博士 / 國立臺灣大學 / 海洋研究所 / 106 / Submarine groundwater discharge (SGD), comprising of terrestrial fresh groundwater flowing from the aquifers and seawater infiltrating through the permeable sediments, is an unignorable component of hydrological cycle and carries high concentrations of associated dissolved materials (e.g. nutrients, trace metals, dissolved inorganic carbon and even pollutants) into the coastal ocean that might change the seawater quality and further impacts on biogeochemical systems.
Due to its particular location, the Taiwan Strait (TS) serves as a receiver for ample terrestrial materials (e.g. sediment and nutrients) from Taiwan in the east and the Mainland China in the west as well as a conduit for exchange of water and nutrients between the East China Sea (ECS) shelf in the north and the South China Sea (SCS) basin in the south. These nutrients from the TS influence the ecological system functioning in the ESC, where is surrounded by several highly populated cities. Although fluxes of terrestrial materials via surface runoffs into the TS have been fairly reported, those via underground flows (i.e. submarine groundwater discharge, hereinafter referred to as SGD) are rarely studied. To address this problem, we investigated the appearing of SGD in the TS through clarifying souring and sinking transports of radium isotopes.
Spatial distributions of 228Ra and 226Ra in the Taiwan Strait were observed in both spring and summer during 2009–2012 and revealed strong seasonality in response to wax and wane of the cold, brackish, and Ra-enriched water in the west and the warm, saline, and Ra-depleted water in the east. The overall picture of 228Ra and 226Ra clearly indicated their sources along the coast of China and eastward transport across the strait. Based on 228Ra mass balancing in the boundary-defined boxes off the estuaries of Minjiang and Jiulongjiang, respectively, the benthic diffusion contributed less than 7 % to the total 228Ra input and the river provided 24–44%. The 228Ra remainder was over 50 %, which was thought to be supplied by SGD. On average, SGD fluxes of 3.2±1.2×10^7 and 2.6±0.7×10^7 m3 d-1 were estimated to furnish the 228Ra remainder in the regions off the estuaries of Minjiang and Jiulongjiang, respectively. Using the overall SGD rate of 6.1 cm d-1, SGD-derived dissolved nitrogen (DIN) and phosphorus (DIP) input along the Fujian Province coast within 1 km offshore are evaluated to be 17.4×10^6 and 0.59×10^6 mol d-1, respectively, which are comparable to those obtained on the shelf off the Pearl River. It indicates that SGD from the Fujian Province’s coast into the western TS is an unignorable process to supply nutrients.
At the Gaomei Wetland which located at the south of Da-Chia River’s mouth, we estimated the fluxes of SGD and SGD-borne dissolved inorganic carbon (DIC) by using Rn and Ra as tracers. Time-series observations of 222Rn and 224Raex in coastal waters over 2 tidal cycles during dry (May of 2014) and wet (August of 2014) seasons revealed a good response to tidal fluctuation. At wet season, 224Raex activity in coastal waters was high at low tide but low at high tide; whereas an opposite trend displayed at dry season. The much higher 224Raex activity in the low-tide water at wet season maybe not mainly caused by Ra desorption from suspending particles and implies SGD input. The coastal water samples showed high 222Rn at low tide and low 222Rn at high tide. Based on 222Rn mass balance model, we estimated the SGD flux ranging from 0.1 to 47 cm d-1. The slightly high SGD fluxes occur at spring tide of wet season, implying a stronger tidal pumping coupled with a large hydraulic gradient. The overall DIC fluxes through SGD were 1.6×10^6 and 2.3×10^6 mol d-1 in dry and wet seasons, respectively. In spite of being only ~26 % compared to the DIC flux from the Da-Chia River, the DIC fluxes through SGD might have a great effect on the coastal biogeochemical cycle once released to the Gaomei Wetland.
All the reported SGD rates observed in the TS are assembled with their DIN and DIP fluxes. Within 1 km offshore, input of SGD along the western Taiwan’s coast is comparable to those along the Fujian Province’s coast; while inputs of DIN and DIP via SGD along the western Taiwan’s coast are lower than those along the Fujian Province’s coast and plausibly transported to the ECS shelf by northward current that further influences the primary production. Therefore, nutrient input via SGD is an unignorable contribution to the biogeochemical budgets over the TS and the ECS shelf.
| Identifer | oai:union.ndltd.org:TW/106NTU05279002 |
| Date | January 2018 |
| Creators | Feng-Hsin Hsu, 許鳳心 |
| Contributors | Chih-Chieh Su, 蘇志杰 |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | en_US |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 119 |
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