Unraveling roles of dissolved organic matter in the co-occurrence of arsenic and ammonium in Quaternary alluvial-lacustrine aquifers

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Geogenic arsenic (As) and ammonium (NH4+)-contaminated groundwaters are global public concerns, posing threats to human health and ecological security. However, when considering the enrichment of either As or NH4+ in groundwater, the concurrent occurrence of both contaminants warrants further discussion. Herein, we conducted a hydrogeochemical study in the shallow confined aquifer of the central Yangtze River basin to elucidate the key mechanism of coexistence of As and NH4+ through a thorough analysis of groundwater chemistry, inorganic carbon isotope (δ13C-DIC), and dissolved organic matter (DOM). High As and NH4+ groundwater mainly occurs in meanders of the middle reaches of Yangtze River, with maximum concentrations up to 936 μg/L and 45.3 mg/L, respectively. Our findings indicate that the degradation levels and characteristics of DOM play a significant role in the co-occurrence of As and NH4+ with varying concentrations in groundwater. δ13C-DIC signatures show that there was a simultaneous increase in the concentrations of As and NH4+ in groundwater, with the highest concentration of As during fermentative conditions. In contrast, during the progression of OM degradation towards methanogenesis, the NH4+ concentration reached its peak, while the As concentration was lower compared to the fermentation stage. Optical properties of DOM demonstrate that elevated levels of NH4+ result from the mineralization of N-containing organic compounds particularly as the degradation proceeds to methanogenesis stage, while both fermentation and methanogenesis processes significantly contribute to the microbially reductive dissolution of As-bearing iron oxides in groundwater systems. This study presents isotopic and fluorescent evidence to clarify the co-existence of As and NH4+ in Quaternary alluvial-lacustrine aquifers.