Drinking water fluoride and water-improving project in Dali County
Over the last 40 years, the fluoride concentration in drinking water for the residents in Dali County has declined significantly (Table 2). In 1980, the drinking water for all residents were supplied by private household wells, and the fluoride concentration in drinking water was generally higher than 1.5 mg/L, ranged from 0.6 to 11.2 mg/L, and the exceeding rate for safe drinking standard approached 100%. By 2010, most villages of the county had completed water improvement, but due to inadequate water source survey, poor water quality stability and engineering problems, the exceeding rate of fluoride concentration for safe drinking was still as high as 93.8%, with a range of 0.4–6.1 mg/L. From 1980 to 2010, after water improvement, the median fluoride concentration in drinking water decreased from 2.40 mg/L to 1.26 mg/L and the exceeding rate for safe drinking decreased to 93.8%. The results of sampling analysis in 2018–2019 showed that the range of drinking water fluoride further decreased to 0.4-4.0mg/L, with the median value declined to 1.20 mg/L and exceeding rate declined to 83.7%.
Table 2
Statistics on the villages within various drinking water fluoride concentration range
fluoride concentration (mg/L)
|
[0,1.0]
|
(1.0,1.5]
|
(1.5,2.0]
|
(2.0,3.0]
|
(3.0,4.0]
|
(4.0, ∞)
|
Samples higher than the standard (> 1.0 mg/L)
|
1980
|
2
|
6
|
37
|
152
|
43
|
35
|
99.3%
|
2010
|
18
|
211
|
36
|
11
|
10
|
3
|
93.8%
|
2018–2019
|
47
|
215
|
6
|
16
|
4
|
1
|
83.7%
|
Spatial analysis of fluoride concentration in drinking water had been conducted (Fig. 2) to help analyze the drinking water in different areas. In 1980, high fluoride drinking water distributed all over the county, and ultra-high fluoride drinking water (> 5.0 mg/L) mainly distributed in the low-lying areas to the north of Luohe River. In 2010, most towns to the north of Luohe River benefited from the water-improving project and the drinking water fluoride concentration mostly became lower than 2.0 mg/L. The ultra-high fluoride water (> 5.0 mg/L) still concentrated in the same area, but the scope reduced significantly. To the south of Luohe River, comparations with 1980 showed that the fluoride concentration of drinking water in Xiazhai did not have obvious changes, while that in Shicao decreased significantly. Up to now after the upgrading and maintenance of the water-improving project, the fluoride concentration in drinking water of most villages had been controlled below 1.5 mg/L (Table 2), and those remaining villages with high fluoride drinking water are mainly located in the towns of Boshi and Xiazhai.
The above changes of fluoride concentration in drinking water mostly depend on the water-improving project of the county. In the late 1980s, the Dali government began to reduce the hazards of high fluoride in drinking water by carrying out low-fluoride drinking water supply projects, which mainly included the development of low fluoride groundwater for new water sources and the construction of water supply facilities (water tanks and pipes) for households. Up to now, Dali County had successively built one large water supply station (Yuhong Station in the town of Duanjia) and six medium or small ones (water supply capacity of less than 10000 people), three of which are located in fluorosis areas (Xiyuan Station in the town of Qiangbai, Xiazhai Station and Shicao Station). Although the drinking water in Dali County is all directly from groundwater sources, the different situations of the main water sources have caused the different drinking water fluoride. The water source developed by Yuhong Station is the “380” karst water (Ordovician limestone karst fissure water with a water level elevation of about 380 meters) with an average fluoride concentration about 1.2 mg/L, and this station supplies water to more than 200 villages to the north of Luohe River. The water source of Xiazhai station is mainly phreatic water with a depth of 80-100m, and the average fluoride is around 2.4 mg/L. It supplies water to 12 surrounding villages. The water source of Qiangbai and Shicao station is phreatic water of aeolian sand areas, with average fluoride concentration of 1.13 mg/L, supplying water to 34 surrounding villages. In addition, there are 9 villages using disperse water supplies, and the water sources are mainly local self-drilled wells.
