BACKGROUND The North Henan Plain is located in the middle and lower reaches of the Yellow River, and both high arsenic and fluoride groundwater exist. However, the coexistence mechanism of arsenic and fluoride in this sporadic distribution area is unclear.

OBJECTIVES To investigate the spatial distribution characteristics and formation mechanism of arsenic and fluoride in shallow groundwater in the North Henan Plain.

METHODS 332 groups of shallow groundwater samples were collected and analyzed in the North Henan Plain. Atomic fluorescence spectrometry was used to determine arsenic content, and ion chromatography and inductively coupled plasma emission spectroscopy were used to determine the content of fluoride and other cation-anions. Based on the spatial distribution of arsenic and fluoride, combined with hydrochemical diagrams and factor analysis, three main factors affecting the evolution of groundwater in this area were extracted, and the formation mechanism of high arsenic and high fluoride groundwater in this area was discussed.

RESULTS The concentrations of arsenic and fluoride in groundwater were 0.0001-0.1900mg/L and 0.13-4.94mg/L, respectively. The high-arsenic groundwater was mainly distributed in the vertical depth of 15-80m in the front alluvial-diluvial depression of Taihang Mountain and the Yellow River flood fan. The high-fluoride groundwater was mainly distributed in the vertical depth of 7-100m in the modern channel influence zone of the Yellow River. Evaporation and concentration, mineral dissolution/desorption, and redox environment were the main factors controlling the evolution of groundwater in this area. Fluoride had loads of 0.214 and 0.743 in factor F1 (evaporation and concentration) and F2 (mineral dissolution/desorption), respectively. High concentration of F appeared in groundwater with low concentration of Ca²⁺, and the concentration of F was positively correlated with ρ(Na⁺)/ρ(Na⁺)+ρ(Ca²⁺). The strong evaporation and concentration in the modern channel influence zone of the Yellow River contributed to the dissolution of fluorine-containing minerals, and the irrigation of the Yellow River water increased the concentration of Na⁺ in groundwater, which further enhanced the dissolution. Arsenic had a load of 0.728 in factor F3 (redox environment). Arsenic was positively correlated with Fe²⁺ and NH₄⁺, and negatively correlated with NO₃^- and SO₄^2-. The lower Eh corresponds to the higher arsenic concentration. The reductive environment in the front alluvial-diluvial depression of Taihang Mountain and the Yellow River flood fan was favorable for the reductive dissolution of arsenic containing iron oxides/hydroxides, resulting in the formation of the high-arsenic groundwater. The desorption of arsenate/arsenite/fluoride in the form of anions on the mineral surface caused by the increase of pH value was favorable for the coexistence of arsenic and fluoride in groundwater. However, the correlation between arsenic and fluoride in groundwater in this area was not significant. The high concentration of calcium ions in the high arsenic region was not conducive to the enrichment of fluoride. In contrast, the weak reducing conditions in the high fluoride region were not conducive to the dissolution of arsenic-containing iron oxides/hydroxides.

CONCLUSIONS The results clarify the coexistence mechanism of arsenic and fluoride in the North Henan Plain, and enrich the theoretical system of co-contamination of groundwater with high arsenic and fluoride.