Hydrochemical characteristics of groundwater in Tongchuan City, China

In order to reveal the formation mechanism of hydrochemical characteristics in Tongchuan City, we collected and detected 39 samples of underground water. Correlation analytical method, hydrochemical method and ion ratio coefficient method were employed to investigate the hydrochemical characteristics, influencing factors and principles of changing. Results demonstrate that the main factors, controlling the shallow water salinization, are the SO4, NO3, Cl and K. The contents of SO4, NO3 and Cl are greatly different and other indexes are relatively stable. The values of γNa/γCl demonstrate that the Na is released from the soil aquifer in the runoff process. There is an exchange between the Ca in water and Na in soil, which leads to γNa>γCl. As the exchange time in deep water is longer than that of shallow water, the exchange and adsorption of ions are more sufficient. The value of γNa/(γNa+γCl) demonstrates that with increasing of ground water depth, the level of cation exchange enhances, which leads to the dominant cation turning to Ca from Na. The value of γHCO3+γSO4/γCa+γMg demonstrates the shallow water is mainly from atmospheric rainfall and the influence of cation exchange is more obvious on deep water. The types of shallow water in Tongchuan City are mainly HCO3·SO4-Ca. Its classification is relatively complex. Whereas, the deep water is mainly HCO3-Na and the category of underground water is simple. As for shallow water, the content of alkaline earth metal is larger than that of alkali metal. For the deep water, the content of weak acid group is larger than that of strong acid group. In the region of upper reaches, the features of both shallow and deep waters are mainly weak acid and alkaline earth metal. The chemical composition mainly consists of carbonate leaching. With the flowing of underground water, the shallow water turns to be strong acid and alkaline earth metal and the deep water turns to be weak acid and alkali metal.


INTRODUCTION
The composition and distribution of hydrochemistry are direct proofs to study the formation and change of underground water.They are basic principles for exploitation and planning.The composition and concentration distribution of hydrochemistry are formed in the development of geological history.Some geochemical conditions play important roles in the process of formation, such as the topographical distribution, meteorological and hydrological changes, geological structures, changes of hydrological conditions and human activities.Through hydrochemical information, we can explore the features of occurrence environment, runoff channels and material exchanges, which can help us to reveal the cycle principles of underground water (Florian, 2011;Ravikumar et al., 2010).
Tongchuan City is an industrial city with plenty of coal mines.Its coal and building-material industries are highly developed.So it is an important industrial base for energy and raw materials in northwest of China.In the process of development, there are large populations settled down.What is more, the scales of industry and agriculture, commerce and tourism have increased rapidly.These changes have propelled a high demand for underground water.However, the underground water has been seriously destroyed, which has affected the living water, industrial water and agricultural water.The study of underground water hydrochemistry is the main contents of water resources quality evaluation, which has an important meaning in water resources utilization and management together with the protection of the ecological environment.
In this paper, we collected underground water with the principle of scientific system.Then, relative hydrogeochemical parameters were studied.Descriptive statistics, correlation analysis, ion ratio coefficient method and Piper diagrammatic method were employed to study the characteristics of spatial and temporal variability together with the evolution rules.We also revealed the hydrochemical procedure of underground water quality evolution, which provides scientific proof for water resources protection and rational development.

Overview of the research area
Tongchuan City is located in the transitional zone of Guanzhong Plain to loess plateau of Shanxi Province (E108°34′~109°29′, N34°50′~35°34′), with an area of 3882 km 2 .Tongchuan City has a warm temperate continental climate, which is dry and cold in winter and with scarce rain and snow.Affected by the warm moist air mass of Pacific Ocean, it is hot and humid with relative abundant rain in the summer.There are usually droughts in spring and water-logging in autumn.The topography of Tongchuan City is north-west high and south-east low, which results in the obvious difference of climate.The average precipitation is 555.8~709.3mm and the average temperature is 8.9~12.3°C.

Collection and analysis of samples
In August 2011, we collected 39 samples of underground water in Tongchuan City, which consists of 25 groups of shallow water (well total depth less than 30 m) and 14 groups of deep water (well total depth more than 100 m), which are shown in Figure 1.The parameters to be detected include HCO 3 -, SO 4 2-, NO 3 -, Cl -, Ca 2+ , Na + , Mg 2+ , K + and total dissolved solids (TDS).The detection of samples was carried out in underground mineral water and environmental monitoring center of the Ministry of Land and Resources.The equipments used are as follows: flame atomic absorption spectrophotometry was employed to detect the concentration of K + and Na + ; Ion chromatography was used to detect SO 4 2-and Cl -; ethylenediaminetetraacetic acid (EDTA) titrimetry was used to study HCO 3 -; Gas phase molecular absorption spectrometry was used to detect NO 3 -; The value of TDS is the sum of ion concentration; we use SPSS18.0 and AquaChem3.7 were used to analyze the data.

