Evaluation of fresh groundwater contributions to the nutrient dynamics at shallow subtidal areas adjacent to metro-Bangkok

摘要

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 levels due to over-pumping suggests a decrease of the fresh groundwater flux into the sea. In this study, time-series resistivity monitoring under the seabed adjacent to Bangkok city did not actually show any evidence of fresh groundwater fluxes, and 8D and 518O signatures in porewater also followed this phenomenon. Consequently, the .observed upward water flux can be mainly attributed to recirculation of the overlying water. 515N and 818O 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.