Rui-ping Liu ,Hua Zhu Fi Liu,Ying Dong Ray M El-Wardany,

a Xi’an Center, China Geological Survey, Ministry of Natural Resources, Xi’an 710054, China

b Key Laboratory of Subsurface Hydrology and Ecological Effects in Arid Region, Ministry of Education, Xi’an 710054, China

c Key Laboratory for Geo-hazards in Loess Area, Ministry of Natural Resources, Xi’an 710054, China

d Field Base of Scientific Observation of Shaanxi Tongguan, Ministry of Natural Resources, Xi’an 710054, China

e Dalian ocean and fishery comprehensive administration supervision lochus, Dalian 116000, China

f Assiut Branch, Al-Azhar University, Assiut 71524, Egypt

Keywords:Fluoride Groundwater Human health risks Loess Plateau Hydrogeological survey engineering Dali County Shaanxi Province China

ABSTRACT This study aims to investigate the mechanisms and health risks of fluoride enrichment in groundwater in the Loess Plateau,China.By taking Dali County,Shaanxi Province,China as an example,this study obtains the following results through field investigation and the analyses of water,soil,and crop samples.(1) The groundwater can be divided into two major types,namely the Quaternary pore-fissure water and Karst water.The Karst area and sandy area have high-quality groundwater and serve as the target areas for optional water supply.The groundwater in the study area is slightly alkaline and highly saline.Meanwhile,high-fluoride groundwater is mainly distributed in the loess and river alluvial plains in the depression area of the Guanzhong Basin and the discharge areas of the groundwater,with the highest fluoride concentration exceeding seven times the national standard.(2) Fluoride in groundwater mainly originates from a natural source and human activities.The natural source refers to the fluoride-bearing minerals in rocks and soil,and the fluoride from this source is mainly controlled by natural factors such as climate,geologic setting,pH,specific hydrochemical environment,ion exchange,and mineral saturation.Human activities in modern life can be further divided into industrial and agricultural sources primarily.(3) The health risks of fluoride contamination are very high in the Loess Plateau,especially for children compared to adults.Meanwhile,the risks of fluoride exposure through food intake are higher than those through drinking water intake.The authors suggest selecting target areas to improve water supply and ensure the safety of drinking water in the study area.Besides,it is necessary to plant crops with low fluoride content or cash crops and to conduct groundwater treatment to reduce the fluoride concentration in drinking water.These results will provide a theoretical basis for safe water supply in the faulted basin areas in the Loess Plateau.

1.Introduction

Undoubtedly,groundwater is a crucial water source for people’s livelihood,industry,and agriculture in the modern society in the whole world,especially in arid and semi-arid regions with scarce and irregularly distributed surface water and precipitation (Li PY et al.,2018).However,groundwater is facing a progressively immediate threat owing to the expeditious expansion of industry,rapid population growth,and extensive agricultural activities.Moreover,the physical and chemical characteristics of groundwater are increasingly subject to natural and human conditions.Therefore,the main priority is to understand the impacts of water-rock interactions and human activities on element concentration (Chidambaram S et al.,2018; Hui Q et al.,2020).Fluorine (F) is an essential element for human health,especially for bone development and enamel creation (Subba Rao N et al.,2017).One of the most critical problems in the world is the fluoride concentration in drinking water since a low level of fluoride in drinking water can lead to tooth decay and a high fluoride concentration will cause dental and skeletal fluorosis [World Health Organization (WHO et al.,2017; Ali S et al.,2016).Previous studies on fluoride in groundwater primarily have revealed the exact genesis and hydrogeochemical characteristics of high-fluoride groundwater,the geochemical processes controlling fluoride distribution in groundwater,and conceivable effects of high-fluoride groundwater on health.Ali S et al.(2016) reviewed global researches on the sources and dynamics of fluoride and the correlation between fluoride and water from other global perspective regions.They concluded that the fluoride concentration in water bodies is frequently higher in arid and semi-arid regions in Asia than in other regions,which is related to abounding sodium,arsenic chloride,and bicarbonate.Moreover,they reported that the fluoride in water primarily originates from geological sources(mainly including fluoride-bearing minerals in rocks and sediments) and human-related sources (pesticides and industrial wastes).Borgnino L et al.(2013) investigated the factors that control the release mechanisms of fluoride from sediments in central Argentina through experiments and concluded that fluoride-bearing facies are dominated by fluoride dissolution and fluoride desorption,respectively under acidic and alkaline conditions.Wei C et al.(2016)investigated the fluoride concentration in the Quaternary sedimentary aquifers and Tertiary clastic aquifers in semi-arid regions in northwest China.As they concluded,the hydrolysis of fluoride-bearing minerals serves as a more vital control process for the fluoride concentration in Tertiary clasts,while the evaporation and competitive adsorption of boron and HCO3are more significant for fluoride enrichment in the Quaternary.

