BACKGROUNDArsenic content in shallow groundwater of the Yellow River alluvial fan plain exceeds the standard. A comprehensive understanding of the arsenic enrichment mode driven by the nitrogen cycle of shallow groundwater in the northern Henan Plain is essential for the sustainable use of groundwater resources and the health of residents. The main area of the North Henan plain is the Yellow River alluvial fan plain. The sedimentary environment in the northern Henan plain is complex, and is influenced by the Yellow River breach, diversion and oscillation, as well as alluvial diluvial in the mountainous area around the basin. The distribution, migration and release mechanism of As, NH₄⁺ and NO₃^- are quite different under different sedimentary environment conditions. The joint enrichment mechanism of the three is still unclear, which is worth further study.

OBJECTIVESTo investigate the effect of nitrogen cycling on arsenic migration and enrichment in groundwater in the Northern Henan Plain.

METHODS513 shallow groundwater samples were collected from the northern Henan Plain. Atomic fluorescence spectroscopy was used to determine the arsenic content, atomic absorption spectroscopy and ion chromatography, and other methods for major and trace element analysis. The correlation between nitrate, ammonia nitrogen and arsenic was investigated, and the influence of nitrogen cycle on the migration and enrichment of arsenic in groundwater was studied.

RESULTSThe over-standard rate of arsenic concentration in shallow groundwater in the study area was 17.3%. The occurrence and transformation modes of nitrogen under different depositional environmental conditions were important driving factors for arsenic enrichment. A large amount of NO₃^- was produced by nitrification in the groundwater of the alluvial fan in the piedmont alluvial fan, which had the average concentration of 9.3mg/L and was the highest in the district. At the same time, the concentration of arsenic was the lowest in the district, with the average of 1.3μg/L. The good negative correlation between NO₃^- and As concentration indicated that nitrification produced a large amount of NO₃^-, which was not conducive to the dissolution of arsenic-containing iron oxide. The depressions in front of the alluvial fan and the Yellow River crater fan with higher NH₄⁺ content were the gathering places of high-arsenic groundwater. The average concentration of arsenic in groundwater from these two areas was 49.7μg/L and 18.9μg/L, respectively, and the exceeding rate reached 87.5% and 71.4%. The good positive correlation between arsenic content and NH₄⁺ in groundwater indicated that the process of denitrification and dissimilative reduction of nitric acid to ammonium (DNRA) consumed NO₃^- in groundwater and generated a large amount of NH₄⁺, which promoted the reduction and dissolution of iron oxides that adsorbed arsenic, forming an environment rich in arsenic.

CONCLUSIONSThe nitrogen cycle plays an important role in the migration and enrichment of arsenic. The migration and enrichment mode of arsenic driven by the nitrogen cycle provides a scientific basis for the treatment and supervision of groundwater with high arsenic content.