Cause of shallow high-fluoride groundwater in Nanyang Basin

(1) Overview of the causes of fluorine distribution in shallow groundwater

Through the summary and analysis of the distribution characteristics of fluorine in shallow groundwater in the Nanyang Basin plain area, it can be seen that the basin periphery is close to Low-fluoride groundwater is widely distributed in mountainous areas and hilly areas, medium-fluoride groundwater is mainly distributed in the middle of the basin, and high-fluoride groundwater is mainly distributed in the center of the basin plain area near the opening of the basin and in local low-lying areas of the central plain. There are specific reasons for this macroscopic distribution characteristic.

As a relatively independent and relatively complete tectonic geological unit, the Nanyang Basin's unique topography and landform characteristics provide natural and favorable conditions for the formation and development of a groundwater flow system with complete sources and sinks within the basin. The eroded mid-low mountainous areas and hilly areas around the Nanyang Basin constitute the source area of ??groundwater in the basin, and different locations in the central plain area of ??the basin constitute the source areas and sink areas of various levels of groundwater flow systems.

An analysis of the climate characteristics of the Nanyang Basin shows that in summer, due to the interaction between the continental thermal low pressure and the Pacific subtropical high pressure, the subtropical high pressure carries a large amount of water vapor into Henan Province along with the southeasterly wind. Affected by the topography, when the moist and hot vapor from the western Pacific encounters the eastern branch of the Funiu Mountains in the northern part of the Nanyang Basin, the water vapor climbs and easily forms precipitation, thus forming an uneven spatial distribution pattern of precipitation in this area. Therefore, although the Nanyang Basin is in the transition zone between the semi-humid zone and the humid zone on a macro scale, the precipitation is relatively abundant. However, if the scale is reduced, different regions within the Nanyang Basin will show certain climate differences, and the uneven spatial distribution pattern of precipitation and evaporation is a specific manifestation of this. It can be seen from the distribution of multi-year average precipitation in the Nanyang area that the ridges in the east of the Nanyang Basin, the mountainous areas in the north and west, and the piedmont areas have more precipitation than the plain areas, and the precipitation is relatively abundant. Due to the large depth of the groundwater level and the coarse lithological conditions that are not conducive to the rise of capillary water, the amount of evaporation is small, and the strong leaching effect of atmospheric precipitation continuously takes away the fluorine in the soil in this area, thus depleting the soil of fluorine. In turn, the ability to regulate fluorine in groundwater decreases. The replacement of groundwater with higher frequency causes more and more fluorine to migrate with the groundwater, so that the water and soil environment in these areas has a lower fluorine background. This is also the reason why large areas of low-fluoride groundwater appear in the vast piedmont areas relatively close to groundwater source areas.

As far as the groundwater flow system is concerned, because the piedmont zone is located closer to the source area of ??groundwater - the surrounding mountainous areas than the plain areas, the flow of groundwater is shorter, and the water-rock interaction The degree of fluorine is not sufficient, and the topography and landform conditions are not conducive to the collection of groundwater, and the evaporation effect is relatively weak. These are enough to cause the fluorine content in the groundwater in the piedmont zone to be relatively low. On the contrary, because the plain area is far away from the source of groundwater, the groundwater flow is greatly lengthened, the water-rock interaction is relatively sufficient, and the fluorine content in the groundwater is relatively high. Coupled with the effect of evaporation and concentration, the fluorine content in the groundwater in the plain area is relatively high. higher.

As a regional groundwater sink, the central plain area of ??Nanyang Basin continuously receives recharge from surface water and groundwater from surrounding mountainous areas, so fluorine that migrates with groundwater continuously accumulates here. Due to the relatively low precipitation in this area and the relatively flat terrain, the groundwater flow rate is relatively slow and the water circulation cycle is relatively long. At the same time, the shallow groundwater level in the plain area is buried deep and the thin rock is conducive to the rise of capillary water. These characteristics lay the foundation for the strong evaporation and concentration in this area. Therefore, the continuous accumulation of fluorine-containing substances caused by the extensive fluorine sources in the mountainous areas around the Nanyang Basin, as well as the strong evaporation and concentration caused by the shallow groundwater depth and relatively sluggish groundwater flow in the plain area, have caused the groundwater in this area to The fluorine content gradually increases, resulting in a larger distribution area of ??medium-fluorine groundwater, and also creates an ideal environment for the formation of high-fluorine groundwater in local areas.