The Loess Plateau in China is covered by the largest and thickest loess deposits in the world (Li PY et al.,2018a).It is highly significant since it supports a vast number of Chinese people with crops.It has an arid and semi-arid climate,suffering limited and intermittently distributed surface water resources and precipitation.Therefore,the inhabitants in this region mainly depend on groundwater for their livelihood.The groundwater in loess areas tends to suffer poor quality and contain a high content of salts and other toxic elements,which is closely related to human health.High-fluoride groundwater is widely distributed in loess areas in China(Fig.1).However,studies on the impacts of high-fluoride groundwater on health in loess areas in China are very limited.Therefore,the foremost purposes of this study are to delineate the occurrence and spatial distribution of fluoride in groundwater in typical areas in the Loess Plateau,to determine geochemical influencing and control mechanisms of the fluoride concentration in groundwater,and to quantify the impacts of fluoride in groundwater on human health.This study will provide a theoretical basis for improving local groundwater quality.

2.General environment

2.1.Physical geographical and hydrometeorological characteristics

Dali County is 125 km away from Xi’an City,Shaanxi Province,China.It lies in the confluence of the Yellow River,Luohe,and Weihe rivers in the eastern part of the Guanzhong Plain,Shaanxi Province and is surrounded by the three rivers.It is bounded by Shayuan Area in the south and Lianshan Area in the north and can be divided into five major areas according to different landforms,namely loess tableland area,alluvial plain (terrace) area,sandy area of eolian sand,alluvial sand area,and eroded tectonic depression area.It has a warm temperate semi-arid continental monsoon climate,with an average multiyear temperature of 14.4°C and average multiyear precipitation of 514 mm.The precipitation in this county features uneven spatial-temporal distribution.It generally occurs from July to September and gradually decreases from northwest to southeast.The average multiyear evaporation is 1100-1400 mm and tends to increase from west to east and from south to north.The Yellow River in the north flows from the eastern part of Jinshuigou to the Tongguan Port together with Weihe River,with a length of the main stream of 47.65 km and an average annual runoff of 316×109m3.The Weihe River rises in Zhangjia Town in the west and flows to Tongguan County to join the Yellow River.The length and drainage area of main streams in Dali County are about 84 km and about 45 km2,respectively.The Luohe River rises in Beidang Village and turns southward to Chaoyi Town and then southeastward into the Weihe River,with a current length of 121.5 km and a basin area of 26.7 km2(Liu RP et al.,2009a,2009b).

2.2.Hydrogeological characteristics

According to strata,structures,and burial conditions,the groundwater in Dali County mainly includes Quaternary porefissure water and karst water and the former can be further divided into pore phreatic water and pore confined water.

The strata bearing the pore phreatic water include Upper Pleistocene alluvial gravels,Middle Pleistocene alluvial strata,and Middle Pleistocene loess-like loam.The Middle Pleistocene fluvial deposit Q2al+lcovers the Middle Pleistocene eolian layer Q2eoland alluvium Q2alto the north of the Luohe River.It is composed of interbeds consisting of clay,sands,and gravels,with a thickness of 50-100 m.The Q2eolis mainly distributed in the loess tableland and the fourth-order terrace of the Weihe River.It consists of brownish-yellow loess interbedded with multiple paleosol layers,with a thickness of 90 -100 mm.The Upper Pleistocene eolian layer Q3eolcovers the tableland surface,with a thickness of 10-15 m.The Upper Pleistocene alluvium Q3allies under the eolian layer of Weihe and Luohe terraces.It has a burial depth of 50-60 m and consists of silty clay,sands,and gravels.The Holocene Q4is distributed in the floodplain and terrace areas of the Yellow River and Weihe rivers and consists of silt,silty clay,sands,and gravels,with a thickness of 10-30 m.

