4.1. Chemical Compositions of Groundwater
All of the parameters were statistically analyzed to provide general chemical characteristics in groundwater. Table 2
exhibited the minimum, maximum, mean, and the standard deviation for each parameter. The permissible limits for drinking purposes set by World Health Organization [12
] and Chinese regulation [43
] were involved.
Groundwater in this area was slightly alkaline to neutral, as the recorded pH values ranged from 7 to 8.5, with a mean value of 7.8. The pH values were within the permissible limits (6.5–8.5) set by WHO and the Chinese standards at all sites. The TDS parameter is generally used as a measure of water palatability, because high TDS may distort the taste of the water [11
]. Freshwater with TDS < 1000 mg/L is regarded as being suitable for drinking [11
]. In the study area, the TDS values ranged from 468 to 2560 mg/L with a mean of 1225 mg/L, and the results indicated that 62% of groundwater samples exceeded the permissible limit for drinking purpose.
The concentration of cations was in the order of Na+
. The sodium values in groundwater ranged from 58.5 to 691 mg/L. Fourteen samples had high levels of Na+
, exceeding the permissible limit for drinking purposes, based on the WHO standards (>200 mg/L). Sodium was dominant, but highly variable, because the standard deviations were larger than the mean value (Table 2
). Calcium and magnesium are common elements in water. The concentrations of Ca2+
were in the ranges of 30.9–255 mg/L and 21.3–116 mg/L, respectively. A total of 92% and 96% of the samples had higher values of Ca2+
, which were beyond the acceptable limits of WHO (75 mg/L, 30 mg/L), respectively. This implies that hard water (caused by compounds of Ca2+
) may contribute to scaling in boilers and industrial equipment. The concentration of K+
was quite lower, ranging from 1.72 to 16.3 mg/L in the study area. Higher K+
levels (>10 mg/L) were observed in the three samples.
Anions dominances of the groundwater were observed as SO42−
. Chloride is an extremely stable element in water, which may be derived from weathering, the leaching of sedimentary rock and soil, and domestic effluents [44
]. The observed concentration of Cl−
was between 49.7 and 553 mg/L, with a mean of 148 mg/L. The majority of the groundwater samples were suitable for drinking, but the Cl−
levels in six samples were beyond the permissible limit (250 mg/L). SO42−
concentrations were from 82 to 986 mg/L in the study area. The maximum permissible limit for sulfate was 250 mg/L. A total of 58% of the samples were above the threshold for drinking purposes. Interestingly, the groundwater samples showed large variations in both SO42−
), and abnormally high concentrations of SO42−
were measured in the W1, W34, W41, and W45 samples. The observation may imply the adverse impact of sewage or effluent on groundwater quality. These findings were consistent with the results of the hydrogeochemical characteristics of groundwater carried out in the alluvial plain [9
]. The levels of HCO3−
in the groundwater samples were between 170 and 661 mg/L, with a mean of 459 mg/L. Results showed that 90% of the groundwater samples had higher levels of HCO3−
, exceeding the permissible limit (>300mg/L).
As an essential trace element in human body, fluoride concentration in drinking water should not exceed 1.5 and 1.0 mg/L, as set by WHO and the Chinese standards, respectively. Long-term exposure to high-fluorine water may cause adverse effects such as dental or skeletal fluorosis [45
]. In the study area, the concentrations of fluoride ranged from 0.11 to 6.33 mg/L, with a mean of 0.85 mg/L. Four and 11 samples located in the southern part of the region were beyond the acceptable limits of 1.5 and 1.0 mg/L, respectively. This may be probably attributed to the dissolution of fluoride bearing mineral.
The nitrate concentration of groundwater in agricultural regions reached in high levels in recent decades, due to the intensive application of chemical fertilizer [13
]. As shown in Table 2
, the NO3−
-N concentrations in the groundwater samples ranged from 2.66 to 103 mg/L. Thirty and 14 samples showed higher concentrations when compared to acceptable limits for drinking purpose set by WHO (10 mg/L) and Chinese regulations (20 mg/L), respectively. The nitrate concentrations in W32 and W37 even reached to 67.9 and 103 mg/L, respectively. Both irrigation activities and fertilizer application may be likely to create a blanket non-point source of nitrate, even though there was no significant correlation between the nitrate level and the land use in the study area.
4.2. Hydrogeochemical Facies
The Durov diagram [40
] is a useful graphical tool for representing hydrogeochemical data. As shown in Figure 2
, the majority of the samples, with respect to cations, were observed in B (no dominant), and filed in the left triangle. The cations in the groundwater were predominated by the mixed type. Only three samples were plotted in the field of D (sodium type in Figure 2
), suggesting the dominance of (Na + K) in the groundwater. On the contrary, distinct difference was detected in anions for the groundwater. The red filled circles in the field of E belonged to the bicarbonate type, indicating the dominant anion of HCO3−
. These samples were mainly characterized by fresh water (TDS < 1000 mg/L), which were located in the recharge zone of the aquifer (the southern part of the study area). The other samples (blue and green filled circles) belonged to the brackish water group with the sulfate and mixed water type, indicating the combined influences of evaporation, water–rock interaction, and/or human activities.
