臺灣地區水體中水氡之空間分布初探

Abstract

本研究旨在以放射性同位素示蹤劑原理建立調查陸域地下水和海底地下水輸出的方法。研究以臺灣陸域上的臺北盆地、蘭陽平原和屏東平原為例，進行地表及地下水氡活度分析。而在海域調查方面，則以龜山島東側海域為研究案例，於海研二號1792及1871航次中進行海表水氡活度分析，並配合變頻聲納和多音束水深探測及水文流場分析，證實方法的實用性與穩定性。 本研究利用RAD7（Durridge Company Inc.）氡氣偵檢儀，發展RAD H2O方法測量陸域水體中的氡氣。臺北盆地、蘭陽平原及屏東平原之地下水氡活度皆在1000 Bq/m3以上，然而三地之水文地質條件不盡相同。臺北盆地地表與地下水氡活度差異甚大，推測可能導因於兩者之間相隔著厚層阻水層而不利交換。蘭陽平原之地下水及湧泉水氡活度皆高，並且由平原之西北側往沿海遞減，顯示全區為由雪山山脈高壓含水層出露後形成的淺層地下水，並不斷往沿海地區輸出。屏東平原之地下水氡活度以潮州斷層沿線最高（約15000至30000 Bq/m3），而沿海地下水氡活度仍有4000 Bq/m3，顯示陸域地下水體直接入海的可能性極高；此外，本區地表水氡活度與地下水氡活度相當，且較山區地表水高，代表平原內地表及地下水之間有良好的交換作用。 在海域調查方面，本研究利用變頻聲納（chirp sonar）及EK60漁探儀辨識出龜山島東側淺海海域及東北側水深約200公尺處存在多處海底熱液活動，而多音束測深（multi-beam）資料亦顯示這些位置的海底地形較為崎嶇起伏。調查期間於海研二號1792航次先以RAD AQUA方法分析海表水氡活度，發現水氡相對高值與熱液活動位置吻合。而後於海研二號1871航次中，規劃網格狀測線並維持船速3節使用RAD AQUA方法測量，配合水文流場資料可更準確對應海底熱液活動位置，證實此方法於海域調查中可準確探知水中氡氣源區位置。

This study aims to build up the analysis method of radon-in-water to survey groundwater and submarine groundwater discharge (SGD). Radon (222Rn), a radioactive gas, is one of the most extensively used naturally occurring tracers for groundwater and SGD since it is enriched in groundwater, easier to measure by RAD7 (Durridge Company Inc.), and it is inactive in biological activities. In Taipei Basin, Lanyang Plain and Pingtung Plain, radon activities in groundwater are higher than 1000 Bq/m3. However, the hydrogeology conditions in three regions are not the same. The difference of radon activity between surfacewater and groundwater in Taipei Basin is significant. It is probably caused by thick aquitard, blocked the exchange between two water masses. Radon activities of groundwater and springs in Lanyang Plain are high, and descending from northwest to coastal area. High-pressure aquifer of Hsuehshan Range is the major source of groundwater in this region. In Pingtung Plain, the highest radon activity in groundwater is along the Chaozhou Fault (around 15000 -30000 Bq/m3). At coastal area, the radon activity in groundwater is as high as 4000 Bq/m3. It indicates the terrestrial groundwater may directly discharges into the ocean. In addition, the radon activity of the surfacewater in plain area is higher than the mountain area, It implies that a well connectivity between surface and groundwater in Pingtung Plain. Kueishantao (KST) is a young volcanic islet located off northeastern Taiwan and at the southwesternmost part of the Okinawa Trough. Submarine hydrothermal systems are ubiquitous near the coast of KST. The spatial distribution of radon in surface seawater, in conjunction with chirp sonar, scientific sounder system (EK60) and multi-beam bathymetric survey works conducted on OR2-1792 and 1871 cruises. Most of hydrothermal vents identified by chirp sonar and EK60 profiles are located at shallow waters within 20m water depths and off northeastern area (water depths around 200m) and the radon activity in surface seawater is also higher than adjacent seas. Our results confirm the feasibility of RAD H2O and RAD AQUA methods in tracing the process of groundwater and submarine hydrothermal activity in Taiwan area.