The strata bearing the pore confined water include Holocene alluvial gravels,the alluvial-lacustrine silty fine sands in the upper part of the Lower Pleistocene,and Lower Pleistocene-Upper Pliocene sands.A small amount of Lower Pleistocene fluvial deposit Q1al+lis exposed at the bottom of the Jinshui Gully in the north.It is composed of grayishyellow clay and multiple fine-grained sand layers,with a thickness of more than 200 m in general.

Karst water is mainly distributed in the northern part of Dali County,and its aquifers are composed of Cambrian-Ordovician carbonate rocks.

The recharge area of the groundwater in Dali County is the Beishan Mountain,and the discharge areas include the alluvial plains of Weihe river and Luohe rivers and the tectonic depression area.The discharged water flows into the Yellow River,Weihe river,and Luohe rivers through mountainous areas,loess tablelands,and alluvial plains.The recharge,scale,and drainage characteristics of the phreatic water in the area are notably controlled by the factors such as terrain and climate.The groundwater is mainly recharged through the infiltration of atmospheric precipitation and diversion-based irrigation and is discharged through vertical evaporation and runoff to adjacent areas.To the north of the Luohe River,the underground runoff flows from north to south along the terrain.In the eolian sand area to the south of Luohe River,it flows from the middle part of the eolian sand area to the Luohe and Weihe rivers in dry seasons but is replenished by two rivers and flows from two rivers to the eolian sand area in wet seasons (Liu RP et al,2009a,2009b).

Fig.1.Spatial distribution of fluoride in groundwater.a-spatial distribution of fluoride in groundwater in Dali Country; b-Geographic location of Guanzhong basin in the eastern hemisphere; c-Geomorphological map of loess plateau (Zhang MS and Liu J,2010); d-spatial distribution of fluoride in groundwater in the Guanzhong Basin.

3.Materials and methods

3.1.Sampling and measurement

In this study,23 groundwater samples were collected from private wells,monitoring wells,and boreholes (G1 to G23) in July,2017.Meanwhile,17 surface water samples were collected from boreholes (G24 to G40) in the low-lying land and rivers for comparison purposes.To assess the risks to human health,23 groups of local crops including corn,sweet potato,apple,jujube,pumpkin,and peanut were collected near the groundwater sampling positions (Fig.1).

The sampling procedures and sample preservation and handling followed relevant national standards issued by the Ministry of Environmental Protection of China in 2009.Meanwhile,sampling rules were considered.For instance,containers were rinsed and washed three times using the water to be sampled before sampling.All samples were analyzed in Northwest Mineral Resources Supervision and Testing Center,Ministry of Natural Resources.The physicochemical indices to be analyzed included pH,total dissolved solids(TDS),total hardness (TH),major ions (Na+,K+,Ca2+,Mg2+,Cl−,SO42−,HCO3−,and CO32−),and fluoride content in crops and water samples.Among them,the temperature,color,odor,taste,conductivity,Eh,and pH of the groundwater samples were measured on the sampling sites,Ca2+,Mg2+,Na+,and K+were analyzed using ICP spectroscopy,and Cl−,HCO3−,and CO32−were determined using ion chromatography.Meanwhile,SO42−and F−were analyzed using the specific gravity method and ion-selective electrode method,respectively.

3.2.Methods

Hydrochemical components were compared with Chinese national groundwater quality guidelines and WHO guidelines to assess their suitability for drinking water (Table 1).Based on the assessment results,F−was selected to perform the noncarcinogenic health risk assessment using the model recommended by the U.S.Environmental Protection Agency(USEPA) and parameter values suitable for the study area.In terms of exposure pathways of fluoride,drinking water intake and food intake were considered while the dermal contact was ignored in this study since dermal contact usually poses low risks to health.The assessment model of intake-induced risks to health is as follows:

Where,HQdenotes the hazard quotient of noncarcinogenic health risks,andHQ> 1 means high potential health risks that are unacceptable for adults and children.

