitude of groundwater discharge are scarce, because measurements cannot be performed easily and sites of discharge are not always obvious—groundwater is the “invisible pathway” between land and sea. Since the chemical composition of groundwater and its rate of discharge determine the net effects on the overall chemical budget of receiving waters, the contribution will be significant when either flow rates or the ratio of components in groundwater relative to the surface waters is sufficiently high (Moore, 1996, Moore, 1999, Zektser, 2000, Burnett et al., 2003, Buddemeier, 1996).
The direct discharge of groundwater into the coastal zone, called “submarine groundwater discharge” (SGD) is now recognized as a significant, but poorly quantified, pathway between land and sea. As such, SGD acts as a source of inorganic and organic carbon, nutrients, and other dissolved species to coastal waters and ecosystems. The term “groundwater” as used here may be either meteorically derived fresh water, seawater that has infiltrated the subsurface and reacted with aquifer solids (referred to subsequently as “saline groundwater”), or a combination (Burnett et al., 2003). SGD is very difficult to quantify because of its diffusive nature—perhaps the main reason the process had been ignored by coastal scientists for so long. Significant progress has been made in the last decade in advancing our understanding of the forces and components of SGD as well as advances in measurement techniques, protocols that can be applied elsewhere, and models (Moore, 1996, Moore, 1999, Burnett et al., 2002, Taniguchi et al., 2002, Burnett et al., 2003, Slomp and Van Cappellen, 2004). A review of all SGD research performed as of a few years ago showed that there is a significant lack of data for regions in Africa, South America, and Southeast Asia (Taniguchi et al., 2002). One conclusion of that review was that groundwater transport could be particularly important in Southeast Asia because of the wet humid climate and excessive limestone terrains that are conducive to rapid underground flow.
We report here our initial assessments of seepage rates from two areas of the Upper Gulf of Thailand. We will show that while the fresh groundwater discharge component is generally low in this region, saline groundwater fluxes of both inorganic and organic forms of nitrogen and phosphate are substantial. Our estimates indicate that these fluxes are on the same order as those from the Chao Phraya River, the principal source of fresh water discharge to the Gulf of Thailand.
Section snippets
Chao Phraya River
This investigation was focused on the lower reaches of the Chao Phraya River (the “River of Kings”), its estuary, and two near-shore areas of the Upper Gulf of Thailand. One of these coastal areas is on the eastern side (Sri Racha) and the other to the west (Hua Hin) of the Upper Gulf (Fig. 1). The Chao Phraya Estuary was the site of recent investigations concerning natural radioisotopes (Nozaki et al., 2001, Dulaiova et al., 2006). Based on the amounts of radium observed, Nozaki suggested that
Upper Gulf of Thailand
The Gulf of Thailand is a semi-enclosed tropical sea located in the South China Sea (Pacific Ocean), surrounded by Malaysia, Thailand, Cambodia and Vietnam. The Gulf covers roughly 320,000 km. It is relatively shallow; with a mean depth of 45 m, and a maximum depth of only 80 m. The Upper Gulf, where this work was performed, has an inverted U-shape and is the catchment basin of four large rivers on the northern side (Maeklong, Thachin, Chao Phraya and Bangpakong Rivers) and two on the western
River–Gulf of Thailand surveys
Our initial survey of the river–estuary–Gulf was conducted during the dry season, January 23–26, 2004. The average discharge of the river, as measured at the Chao Phraya Dam (also known as the Chainat Dam; 277 km upstream from river mouth), was 35 m/s and that on another branch of the river at Rama VI Dam (198 km) was 12 m/s giving a total river discharge of 47 m/s during that period. Our second survey was performed July 17–19, 2004 during the wet season. The average total discharge measured at
River–estuary patterns
The difference in fresh water flow had a dramatic effect on the distribution of surface salinity in the river (Fig. 3). During January, there was significant salinity (1) in the surface waters upstream of the city of Bangkok, over 40 km from the mouth of the river. In July, with much higher discharge, we did not observe that level of salinity in the surface waters until we were within a few kilometers of the river's mouth. Differences in salinity at the same location during the same survey were
Summary
Our results have shown that seepage of mostly saline groundwater into the Upper Gulf of Thailand can contribute meaningful amounts of biogeochemically important species. Although our estimates are admittedly very rough because of the limited amount of seepage flux data currently available for the area, we can show that the amounts of inorganic and organic nutrients added via seepage are at least comparable in magnitude to that derived from the Chao Phraya River. That river is the largest source
Acknowledgements
The authors thank W.S. Moore and one anonymous reviewer for their very useful comments on an earlier version of this paper. The investigators thank the National Science Foundation (Award no. OCE-0350514), the Japan Society for the Promotion of Science (Award no. 13304036), SARCS (the Southeast Asia Regional Committee for START; System for Analysis, Research, and Training) and the Asia-Pacific Network (APN 2004-16-NSY) for the financial assistance that made this research possible. We also thank
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