Since the Nanyang Basin plain area is a high-yield area for food crops such as wheat, corn, cotton, and tobacco in Henan Province, agricultural irrigation is relatively common, and human influence can also increase the evaporation of water in the area, which in turn leads to soil degradation in the area. The accumulation of salt and fluorine in the surface soil lays the foundation for further migration into groundwater.

Furthermore, based on the very few distribution characteristics of deep high-fluoride groundwater and the available hydraulic connection data between deep and shallow groundwater, it can be concluded that the contribution of fluorine contained in deep groundwater to shallow groundwater is Limited, only a few areas have a significant impact on the fluorine content in shallow groundwater due to the recharge of deep fluorine-containing groundwater. It is further concluded that most of the fluorine in shallow high-fluorine groundwater is related to the enrichment of elements related to the salinization effect of the earth's shallow continents.

(2) Analysis of factors affecting the enrichment of fluorine in shallow groundwater

1. Climatic conditions

Climate conditions have an impact on the fluorine content in groundwater It has a certain impact. It can be seen from the phenomenon that fluorine is mostly distributed in arid and semi-arid areas. The arid and semi-arid climate affects the fluorine content in groundwater by promoting the evaporation and concentration of groundwater, which is conducive to promoting the evolution of groundwater to high fluoride.

The Nanyang Basin in Henan Province is located near the climate dividing line between the north and south of my country. It is a transition zone from subtropical to warm temperate zones and has a typical continental monsoon semi-humid climate.

In winter, under the control of polar cold high-pressure air masses, the eastern Pacific is occupied by the Aleutian low pressure. The air pressure situation is from west to east, with northerly winds prevailing and little rain. In summer, under the control of continental hot low-pressure air masses, the Pacific subtropical high pressure Advancing to the northwest, the air pressure situation changes from east to west, the wind direction is southerly, the water vapor is abundant, and the precipitation is larger. The multi-year average temperature is 15.1℃, with the lowest temperature in January, with an average temperature of 1.4℃. July is the highest, with an average temperature of 27.4℃. Winters are cold with northeasterly winds, and summers are hot with southwesterly winds. The multi-year average precipitation in the study area ranges from 678.1 to 967.8mm, with the maximum precipitation being 1984.9mm (1964) and the minimum precipitation being 411.7mm (1976). Precipitation is mostly concentrated from June to September, accounting for more than 60% of the total annual precipitation, up to 68%. The precipitation is the smallest from December to March of the following year, accounting for about 4.5% of the total precipitation throughout the year. The multi-year average evaporation in the study area is 1725.7~1879.5mm, the multi-year average monthly maximum evaporation is 303.4mm, and the monthly average minimum is 61.1mm. Evaporation is largest from May to August, accounting for about 25% of the annual evaporation; it is smallest from November to February, accounting for 16.4% of the year. In addition to obvious monthly, seasonal and annual changes in precipitation, there are also obvious zonal characteristics. The vast mountainous area near Sikeshu Township in the north of Zhenping County and the area near Tanghe County are two relatively concentrated precipitation areas, with the average annual precipitation exceeding 870mm. The southwest part of Zhenping County, including Jiasong Town, Xiaji Township, Peiying Township and Zhaoji Town, has relatively small annual average precipitation, ranging from 700 to 720mm. The average annual precipitation in the vast plain area is in the middle, ranging from 720 to 800mm. Evaporation decreases as precipitation increases, and conversely increases as precipitation decreases.

It can be seen from the zonal differences in the distribution of annual average precipitation (Figure 5-21), whether it is the high-fluoride groundwater area in the south of Waizi Town, the high-fluoride groundwater area near Peiying Township, or the Guanzhuang- In the high-fluorine groundwater distribution area from Shi'an Town to Lihepu Town, in addition to seasonal differences, the average annual precipitation in the zone is relatively small. This phenomenon is especially true near Peiying Township. obvious. Therefore, the strong evaporation concentration effect and the climate characteristics of relatively large evaporation provide favorable conditions for the formation of high fluorine groundwater in these areas.

