| dc.description.abstract | 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. | en_US |