BACKGROUNDA typical irrigation area of the downstream Yellow River (Henan) is an important agricultural planting area in the northern Henan Plain. The shallow water quality in this area is generally poor, which is often used for crop irrigation or livestock drinking, posing a risk to human health. Therefore, the study on the variation characteristics and mechanism of arsenic and fluorine content in groundwater in this area will help to improve the level of understanding of pollution in this area.

OBJECTIVESTo investigate the spatial variation characteristics and mechanism of arsenic and fluorine concentration.

METHODSBased on 327 groups of shallow groundwater samples collected in 2010 and 2020 in the irrigated area, the distribution of arsenic and fluorine in the groundwater of the irrigated area was analyzed, and compared to the evolution characteristics of arsenic and fluorine in the shallow groundwater of the irrigated area in the last ten years.

RESULTSIn 2020, the number of samples with high arsenic concentration (>10μg/L) and high fluorine concentration (>1mg/L) in shallow groundwater accounted for 26.1% and 26.06%, respectively. The high-arsenic groundwater was distributed in the sedimentary environments with interbedding structure of sediment, such as the Taihang Mountain depression and the Yellow River alluvial plain, with strong reducibility, poor groundwater runoff and strong cation exchange, resulting in high concentration of Ca²⁺ in the environment. Arsenic concentration increased in 31.8% of water samples and decreased in 36.7% of water samples in the last ten years. The increase of arsenic content was caused by the dissolution and release of manganese oxide due to the enhanced reducibility of groundwater. The variation of water level caused by agricultural irrigation and water replacement in different areas during the last ten years was a potential factor of arsenic concentration change. The high-fluorine groundwater was mainly distributed along the Yellow River in Xinxiang and Puyang, Henan Province. The concentration of fluorine ion was affected by the dissolution of calcareous minerals such as fluorite in the sediments, which made the high-fluorine groundwater appear in the low-calcium environment. In the last ten years, the study area with decreased fluorine ion concentration accounted for 60.2%, whereas areas with increased fluorine ion concentration accounted for 32.1%. The overall change trend was good, but the fluorine ion concentration in the high fluorine area continued to increase. Changes in fluorine concentration were also affected by changes in Ca²⁺, with further increases (decreases) as Ca²⁺ concentration decreased (increased). Elevated fluorine in groundwater was distributed along the Yellow River, affected greatly by the Yellow River water supply. So, the groundwater flow condition was good and the cation exchange was weakened, reducing the Ca²⁺ content. At this time, the arsenic content in the groundwater was less affected by the environment, so the study area with fluorine increase showed an inverse relationship between arsenic and fluorine in terms of regional distribution and evolution.

CONCLUSIONSThis study will provide support for rational utilization of groundwater in the study area.