宜蘭平原地下水之地球化學特徵

Abstract

過去在宜蘭平原已進行相當多地質調查、地球物理分析、地層對比等等工作，來研究平原的水文地質特性，了解地層分布及地下水分層架構。在現有多種完整的地質資料下，本研究嘗試利用氣體地球化學的方法，分析宜蘭平原地下水氣體成分，配合乾濕季採樣，並利用前人研究的資料作輔助，觀察氣體及其他水化學在此區域的變化情形，並嘗試探討與地質環境的相關性以及可能地下流體來源。 宜蘭平原地下水監測井總共三十七口，其中有五口自流井和三口明顯含甲烷氣井。採集時間配合宜蘭乾濕季，於2013年4月與9月進行完整水化學及溶解氣採樣。現地記錄井水狀況、分析水氡活度，於實驗室內分析溶解氣、氦同位素、陰陽離子與氫氧同位素等成分。 由現地記錄發現，濕季水溫皆高於乾季，西部靠山區井水溫度低於東部；pH值大約介在6-8之間；東部靠海部分監測井導電度、氯離子與硫酸根離子濃度皆高，推測已有海水混染。氦同位素分析結果顯示，平原中南側有含地函、地殼、及空氣飽和水三個不同來源端成分之訊號，暗示地函源流體經由破裂帶（或三星斷層）向上遷移至平原淺處。此地區地下水所含溶解氣大約分成三類：富集氮氣、氮氣-甲烷雙重富集與富集甲烷氣，其中三口水井含有明顯甲烷氣並有大量氣體冒出，分析結果有十一口井樣品之溶解甲烷含量大於30%，其中只有一口井位於平原西北部，其他皆集中在平原中部至南部，研究結果顯示平原地下水中甲烷氣來源主要微生物成因。宜蘭平原地下水之氡氣活度在 800-10,000 Bq/m3 之間，近山區水氡活度高於平原中心並向沿海遞減，主要高活度集中在平原西部與南側，西部靠近山腳地區井水大多呈氧化環境與高溶氧量；南部高水氡活度則可能因受厚泥層影響，西部地下水受阻，推測水氡來源非西部高濃度水氡補助，應為圍岩中所含鈾釷元素蛻變而來的氡氣。整體而言，宜蘭平原中南部之水氡、一般溶解氣及氦氣皆有較大的活度與數值。 而在水化學成分方面，陰離子分析結果可以分成三大類，大多以碳酸氫根特徵為主，西部鄰近山區部分點位則有硫酸經根離子群特徵，只有被海水混染之井位含有高氯離子成分。氫氧同位素分析結果大部分皆落於宜蘭平原天水線上，唯大幅井之樣品可能因為嚴重海水混染而導致其分析結果偏離了天水線。此外，本研究分析結果顯示，宜蘭平原之地下水成分明顯受區域降雨及季節性氣團影響。

Ilan Plain is located at the northeast Taiwan and has been tectonically spreading due to the westward back-arc rifting of the Okinawa Trough. In previous studies, geological survey, geophysical research, and stratigraphic correlation were conducted to rebuild the aquifer systems in the Ilan Plain. However, few gas geochemical data of ground water are available. In this study, it is first time to systematically analyze the gas geochemistry of ground water from 37 wells to recognize the gas sources/components in the Ilan Plain. Water samples can be classified as three main groups, most samples are classified as sulfate and bicarbonate groups, few samples belong to chloride group due to the contamination of sea water. Based on the major dissolved gases compositions, the ground water samples can be divided into two groups, i.e., CH4-enriched group and N2 enriched group. Ten out of total 37 wells show CH4-enriched affinities with CH4 proportions of 30-50%, in which three wells exhibit very high CH4 contents, up to 70-90%. One of the CH4-enriched wells are located in the northwest, and the remains are in the southeast of Ilan Plain. The dissolved radon concentrations are in the range of 800-10,000 Bq/m3 in the studied area. It is interesting to note that the radon actives are higher in the west mountain areas and gradually decreasing toward the east coast areas. Meanwhile, the oxidation-reduction potential and dissolved oxygen data show positive correlations with the radon actives. It implies that the recharge of the ground water in the Ilan Plain may be from the Hsuehshan Range in west. Furthermore, elevated helium isotopic results suggest that mantle component may play an important role for the gas sources in the southeast and center of the Ilan Plain, where may be corresponding to the extensional structure or the suspected faults in the Ilan Plain.