4.4. Groundwater Suitability for Drinking
EWQI was used to assess groundwater quality for drinking purposes in the study area. The role and contribution of each parameter in water quality assessment were described, with the weights based on entropy information. Parameters with a maximum entropy weight and a minimum entropy value had the most effect on water quality. As shown in Table 3
and TDS had more significant effects on groundwater quality in the study area, because these ions had the lower entropy values and higher entropy weights. The entropy weights were 0.159, 0.131, 0.13, 0.119, and 0.107, respectively. Fluoride had the lowest entropy weight (0.057), indicating its minimal influence on drinking water quality. The influence of the parameters on water quality decreased in the order: Mg2+
> TDS > Na+
-N > F−
The calculated EWQI values of groundwater were between 45.6 and 231, with a mean value of 112. Of the 50 samples, one sample (W44) was of extremely good quality, and 25 were of good quality, which were suitable for human consumption. However, 48% of the groundwater samples cannot be directly used for drinking purposes. Eighteen samples had medium quality which should be used for drinking purposes before proper treatment. Besides, two samples (W32 and W45) were of poor quality, and four samples were classified as extremely poor-quality water (W1, W34, W37, and W41). The spatial variation of groundwater quality for drinking purpose was shown in Figure 8
a. As mentioned above, the greater concentrations of SO42−
were measured in W1, W34, W41, and W45, and the samples of W32 and W34 were characterized by higher concentrations of NO3−
-N. The greater EWQI values reflected the poor water quality in these sampling locations. Given the scattered points, human activities may probably be responsible for water quality deterioration in these sampling sites. Therefore, more attention should be paid on the variation of sulfate, chloride, and nitrate concentrations in further groundwater management.
Currently, the Chinese government has taken efforts to address the situation by implementing various technical and financial programs to ensure drinking water safety. A major policy plan on water pollution control and clean-up (“Water Ten Plan) was adopted in 2015 [65
]. Due to the government-led improvements in water supply and safe drinking water initiatives, it has had a profound influence on the health and livelihoods of millions of Chinese people and the environment [66
]. In 2017, the addition of 54 chemical parameters was made to the revised national standard for groundwater quality [43
]. In this context, findings in this study will contribute to a better understanding of groundwater sustainability in further water management and protection.
4.5 Groundwater Suitability for Irrigation
As the suitability of groundwater for irrigation purposes also depends on its chemical constituents, the salinity hazard (TDS), SAR, Na%, and PI were used to assess the water quality for irrigation. According to the calculated parameters, the groundwater can be classified into various classes, which are shown in Table 4
Given the intense evaporation and shallow water depth, salt is prone to accumulate in soil in the study area. It may induce salinity hazards, because the roots are unable to absorb water with high concentrations of salt in soil. In general, water salinity is measured by the EC (electric conductivity) or TDS [68
]. As shown in Table 4
, only one sample was good for irrigation, due to the relative lower values of TDS. The majority of samples (36 samples, 72% of the total samples) were permissible for irrigation (Figure 8
b), but nine and four samples belonged to doubtful and unsuitable categories, respectively. This is mainly attributed to the intense evaporation and intensified irrigation activities in the region.
SAR is also considered to be an important parameter for determining the suitability of groundwater for agricultural use. It can be used to indicate alkali/sodium hazards to crops, because the parameter can express the degree of cation exchange reactions in soil when sodium replacing the absorbed magnesium and calcium occurs [69
]. Based on the classification by Richards [70
], water with SAR values of less than 18 indicated a suitability for irrigation, which did not have, or had a low sodium hazard. The range of 18–26 indicated that the water was harmful for almost all types of soil. If the SAR value was beyond 26, the water was unsuitable for irrigation. The SAR values were in the range of 1.19–12.76 for all of the groundwater samples in the study area. As shown in Table 4
and Figure 8
c, 96% of samples belonged to the excellent category, and the remaining samples belonged to the good category.
The sodium percentage is an important factor to indicate the sodium hazard in irrigation water. Sodium tends to displace magnesium and calcium ions in the soil when the concentration of Na+
is high in irrigation water. This exchange process decreases the permeability in soil, and eventually restricts the air and water circulation [71
]. Thus, irrigation water with a high Na % may deteriorate the soil structure and reduce its aeration and permeability, causing adverse impacts on crop growth [72
]. Wilcox [73
] proposed the classification of water quality for irrigation water (Table 4
). As shown in Table 4
and Figure 8
d, the majority of the groundwater samples were in the permissible category for irrigation purposes. Only six samples had 60% to 80% sodium, showing adverse effects on soil permeability and texture. Two samples (W45 and W48) were harmful for crops, because of their unsuitable sodium percentages.
In addition, PI values can present the permeability of the soil that is affected by Na+
, and HCO3−
, and the soil type. Doneen [74
] proposed a permeability index (PI) and classified it into three categories (Table 4
). Based on the classification, water with >75% (Class I) PI was good for irrigation, 25–75% (Class II) was suitable, and <25% (Class III) was unsafe water. The decrease of PI has harmful effects on plant growth. It can be seen form Table 4
and Figure 8
e that all the samples were suitable for irrigation in the study area. A total of 14% and 86% of the groundwater were designated as Class I and Class II, respectively.
A comparison of the spatial variation of these indices indicated that majority of the samples were permissible for irrigation purposes. However, 26% samples were doubtful and unsuitable for irrigation, because of the high salinity in the water. Therefore, more attention should be paid on groundwater quality monitoring (particularly for water salinity) in the Zhongning area, for ensuring dependable and affordable groundwater, and protecting the quantity available for future use.