Figure 5-21 Multi-year average precipitation contour map in Nanyang area (unit: mm)

2. Groundwater table depth and vadose zone lithology

< p>Obviously, the drier the climate and the smaller the relative humidity, the more intense the evaporation will be. In addition to climate, the depth of the groundwater table and the lithological conditions of the vadose zone can also affect the concentration of soil and groundwater chemical components by affecting evaporation and concentration. Therefore, climatic conditions, groundwater table depth, and vadose zone lithological conditions can cooperate with each other to affect the fluorine content in groundwater by affecting evaporation.

During the evaporation process, only groundwater within a suitable burial depth range can continuously supply soil moisture through capillary water generated by capillary phenomena, thus maintaining the continuous progress of soil evaporation. If the depth of the groundwater table exceeds the range of the maximum capillary rise height corresponding to a certain soil lithology, the capillary water supplied by the groundwater to the soil will be out of touch with the soil water. This phenomenon is very unfavorable to the continued evaporation.

Relevant research shows that the shallower the phreatic surface is buried, the more intense the evaporation will be. Figure 5-22 shows the relationship between the phreatic stable evaporation intensity and the evaporation amount and the water level and depth. It can be seen that within a certain range, as the phreatic depth continues to increase, the evaporation intensity increases slightly; when it reaches a certain depth, as the phreatic depth continues to increase, the evaporation intensity begins to decrease until it reaches zero. When the water level is less than 2m deep, the evaporation increases significantly as the water level becomes shallower; when the water level is greater than 2m, the evaporation decreases significantly.

Figure 5-22 The relationship curve between phreatic stable evaporation intensity and evaporation amount and water level depth

The influence of vadose zone lithology on groundwater evaporation is mainly through its effect on capillary rise height and speed The impact is carried out. The maximum capillary rise height of vadose zone rock and soil is shown in Table 5-9. The maximum capillary rise height of medium-fine sand is small, much lower than the maximum capillary rise height of clayey soil. The capillary rise height of clayey soil is larger, but the capillary rise speed is low. The vadose zone composed of sub-clay between the two has a large capillary rise height and fast rising speed, and the intensity of phreatic evaporation is high.

Table 5-9 Maximum capillary rise height of soil

(According to Xilin Bekchulin, 1958)

The area of ??shallow high-fluorine groundwater in Nanyang Basin The depth of underground water table is shown in Table 5-10. It can be seen from Table 5-10 that the groundwater burial depth ranges from 0.4 to 5.1m, and the range of changes is large, but it is within the range of the maximum capillary rise height of clay and sub-clay in this area. The vadose zone in the plain area of ??the Nanyang Basin is mainly composed of subclay, clay soil, a small amount of silty sand, and the staggered superposition of these lithological soils, which is conducive to supporting the rise of capillary water. However, the depth of groundwater near Peiying Township and Guanzhuang Town, where shallow high-fluorine groundwater is distributed in the area, is relatively small. The combination of relatively dry climate conditions, specific lithology and groundwater table depth conditions in the Nanyang Basin It is by promoting the continuous evaporation and concentration that the fluorine content in the water and soil environment of the area is affected.

Table 5-10 The depth of groundwater in the area where shallow high-fluorine groundwater is located in the Nanyang Basin

It can also be seen from the sedimentary differentiation rules of rock and soil in the alluvial plain. The particle size deposited in the piedmont zone on the edge of the plain area is relatively coarse, and the underground water table is buried deep, which is not conducive to the rise of capillary water. In addition, the climate characteristics of high precipitation and large precipitation result in relatively small evaporation, so the piedmont The groundwater in the area has low salinity and low fluorine content. On the contrary, toward the center of the plain area, factors such as better-sorted fine-grained sediments, shallower groundwater depth, and relatively arid climate are conducive to promoting the evaporation and concentration of groundwater, thereby promoting an increase in fluorine content.

3. Topographic conditions

Topography is an important condition that affects the migration and enrichment of fluorine in shallow groundwater. It mainly affects the concentration of fluorine in groundwater by shaping hydrodynamic conditions. Migration and aggregation processes.