The daily average exposure doses per unit weight (CDi)through drinking water and food routes are expressed usingCDidandCDif,respectively.They can be calculated as follows:

4.Results and discussion

4.1.Hydrochemistry and water quality assessment

The statistics of the water samples and the guidelines used for drinking purposes are listed in Table 2.Both the groundwater and surface water samples were slightly alkaline,with pH values of 7.45-8.49 (average: 7.97) and 7.34-9.27(average: 7.85),respectively.The TDS values of the groundwater and surface water were 388 -8360.26 mg/L(average: 2265.82 mg/L) and 711.62 -38417.83 mg/L(average: 4713.84 mg/L),respectively.Therefore,both the groundwater and surface water near the Yellow River,Wiehe River,and Shayuan area are freshwater.The average concentrations of Na+K,Ca2+,Mg2+,Cl−,SO42-,and HCO3−in the groundwater were 520.88 mg/L,43.51 mg/L,96.87 mg/L,288.89 mg/L,630.04 mg/L,and 535.01 mg/L,respectively,and those in the surface water are 1201.57 mg/L,56.20 mg/L,185.35 mg/L,1100.41 mg/L,1527.69 mg/L,and 362.74 mg/L respectively.Moreover,the cations in both the groundwater and the surface water were dominated by Na+and K+,followed by Ca2+and Mg2+.The anions in the groundwater mainly included SO42−and HCO3−,followed by Cl−,while the anions in surface water were dominated by Cl−and SO42−due to the evaporation of surface water.The fluoride concentrations in the groundwater and the surface water were 0.01-7.0 mg/L and 0.51-1.8 mg/L,respectively.According to Chinese national drinking water guidelines,only about 21.7% of the groundwater samples and 41% of the surface water samples are suitable for drinking.Furthermore,the groundwater and surface water in the loess plateaus and alluvial plain in the study area showed poor water quality and high concentrations of main hydrochemical components and F−.From the epidemiological point of view,excessive fluoride in drinking water will increase the risks of dental fluorosis and skeletal fluorosis.Therefore,it is necessary to conduct water treatment to reduce the fluoride concentration in the drinking water.

Table 1.Parameters in the health risk assessment model.

Table 2.Statistical analysis of physicochemical indices of samples against drinking water guidelines.

4.2.Genesis and distribution of fluoride in groundwater

4.2.1.Fluorine sources

The fluoride in groundwater is mainly sourced from fluoride-bearing minerals in rocks and soil.The high-fluoride groundwater in Dali County is primarily distributed in the loess area and the areas interbedded with sandy clay and sandstones.According to Fig.1,the loess area is represented by the Middle Pleistocene deposits (Q2) and the Upper Pleistocene eolian layer (Q3eol).Q2deposits cover the tablelands in the north and the upper parts of the second-,third- and fourth-order terraces in the middle area.They are relatively loose and the loess-like loam in them is mainly composed of 6-7 paleosol layers and thin-laminated siltyfine-grained sands.Q3eolcovers the part to the north of the Luohe River,with a thickness of 10-15 m.The loess contains fluoride-rich biotite,muscovite,hornblendes,tourmalines,and a large number of clay minerals.The abundance of fluoride reaches 179-49 mg/kg and the soil layers are mostly alkaline.The soil is rich in calcium and sodium,which is beneficial to the enrichment of fluoride.Q2aland Q3alalluviums lie under the eolian layer of the Luohe terrace,with a burial depth of 50-60 m.They are rich in micas,feldspars,calcites,calcium carbonates,and soluble salts in the second-,third-,and fourthorder terrace areas of the Weihe River in the middle part.The Holocene is located in the floodplain and first-order terrace area.It is sandy in general and mainly consists of mediumfine-fined quartz and feldspars.Meanwhile,the gravel components in the Holocene part mainly include sandstones,limestones,and calcareous nodules.The smaller the particles,the higher the water-soluble fluoride content.Therefore,the fluoride content is higher in clay and loam soil than in clayey silt,and the fluoride content in shales is notably higher than that in sandstones.

