NITROGEN CYCLING IN HIGH ARSENIC GROUNDWATER SYSTEM

Co-occurrence of elevated NH₄⁺ and arsenic (As) has been widely found in anoxic aquifers, which was naturally sourced from organic matter degradation coupled with Fe(III) oxide reduction. Anaerobic NH₄⁺ oxidation coupled to Fe(III) oxide reduction (Feammox) may also contribute to As and Fe(II) mobility. Knowledge of co-cycling Fe(III)/Fe(II) and nitrogen species is crucial on As enrichment processes, which has been not well documented yet.

In deep groundwater along an approximate flow path from the alluvial fan to the flat plain in the Hetao Basin, China, dissolved NH₄⁺, Fe(II), and As concentrations consistently increased, and ORP values and dissolved NO₃^- concentrations decreased, accompanied by the increases in δ¹⁵N-NO₃ and δ¹⁸O-NO₃ values and decrease in δ¹⁵N-NH₄ values. Isotopic data suggest that NH₄⁺ were mainly sourced from organic nitrogen mineralization, which was thereafter processed to nitrification pathway in the alluvial fan. Nitrification of NH₄⁺ contributed to groundwater NO₃^- in the alluvial fan, which was discharged into the transition area. Values of δ¹⁵N-NO₃ and δ¹⁸O-NO₃ were positively correlated in deep groundwater from the transition area and the flat plain, both of which were negatively correlated with NO₃^- concentrations following Rayleigh fractionations, supporting the occurrence of denitrification. In these two zones, δ¹⁵N-NH₄ values became gradually enriched as NH₄⁺ concentration decreased. However, changes in δ¹⁵N-NO₃ and δ¹⁵N-NH₄ values were unassociated, possibly indicating that NH₄⁺ oxidation was coupled to Feammox process, rather than anammox process. Higher δ¹⁵N-NH₄ values in the transition area than those in the flat plain possibly suggested stronger Feammox rates in the former one, where lower dissolved As concentrations were observed. Higher Feammox rates were typically related to lower organic matter degradation rates. Therefore, the Feammox process may contribute slightly to Fe(II) and As enrichment. Higher As concentrations were related to stronger organic matter degradation and Fe(III) oxide reduction, leading to higher NH₄⁺ and Fe(II) but lower NO₃^- concentrations, accompanied by lower δ¹⁵N-NH₄ and higher δ¹⁵N-NO₃ values. The study provided a clear picture on co-cycling processes of dissolved nitrogen species and As behavior in groundwater systems.