As we all know, the chemical composition of groundwater is the result of various hydrochemical interactions between groundwater and the geotechnical media flowing through it. The chemical effects of groundwater mainly include filtration, concentration, decarbonation, desulfurization, alternating cation adsorption, etc. The chemical composition of groundwater changes continuously with the flow process, showing a regular distribution. Relevant studies have shown that the chemical composition at any point in the groundwater flow system mainly depends on the following factors: ① the chemical composition of water when it is input from the outside; ② the process and flow rate of groundwater; ③ the media encountered during the flow process; ④ during the flow process various water chemical reactions that occur.

As far as groundwater flow systems are concerned, the hydrochemical characteristics of groundwater flow systems at different levels are quite different. When other conditions are the same, the longer the groundwater stays in the rock formation, the more water it will dissolve from the surrounding rock and soil. The more components are obtained through filtration. The water in the regional flow system travels over a long distance, has a slow flow rate, and contacts many rock formations, so the chemical composition of the groundwater in the discharge area is relatively complex. The water in the local flow system has a short distance and a fast flow rate, so the chemical composition of the groundwater in the discharge area is relatively simple. In the case of complex terrain, the chemical composition of groundwater in local areas becomes the superposition of the effects of the local flow system and the regional flow system, as shown at the location of point d in Figure 5-23.

Figure 5-23 Schematic diagram of groundwater flow system

(Modified after Wang Dachun et al., 2006)

The topographic and geomorphological conditions are precisely determined by the hydrodynamic field The shaping thus affects the flow of groundwater, providing a basis for the formation of different levels of groundwater flow systems.

From the distribution map of fluorine in the shallow groundwater of the Nanyang Basin (see Figure 5-22), it can be seen that from the hilly and low mountainous areas on the edge of the Nanyang Basin to the gentle accumulation plains, the shallow groundwater Fluorine content shows regular changes. The fluoride content in shallow groundwater in hilly and low mountainous areas is low, most of which are less than 0.5mg/L. The fluorine content of shallow groundwater in denudation hills begins to increase. Both low-fluorine and medium-fluorine groundwater are distributed, showing signs of transition from low-fluorine to medium-fluorine groundwater. For example, the fluorine content in Tangying Village, Linpa Town, Dengzhou City has reached 1.0 mg. /L. In the gentle accumulation plain area in the south-central part, high-fluoride groundwater appears in patches. Among the eight shallow high-fluoride groundwater sample points in the study area, except for the water point in Tangying Village, Linpa Town, the other water points are distributed in In the gentle accumulation plain area, the highest point appears near Gaozhuang Village, Lihepu Town, Xinye County, with a maximum of 2.90mg/L.

As mentioned before, the Nanyang area is a semi-enclosed basin surrounded by mountains on three sides in the west, north and east and opens sloping towards the south. Erosion and denudation mountains are mainly distributed in the north and west, and denudation hills are mostly distributed in the east. There are also low mountains and hills in the southwest and southeast regions, and the central region is a large area of ??alluvial plains. The transition zone between the plains and the mountains is scattered with ridge-shaped and drum-shaped hills. The basin is surrounded by mountains on three sides and the terrain is high. The central plain area is open in the north and gradually shrinks to the south. The low-lying and flat topographic features prompt the groundwater and mineral elements contained in the water in the study area to converge and accumulate in the low-lying areas (Figure 5-24) . Due to the continuous evaporation and concentration, high mineralization and high fluorine groundwater are easily formed in the central plain area.

Figure 5-24 Hydrodynamic diagram of fluorine migration and accumulation

Obviously, the above analysis is for the Nanyang Basin, and not all local areas. For example, in the central plain area of ??Nanyang Basin, high-fluoride groundwater is only distributed in patches in the basin, while shallow groundwater with medium or even low fluoride is distributed around the high-fluorine groundwater distribution area. The reason is closely related to the topography.

Through further analysis of the topographic and landform characteristics of the plain area of ??the Nanyang Basin, it can be seen that the vast plain area in the central part of the basin is not all flat terrain, but there are denudation ridges and wavy hills scattered around the periphery of the plain area. and the Lonely Hills. This staggered distribution of plains, hills and low hills further creates the undulating and complex topography characteristics of the plain area of ??the Nanyang Basin. For local areas in the Nanyang Basin, the migration and accumulation of fluorine in shallow groundwater are not only controlled by macro-regional topography, but also related to micro-topography conditions. The existence of micro-topography causes certain differences in fluorine content even in different locations within the shallow high-fluorine groundwater distribution area.

