以水中鐳同位素之源匯傳輸探討臺灣海峽沿岸海底地下水滲流之現象

Submarine Groundwater Discharge along the Coast of the Taiwan Strait indicated by Source and Sink Transport of Radium

許鳳心(Feng-Hsin Hsu)

指導教授 ： 蘇志杰

國立臺灣大學/理學院/海洋研究所/博士(2018年)

https://doi.org/10.6342/NTU201800592

摘要

受到陸域水頭壓力差異、潮汐波動或其他動力的驅使，沿岸海域地下含水層中的地下水及海床內部孔隙水會穿越海陸交界進入海洋，此種現象稱為海底地下水滲流(Submarine Groundwater Discharge，簡稱SGD)。地下水入海總量雖可能低於地表逕流，卻是水循環中不可忽視的一個環節，亦為陸源化學物質進入海洋的途徑之一。因流經地層中淡海水混合區域溶解大量元素物質，海底地下水滲流通常具有高濃度溶解性物質，甚至營養鹽濃度都可能遠高地表逕流。當海底地下水滲流進入近岸海域後，極可能改變海水性質，進而衝擊海域生態及影響元素生地化循環。
臺灣海峽雖面積不大、水亦不深，卻位居要津。地表侵蝕速率極高的高山島嶼(臺灣)居於東；幅員最遼闊的陸地(歐亞大陸)依於西；西太平洋面積最廣的邊緣海盆(南海)、邊緣陸棚(東海)各鄰於南、北；如此獨特布局造就臺灣海峽的重要性。臺灣海峽不僅為南海海盆與東海陸棚之間水體及物質交換的重要管道，自東西兩岸輸入的陸源物質更是調控此兩大邊緣海域生態系統的重要因素之一。故此，釐清臺灣海峽所有陸源物質輸入之通量有其必要性，尤其是溶解態物質，營養鹽、溶解態有機碳、溶解態無機碳…等。臺灣海峽的溶解性物質輸入通量之研究以往僅聚焦兩岸地表逕流，但沿岸海底地下水滲流的輸出往往被忽略，資料非常稀少。本研究利用天然放射性鐳同位素(228Ra、226Ra、224Ra)作為示蹤劑，探討臺灣海峽沿岸海底地下水滲流入海之水量及相關溶解性化學物質之通量；並彙整過往已發表數值，進一步比對河川輸入水量來釐清海底地下水滲流的影響力。
本論文第一部份針對臺灣海峽水體中鐳同位素(228Ra、226Ra、224Ra)活度的空間分布進行討論，結果顯示大陸福建沿岸的泥質斜積層為226Ra、228Ra、224Ra的首要來源，並自海峽西岸往臺灣方向逐漸降低。海峽西側為往南流低溫、低鹽、高活度鐳的大陸沿岸水；海峽東側為往北流高溫、高鹽、低活度鐳的南海水(混合黑潮支流水)；推測此二水團隨每年季風更迭，南北折衝，此消彼長，是台灣海峽水體中鐳同位素時空分佈的主要控制因子。透過於閩江口、九龍江口外海各自建立228Ra的質量守恆模式(Mass Balance Model)：釐清228Ra的收支通量，並估算海底地下水滲流之通量。閩江口外海區域的估算結果平均為8.7±3.7 ×10^9 m3 d-1，此通量約為閩江年逕流量的1/5倍；九龍江口外海區域的估算結果平均為9.4±2.7 ×10^9 m3 d-1，此通量約為九龍江年逕流量的3/5倍。利用估得之平均海底地下水滲流速率6.1 cm d-1，簡單計算大陸福建省545公里海岸線之1公里以內海域經由海底地下水滲流輸出之溶解態無機氮及磷分別為17.4×10^6及0.59×10^6 mol d-1。此通量與前人在珠江(中國年逕量排名第二高的河川)外海陸棚區域所得之結果不相上下；由此可見，大陸沿岸的海底地下水滲流進入台灣海峽西側對於營養鹽的貢獻是不可忽略。
本論文第二部份針對臺灣西部沿岸(臺灣海峽東側)進行地下水入海調查，以臺中地區大甲溪河口的高美溼地為例。透過計測沿岸水體中短壽命鐳同位素(224Ra)及氡氣(222Rn)，探討海底地下水存在的可能性。結果顯示乾、濕季水體中超量224Ra活度時序變化呈現相反趨勢，表示兩季節主要來源明顯不同。乾季主要為水體中顆粒表面脫附的超量224Ra；濕季低潮位時，水體中異常高值的超量224Ra活度並非單純由脫附自水體中顆粒表面所造成的，極可能包含海底地下水滲流匯入的部分。經由孔隙水中氫氧同位素值(δ18O、δD)發現海底地下水滲流主要為再循環海水。通過建立水體中氡氣質量守恆模式，估算得到海底地下水滲流通量為0.1－47 cm d-1，並隨著潮位變化。當高潮位時，海底地下水滲流通量較低；當低潮位時，海底地下水滲流通量較高。海底地下水滲流同時挾帶1.6×10^6 mol d-1(2014年5月)、2.3×10^6 mol d-1 (2014年8月)的溶解態無機碳進入高美濕地，影響初級生產力。
最後集結已發表兩岸輸入臺灣海峽的海底地下水滲流及所挾帶的溶解態無機氮、磷之通量。大陸福建沿岸1公里以內海域地下水入海之通量雖遠不及閩江與九龍江，但所挾帶之溶解態無機氮、磷輸出通量與河川輸出量不相上下。相較之下，臺灣西部沿岸1公里以內海底地下水入海所挾帶之溶解態無機氮、磷卻低於臺灣西部河川總輸出，且可能因強勁往北的海流而傳輸至北邊的東海陸棚。無論高低，海峽兩岸海底地下水滲流總匯入的溶解態無機氮、磷通量在生地化循環的收支平衡都是不可忽略的環節。

關鍵字

台灣海峽 ； 海底地下水滲流 ； 鐳同位素 ； 閩江口 ； 九龍江口 ； 高美濕地 ； 大甲溪口

並列摘要

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.

並列關鍵字

Radium isotopes ； Taiwan Strait ； submarine groundwater discharge ； Minjiang ； Jiulongjaing ； Gaomei Wetland ； Da-Chia River

參考文獻

Alling, V., D. Porcelli, C.-M. Mörth, L.G. Anderson, L. Sanchez-Garcia, O. Gustafsson, P.S. Andersson, C. Humborg, 2012. Degradation of terrestrial organic carbon, primary production and out-gassing of CO2 in the Laptev and East Siberian Seas as inferred from δ13C values of DIC. Geochimica et Cosmochimica Acta 95, 143–159.

Basu, A.R., S.B. Jacobsen, R.J. Poreda, C.B. Dowling, P.K. Aggarwal, 2001. Large groundwater strontium flux to the oceans from the Bengal Basin and the marine strontium isotope record. Science 293, 1470–1473.

Beck, A. J., J. P. Rapaglia, J. K. Cochran, H. J. Bokuniewicz, S. Yang, 2008. Submarine groundwater discharge to Great South Bay, NY, estimated using Ra isotopes. Marine Chemistry 109, 279–291.

Beck, A. J., J. K. Cochran, S. A. Sañudo-Wilhelmy, 2010. The distribution and speciation of dissolved trace metals in a shallow subterranean estuary. Marine Chemistry 121, 145–156.

Beneš, P., P. Strejc, 1986. Interaction of radium with freshwater sediments and their mineral components. Journal of Radioanalytical and Nuclear Chemistry 99, 407–422.