27. Evaluation of Fresh Groundwater Contributions to the Nutrient Dynamics at Shallow Subtidal Areas Adjacent to Metro-Bangkok

Both submarine groundwater discharge (SGD) and the Chao Phraya River are major agents of nutrient supply into the Gulf of Thailand. With the development of the city of Bangkok, however, lowered groundwater flux into the sea. In this study, time-series resistivity moitoring under the seabed adjacent to Bangkok city did not actually show any evidence of fresh groundwater fluxes, and –D and – ¹⁸O signatures in porewater also followed this phenomenon. Consequently, the observed upward water flux can be mainly attributed to recirculation of the overlying water. – ¹⁵N and –¹⁸O values in nitrate suggested that nitrate was mainly supplied via the river, and rapidly reduced in the surface suboxic sediment, while re-mineralized ammonium and phosphate were substantially released into the overlying water. River water derived nutrient could be still important as original sources of organic matter, even at the area where high amounts of SGD is observed.