The water-improving project has improved the quality of drinking water and strengthened the unified management of drinking water supply through the development of low fluoride water sources and the construction of water improvement pipelines. However, the later management, maintenance, operation and the stability of water sources are also important factors affecting the water-improving effect. Our investigation on the water-improving project found that, even though all the villages in fluorosis areas of Dali County have constructed water-improving project, but the controllable rate of the project is lower than 10%. The investigation on residents' cognition of tap water showed that, in 27 villages, at least 50% of the residents had given up using the terminal tap water of the water-improving project because of some water supply problems, such as the poor water quality, bad taste, intermittent water supply or unreasonable charges. These negative feedbacks mainly came from the villages served by Yuhong Station (Fig. 3), mainly distributing in the town of Boshi. Comparing the spatial distribution of fluoride in drinking water and the investigation of water-improving project, it can be found that the villages with negative feedbacks correspond highly with the villages with high fluoride in drinking water. For those villages supplied by local water sources such as Xiazhai, Qiangbai, Shicao and these disperse water sources, the fluoride concentration of local ground water is crucial.
Statistical changes of endemic fluorosis in Dali County
From 1980 to 2017, the endemic fluorosis in Dali County has been greatly improved, including the narrowed scope of fluorosis area, the decreased number of fluorosis population and the weakened degree of the fluorosis symptoms (Table 3). The census on fluorosis in Dali County in 1980 showed that, among the 346 surveyed villages, the proportion of fluorosis prevalent villages was 83.5%, including 1 slight fluorosis village, 12 moderate ones, 167 severe ones, and 109 extra severe ones. Furthermore, in 1980, Dali County had counted 284,536 dental fluorosis patients, and the average prevalence of dental fluorosis was as high as 81.2%. In 2010, the proportion of fluorosis villages decreased to 75%, and the number of severe and extra severe fluorosis villages dropped significantly. In this year, a total of 14,022 children aged 8–12 were surveyed, and 4,268 children suffered from dental fluorosis, with the average prevalence of dental fluorosis decreased to 30.4%. By 2017, a total of 181 villages and 18,427 children were investigated, of which 130 villages and 4,732 children had fluorosis effected, the fluorosis villages proportion and dental fluorosis prevalence continuously decreased to 71.8% and 25.7%. It can be seen that fluorosis in Dali has been well controlled during the past 40 years, however, the prevalence is still relatively high compared to some other regions in northwestern China. For example, Xiji County and Yanchi County in Ningxia Province are also typical endemic fluorosis areas in Northwest China, but the prevalence of dental fluorosis in children there are 459.74/100,000 and 3.3115/100,000 respectively (Li et al. 2020), much lower than the prevalence level in Dali County.
Table 3
Survey results of villages and population affected by fluorosis
|
Total investigated villages (n)
|
Fluorosis affected villages (%)
|
Slight villages
(%)
|
Moderate villages (%)
|
Severe villages
(%)
|
Extra severe village (%)
|
1980
|
346
|
289(83.5%)
|
1
|
0.3%
|
12
|
3.5%
|
167
|
48.3%
|
109
|
31.5%
|
2010
|
296
|
222(75.0%)
|
177
|
59.8%
|
25
|
8.5%
|
13
|
4.4%
|
7
|
2.4%
|
2017
|
181
|
130(71.8%)
|
110
|
60.8%
|
16
|
8.8%
|
1
|
0.6%
|
3
|
1.7%
|
|
Total surveyed population
|
Type of respondents
|
Prevalence
|
Population with dental fluorosis
|
Total
|
Normal
|
Suspicious
|
Extremely mild
|
Mild
|
Moderate
|
Severe
|
1980
|
350,284
|
All ages
|
81.20%
|
284,536
|
n.a.
|
n.a.
|
n.a.
|
n.a.
|
n.a.
|
n.a.
