Effect of clayey sediment compression on fluoride enrichment in the Quaternary groundwater system of Cangzhou Plain, China.

Groundwater is an important global source of drinking water. Long-term intake of groundwater with high levels of fluoride (F^– >1.5 mg/L) may result in waterborne fluorosis, including dental fluorosis and skeletal fluorosis [1], [2]. High fluoride groundwater has become one of the most serious geological environmental issues in the world, with reports from many countries such as India [3], Pakistan [4], South Africa [5], South Korea [6], Nordic Europe [7], and China [8]. Approximately 26 million people in 29 Chinese provinces suffer from high fluoride groundwater [9].

Natural fluorine is mainly present in igneous, volcanic, and metamorphic rocks, with a minor amount found in deposits rich in fluoride mother rocks [7]. Fluorine can occur in crystalline structures by substituting OH^– due to the similarity in charge and radius [10]. The Na-HCO₃ type water with a low Ca concentration is advantageous for fluorite dissolution and ion exchange between OH^– and F^– [11], [12]. Chemical weathering and dissolution of fluorine-containing minerals can lead to the release of fluorine into groundwater [13]. Under the influence of solution pH, poorly crystalline phases rich in Al/Fe in sediments, such as hematite, can serve as effective components for adsorbing or releasing fluorine [14].

The causes of high fluoride groundwater mainly include leaching-enrichment type, evaporation-enrichment type, and compression-release type. The leaching-enrichment type represents that fluorine enters groundwater from fluorine-containing minerals through complex hydrogeochemical reactions such as chemical weathering and dissolution, then accumulates in the discharge area with groundwater flow [15], [16]. The evaporation-concentration type is generated by the strong evaporation of shallow groundwater in arid/semi-arid climate conditions, such as in the Yellow River’s lower flood plain in northern China [17]. The compression-release type refers to a compression process of clayey sediments causing the high fluoride pore water in the clay layer to be released into adjacent aquifers and land subsidence [2]. The first two types of high fluoride groundwater have been widely reported. However, the effect of clay layer compression on the enrichment of groundwater fluoride is frequently disregarded due to its low hydraulic conductivity and limited water supply. On the one hand, researchers found that the recharge amount of clay layer in the severe land subsidence area of Cangzhou Plain (CZP) accounted for 57.6 % of the groundwater in the deep confined aquifer based on GIS spatial analysis and statistical records [18]. On the other hand, clayey sediment contains absorbed fluoride besides fluorine-containing minerals, such as scattered fluoride spread on the surface of clay particles. Studies have shown that these sediment fluorides may migrate into pore water and subsequently enter the aquifer by releasing pore water during compression [10]. In recent years, the phenomenon of deteriorating groundwater quality due to the compression-release of clay pore water has occurred frequently around the world, such as the release of As [19], [20], ammonium [21], heavy metals [22], etc. However, the degree of impact of clay fluorine release on groundwater remains unclear.

The North China Plain (NCP) has the most severe land subsidence in China. The severity area of subsidence in NCP reaches 80 % of the entire country, and it is still in a relatively rapid development stage overall ([24], [25], [23]). Herrera-García et al. [26] discovered that China accounts for about 11 % of the world’s 12 million km² potential subsidence area, which is mainly distributed in the NCP, and they predicted that the NCP would still be a severe subsidence area worldwide in 2040. Clay compression induced by groundwater overexploitation is the primary cause of land subsidence in the NCP [27]. The high fluoride groundwater induced by clayey sediment compression in the NCP has been preliminarily revealed. Xue et al. [28] determined the hydraulic connection between Quaternary compressible clay pore water and aquifer using ²H, ²O, and ⁸⁷Sr/⁸⁶Sr isotopes. Li et al. [8] discovered that the compression of clay layers may induce the release of fluorine into aquifers with pore water through hydrochemistry analysis and simulation. The eastern part of the NCP has experienced several transgressions [29]. Because clay minerals can “fix” the marine element fluorine through adsorption and release it into the aquifer during clayey sediment compression [30], the phenomenon is more common in coastal areas. CZP, as a coastal area with severe land subsidence in the NCP, had a maximum accumulated subsidence of 2.67 m from 1975 to 2013. Meanwhile, the highest concentration of groundwater fluoride was up to 5.59 mg/L [10], [31], posing a serious threat to the drinking water safety of local populations. It is critical to evaluate the impact of clayey sediment compression on the high fluoride groundwater formation in CZP.

Based on the long-term monitoring data of land subsidence and the hydrochemical analysis of plenty of groundwater samples collected at different depths in the CZP, this study can now identify and evaluate the impact of clayey sediment compression on groundwater fluoride enrichment. This study aims to (1) identify the sources and main influencing factors of groundwater fluoride in different Quaternary aquifers and (2) evaluate the contributions of compression-release of clay pore water on the groundwater fluoride enrichment in the deep confined aquifer in CZP. The present study’s findings can provide a new methodology and evidence to distinguish the released fluoride from clayey sediments, which has previously been neglected in understanding the formation of high fluoride groundwater.