4.2.2.Geological structure

The fluoride content in major basins in the Loess Plateau is very high (Fig.1b),especially in the Guanzhong Basin.Dali County is located in the northeastern depression area of the Wiehe faulted basin and is a part of the Wiehe graben(Fig.1).The geological structure of Dali County is characterized by the fault-block uplift in the north,the stepfault slope in the middle,and deep-seated areas of a graben structure in the south and east.Fluorine tends to be highly enriched in deep faults,such as the Shuangquan fault along the front edge of the Loess Plateau.In this fault,general rising springs are exposed,steep loess wall linearly extends,and compression fracture surfaces are visible.As a result,the underground runoff is prevented from overflowing the surface but is permanently exposed to springs.Fluorine is enriched in groundwater in Shuangquan Town due to evaporation and concentration and it continues to migrate on both sides of the fault due to the same water-bearing ambiance.

4.2.3.Climate and terrain

The regions with high-fluoride water suffer a long-term arid climate,little rain,and high evapotranspiration,which lead to the slow circulation of fresh water and long residence of groundwater in aquifers.Furthermore,the residence time of groundwater in weathered aquifers becomes longer due to the low head conduction of the groundwater.All these promote the dissolution of fluoride-bearing minerals and the ion exchange between F-and OH−in weathering products,leading to extra enrichment of F-in groundwater.The fluoride content in groundwater gradually increases in the low and middle parts and piedmont alluvial fan area of the Huanglong Mountain in the Loess Plateau.Dali County is located in a flat or low-lying drainage area.Fluoride ions and other chemical elements are enriched in shallow groundwater in the county owing to weak groundwater runoff,poor water alternation,shallow water level,and strong evaporation.

4.2.4.Hydrogeological conditions

The fluoride content in phreatic water is notably related to the structure types of aquifers,water conductivity,and the burial depth of groundwater,runoff condition,recharge and drainage types,and the lithology of vadose zones.The highfluoride groundwater in Dali County is generally formed in multi-layer water-bearing structures.The water conductivity coefficientkin the loess tableland area is 0.11-0.95 m/d,indicating poor water conductivity.Meanwhile,Fanjia Town has a capillary rise height of 2.5-3.8 m,a burial depth of groundwater of 1-5 m,slow horizontal runoff,and strong evaporation,leading to a high fluoride content of greater than 4 mg/L in groundwater.

4.2.5.Hydrogeochemical processes

(i) Dominant zones of hydrogeochemical process

The Gibbs diagram was used to further understand the fluoride sources of water samples.As shown in Fig.2,all water samples fell within the dominant zone of rocks and evaporation,indicating that chemical properties of the groundwater and surface water in the study area are controlled by rock weathering,water-rock interactions,and evaporation.The water-quality evolution mechanisms of high-fluoride water and low-fluoride water are quite different,indicating that the natural process dominates the water-quality evolution in the study area.Furthermore,the fluoride in groundwater in the study area increases from northwest to southeast,which is consistent with the flow direction of groundwater (Fig.1a).The groundwater with a fluoride content below the national standard limit is distributed in the sandy area and the Yellow River beach area to the south of the Luohe River.About 20%of areas in Dali County are classified as moderate-fluoride areas,with a fluoride concentration in groundwater of 1.0-1.5 mg/L,which is higher than the national standard limit.The rest parts of the study area are high-fluoride areas,with the fluoride concentration in groundwater higher than the WHO guideline limit.High-fluoride groundwater is distributed in the discharge areas of the Guanzhong Basin(Fig.1c).