|
2010
|
14,022
|
Children
|
30.40%
|
4,268
|
9,754
|
2,232
|
1,279
|
658
|
91
|
8
|
2017
|
18,427
|
Children
|
25.70%
|
4,732
|
13,695
|
2,517
|
1,397
|
746
|
63
|
9
|
Based on the calculation formula and the counted number of children with various symptom grades, DFI of children in 2010 and 2017 were calculated (Table 4). In 2010, the DFI of the villages ranged from 0.00 to 2.22, and the average DFI of the whole county was 0.29, while that of 2017 was 0.00–2.00, with the average DFI decreased to 0.24. According to the epidemic degree corresponding to DFI, in 2010 and 2017, Dali County as a whole belonged to the negative epidemic area. It can be considered that although the dental fluorosis prevalence of children in Dali County is high, most of the children have very mild or mild symptoms of dental fluorosis. However, the few villages with a high concentration of fluorosis cases with severe symptoms cannot be ignored. In 2010, there were two villages with the grade of “significant epidemic”, including the village of Da’er in Zhaoyi and the village of Xichengnan in Chengguan (central town), with the DFI fluorosis of 2.03 and 2.22, respectively. More than half of the children surveyed in these two villages showed mild and moderate symptoms of dental fluorosis, that is, chalky and discolored teeth (the tooth surface lost normal luster, opaque plaques, and even yellow or tan) and 9 cases of severe symptoms (the teeth of the tested objects have obvious defects) were detected. By 2017, the overall prevalence of dental fluorosis had decreased, but the number of the severe symptoms cases was still as high as 8, indicated that the remission of endemic fluorosis was limited.
Table 4
Villages with different DFI in Dali County
Range of DFI
|
[0,0.4]
|
(0.4,0.6]
|
(0.6,1.0]
|
(1.0.2.0]
|
(2.0,3.0]
|
(3.0,4.0]
|
Corresponding popularity
|
negative
|
verge
|
mild epidemic
|
moderate epidemic
|
relatively significant epidemic
|
significant epidemic
|
2010
|
Villages
|
n = 222
|
n = 33
|
n = 24
|
n = 15
|
n = 2
|
n = 0
|
Proportion (%)
|
75%
|
11%
|
8%
|
5%
|
1%
|
0%
|
2017
|
Number of villages
|
150
|
n = 14
|
n = 13
|
n = 4
|
n = 0
|
n = 0
|
Proportion (%)
|
83%
|
8%
|
7%
|
2%
|
0%
|
0%
|
Spatial distribution and influencing factors of dental fluorosis
In this study, spatial visualization of the distribution of fluorosis in Dali County was carried out to analyze the changes of fluorosis areas (Fig. 4). In 1980, the villages with severe and extra severe fluorosis widely distributed all over the county, the towns of Chengguan (central town), Anren, Zhaoyi, Bayu and Buchang were the main fluorosis areas with the prevalence higher than 90%. In 2010, the severe fluorosis areas were mainly distributed in the towns of Zhaoyi, Boshi and Xiazhai, while the fluorosis prevalence of other areas had been basically controlled under 40%. The towns of Hujia and Shicao became obviously slight fluorosis areas, and many villages here even achieved zero detection rate of dental fluorosis. In 2017, the severe fluorosis villages were mainly distributed in Boshi and Zhaoyi, the moderate fluorosis villages were mainly distributed in Shicao and Xiazhai, and the slight ones in Buchang, Bayu and almost half of the areas to the north of Luohe River.
By comparing Fig. 4 and Fig. 2, it can be seen that the spatial distribution and changes of endemic fluorosis are highly consistent with the fluoride concentration in drinking water. The results of correlation analysis between the prevalence of dental fluorosis and the fluoride concentration in drinking water (Table 5) showed that the dental fluorosis prevalence and DFI for children in 2010/2017 both had significant positive correlation with the drinking water fluoride. In 1980, the prevalence on dental fluorosis for did not show a significant water-affected characteristic, but the correlation between the prevalence of skeletal fluorosis and water fluoride was quite high. The analysis results showed that the distribution of the fluorosis prevalence in local population was not completely consistent with the distribution of fluoride concentration in drinking water, which indicated that fluoride concentration in drinking water is an important factor affecting the prevalence of fluorosis, but not the only one.
