2.1. Study Area
Mount Emei is located in Leshan City, Sichuan Province, China. It is one of the four famous Buddhist Mountains and is a Chinese national key cultural relics’ protection unit, national key scenic spots, and has been listed on the UNESCO World Cultural and Natural Heritage List since 1996. It is a typical block mountain formed by the Himalayan movement and the accompanying uplift of the Qinghai–Tibet plateau. The elevation of the western mountain area is over 3000 m above sea level. The annual average rainfall in the mountainous area is 1786 mm.
The study area of the Gaoqiao diluvial fan is located at the eastern foot of Mount Emei, as shown in
Figure 1. It is 3 km away from the top of Mount Emei, and the elevation on average is 450 m above sea level, belonging to the prospective reserve zone of Mount Emei. Luomuzhen is a secondary scenic spot located in the middle of the Gaoqiao diluvial fan.
The study area is approximately 16 km
2 and the longitudinal length NW–SE extending is about 7 km. The Quaternary stratum (Q
4, Q
33, Q
32, Q
31, etc.) is a widely horizontal distribution. The deposits in the study area are mainly basalt, sandstone, and dolomite, with sand and clay intermixed. The thickness of the accumulation layer is large, and the stratum separation in the vertical direction is poor. The rich groundwater of the study area comes from rainfall and lateral recharge from the mountainous area and discharges from the middle and edge of the fan with wells and overflow springs. The average shallow groundwater level in the study area is shown in
Figure 2.
From the top to the edge, the fan is divided into three parts according to the gravel particle size of the material composition. In the top area, the gravel particles were typically about 20–50 cm in diameter; in the middle area, the size was much smaller such as gravel, sand, and silt; and in the edge area, the smallest compositions were found such as sub-sandy soil, and sub-clay. The material compositions of the three parts of the fan are shown in
Figure 3.
The groundwater of the Gaoqiao diluvial fan provides a water source for industrial, agricultural, and residential use. Before 1990, there was a phosphate fertilizer plant, lime mine plant, and cement plant, forming industrial pollution sources, which resulted in the deterioration of groundwater quality [
3]. The phosphate fertilizer plant had produced calcium, magnesium, phosphorus, and potassium fertilizer from phosphorite ore with dolomite as a cosolvent, coking coal, and white coal by the blast furnace method since 1964. The annual output had increased from 20,000 tons in 1964 annually to 130,000 tons in 1984. Due to the high fluorine content of the raw materials and fuels, the backwardness of the production technology, a low chimney, and the absence of exhaust gas and water recovery equipment, a large amount of fluoride was discharged into the ecological and groundwater environment during the production process from the plant. The content of fluorine in the wastewater, measured by the environmental protection department in 1978, was as high as 45 mg/L. Coercive measures had been taken to eliminate fluorine in the wastewater of the plant from 1980 to 1996. However, the “dental fluorosis” of the villagers in Yucun, 100 m downstream of the plant, was still showing heavy pollution of fluorine in the groundwater from 1980s. All the industrial pollution sources had been banned since 1996 and the groundwater environment was slowly recovering.
2.2. Sample Collection and Analysis
The historical water quality data of hydrochemical components in 1995 was obtained from paper [
24]. A total of eight groups of water samples were collected from the shallow groundwater of the fan in August 2016. The points were sampled at a similar area of the fan between 1995 and 2016, shown in
Figure 2. The sampling points were distributed in the vertical and horizontal sections from the recharge area to the discharge area of the Gaoqiao diluvial fan according to the flow direction, as shown in
Figure 3. The hydrological characteristics of sampling points were shown in
Table 1. During the process of filtering samples, blank samples (deionized water), and parallel samples were taken into consideration to control the reliability.
Water samples were tested by SiChuan Tianshengyuan Environmental Limited Corporation. The test method used to detect the presence of K+, Na, Ca2+, Mg2+, and Fe was inductively coupled plasma atomic emission spectrometry; the method used to detect F, Cl−, and SO42− was ion chromatography; and the method used to detect HCO3− was the acid–base titration. The measurement method used to obtain the total dissolved solid was a weighing method. The total hardness (measured by CaCO3) was titrated with an EDTA (Edta direct titration method) titration method, and the test method for ammonia nitrogen (measured by N) was nashi reagent spectrophotometry.