(ii) Impacts of pH

In acid water with a low pH,fluoride ions react with hydrogen ions to form hydrofluoric acid,which dissolves silicon dioxide and silicate rocks to produce gaseous silicon fluoride.This will reduce fluoride in groundwater and is not conducive to fluoride enrichment.In addition,fluoride ions(F−) can react with calcium ions (Ca2+) to form insoluble calcium fluoride (CaF2),and the reaction formula is as follows:

Acid water with a low pH reduces the reactant F−and promotes F−migration,which is not favorable for fluoride enrichment.In contrast,groundwater with a high pH can dissolve aluminosilicate minerals.The hydrolyzation of alkali metals will increase water alkalinity,promote the dissolution of fluorosilicate minerals,release the fluoride in rocks,and increase the fluoride content in groundwater.Therefore,the higher the pH of groundwater,the more beneficial to fluoride enrichment (Fan JJ et al.,2008).According to investigations,the pH of 7.45-8.49 for groundwater and 7.34-9.27 for surface water in the study area can inhibit fluoride adsorption and improve the dissolution of fluoride-bearing minerals.As shown by the Pearson correlation matrix,there was a significant positive correlation between fluoride and HCO3−(r=0.603) and pH (r=0.301),and a significant negative correlation between fluoride and calcium ion (r=0.558; Table 3).Therefore,it can be inferred from the Pearson correlation matrix that fluoride-rich groundwater is highly alkaline and low-hardness (Narsimha A et al.,2018).In addition,the enhanced ion exchange between fluoride ions and hydroxide ions (illite and mica) in the case of a high pH will also increase the fluoride concentration in groundwater.In an alkaline environment,hydroxyl can replace exchangeable fluoride in fluoride-bearing minerals (such as biotite and muscovite; Singh CK et al.,2013),thus leading to an increase in fluoride content of groundwater.The exchange between OH−and apatite/biotite/muscovite/chrysotile is as follows:

Fig.2.Gibbs diagram showing the control mechanisms of natural water chemistry.

(iii) Hydrochemical environment

The enrichment of fluoride in water is controlled by a specific hydrochemical environment.A Piper diagram (Fig.3)was plotted to understand the impacts of the hydrochemical facies on the fluoride enrichment in water.As shown in Fig.3,high-fluoride groundwater was mainly distributed in areas with Na+and HCO3−contents (meq/L) > 80%.In the case of a high content of Ca2++ Mg2+,the fluoride in groundwater was low and hydrochemical types in the groundwater included HCO3-Na,HCO3-Ca (Mg),SO4-Ca (Mg),and SO4(Cl)-Na.The piper diagram indicates that F−enrichment is usually associated with high HCO3−and Na+concentrations in water(Fig.3),while a high Ca2+concentration tends to lower the F−concentration in water.Therefore,fluorite (CaF2) can be dissolved and F−is released into groundwater with Na-bearing salts,or alternatively F−can react with Ca2+to form fluorite(Fig.5).

A hydrochemical environment is primarily affected by the concentrations of major ions.As shown in the Pearson correlation matrix (Table 3),SO42−was significantly correlated with Mg2+(r= 0.991) and the saturation index was greater than 0,indicating that dolomite precipitation in the karst areas is an essential process in the groundwater system.Moreover,the ion exchange between calcium ions and sodium ions may lead to fluoride enrichment in natural water.As mentioned above,high-sodium and low-calcium water are favorable for fluoride enrichment,and thus the changes in sodium and calcium concentrations caused by ion exchange will inevitably affect fluoride behavior.In this study,the binary graph of Na+−Cl−vs.(Ca2++Mg2+)−(HCO3−+ SO42−)was utilized to understand the cation exchange process (Subba Rao N,2017).As verified in Fig.4a,there was a linear relationship between Na+−Cl−and (Ca2++Mg2+)−(HCO3−+SO42−),with a slope of 2.26 (r= 0.97).This indicates that cation exchange plays an important role in regulating the chemical evolution of natural water.The positive correlation between F−and Na+/Ca2+ratio indicates that the cation exchange between Na+and Ca2+affects the fluoride enrichment in the study area (Fig.4b).The ion exchange equation is expressed as follows:

Table 3.Pearson correlation matrix of physicochemical parameters.

Fig.3.Piper diagram showing the hydrochemical characteristics of natural water.