Table 5
Correlation between drinking water fluoride and fluorosis indexes
|
|
1980
|
2010
|
2017
|
Water F−and dental fluorosis prevalence
|
R value
|
0.098
|
0.283*
|
0.248*
|
P value
|
0.103
|
0.000
|
0.000
|
Water F−and DFI
|
R value
|
n.a.
|
0.274*
|
0.293*
|
P value
|
n.a.
|
0.000
|
0.000
|
Water F−and skeletal fluorosis prevalence
|
R value
|
0.456*
|
n.a.
|
n.a.
|
P value
|
0.000
|
n.a.
|
n.a.
|
The water-improving project in Dali County has significantly reduce the fluoride concentration in drinking water by developing low fluoride water sources for centralized water supply, thus made great contributions on the controlling of endemic fluorosis. However, the effect of water improvement project is limited due to some reasons. In 2017, the prevalence of dental fluorosis in Dali County was 25.7%, still at a relatively high level. By observing the distribution of the prevalence rate of fluorosis in Fig. 4, it can be found that the current distribution of major fluorosis areas is closely related to the residents’ negative feedbacks on water-improving project, indicating that the service quality of the water-improving project and residents' cognition and choice of drinking water can seriously affect the prevalence of fluorosis This kind of fluorosis villages are mainly concentrated in the towns of Boshi and Zhaoyi to the north of Luohe River. To the south of Luohe River, the water sources of the water-improving project are the locally developed low-fluoride groundwater wells, therefore the stability of groundwater quality becomes an important factor affecting the fluorosis. For example, in Xiazhai, high fluoride concentration (mean value ≈ 2.4 mg/L) in groundwater is the key factor for the high prevalence of fluorosis.
Occurrence and distribution of fluoride in groundwater
In 1980 and before, the residents of all villages and towns in Dali County directly drank the local groundwater. The later water-improving project is also to develop groundwater sources and then supply water in a centralized way. Therefore, the concentration and distribution of fluoride in groundwater is critical to drinking water safety.
General chemistry and fluoride status of groundwater
Groundwater hydrochemistry is very important for understanding the interaction between groundwater and various aquifer minerals and the change of chemical composition (Su et al. 2015). The hydro-chemical analysis results of Dali County groundwater sampling points in this study are shown in Table 6. The pH variation range of groundwater samples is 7.7–8.5, with an average value of 8.14. The groundwater in the study area is generally neutral to moderately alkaline. The concentration of Na+ in cation is the highest, followed by Mg2+ and Ca2+, and the content of K+ is the lowest. Among anions, the content of SO42- is the highest, followed by HCO3- and NO3-, and the content of Cl- is the lowest. This result is similar to the hydro-chemical characteristics of high fluoride groundwater in the adjacent Hancheng of Guanzhong Plain and Yuncheng basin (Liu et al. 2008;Ji et al. 2020). TDS (Total Dissolved Solids) determines the quality of drinking water, and is one of the most important characteristics of groundwater. The TDS contents in groundwater in the study area are in the range of 251-5510mg/L. Among them, 57.1% of the groundwater samples have TDS < 1000mg/L, which are lower than the limit of the sanitary standard for drinking water. 42.9% of the samples have TDS between 1000-10000mg/L, indicating that the groundwaters in the study area are saline. The concentration of Na+ in cation is the highest, followed by Mg2+ and Ca2+, and the content of K+ is the lowest. Among anions, the content of SO42- is the highest, followed by HCO3- and NO3-, and the content of Cl- is the lowest. This result is similar to the hydro-chemical characteristics of high fluoride groundwater in the adjacent Yuncheng basin (Li et al. 2015). The concentration of SO42- and NO3-in the groundwater are both at a high level, which may be attributed to the use of pesticides and fertilizers in wide agricultural land in the study area (Ren et al. 2021). The fluoride concentration in the groundwater ranges from 0.58-4.08mg/L, the mean value is 2.07mg/L, the median is 1.84mg/L, and the coefficient of variation is 0.46. In 63.5% of groundwater samples, the fluoride concentration exceeded the WHO allowable limit of fluoride in drinking water (1.5mg/L), and the proportion exceeding the allowable limit in drinking water in China (1.0mg/L) was as high as 90.5%.