Descriptive statistics
The negative ions of underground water in Tongchuan City mainly consist of HCO 3 -, SO 4 2-, NO 3 -and Cl -(as shown in Table 1).Especially, the concentration of HCO 3 is the most high (in the range from 213 to 621.7 mg/L), with an average value of 353.11 mg/L.The contents of other ions are as follows: SO 4 2-(in the range from 13.18 to 897 mg/L) has an average value of 221.95 mg/L; NO 3 -(in the range from 6.98 to 482.5 mg/L) has an average value of 96.61 mg/L; Cl -(in the range from 5.25 to 208.3 mg/L) has an average value of 53.06 mg/L.The positive ions of underground water mainly consist of Ca 2+ , Na + , Mg 2+ and K + .Especially, the concentration of Ca 2+ is most high (in the range from 12.71 to 457.2 mg/L), with an average value of 129.32 mg/L.The contents of other ions are as follows: Mg 2+ (in the range from 15.64 to 126.5 mg/L) has an average value of 42.99 mg/L; K + (in the range from 0.31～31.6mg/L) has an average value of 3.21 mg/L.The value of TDS is in the range between 305.8 and 2343 mg/L, with an average value of 821.59 mg/L.For all the samples, 71.79% are fresh water and 28.21% belong to the brackish water.From the variation coefficients of components, we can conclude that NO 3 has the highest variation coefficients in negative ions, which suggests that there is a large distribution in the contents of NO 3 -.There are also a large variation in the contents of SO 4 2-and Cl -.However, the variation coefficient of HCO 3 -is the smallest, which suggests that its content is relative stable.In the four positive ions, K + has the highest variation coefficient, which suggests it has a wider distribution difference than others.The changing degree of negative ions is larger than that of positive ions.
There are some differences between the hydrochemical characteristics of shallow water and deep water.As for the negative ions, the concentration of HCO 3 -and NO 3 show no obvious difference between the shallow water and deep water; the concentrations of SO 4 2-and Cl -are 3 and 2 times higher than that of deep water respectively.Compared with deep water; the proportions of HCO 3 -and NO 3 -in negative ions are lower and the proportions of SO 4 2-and Cl -are higher.As for positive ions, the average contents of Ca 2+ , Mg 2+ and K + are all higher than that of deep water.However, the concentration of Na + is lower than deep water.Ca 2+ possess the maximum percentage in shallow water, which is 54.86% and Na + is the maximum one in deep water, which is 55.47%.What is more, the average value of TDS in shallow water is 1 times more than that of deep water, which suggests that there is a high ion content in the shallow water.
Generally, the ion variation coefficient of shallow water is higher than that of deep water.Among them, K + has the biggest change, which suggests that there is a more obvious variation in shallow water than that in deep water.This is caused by the aquifer medium, topography, hydrology-weather conditions and human activities.There shallow water, which demonstrates that they have relatively stable features.Among them, HCO 3 -has a is a smaller variation for HCO 3 -, Ca 2+ , Na + and Mg 2+ in maximum average value, with a minimum variation coefficient, which suggests that it has a higher relative contents and it is the main negative ions.There are higher variation coefficients in SO 4 2-, NO 3 -, Cl -and K + , which suggests they are sensitive to the change of environment, with bigger change in shallow water.In deep water, there are bigger variation coefficient in SO 4 2-, NO 3 -and Cl -.We can conclude that they have a bigger contents change and a smaller variation in shallow water.