(iv) Saturation indices of minerals

The dissolution or precipitation of minerals depends on the saturation indices of minerals,which are used to denote the saturation state of water relative to minerals and indicate the dissolution or precipitation behavior of minerals.In this study,the saturation indices of dolomite,calcite,gypsum,and fluorite were calculated to understand the impacts of mineral saturation on fluoride enrichment.The dissolution behavior of these minerals was notably interdependent since they all contain calcium ions.The low solubility of gypsum will lead to the high solubility of fluorite (Xiao J et al.,2015).As shown in Fig.5,the saturation index of calcite varied from 0.2 to 1.26,and that of dolomite ranged from 0.29 to 3.19,indicating that dolomite and calcite are supersaturated or close to saturation.The further addition of calcium ions will induce the supersaturation and precipitation of calcite.The saturation index of gypsum was −2.35-0.97,indicating that gypsum is unsaturated.This implies that gypsum can continue to dissolve and release Ca2+into groundwater,which will affect the fluoride enrichment in natural water.The saturation index of fluorite ranged from −5.08 to −0.71,indicating that fluorite is unsaturated.Meanwhile,fluorite dissolution contributes to an increase in fluoride content.This can be confirmed by the logarithmic positive correlation between fluoride and fluorite saturation index (R2= 0.83).

4.2.6.Human activities

By all odds,human activities can also affect fluoride enrichment.Pumping a large amount of confined groundwater in the Guanzhong Plain caused unconfined water (high fluoride) to leak into the confined aquifers (Li PY et al.,2014).Recently,fluoride enrichment is mainly controlled by natural processes and human activities.Human-related sources of fluoride can mainly be divided into industrial and agricultural sources.Fluorine-bearing wastewater or waste residue is frequently discharged in the industrial production of products such as phosphate fertilizer,glass,chemical industry,bricks,and tiles,which directly or indirectly brings fluoride into the soil through the processes such as precipitation,infiltration,and leaching.In addition,fluoride-bearing phosphate fertilizer,pesticides,and irrigation water in modern agricultural activities also increase the fluoride content in soil,thus affecting the fluoride content of local or regional shallow groundwater.

Fig.4.Bivariate plots showing ion exchange (a) and its impacts on fluoride enrichment (b).

Fig.5.Relationships between F− and saturation indices of calcite,dolomite,gypsum,and fluorite.

4.3.Health implication of fluoride in food and water

As proposed by the WTO,fluoride mainly affects human health in three ways,namely drinking water,skin contact,and food intake.This study focuses on the risks to the health of adults and children caused by excessive fluoride in drinking water and food.The fluoride content in some crops was investigated and monitored in this study (Table 4).It can be noticed that the fluoride content in jujube,sweet potato,corn,apple,pumpkin,and peanut was roughly the same and was about 5-12 times higher than the pollutant limits of food(Centers for Disease Control and Prevention,2005).The noncarcinogenic health risks of fluoride for adults and children were assessed (Tables 4,5).According to the assessment results,the HQ values of fluoride in the six foods (except for peanuts) were all greater than 1,and the HQ values for children were 1.01-1.86.The average HQ values for children were in the order of sweet potato (1.64) > pumpkins (1.61) >apples (1.28) > corn (1.26) > jujube (1.18).This indicates that the risks of fluoride through food intake are very high for children but can be ignored for adults.Table 5 shows that the HQ values of fluoride in groundwater were 0.01-8.81(average: 2.77) and 0.01-4.11 (average: 1.29),respectively for adults and children,while the HQ values of fluoride in surface water 0.01 -2.26 (average: 1.18) and 0.3 -1.06(average: 0.55),respectively for adults and children.This indicates that the risks of fluoride through drinking water intake are acceptable in some areas in the Shayuan area but are too high to be ignored in most areas.The risk degree of a pollutant to health is determined by several factors,namely pollutant concentrations,reference dose of non-carcinogenic pollutants,and body weight.High pollutant concentrations,low reference dose,and low body weight will lead to high risks.Briefly,the non-carcinogenic health risks of fluoride for adults and children are very high in the study area (Fig.6).According to the statistics of the food intake as per the dietary guidelines of Chinese residents,the average HQ values of fluoride in food (excluding meat intake) are 6.01 and 3.01,respectively for children and adults,and the average HQ values of fluoride in drinking water are 1.97 and 0.92,respectively for children and adults.This confirms that the health risks of fluoride through food intake are higher than that through drinking water intake,and the health risks of fluoride are higher for children than for adults.As for the prevention and control measures of fluoride in groundwater,it is necessary to replant crops with low fluoride content or cash crops.Meanwhile,groundwater treatment should be conducted to reduce fluoride concentration in drinking water.

