Sulfammox driving sulfate reduction and thioarsenate formation in groundwater: evidence from multiple isotopes

Abstract: The mechanisms governing sulfur cycling and its impacts on arsenic speciation and enrichment in groundwater remain poorly understood. This gap was investigated by using multi-sulfur isotopes (including δ34S-SO4 and δ34S-H2S), nitrogen isotopes (δ15N-NH4), and molecular characteristics of dissolved organic matter (DOM) in high-arsenic groundwater from the Datong Basin, China. Results show a concurrent increase in total arsenic concentrations (from 0.14 to 700 μg/L) and thioarsenate proportions (up to 92%) from the alluvial fan (Zone I) to the flat plain (Zone III). This trend was accompanied by increased H2S concentrations (from <1 μg/L to 2920 μg/L) and δ34S-SO4 (from 2.2‰ to 64.5‰), but decreasing DOC/NH4+ molar ratios. The significantly negative correlations between DOC/NH4+ molar ratios and H2S concentrations and δ34S-SO4 values supported sulfate-driven anaerobic ammonium oxidation (Sulfammox) as a key SO42- reduction pathway. This process was prominent in Zone III with limited DOM biodegradability, low DOC/NH4+ molar ratio, and high NH4+/SO42- molar ratios. High H2S concentration in Zone III was responsible for elevated arsenic concentrations and high proportions of thioarsenates. These findings were the first to highlight the critical role of NH4+ acting as electron donors in promoting Sulfammox processes in groundwaters, elucidating a novel mechanism of arsenic enrichment in groundwater.
Datasets on groundwater geochemistry, thioarsenate species, multi-sulfur and nitrogen isotope values, and FT-ICR MS results.