In other words, the staggered distribution of different types of landforms in the area has different levels of impact on local areas in the plain area by shaping groundwater flow systems with different levels. It is the existence of this influence that causes differences in the distribution of fluorine in shallow groundwater in local areas.

In the Nanyang area, the groundwater in some hilly areas also has high fluoride. The reason for this phenomenon is that the sides of the hilly areas are often rivers or ancient river channels, and their lithology is often sand and gravel, and the aquifers seep into it. It has good performance, but hilly land is often made of clay soil with fine particle size. The permeability of the aquifer is poor, but the capillary rise height is large. This causes water and salt to move from low-lying areas with good permeability to low-lying areas with poor permeability under drought and evaporation conditions. As the hillock migrates, salt accumulates near the surface of the hillock; when precipitation occurs, the salt in the vadose zone infiltrates into the groundwater with the precipitation. However, due to the poor permeability of the hillock, the water and salt cannot be smoothly discharged to both sides, causing the hillock to be in trouble. Fluorine content is high in groundwater. Figure 5-25 shows the water and salt migration pattern under evaporative precipitation conditions in the hilly area.

Figure 5-25 Schematic diagram of water and salt migration model in hilly areas

In order to study the impact of micro-topographic conditions on the distribution of fluorine in shallow groundwater in different sections of local areas, a large sample was selected. The three sections of Yangying-Huangyigou, Quling-Liuyan and Liupo-Xiaguanying were analyzed in more detail, and a schematic cross-section diagram of the groundwater flow system was drawn, as shown in Figure 5-26 to Figure 5-28. .

Figure 5-26 Schematic diagram of salt migration section in Dayangying-Huangyigou section

Figure 5-27 Schematic diagram of salt migration section in Quling-Liuyan section

Figure 5-28 Schematic diagram of salt migration section in the Liupo-Xiaguanying section

(1) Figure 5-26 is the Dayangying-Huangyigou section, starting from the left in Dengzhou Dayang Ying, to the right of Huangyigou Village, Guotan Town, Tanghe, is the distribution area of ??high fluorine groundwater near the opening of the basin in the central plain area. This area constitutes the sink area of ??the regional flow system of the Nanyang Basin. The section extends nearly east-west, with a horizontal distance of about 48km and a vertical height difference of 0 to 8.0m. Larger rivers such as Baihe River, Lihe River, Jianhe River and Tanghe River flow through it from west to east, and there are also many ditches and artificial irrigation canals in between. According to the detection results of groundwater fluoride ion content near the profile line, the groundwater fluoride content in the area east of Tanghe River and the area west of Baihe River is low, which is a low-fluorine groundwater distribution area of ??less than 0.5mg/L. The area from Mengying to Lihepu Town is a distribution area of ??medium-fluoride groundwater. From Lihepu Town to Zhangzhuang, there is shallow high-fluoride groundwater with a fluorine content of up to 2.9 mg/L, which is the highest in the Nanyang Basin groundwater fluorine content. most.

As can be seen from Figure 5-26, the terrain characteristics of this area are high on both sides and low in the middle. There is a local uplift phenomenon in the middle section of the section from Lihepu Town to Zhangzhuang. The local height difference of the micro uplift is 0 to 3.0m. The area from Mengying to Lihepu Town is a depression with a local height difference of 0 to 2.0m.

It can be seen from the depth of groundwater level in Figure 5-26 that the section from Dayangying to Baihe in the west, the section from Lihepu Town to Zhangzhuang in the middle, and the section from Jiamiao to Huangyigou in the east The groundwater level in this section is relatively deep and the vadose zone is thick, ranging from 5 to 8m. The depth of groundwater level in other areas is 4 to 6m.

Local groundwater level changes caused by topographic relief and the hydraulic connection between rivers and groundwater also cause lower-level flow systems to be nested within the local groundwater flow system. It can be seen from the changing trend of groundwater levels that the area from Mengying to Zhangzhuang constitutes the sink area of ??the local groundwater flow system within the profile.

