| 摘要: | 近期氣候變遷的衝擊雖讓地下水資源的開發與使用愈發重要，但全球各地的地下水常遇到與點源污染排放無關的砷濃度過高的問題，包括台灣西南沿海及蘭陽平原中下游地區。台灣高砷濃度的地下水經調查後，常發現有水溶氣甲烷共存的現象，特別是蘭陽平原，因此被推斷砷在含水層中的遷移與甲烷的存在具有一定的關聯性，但時至今日仍未被直接驗證。為了解 “現地甲烷厭氧氧化” 與 “砷自沉積物釋出” 兩者間的連結，實驗室先前藉由現地調查與縮模驗驗、並運用地質化學模擬與分子生物探測等技術後發現，蘭陽平原地下水中微生物所主導的鐵還原溶解作用，是砷自含水層礦物相釋出的主因，而甲烷雖然極可能在此程序中扮演著電子供體的角色，但與非甲烷的現地溶解性有機質相比，其真正的貢獻程度為何，仍需進一步確認。有鑑於此，本研究重複之前的調查手法，但修正培養實驗的設計，並針對現地地下水甲烷除外的溶解性有機質的物種組成，以三維螢光光譜進行追蹤，以釐清三價鐵與五價砷被還原時，實質的生物性甲烷厭氧氧化程度。現地調查的結果顯示，蘭陽平原武淵採樣處的地下水仍維持著高(三價)砷、高(二價)鐵、高甲烷與低溶氧的特徵，說明該處地下水依舊保有厭氧狀態。水樣經三維螢光光譜分析後發現，該地下水的有機物組成是以不易被微生物降解的腐植物質為主，此特性恰好呼應現地下水生化需氧量較低的檢測結果。本研究也將部署於現地井水兩個月後長有生物膜的羊毛絨、自行合成的水鐵礦，以及所採集的現地地下水，組合進行為期七個月的縮模培養試驗。培養後發現，曝氮氣移除甲烷的實驗組(N2)在試驗第130天的螢光光譜雖有微生物代謝產物色氨酸的螢光訊號，但該組別卻無觀察到顯著的溶解態三價砷與二價鐵的生成；而在曝過飽和甲烷的實驗組別，經培養200天後的水溶液相中的三價砷與二價鐵皆顯著增加，且頂空甲烷的濃度顯著衰減，此結果有別於滅菌組與N2組，顯示該系統的微生物具有厭氧甲烷氧化並呼吸鐵礦的能力，使得原本吸附在鐵礦的五價砷藉此還原成三價砷而釋出至水溶液相中。上述的現象也同樣在以甲烷做為唯一碳源的人工合成地下水的培養過程中觀察到。不僅如此，甲烷濃度的衰減也反應在厭氧甲烷氧化功能性基因pmoA與mcrA的表達上，說明甲烷的減少確實是因生物性作用所導致；生物膜次世代定序的結果更進一步說明厭氧甲烷氧化菌Methanoperedens，很可能是利用甲烷做為電子供體並造成砷與鐵還原溶解的關鍵菌群。最後，統計分析的結果再次表明Methanoperedens與水溶液相中砷與二價鐵具顯著相關性 (p < .05)。綜上所述，本研究的結果明確指出甲烷相較於其他非甲烷的溶解性有機物，具有更高驅動武淵地區砷自含水層釋出的潛勢。;Recent climate change impacts have heightened the importance of groundwater development and utilization. However, elevated arsenic concentrations unrelated to point source pollution discharge remain a challenge in groundwater worldwide, including the southwestern coast and middle-lower reaches of the Lanyang Plain in Taiwan. Investigations into high arsenic concentrations in Taiwanese groundwater often reveal co-occurrence with dissolved methane, particularly in the Lanyang Plain. This suggests a potential correlation between arsenic migration within aquifers and the presence of methane, though direct verification has not been established to date. To elucidate the connection between in-situ methane anaerobic oxidation and arsenic release from sediments, our laboratory previously conducted field surveys, scaled modeling experiments, and utilized techniques such as geochemical simulation and molecular biological detection. Results indicated that microbial-mediated iron reduction dissolution in Lanyang Plain groundwater is the primary driver of arsenic release from aquifer minerals. While methane likely acts as an electron donor in this process, its true contribution relative to non-methane dissolved organic matter remains to be conclusively determined.
In light of this, this study replicates previous survey methods with modifications to experimental design, focusing on the composition of dissolved organic matter excluding methane in local groundwater. Tracking is conducted via three-dimensional fluorescence spectroscopy to clarify the extent of substantive biological methane anaerobic oxidation during trivalent iron and pentavalent arsenic reduction. Field surveys reveal that groundwater samples from the Wuyuan sampling site in the Lanyang Plain maintain characteristics of high trivalent arsenic, bivalent iron, methane, and low dissolved oxygen, indicating ongoing anaerobic conditions. Analysis of water samples via three-dimensional fluorescence spectroscopy reveals a predominance of recalcitrant humic substances in organic matter composition, aligning with low biochemical oxygen demand detected in groundwater. This study also deploys a seven-month scaled cultivation experiment involving combinations of woolen, biogenic iron, and locally collected groundwater with biofilm growth after two months in-situ. Results indicate that in the nitrogen-exposed methane removal experimental group (N2), although fluorescence signals of microbial metabolite tryptophan are observed on the 130th day of the experiment, no significant generation of dissolved trivalent arsenic and bivalent iron is observed, contrasting with the saturated methane exposure group where significant increases in trivalent arsenic and bivalent iron are observed after 200 days of cultivation, accompanied by significant reduction in headspace methane concentration. This differs from the sterilized and N2 groups, suggesting microbial capability for anaerobic methane oxidation and respiration of iron minerals in this system, leading to reduction of pentavalent arsenic adsorbed on iron minerals to release trivalent arsenic into the aqueous phase. Similar phenomena are observed in the cultivation process of artificially synthesized groundwater with methane as the sole carbon source. Furthermore, methane concentration reduction is reflected in the expression of functional genes pmoA and mcrA for anaerobic methane oxidation, indicating microbial involvement in methane reduction. Subsequent sequencing of biofilm generations further indicates Methanoperedens, a key microbial group likely utilizing methane as an electron donor to drive arsenic and iron reduction dissolution. Finally, statistical analysis results once again demonstrate significant correlation (p < .05) between Methanoperedens and trivalent arsenic and bivalent iron in the aqueous phase. In summary, the results of this study clearly indicate that methane, compared to other non-methane dissolved organic matter, holds a higher potential to drive arsenic release from the aquifers in the Wuyuan area. |