Correlation statistics
Correlation statistics can be used to reveal the similarity,     mainly controlled by SO 4 2-, Cl -, Ca 2+ , Na + and Mg 2+ .There are perfect positive correlations between Ca 2+ , Mg 2+ and SO 4 2-, Cl -, which suggests that Ca 2+ and Mg 2+ are mainly from sulfate and chlorate.There is a strong positive correlation between Na + and Cl -, which suggests that Na + and Cl -are mainly from chlorate.There is also positive correlation between K + and NO 3 -, which demonstrates that they are from the same source (Dudeja et al., 2011).There is a strong positive correlation between TDS and Mg 2+ and it also has some relationships with SO 4 2-.The effects of other indexes are relatively slight.All of the above results demonstrate that in the deep water TDS is mainly controlled by Mg 2+ and SO 4 2-.Consistent with shallow water, there are perfect positive correlations between Ca 2+ , Mg 2+ and SO 4 2-, Cl -in deep water.Different from shallow water, there is no positive correlation between Na + and Cl -.There is also no relationship between K + and other ions in deep water.samples of underground water in Tongchuan City can be divided into 22 types as shown in Table 3.There are various categories of hydrochemical types.Among them, the type HCO 3  3.We can conclude that the types of water chemistry of shallow groundwater are complex and the deep water is relatively simple.Generally, the hydrochemical types of shallow water are simpler than that of deep water.This is caused by the longer time of leaching and replacing, which makes the composition of water more complex.The adverse consequences are caused by the contamination of shallow water.

Features of the hydrochemical evolution
We obtained the Piper trilinear chart using software of 'information system of groundwater pollution investigation' as seen in Figure 5.It can be concluded that there are obvious differences in the chart of shallow and deep waters.In the shallow water, the positive ions mainly consist of Ca 2+ and the concentration of Na + and K + are relative low.In deep water, the positive ions mainly consist of Na + and K + .The concentration of Mg 2+ and Ca 2+ are relative low.The negative ions of shallow water

Hydrochemical type Sample number Ratio (%)
Shallow groundwater Deep groundwater mainly consist of HCO 3 -and SO 4 2-.The concentration of Cl -is relatively low.In deep water, the negative ions mainly consist of HCO 3 -and the concentration of Cl -and SO 4 2-are relatively low.Along the direction of flow, the change of negative ions in deep water is not obvious.The ratio of positive ions increases and the concentration of Ca 2+ and Mg 2+ decrease.With the increase of runoff channels, the Na + in water-bearing media has exchanged with Ca 2+ and Mg 2+ in water.The type of underground water turns to be HCO 3 -Na from HCO 3 -Mg•Ca•Na and HCO 3 -Ca•Na.Along the direction of flow, there is no obvious exchange in the positive ions of shallow water and the change of negative ions are obvious.With the increasing of SO 4 2-, the concentration of HCO 3 -decreases and the concentration of Cl -is almost the same.The increasing of SO 4 2-means the contamination is more serious along the flow, which has a relationship with the city and industrial zone in the downstream region.The types of water turn to SO 4 -Ca•Mg, SO 4 •HCO 3 -Ca•Na from HCO 3 -Ca, HCO 3 -Ca•Mg.
Generally, as for underground water, the containment of alkaline earth metal is larger than that of alkali metal.For the deep water, the containment of weak acid is larger than that of strong acid.In the upstream area, both the carbonate hardness of shallow and deep waters are higher than 50%.The feature of water is mainly weak acid and alkaline earth metal, which suggests that the formation of water is mainly from carbonate leaching.With the flowing of water, carbonate hardness of deep water becomes higher than 50% and the content of alkali metal is larger than alkaline earth metal.The underground water changes into weak acid and alkali metal.

