BACKGROUND Groundwater is the main water source in the Northwest Shandong Plain Province. It is necessary to find out the causes and connections of high fluorine and high iodine groundwater, which can provide reference for local drinking water safety and solving endemic diseases. At present, there is a lot of research on shallow high fluorine ground water in northwest Shandong, but little research focuses on the cause of deep high fluorine ground water, and the relationship between high fluorine groundwater and high iodine groundwater.

OBJECTIVES (1) To reveal the spatial distribution characteristics of fluorine and iodine in groundwater in the Northwest Shandong Plain; (2) To speculate the formation mechanism of high fluorine groundwater; (3) To identify the key hydrogeochemical processes of iodine enrichment in the groundwater system; (4) To explore the relationship between high fluorine and high iodine in groundwater.

METHODS Sediment samples were collected to investigate the lithology, mineral composition and elementary composition of the Northwest Shandong Plain. A total of 326 shallow (0-55m), middle (55-225m) and deep (>225m) groundwater samples were also collected. Hierarchical clustering analysis was applied to classify groundwater based on 18 water chemical parameters (pH, dissolved total solids, sulfate, chlorine, iron, volatile phenols, iodine, fluorine, arsenic, lead, calcium, magnesium, potassium, bicarbonate). The correlation analysis of water chemical parameters and sediment compositions was executed to investigate the causes of high fluorine and iodine groundwater. Furthermore, the calculation of the saturation index of fluorite further helped to determine the main formation mechanism of high fluorine groundwater. The water chemistry type of groundwater was obtained by drawing the piper three-line diagram, and combined with pH and other indicators, the main mechanism of the enrichment of deep high fluorine groundwater was studied. The piper three-line diagram and TDS relationship diagram were drawn to confirm the evaporation concentration mechanism of shallow high iodine groundwater. In addition, the hydrogeological conditions of the study area were analyzed, and the strong reducing environment of deep groundwater in the study area was judged by combining redox environmental sensitive factors, and the main reason for iodine enrichment in deep groundwater was obtained.

RESULTS The groundwater samples are divided into high fluorine groundwater, high iodine groundwater, and high fluorine and high iodine groundwater by hierarchical clustering analysis. High fluorine groundwater and high iodine groundwater have great similarities, and they are primarily the largest cluster. The high fluorine groundwater is mainly distributed at a depth of 0-40m, 50-110m and 225-305m below the ground surface, with the maximum value (13.7mg/L) occurring at a depth of 110m underground. The high iodine groundwater enriches at 0-10m, 55-65m and 225-285m deep, with the maximum value (4.6mg/L) appearing at 234m underground. Fluorine and iodine in groundwater are often associated, i.e., high fluorine groundwater also has higher iodine content. This is because the enrichment environmental conditions conducive to the two are similar. Strong evaporation concentration and leaching are the main processes leading to high fluorine and high iodine groundwater in the Northwest Shandong Plain. The fluorine-containing minerals and iodine-containing sediments in the shallow groundwater were continuously dissolved, which increased the content of fluorine and iodine in the groundwater. When rainfall occurs, fluorine and iodine in the soil dissolve or leach into the groundwater, resulting in its enrichment. In addition, the alkaline groundwater environment and high HCO₃ ⁻ content can promote the dissolution of fluorine-containing minerals and the enrichment of iodine. The strong reducing groundwater environment and slow flow rate of deep groundwater can be conducive to fluorine enrichment. High fluorine groundwater and high iodine groundwater have similar chemical types, with the main anions of HCO₃ and Cl, and the main cation of Na. In a weakly alkaline environment (pH=7-10), high fluorine and high iodine groundwater is formed through leaching and ion exchange.

CONCLUSIONS The dissolution of fluorine-rich minerals such as fluorite (CaF₂), evaporation and ion exchange might be the dominant processes controlling fluorine mobilization in groundwater. The iodine in groundwater mainly comes from marine organisms and organic matter in sediments in the Northwest Shandong Plain. Evaporation, leaching and the reduction dissolution of ammonia nitrogen and sulfide are likely the primary processes leading to iodine enrichment. The weakly alkaline and reducing groundwater environment and the presence of organic matter are all important factors in the formation of high iodine groundwater. Generally, high fluorine groundwater also has high iodine content due to the similar formation mechanisms.