Table 6
Statistics of physico-chemical parameters of groundwater samples in Dali County
|
Max
|
Min
|
Mean
|
SD
|
Variability
|
pH
|
8.5
|
7.7
|
8.14
|
0.17
|
0.02
|
K+ (mg/L)
|
180.20
|
1.23
|
8.06
|
25.30
|
3.14
|
Na+ (mg/L)
|
2193.63
|
44.13
|
353.61
|
348.44
|
0.99
|
Ca2+ (mg/L)
|
214.00
|
5.43
|
36.78
|
39.51
|
1.07
|
Mg2+ (mg/L)
|
439.48
|
6.20
|
65.69
|
71.14
|
1.08
|
NO3-(mg/L)
|
636.29
|
10.47
|
109.61
|
142.79
|
1.30
|
HCO3- (mg/L)
|
1793.30
|
20.74
|
371.08
|
341.30
|
0.92
|
Cl- (mg/L)
|
2596.21
|
24.22
|
263.66
|
430.30
|
1.63
|
SO42- (mg/L)
|
3866.48
|
37.63
|
383.91
|
593.56
|
1.55
|
F- (mg/L)
|
4.08
|
0.58
|
2.07
|
0.95
|
0.46
|
TDS (mg/L)
|
5150
|
251
|
1167.06
|
918.18
|
0.79
|
EC (µs/cm)
|
7750
|
373
|
1833.06
|
1398.75
|
0.76
|
The distribution of fluoride in groundwater in the study area is shown in Fig. 5. The fluoride in groundwater in the whole county is generally higher than 1.0mg/L, of which the high value center is located in the towns of Buchang, Boshi Township and Zhangjia, followed by Zhaoyi and Xiazhai; Low value centers are located in the towns of Bayu and Shicao. Compared with the geomorphic features of the county, it can be found that high fluoride water is mainly distributed in the second terrace and tectonic depression of Weihe River, while the fluoride concentration of groundwater around the Sandy Area is low, close to the safe drinking standard.
Fluoride sources and influencing factors for enrichment
Fluoride sources. Fluoride rich strata are the material basis for the formation of high fluoride groundwater. In the study area, the loess plateau and the Weihe River & Luohe River terraces have thick loess and clay layers, which contain mica, amphibole, tourmaline and more clay materials. The average soil fluoride content in the loess plateau and terraces of Weihe River & Luohe River is about 619.20 mg/kg (Ke et al. 2010), and the rich fluoride in the soil is the main source of fluoride in the groundwater (Liu et al. 2008)。
Geomorphologic characterization. The topography affects the direction of groundwater runoff, thus controlling the fluoride in natural groundwater (Brindha and Elango 2013). Generally, low fluoride is distributed in mountains, while high fluoride is distributed in plain depressions (Li et al. 2021;Su et al. 2021). The spatial distribution of groundwater fluoride in Dali County also conforms to this law. When the groundwater reaches the low and gentle terrain, the flow of underground runoff is getting slow, and the residence time of the groundwater in the weathered aquifer becomes longer (Arveti et al. 2011), thus promoting the dissolution of fluoride minerals and the ion exchange between F- and OH- in the weathering products, leading to the increase of fluoride concentration in the groundwater (Liu et al. 2021). In Dali, the overall terrain is high in the north and low in the south, which can make the fluoride migrate to the low-lying place with the flow of groundwater (Li et al. 2019). The groundwater stagnation area dominated by flat terraces and depressions provides a place for the enrichment of fluoride, therefore causing the higher fluoride concentration.