The Tang River and the Bai River flow from north to south in the east and west of the profile respectively. From the rock and soil sedimentation differentiation rules in the river distribution area, it can be seen that there are mostly coarse-grained silty sand deposited near the river, making the The hydraulic connection between groundwater and surface water is closer and more susceptible to the influence of rivers, which is not conducive to the accumulation of salt. In addition, these two areas also constitute the source area of ??the local flow system, resulting in less fluorine content in the groundwater. The area from Mengying to Zhangzhuang is not only the sink area of ??the regional groundwater flow system, but also the potential sink of the local groundwater flow system. Fluorine that migrates with groundwater in distant and surrounding areas gathers here at the same time, so the fluorine content in the groundwater begins to increase.

The reason why the fluorine content in the groundwater from Lihepu Town to Zhangzhuang seriously exceeds the standard, This is mainly because the groundwater burial depth in this area is greater than that between Mengying and Lihepu Town, and the vadose zone is thicker. Under drought conditions, groundwater continuously supplies capillary water to the soil for evaporation. The thick vadose zone enables salt to accumulate in the upper part that supports capillary water, thus providing conditions for the storage of soil salt, and fluorine is therefore stored in the soil. In the air zone. Since the relatively high terrain is more conducive to evaporation, the soil water and capillary water movement trends in the surrounding areas also tilt here when evaporation is strong, leaving behind salt as the evaporation occurs. During precipitation, the leaching of soil by atmospheric precipitation causes fluorine in the soil surface and part of the fluorine stored due to evaporation in the vadose zone to migrate with salt to the groundwater in the lower part of the vadose zone, thus causing a relative increase in the fluoride ion content in the groundwater. Therefore, a large amount of fluoride ions accumulated in the vadose zone soil along with salt constitute to some extent the "fluorine source" of the groundwater, resulting in a higher fluorine content in the groundwater than in surrounding areas.

The relatively thin vadose zone makes the groundwater fluorine content in the area from Mengying to Lihepu Town, which is also the sink area of ??the groundwater flow system, relatively moderate, making it a distribution area of ??medium-fluorine groundwater. Due to the thin thickness of the vadose zone, under conditions of intense evaporation, the supply of soil moisture by capillary water can easily reach the surface, and fluorine can easily accumulate on the surface. Under the influence of atmospheric precipitation, the fluorine accumulated on the surface is relatively easy to migrate into the groundwater, and the influence of surface runoff from ditches and Lihe Rivers has caused this phenomenon.

(2) Figure 5-27 is the Quling-Liuyan section, which crosses the shallow high-fluorine groundwater distribution area near Peiying Township, Dengzhou. This area is distributed in the northwest of the Nanyang Basin plain area. The marginal zone in the lower part, which is close to the hilly land, is not only the sink area for shallow groundwater in this area, but also constitutes the source area of ??groundwater in the center of the plain area in the distance. The horizontal distance of the section cutting is about 20km, and the local terrain height difference is 0~4.0m. The section has Tuanhe River and Yanling River flowing from west to east and Yanling River from northwest to southeast. In addition, there are a few ditches. It can be seen from the distribution of fluorine content in groundwater on the profile that the fluoride ion content in groundwater near Tuanhe River and Yanling River is low, indicating that it is low-fluoride groundwater. The fluoride ion content in the groundwater from Heping Village to Huwan in the middle of the profile is relatively high, exceeding the drinking water standard, ranging from 1.0 to 1.5 mg/L. Other areas have intermediate levels of fluoride in groundwater.

The overall moderate to low groundwater fluorine content in this area is related to the location of the profile. First of all, as the source area of ??the groundwater flow system formed by this area and the central plain area of ??Nanyang Basin, fluorine that migrates with groundwater is continuously transported to the central plain area, thus forming the movement characteristics of fluoride ions that are easy to migrate in this area. Secondly, as a sink for groundwater in surrounding hilly areas, due to the short distance, the water-rock interaction is not sufficient. Although the fluoride ions in the groundwater will accumulate to some extent, their content will not accumulate excessively due to the migration of fluorine, resulting in excessive fluorine content. of groundwater. Therefore, the distribution of medium- and low-fluorine groundwater in this area should be in a dominant position. This can also be seen from the distribution map of fluorine in shallow groundwater in the Nanyang Basin (see Figure 5-2).

