台灣西部近岸水體清除現象：釷-234與鈾-238不平衡的應用

Abstract

本研究利用海研二號（ORII-1248，2004年11月19日至23日），沿台灣西岸進行採樣工作，沿航跡使用表水監測系統觀測表水溫度、鹽度、透光度及螢光連續訊號。除基本水文資料外，採集水樣進行營養鹽、葉綠素a、顆粒態有機碳、初級生產力，以及細菌生產力等參數的分析。本研究目的為探討污染嚴重的台灣西部近岸水體顆粒動力機制。 由螢光和消光係數隨緯度的變化可知，南北海域顆粒組成差異大。以北緯23.67度為分界，台灣西南部近岸海域顆粒組成以生物性顆粒為主，西北部近岸海域顆粒組成則為無機性顆粒支配。高懸浮顆粒濃度導致溶解態234Th活度極低且變化大（0.1-0.75dpm/L），活度低值總出現在輸砂量高的台灣西部五大河口地區。與溶解態234Th活度相較，顆粒態234Th活度於台灣西南部近岸海域變化小，在台灣西北部近岸海域則與溶解態234Th活度呈鏡像分佈關係。本研究使用穩定狀態水平傳送清除模式估算清除現象強烈的台灣西部近岸水體，顆粒去除速率和停留時間，溶解態234Th與顆粒態234Th停留時間分別為<0-16天和<0-10天。

During November 19-23, an intensive sampling expedition in the coastal region off western Taiwan (Cruise #: ORII-1248) was carried out. A total of 27 stations were occupied to collect large-volume seawater for dissolved and particulate 234Th measurements. In addition to the basic hygrographic parameters (T, S, fluorescence and light transmission), separate samples for the measurements of nutrients, particulate organic carbon, primary productivity and bacteria production were also collected. The investigation is aimed to provide information of particle dynamics in the nearshore waters of highly populated west coast of Taiwan. Latitudinal distributions of fluorescence and light transmission evidently indicate that particle compositions in the northern and southern regimes are very different. Divided by Cho-Shuei River system at about 23°40’N, particles in the northern regime are characterized by terrigeneous characteristics whereas those in the southern regime are dominated by biological origin. Resulted from high turbidity, the dissolved 234Th activities are extremely low, ranging between <0.1 dpm/L and 0.75 dpm/L, and show drastic latitudinal variability. Low dissolved 234Th activities are usually associated with the five river systems reported with high particulate loads. Particulate 234Th activities showed less drastic change than dissolved 234Th in the southern regime (average value of ~0.3 dpm/L), whereas those in the northern regime are mirror-imaged with dissolved 234Th activities. Large deficiency of 234Th relative to 238U indicates an active scavenging-removal phenomena prevailing in the nearshore waters. An advection-scavenging-removal model was be adopted to derive scavenging and removal rates of 234Th. Dissolved 234Th residence time and particulate 234Th residence time are <0-16 days and <0-10 days, respectively.