Climate. Rainfall and evaporation intensity are the main climatic conditions affecting the fluoride content in groundwater, which play an important role in the migration and accumulation of fluoride. Dali County has a typical warm temperate continental monsoon climate, with the same period of rain and heat, and the overall climate is relatively dry (Wu et al. 2017). The rainy season lasts from July to September, and the precipitation exceeds 50%. After groundwater reaches flat low-lying area with runoff, the strong evaporation caused by the dry climate in this area provides a motive force for the concentration of water fluoride, which will aggravate the accumulation of fluoride in low-lying areas. In such dry climatic conditions, dissolved constituents including Ca2+ present in groundwater get concentrated and precipitated by evaporation which eventually leads to the salination of groundwater (Panwar et al. 2015). In addition, the study area is characterized by concentrated rainfall in summer. The infiltration caused by a large amount of rainfall in a short time will strengthen the leaching effect, and promote the fluoride in the shallow loess to infiltrate and percolate with rainwater in vertical and lateral groundwater flow (Raju 2017), caused the increased fluoride in groundwater. The Gibbs diagram can be used to evaluate the influences of rock weathering, evaporation and precipitation on water chemistry in this study (Fig. 6). It can be found that most of the sample points are concentrated in the upper right corner of the figure, indicating that evaporation crystallization has a greater impact on the chemical process of groundwater in the study area, and is the most significant natural mechanism governing the groundwater evolution in the study area. Some samples are located in the gray shadow area, which proves that the rock weathering in the study area has an impact on the fluoride enrichment in groundwater, but the fluoride concentration in this type of groundwater is relatively low, generally not more than 2.0mg/L.
Hydro-chemical characteristics. Fluoride ion concentration in groundwater is closely related to hydro-chemical type. Piper diagrams is widely used in the researches of groundwater chemical types, and can effectively reflect the hydro-chemical type information of water bodies (Liu et al. 2020). As shown in Fig. 7, most of the groundwater samples are concentrated in areas ② and ④, indicating that the local groundwater are mainly classified as HCO3-Na and SO4·Cl-Na types, which are basically sodium rich and low calcium water. When the concentration of F- is less than 1.0mg/L, the hydro-chemical type of the sample point is mainly mixed type, and there is no strong and prominent ion. With the increase of fluoride concentration in groundwater, the sample points gradually gather in the direction of NA+ and K+ in the cation triangle, and migrate to the direction of HCO3- in the anion triangle. The high sodium concentration may come from cation exchange and water percolation which introduces sodium from vadose zone into groundwater (Ren et al. 2021). The presence of a large amount of Na+ helps dissolve some minerals rich in fluoride (Guo et al. 2007)The high concentration of HCO3- indicates that the dissolution of carbonate is an important process to regulate the main anions. The high concentration of HCO3- contributes to the formation of a high alkalinity environment, which promotes the dissolution of fluorite in rock and soil layers, and tends to mobilize F- out of weathered rocks and sediments (Su et al. 2015), so as to increase the fluoride concentration in groundwater.
Spatial evolution of high fluoride groundwater
Climate, topography and geomorphology all deeply affect the recharge, runoff and discharge of groundwater, significantly controlling the formation of groundwater with high fluoride concentration (Xiao et al. 2015;Li et al. 2018). According to the above analysis, it can be determined that the formation of high fluoride groundwater in Dali County is mainly caused by rock dissolution and evaporation enrichment. In combination with the natural environment characteristics and hydrochemistry of different areas in Dali County, different groundwater fluoride zones can be divided to better show the spatial evolution mode of groundwater fluoride concentration. The result of spatial analysis shows that the fluoride concentration of groundwater in Dali County has obvious spatial differentiation along the runoff direction, which is similar to the change rule of fluoride in natural groundwater observed by some scholars from the recharge area to the discharge area (Tirumalesh et al. 2007;Vithanage and Bhattacharya 2015). In the north of Taiyuan Basin, which is close to Dali County, Guo et al. (2007) found that the enrichment mechanism of fluoride in groundwater varies along the direction of underground runoff. In the recharge and runoff area, the interaction between groundwater and fluoride rich minerals is the main factor for the increase of water fluoride concentration, while in the discharge area, the evaporation and mixing of karst water have a greater contribution to the enrichment of fluoride in groundwater. Along the direction of underground runoff, Dali County is divided into the following four regions according to the geomorphic characteristics and geological background to describe the formation mechanism and characteristics of groundwater fluoride (Fig. 8).