The existence of local inter-river depressions causes fluorine to accumulate in the area from Heping Village to Huwan. Since this area constitutes a sink area for a local groundwater flow system formed with the surrounding medium-fluorine groundwater distribution areas, the surrounding medium-fluoride groundwater distribution areas Fluorine in groundwater continuously accumulates. Since there are no large rivers nearby, the replacement of groundwater in this area is relatively slow. The shallow depth of groundwater provides favorable conditions for the evaporation of larger amounts of water. Therefore, the influence of the local groundwater flow system further increases the fluorine content in some areas of the region, forming high-fluoride groundwater.

(3) This mutual nesting and superposition phenomenon between groundwater flow systems at different levels and levels can also be reflected from Figure 5-28. The location of this section in the Nanyang Basin is near the center of the central plain area, on the west bank of Baihe River and Lianhe River. The overall flow trend of groundwater is from north to south.

Because it is close to the center of the basin, this area is the sink of the regional groundwater flow system composed of the mountainous area and the plain area of ??the Nanyang Basin. However, it can also be seen from Figure 5-28 that there are also There are smaller levels of local flow systems.

Due to the influence of rivers, the groundwater levels near the Concrete River, Lianhe River and Baihe River are relatively high, forming the source of a local flow system. In addition, there are good groundwater alternating conditions around the river. Therefore, near the river The fluoride ion content in groundwater is low. Groundwater accumulates in the area from Gangzhuang to Houlingpo on the profile. Similarly, due to the accumulation of fluorine that migrates with groundwater around the Nanyang Basin and on both sides of the profile, relatively strong evaporation conditions, and the 5-9m thick vadose zone, the fluorine Affected by factors such as storage and release, the fluorine content in groundwater from Gangzhuang to Houlingpo exceeds the standard.

4. The replenishment and drainage relationship between medium-deep and shallow groundwater

Based on the available data, water levels of medium-deep and shallow groundwater were drawn (Figure 5- 29), as shown in Figure 5-29, the basic law of the groundwater level in the Nanyang Basin is that the groundwater level gradually decreases from the piedmont zone near the edge of the basin to the center of the basin. Generally, the shallow groundwater level in the piedmont zone is higher than the mid- and deep-layer groundwater levels. The groundwater level in the plain area is roughly the opposite. The groundwater level in the middle and deep layers is higher than that in the shallow layers. There are two local groundwater artesian areas in the middle of the basin. One is located in the area of ??Waizi Town, Bainiu Township and Yingzhuang Town, and the other is located in the area from Tonghe Township to Lidian Township.

Comparing the distribution maps of fluorine in the deep and shallow groundwater in the Nanyang Basin (see Figure 5-2, Figure 5-3) and related to the location of the artesian area, it can be seen that Waizi Town, Bainiu The artesian areas in the township and Yingzhuang Town correspond to the distribution areas of medium and deep low fluorine groundwater and shallow medium and high fluorine groundwater. It can be seen that the medium and deep groundwater in this area can be clearly and directly connected with the shallow water. The chemical type of deep groundwater in this area is HCO3·Cl-Na·Ca, and the chemical type of shallow groundwater is Cl·HCO3·SO4-Ca. Comparing the chemical types of deep and shallow groundwater, we can also see the close connection between the two. It is Because the medium and deep groundwater continuously recharges the shallow groundwater, the Cl- content in the shallow groundwater increases significantly. Therefore, while the medium-deep groundwater brings a certain amount of fluorine to the shallow groundwater, it also dilutes the fluoride ion content in the shallow groundwater in this area to some extent.

Therefore, under the combined effects of surface environmental factors and low-fluoride groundwater in the middle and deep layers, the shallow groundwater in this area shows a phenomenon of local high-fluoride groundwater surrounded by a large range of medium-fluorine groundwater.

The same phenomenon exists in the artesian area from Tonghe Township to Lidian Township, where the medium and deep groundwater supplies part of the fluorine to the shallow water. The fluorine migrates here with the water from the mountainous areas in the north and the ridge areas in the east. And the fluorine provided by deep groundwater creates the possibility for the formation of large-scale medium-fluorine shallow groundwater in this area.

In addition, the middle and deep groundwater levels near Peiying Township are relatively higher than the shallow groundwater levels. There may also be a hydraulic relationship between deep and shallow groundwater, which in turn causes the high fluorine groundwater in the middle and deep layers to affect the shallow groundwater. fluorine content in it.