Table 4.Fluorine contents of crops and their non-carcinogenic health risks for adults and children.

Table 5.Fluorine contents of drinking water and their noncarcinogenic health risks for adults and children.

Fig.6.Comparison of health risks of fluoride through different intake ways.

High-fluoride groundwater has been reported in many places in the Loess Plateau in China (Subba RN,2017) as well as international loess distribution areas and it is even worse in the Loess Plateau,China due to severe water stress.In this study,about 22.2% of groundwater samples and 50%of surface water samples are unsuitable to drink due to the high fluoride concentration.In addition,more than 75% of non-carcinogenic health risks are caused by the high fluoride in drinking water,thus affecting human health,especially thehealth of children.In the loess and river alluvial plain of the study area,the non-carcinogenic health risk rate caused by high fluoride in drinking water is 100% in children and 78%in adults.Therefore,the main priority should be given to the supply of high-quality drinking water with a low fluoride concentration,which is also the goal of the sustainable management of water resources in the study area (Li PY,2018).Some researchers suggested using fluoride filters to reduce fluoride concentration in drinking water in rural areas of the Loess Plateau (Xiao J et al.,2015).This will be effective for the short term but will add an extra economic burden to the rural people.Given that high-fluoride areas are distributed in loess and alluvial plain areas and that the groundwater and surface water in the karst area and Shayuan area enjoy high quality,the authors suggest establishing central bases of water supply in these two areas to replace the distributed water supply using private pumping wells at present.Up to now,the Yuhong water source has been established in the karst area in the north,and it is necessary to select a target area in the Shayuan area in the south to safeguard the drinking water supply of the whole county.

5.Conclusions

High-fluoride natural water has become a major public health problem in many areas in China,especially in loess areas.In this study,the reasons for fluoride enrichment were revealed according to the hydrochemical characteristics of natural water in the phreatic aquifers in the Loess Plateau in China,and the potential human health risks of fluoride in drinking water and food were quantified.The conclusions are as follows.

(i) The groundwater in the study area is slightly alkaline,and the hydrochemical types in it include HCO3-Na,HCO3-Ca (Mg),SO4-Ca (Mg),and SO4(Cl)-Na.Compared with the drinking water quality standards of China and the WHO,the groundwater in the loess area and the alluvial plain of the Yellow River and Luohe river in the study area suffers a poor water quality.

(ii) As indicated by the analysis of fluoride in soil and the Gibbs diagram,the fluoride in groundwater is mainly related to the local geological setting and high-fluoride groundwater is mainly distributed in the northwestern part of the study area,which is consistent with the groundwater flow direction in this area.

(iii) The control factors of fluoride enrichment in groundwater mainly include specific natural factors such as provenance,geological structure,hydrogeological environment,and hydrogeochemical environment (such as pH,ion exchange,and mineral saturation).Human activities also affect the fluoride content in groundwater.

(iv) Excessive fluoride in drinking water and food will cause health risks in adults and children.According to this study,fluoride poses great risks to human health overall.Meanwhile,children face higher health risks than adults and the risks caused by food intake are higher than those caused by water drinking intake.Therefore,measures should be taken to ensure the health and safety of local residents.

(v) Given that the karst area and Shayuan area enjoy high water quality,the authors suggest establishing a central base of water supply in the Shayuan area in the south,which serves as a measure to provide safe drinking water.

CRediT authorship contribution statement

Rui-ping Liu and Hua Zhu carried out the investigation,experiments,and sampling.Rui-ping Liu wrote the manuscript.Ying Dong and Fei Liu helped Rui-ping Liu conceivably the original idea.El-Wardany RM polished English manuscripts.

Declaration of competing interest

The authors declare that they have no competing interests.

Acknowledgment

This study was funded by the survey projects initiated by the China Geological Survey (DD20189220,DD20211317,and 1212010634713).The authors are also grateful to the experiment and testing lab of Xi’an Center,China Geological Survey for statistical data analyses.