A) Leaching-enrichment zone. This zone is mainly located in the loess platform and the fourth terrace of Weihe River. The loess layer and alluvial proluvial deposits covered on this zone are rich in fluoride minerals. The phreatic aquifer is mainly composed of sand-gravel with strong water yield property. This zone has a strong weathering-leaching processes and the content of Na+ in groundwater has obvious advantages. The dissolution of rocks is the main mechanism of groundwater fluoride enrichment. The hydraulic gradient in this zone is large, leading to a favorite runoff condition, and the amount of groundwater discharged through the runoff is large, so the groundwater updates quickly. The villages and towns in this zone mostly drink the tap water supplied by deep karst water source of Yuhong, and the direct drinking of local groundwater is rare.
B) Runoff-enrichment zone. This zone is mainly concentrated in the second and third terraces of Weihe River. The aquifer structure here is complex, with fine rock and soil particles and poor water permeability. Furthermore, the terrain is low and tends to be flat, and the groundwater runoff is not smooth and converges here, which makes the water rock interaction time longer and the dissolution of fluoride minerals increases. At the same time, because the regional climate is dry and evaporation is strong, the fluoride concentration in groundwater increases sharply. The main mechanism of fluoride enrichment is the dual role of rock dissolution and evaporative enrichment. In terms of groundwater drinking, most villages and towns to the north of the Luohe River in this zone drink Yuhong deep karst water, but some residents also drink the local high fluoride groundwater due to the poor water supply projects. To the south of Luohe River, Xiazhai water source has been developed, which has a high fluoride concentration.
C) Evaporation-enrichment zone. This zone mainly distributed in structural low-lying lands and its surrounding areas, including the township s of Zhaoyi and Boshi. After groundwater runoff reaches this zone, the hydraulic gradient is reduced to about 8°, which is the lowest in the county. After a large amount of groundwater runoff collected here, the buried depth of groundwater becomes shallow due to the low terrain, and the main discharge mode changed from runoff to evaporation. The special hydrodynamic conditions and strong evaporation promote the strong concentration of fluoride in groundwater in this zone, causing the groundwater fluoride concentration generally higher than 2.5mg/L.Although the villages and towns in this zone have conducted water-improving project, the proportion of using self-drilled wells is high, so there is a serious drinking safety problem of the local residents.
D) Runoff zone. This zone mainly distributed in the sand area, involving townships of Shicao and Bayu. The area is a sand dune formed by wind accumulation, with good groundwater permeability and rapid alternating renewal of water bodies. The Na+/(Na++Ca2+) of this zone is at the lowest level in the county. The rich calcium ions can easily combine with fluoride ions to form sediment, thus reducing the fluoride concentration in groundwater. It can be seen from Fig.8 that the TDS and fluoride of groundwater in this zone are both relatively low, which can be considered to be the shallow groundwater distribution area with the best water quality in Dali County.
Temporal changes of groundwater fluoride in 40 years
From 1980 to now, two major changes of the groundwater fluoride concentration had taken place in Dali County. Firstly, the overall groundwater fluoride concentration was significantly reduced. Since 1980, the average value had decreased by 0.92 mg/L, the median value had decreased by 0.58 mg/L, and the exceeding rate of safety drinking had decreased from 99.3–90.5%. Secondly, the distribution center of high fluoride in groundwater had shifted. Over the past 40 years, Boshi and Zhaoyi had gradually evolved into the areas of the county where high fluoride groundwater accumulated. Xiazhai to the south of Luohe River had undergone the same changes, while Shicao and Qiangbai had evolved into low-value areas of groundwater fluoride concentration.