5. Hydrochemical conditions

As mentioned in the introduction, the groundwater space where the groundwater sampling point is located can be regarded as a multivariate and multiphase open system with obvious nonlinearity. Characteristic hydrogeochemical system. In addition to the direct or indirect chemical connections between various material components within the system, such as rock (soil), water, gas, and biomass, there is also an ongoing exchange of matter and energy between the system and the external environment. According to the system theory, since the regional geological background conditions and climate conditions of the Nanyang Basin are relatively stable, the input and output of the regional groundwater system in the Nanyang Basin are relatively stable, and each component within the system can coordinate the intricate relationships between the components through self-organization. Together, they form a unified structural model, thereby resolving the influence of external interfering factors of the system and maintaining the macroscopic stability of the system. The same is true for hydrogeochemical microenvironments with small spatial scales such as groundwater sampling sites.

Therefore, in addition to finding out the rules of fluorine migration and enrichment in the regional geological background of the Nanyang Basin represented by the macro-scale and medium-scale and the zonal local groundwater flow system, it is also important to understand the water chemical microenvironment. The "multiple types" phenomenon should also be taken seriously, because it is the synergistic effect between the various components in the hydrogeochemical microenvironment in which the groundwater is located that is the key to whether the groundwater at the sampling point is high in fluoride. It is certain that the content and existence form of fluoride ions in different water samples are directly related to the differences between the water chemical microenvironments of different sampling points.

Figure 5-29 Isohydrogram of shallow and medium-deep groundwater in Nanyang Basin

Research on groundwater samples in Nanyang Basin shows that H·C·N·S-Ca·Na ·Mg, H·C·S·N-Ca·Mg·Na, H·S·N·C-Ca·Mg·Na and other hydrochemical types have a relatively high probability of high fluoride groundwater. It can be seen that in these water chemical types, the Ca2+ content dominates the cations, and the reasons why the water samples represented by these water chemical types have a relatively high probability of high fluorine groundwater are roughly as follows: ① Due to Among the statistical water chemical types, the sample size of water chemical types with Na+ content dominating in cations is too small, resulting in poor comparability of statistical results for water chemical types with Ca2+ content dominating, that is, these water chemical types may have high fluoride. The probability of groundwater is not high, and the statistical results are exaggerated due to the lack of effective comparison. ② Since the hydrochemical microenvironment in which groundwater is located is an open environment, the hydrochemical state of groundwater at the sampling point is a "transient state" rather than a "final state". In other words, the distance between each water sample and the stable state is This causes the various reaction processes in the water sample to be asynchronous, and the degree of reaction is also different. Some may already be in the complete reaction stage, and some reactions are still very incomplete. As a result, the components in the groundwater are also different, and the fluoride ion content in the groundwater is also different. Similar chemical reaction stages or degrees may also be an important reason for the presence of high fluorine. ③Since the experimental method used this time measures all soluble fluorine in the aqueous solution, including fluoride ions and fluoride complex ions, and when the Ca2+ content is high, the measured F- may be caused by the complex ions. The released fluorine, while the F- content originally present in the groundwater is not high. ④ There are two sets of dominant reaction equations in groundwater, namely CaF2←→Ca2++2F- and NaF←→Na++F-. Their dominance is not outstanding. There are other coordination reactions and synergy between each component. The effect is not complete enough, leaving many possibilities for fluoride ion concentration in groundwater.

The research on the relationship between the differences in water chemical microenvironment and the fluorine content in groundwater will be described in detail in subsequent chapters and will not be introduced in detail here.

It should be pointed out that the controlling effect of water chemical conditions on the accumulation of fluoride ions in the water chemical microenvironment will also be interfered by other factors in the geological environment background. For example, the existence of fluorine-containing mineral deposits can The fluorine content in the surrounding groundwater is very high. In addition, fluoride exchange between soil and groundwater in localized areas with higher levels of highly mobile fluoride may weaken this controlling effect. Even when water chemical conditions have a strong control over the aggregation of fluoride ions in the water chemical microenvironment, it is also related to various environmental conditions. Therefore, water chemical conditions work together with other factors to affect fluoride. content in groundwater.