2.3. The Principle of Five-Element Connection Number SPA in Groundwater Assessment
Set pair analysis (SPA) is used to deal with the problem of determination uncertainty. The essence of this modified uncertainty theory is to treat the certainties and uncertainties as an integrated system. The treatment can be implemented by studying the relationship between the certainty and the uncertainty of an object, which is relative with three aspects of identity-discrepant-contrary (IDC) [
29,
30,
31]. The method proceeds as follows: (1) set pairs of two relative sets in uncertain systems are established; (2) attributes are analyzed and calculated using identity, discrepancy, and contrary; (3) the degree of connection of a pair of combinations can be established according to the different attributes. In SPA, identity, discrepancy, and contrary are transformed under certain conditions, and the evaluation of groundwater environmental quality can be realized through this relationship between interconnection and restriction [
29].
To analyze the characteristics of the two sets (A and B) under a certain background, as well as the similarities and differences, the relationship and the connection degree between A and B is defined as the following:
The discrepancy bi in Equation (1) is rewritten as b1i1 + b2i2 + … + bnin with people’s deeper understanding of the associative coefficient in set pair analysis theory, getting the function with hierarchical structure, i.e., multiple connection number. When n = 3, it is a five-element connection number (L = a + bi + cj + dk + el). The associative components (a, b, c, d, e) have an optimal order, that is, the things represented by a are better than those represented by b, and so on. When the value of i, j, k, and l are not considered, they are only used as markers. When these four values need to be considered, they have both a gain and a decay effect on a. This fully reflects the relationship between the number of connections and the unity of opposites between the components. The method is called five-element connection number SPA.
Here, the hydrochemical characteristics and the complexity of the quality grade of shallow groundwater were studied using a five-element connection number of SPA. The index content and groundwater quality standard of samples were taken as a set pair in groundwater quality evaluation when proper parameters were selected as evaluation indexes. According to the
Groundwater Quality Standards of China (GB/14848-2017) [
32], groundwater quality can be classified into five ranks. Rank III is always chosen as the drinking water quality standard in the groundwater quality evaluation.
The rank values from rank I (excellent quality water) to rank V (extremely poor water) are divided by
S,
F1,
F2,
F3,
P. So, the connection degree is expressed as Equation (2):
where,
N is evaluation value;
μ is the identity degree coefficient;
i,
j, and
k are coefficients of discrepancy degree;
l is the coefficient of contrary degree.
If
a =
S/N,
b = F1
/N,
c = F2
/N,
d = F3
/N,
e =
P/N, that is,
a and
e respect identity and contrary degree and
b,
c, and
d are discrepancy degree. Therefore, Equation (2) can be simplified as Equation (3):
where, the identity degree (
a) and the contrary degree (
e) are relatively determined, while the discrepancy degree (
b,
c,
d) is relatively undetermined.
a,
b,
c,
d,
e and
i,
j,
k,
l, and other parameters are interrelated, interacted, and restricted. The higher the value of
a or the lower the value of
e is, the better the quality of groundwater is.
By analyzing the quantitative relationship between the evaluation index and the groundwater quality grade of each sample, it can be seen that the groundwater quality is at the same level. However, the water environment is different due to a different evaluation index. There are four limiting values between the rank I and rank V water quality indicators, which are considered to be the basis of identity, discrepancy, and contrary values. Therefore, the connection degree plays a key role in the comprehensive assessment of water quality based on SPA, which can be calculated by Equation (4).
where,
S1,
S2,
S3, and
S4 are the limiting values of water quality rank I, II, III, and IV;
x is the measured value of water quality status;
m is the
m-th groundwater sampling point;
p is the evaluation index.
According to the calculation results of Equation (3), the average value of each is taken to obtain the average contact degree (
μm) of the evaluation sample (
m). The specific calculation Equation (5) is as follows:
The correlation degree of different evaluation ranks to the groundwater quality of sampling water in the study area can be obtained according to Equations (4) and (5), and the grade I of groundwater quality can be evaluated. Therefore, the water quality composite grade of the
m sample is:
where
Gm is the complexity of the quality grade of groundwater.
There were eight samples and eight indicators selected as evaluation indexes to evaluate the groundwater environmental quality in the study area by five-element connection number of SPA. The indexes were total hardness (TH), total dissolved solids (TDS), sulfate (SO
42−), chloride (Cl
−), nitrate (NO
3−), fluoride (F), ammonia nitrogen (NH
4+), and iron (Fe), as shown in
Table 2.