For the general decrease of groundwater fluoride concentration in Dali County, it is believed that the main reason is the groundwater dilution caused by the water infiltration of river diversion canal irrigation in recent decades. On the basis of the relevant studies in the Guanzhong Plain Irrigation Area, in the context of the local arid climate with less precipitation, the proportion of modern rainfall to groundwater recharge decreases, and the infiltration of irrigation water has an important contribution to the recharge of shallow groundwater, and the irrigation water is generally with low fluoride concentration, which will dilute the groundwater and thus reducing the concentration of fluoride (Jia et al. 2020;Chen et al. 2021). Since 1997, Dali County has built the Yellow River Irrigation Canal, which directly pumps water from the Yellow River to the regional branch canal irrigation network, and then irrigates the farmland. Over the years, Dali County participated in the construction of several irrigation channels, mainly including Luohui Canal, Donglei Yellow River Water Pumping Project, and Jiaokou Weihe River Water Pumping Project. The well-developed water diversion irrigation projects have delivered large amounts of river water to farmland. Besides, most irrigation canals are not built with lining structure, thus some river water will leak out directly from the canal. According to the statistical data of water resources in Dali County, currently 29% of the total amount of groundwater recharge is occupied by the irrigation water recharge. From 2003 to 2012, due to agricultural irrigation and water diversion canal leakage, the groundwater level in Dali County rose by about 3 meters on average (Zang et al. 2019). The buried depth of groundwater in the loess tableland and Weihe River terrace is decreasing, especially the water level in Weihe River terrace. This can explain the reason why the concentration of fluoride in groundwater in these areas generally has severe drops.
Xiazhai is the area with special groundwater fluoride changes compared with other areas in Dali. This study found that the average fluoride concentration in groundwater in Xiazhai had slightly decreased from 2.19 mg/L (1980) to the current 1.99 mg/L, with the smallest decrease of fluoride concentration among the whole county. We believe that this feature is formed by the joint action of irrigated water recharge and water resources development and utilization. To the south of Luohe River, the fluoride in groundwater is mainly carried by underground runoff from the northern leaching area, so the water fluoride here decreases with the falling groundwater fluoride concentration in the northern region. However, Xiazhai is the most important drinking water source to the south of Luohe River, providing drinking water for about 20,000 residents of more than 10 villages around. Meanwhile, Xiazhai is not within the service scope of irrigation canal project, so the local agricultural irrigation mainly depends on groundwater pumping. For this reason, with the increase demands of residents' normal life and irrigation, the groundwater exploitation in Xiazhai gradually increases and the water volume declined continuously. From the Weinan Water Resources Bulletin in 2019, the average buried depth of groundwater in Xiazhai in 2019 was 12.43m, 1.57m lower than the average buried depth for many years, and 4.39m lower than that in 1985. In arid areas with less precipitation and strong evaporation, due to dilution mechanism, rising water level will reduce the fluoride concentration and falling water level will increase that (Kalpana et al. 2019;Aravinthasamy et al. 2020).Therefore, under the combined effect of the decreased water fluoride caused by irrigation and the increased water fluoride caused by over exploitation, Xiazhai shows a weak change in fluoride concentration of groundwater and a high fluoride level at present. Based on the above situation, the government should focus on the water fluoride concentration and groundwater development in Xiazhai, reasonably control the local groundwater exploitation and strengthen groundwater monitoring. In terms of drinking water supply, new water sources with low fluoride concentration can be developed in the Sandy Area. For medium-sized water supply stations like Xiazhai (served no more than 20,000 residents), technical drinking water fluoride removal facilities can be considered to build locally.