Unraveling influences of nitrogen cycling on arsenic enrichment in groundwater from the Hetao Basin using geochemical and multi-isotopic approaches

Abstract

Sources and co-cycling of nitrogen species in arsenic-prone groundwater remain poorly understood, which could affect arsenic behavior. Here, geochemical and multi-isotopic characteristics of groundwater from various redox environments were investigated to reveal the effects of nitrogen cycling on arsenic mobility in groundwater systems from the Hetao Basin in China. In deep groundwater along an approximate flow path from the alluvial fan (Zone I) through the transition area (Zone II) and to the flat plain (Zone III), a progressive NO3– depletion with gradually increased δ15NNO3 and δ18ONO3 occurred, accompanied by increases in dissolved NH4+, Fe(II), and arsenic concentrations and a decrease in δ15NNH4. Shallow groundwater in the flat plain (Zone IV) covered wider ranges of δ15NNO3 and δ18ONO3 and relatively lower δ15NNH4. Organic nitrogen mineralization contributed primarily to NH4+ release in all zones, and dissimilatory NO3– reduction to NH4+ (DNRA) was an important NH4+ source in Zone IV. While NH4+ loss mainly occurred via nitrification in Zone I, anaerobic NH4+ oxidation was coupled to Fe(III) oxide reduction (Feammox) in Zones II and III and to NO2– reduction (anammox) in Zones II, III, and IV. Groundwater NO3– was reduced via heterotrophic denitrification in Zones II and III, while the DNRA was favored over denitrification in Zone IV with much higher DOC:NO3– molar ratios. Feammox and heterotrophic DNRA increased the dissolved Fe(II) and arsenic concentrations via the enhanced Fe(III) oxide reduction. However, anammox and Fe(II)-fueled autotrophic DNRA decreased their concentrations due to limited Fe(III) oxide reduction and/or enhanced Fe(II) oxidation. The influences of nitrogen cycling on arsenic behavior are mainly mediated by transformations between Fe(III) oxides and dissolved Fe(II). This study provides the first detailed multi-isotopic picture of nitrogen cycling and the relevant effects on arsenic enrichment processes.