Distribution characteristics of hydrochemical type
The underground water of North Ziwuling mountain area is mainly bedrock fissure water.The most common type of water is HCO 3 •SO 4 -Ca.In this area, the water is mainly provided by atmospheric precipitation.The runoff is beyond compare and the stay time is relatively short in aquifer.The water is easy to exchange with the environment and the soluble components, such as Cl -, Na + and K + .Most of the ions are Ca 2+ , HCO 3 -and SO 4 2-, which can form the water type of HCO 3 •SO 4 -Ca.The underground water of Qishui River Valley Area, from south Zhifang to stream outlet, is alluvium pore water.From upstream to downstream, the water type changes into HCO 3 •SO 4 -Ca•Na from HCO 3 •SO 4 -Ca.Especially, the composition of some areas are complex and the water type is HCO 3 •SO 4 -Ca•Na•Mg.The contents of negative ions are relatively stable and the concentration of Na + gradually increases.The value of TDS is also increased.The underground water of Wang River Valley, from south Aipu village to stream outlet, is shallow fissure water.From upstream to downstream, the water type changes into SO 4 •HCO 3 -Ca•Mg from HCO 3 •SO 4 -Ca.
There is an increase in the concentration of SO 4 2-and Mg 2+ .The TDS rises from 621.1 to 1318 mg/L.The underground water of Guangyang River Valley, west of Guangyang town, is alluvium pore water.From upstream to downstream, the water type changes into SO 4 •HCO 3 -Ca•Mg from HCO 3 -Ca.There is a gradual increase in the concentration of SO 4 2-and Mg 2+ .The TDS rises from 458.4 to 1586 mg/L.The underground water of southern Sichuan area is mainly alluvial pore water.Most of the water is deeply confined water and the water type is mainly HCO 3 -Na.The underground water of valley area is mainly HCO 3 -Ca•Na•Mg.The type of underground water is simplex.The TDS is in the range from 400 to 600 mg/L.The concentration of Na + is high, which is mainly affected by loess components.The water yield for gully region of Loess Plateau is small.The sample is relatively shortage.After reading papers, we know that the water type is mainly HCO 3 -Ca•Mg.There are other types, such as HCO 3 •SO 4 -Na•Mg, HCO 3 -Mg•Ca•Na, HCO 3 -Mg•Na•Ca and SO 4 •HCO 3 -Ca.The negative ions are mainly HCO 3 -.The sample of karst water is in relative shortage.After reading papers, we conclude that the water types are mainly HCO 3 •SO 4 -Na•Ca•Mg and HCO 3 •SO 4 -Ca•Na•Mg.The water type of Ju and Qishui Rivers are both HCO 3 •SO 4 -Ca.There are close connections between the water and shallow water, which is an important reason for the high concentration of SO 4 2-.It is also the source of SO 4 2-in karst water.Recently, with the exacerbating of contamination, the name of some area stems from its region.When considering hydrogen nitrate using the method of Shug Kalev, there are six groups of samples that belongs to hydrogen nitrate types.Telling from the distribution, it is mainly focused on the area of southern Chuanyuan and the area along the QI River.The area of southern Chuanyuan is the main farmland area.Compared with loess tableland, it has more water and the land has more fertilizer.The pollution is more serious than that of loess tableland.Above factors may act as the reasons for the forming of hydrogen nitrate type.

Conclusion
The salinization of shallow underground water in Tongchuan City is determined by the SO 4 2-, NO 3 -, Cl -and K + .In deep water, the contents of SO 4 2-, NO 3 -, Cl -are largely different and other indexes are relatively stable.The values of γ Na /γ Cl demonstrate that the process of runoff makes the Na + release from soil and aquifer and there is exchange between the Ca 2+ in water and Na + in soil, which leads to γ Na >γ Cl .Due to long time leaching and replacing, the cation exchange adsorption is very sufficient in deep water.This leads to a higher γ Na /γ Cl value than that of shallow water.The value of γ Na /(γ Na +γ Cl ) demonstrates that with the increasing of underwater depth, the cation exchange adsorption increases, which leads to the advantage ion change into Na + from Ca 2+ .The ratio of γ HCO3 +γ SO4 and γ Ca +γ Mg demonstrates that the shallow water mainly stems from rainfall and the effects of cation exchange are more obvious in deep water.
The type of underground water in Tongchuan City is mainly HCO 3 •SO 4 -Ca.Its hydrochemical types are relatively complex.It is mainly HCO 3 -Na in deep water.As for the shallow water, the content of weak acid is larger than that of strong acid.In the upstream area, the shallow water mainly consists of weak acid and alkaline earth metal.Its components are mainly carbonated leaching.With the flowing of water, the shallow water changes into strong acid and alkaline earth metal and the deep water change into weak acid and alkali metal.

Figure 1 .
Figure 1.Location and detailed land use regionalization of the study area [groundwater sampling sites are shown as triangles (deep groundwater) and circles (shallow groundwater)].

Figure 5 .
Figure 5.The piper diagram for the groundwater samples.

Table 1 .
Summary statistics of the analytical data and groundwater samples of the study area.

Table 2 .
Correlation matrices of hydrochemical parameters of groundwater in the study area.
In shallow water, there is strong positive correlation between TDS and SO 4 2-, Cl -, Ca 2+ , Na + and Mg 2+ .However, HCO 3 -, NO 3 -and K + have no effects on TDS.Above results demonstrate that TDS is (Kshetrimayum and Bajpai, 2012)difference of underground water hydrochemical characteristics(Kshetrimayum and Bajpai, 2012).The Pearson correlation coefficients of shallow and deep waters in Tongchuan City are calculated as shown in Table2.