Imperfections of traditional research methods

There have been many researchers who have realized that the formation mechanism of high fluorine groundwater should be studied from the viewpoint of hydrogeochemistry, and they have already done considerable work, formed some research methods and achieved some results, and there are three kinds of most common research means.

1. Soil column leaching experiment

Zeng Splash Hui, Yang Junyao, et al. will be the study area of unsaturated zone soil profile by a certain proportion of the reduction into a certain diameter of the soil column, and then layered into a Plexiglas cylinder and compaction, the lowest part of the water-bearing fine sand. Distilled water with similar chemical composition and pH value to atmospheric precipitation was generally used as leachate instead of atmospheric precipitation. Finally, the obtained leachate was tested for K+, Na+, Ca2+, Mg2+, Cl-, , , F- and pH, and then analyzed. Through the experiments, Splash Hui Tsang concluded that the main process of atmospheric precipitation infiltration is manifested as the transfer of fluorine and chemical components from unsaturated zone soil body to water. The unsaturated zone soil body is an important source of fluorine supply to shallow groundwater, which has an important influence on the migration and enrichment of shallow groundwater. According to Yang Junyao, the migration of fluorine in the unsaturated zone is dominated by the interphase migration between water and soil, and the vertical displacement caused by hydrodynamic action is relatively weak.

2. Chemical analysis of groundwater samples

In the process of studying fluorine, researchers invariably measured the chemical indexes of the groundwater samples, and measured some major ions contained in the samples as well as the degree of mineralization, pH and so on. It is then common practice to analyze the correlation between F- and various ions or the ratio of certain ions, and to derive correlation coefficients, regression equations, and correlation diagrams between them. Some representative conclusions are: Jin Qiong et al. studied the environmental characteristics of fluorine in the Hexi Corridor area of Gansu Province, indicating that the enrichment and depletion of fluorine are closely related to the water chemistry characteristics, and the generally low calcium-poor alkaline water environment is favorable for fluorine dissolution and enrichment; Liu Ruiping et al. researched the geochemical pattern of water fluorine in the Dali area of Guanzhong Province, and found that Na+, Cl-, and pH in the water chemistry components are positively correlated with fluorine, and Ca2+ is inhibited the increase of fluorine in water; Zeng Ruiping et al. found that F- and the ratio of F- to various ions or certain ions are correlated. and Ca2+ are positively correlated with fluorine, while Ca2+ inhibits the increase of fluorine in water; Zeng Splash Hui et al. studied Xingtai Plain in Hebei, China, and concluded that the preservation conditions of fluorine depend on the chemical composition characteristics of shallow groundwater, in which highly fluoridated groundwater is mostly formed in the water chemistry type which is conducive to the fluorine enrichment and migration, and the correlation coefficients between fluorine and the milligram equivalence percentage of Ca2+ are from -0.458 to -0.749, and the correlation coefficient between the (Na+++Mg2 +)/Ca2+is positively correlated with fluorine; Ding Dan et al. took fluorine ions in the shallow water of the Huaibei Plain as the object of their study, and used the method of statistical analysis to study the chemical environment of fluorine ions' storage, and the results showed that the concentration of fluorine ions increased with the increase of pH in groundwater, but there was no obvious correlation with the K+++Na+concentration, Ca2+concentration, and Mg2+concentration; Guo Tianhui studied the hydrogeology of fluorine in groundwater of the pre-summit of the Faceted Mountains in Lingwu, Ningxia. The hydrogeochemical characteristics of fluorine in groundwater were studied by Guo Tianhui, who concluded that the fluorine content in groundwater was not obviously related to the water chemistry type, but positively correlated with pH and sodium-calcium ratio; Wang Genxu et al. studied the environmental characteristics of fluorine in Northwest Arid Zone, and the results showed that fluorine ions are easy to gather in alkaline environment and showed an approximate positive correlation with the mineralization degree, but it did not reflect the definite positive relationship, and they also concluded that fluorine ions' fugacity environment is also related to hardness, Ca2+ and Na+.

3. Groundwater chemical simulation

The geochemical simulation of groundwater based on the theory of chemical equilibrium is usually carried out by using the equation of mass conservation, based on the thermodynamic model and the water chemical analysis data, to determine the concentration of the forms of the components present in the groundwater, as well as the tendency of the dissolution or sedimentation of the minerals summarized in the water-phase environment. A deeper step is the use of mass balance reaction models to explain the pattern of change in groundwater quality, to study the mixing of groundwater and to predict the evolution of water chemistry.

These research methods have some reference significance to our study of the formation mechanism of high-fluoride groundwater, but these methods still have some theoretical deficiencies.

First, it is based on chemical equilibrium, an equilibrium state. An equilibrium state is a state in which the properties of a system do not change with time under conditions of no external influence. Thus, chemical equilibrium is the conclusion of an isolated system. Its laws are all characterized by chemical reaction equations for the process of its formation and chemical molecular formulas for the products of its formation, which are themselves the result of chemical equilibrium, the end state. This equilibrium state does reflect a possible trend at a given point, but not its essence. However, for the groundwater system, which is an open system, none of the places confronting it is a final state, none of the products of a static, unchanging equilibrium state. In the process of groundwater movement, it constantly receives recharge as well as discharge to the outside world, in addition to the pH value of each place, temperature, and the open conditions of the aquifer at different depths and different parts of the aquifer are not the same, any point to participate in the reaction between the upstream and downstream relationship, that is, this open system has both input and output. It is a dynamic process that always tends to this end state but never reaches it, and it represents a stable dynamic change process formed by local natural conditions over many years, but it is not an equilibrium state. In what position the formation of what substances, it depends on a variety of components, a variety of physicochemical conditions, that is to say, there are a variety of elements of random changes *** with the results of the action.

Secondly, in various experimental processes, the concentration and quantity of reactants, the temperature during the reaction, and the reaction products are known, and the chemical reaction system in the experiment is generally a single physical phase state, with fewer types of reaction elements. However, the chemical reaction system, as an open system, is chemically a multivariate multiphase system. We have not yet been able to distinguish which are physical or chemical processes. So we have no way to analyze the various components of the groundwater itself, their quantities, the formation of the combination of states, fluorine in the form of simple ions or complexes are not yet identified.

Thirdly, there is material exchange between the phases in groundwater, and there is material input or output in the transportation process, so the content of the components in the water samples does not represent all the products of the high-dimensional multi-dimensional multiphase chemical system, and thus it is difficult to provide the complete information of the reaction, which leads to the uncertainty and subjectivity in the results of the study.