Methylated Arsenic Enrichment in Groundwater of Jianghan Plain: Insights from Carbon Isotope and DOM EEMs Analysis

Arsenic poisoning caused by high-arsenic groundwater represents a significant threat to human health. The arsenic methylation process, which converts highly toxic inorganic arsenic into less toxic methylated arsenic species, has the potential to mitigate the environmental risk of arsenic contamination and reduce its toxicity to humans. This process may provide an effective approach to managing arsenic pollution in groundwater. However, current research predominantly focuses on the migration of inorganic arsenic, with limited understanding of the key processes and controlling factors that govern the enrichment of methylated arsenic. In this study, it analyzed shallow groundwater from the Jianghan plain, located along the Yangtze and Han rivers, using inorganic carbon isotope tracing and EEMs of dissolved organic matter. The aim of this study is to elucidate the organic matter degradation pathways that regulate the enrichment of methylated arsenic and to identify the key biogeochemical processes involved. It found that methylated arsenic concentrations in the Jianghan plain ranged from < 0.01 to 444 μg/L, with an average concentration of 30 μg/L. The processes controlling methylated arsenic enrichment differed significantly between groundwater from the Yangtze River and the Han River. Along the Yangtze River, the degradation of high-molecular-weight aromatic organic compounds drives methanogenesis, which in turn promotes arsenic biomethylation and the subsequent enrichment of methylated arsenic in groundwater. Additionally, the sulfate reduction process, associated with the fermentation of organic matter, also supports arsenic biomethylation. In contrast, in the Han River region, the arsenic biomethylation process is primarily driven by the fermentation of small-molecule reactive organic matter.