3.1. Spatial variations of measuring indicators
Figure 3 presents box-and-whisker plots of some water quality parameters of the aquifer under study. Figure 4 presents the spatial evaluation of water quality through isoconcentration maps. Total dissolved solids include salts, minerals, metals, and any other organic or inorganic compound dissolved in water (Adimalla and Qian, 2019; Lal et al. 2020). The lowest concentration of TDS was observed in SP6 (211 mg/L), while the highest concentration (632 mg/L) was observed in SP5. The electrical conductivity showed similar behavior since the minimum value of 341 µS/cm was found in SP6 and the maximum value of 1102.5 µS/cm was found in SP5. Based on the mean value for electrical conductivity (832.5 µS/cm), the groundwater can be classified as permissible (Table 3). The total hardness in SP6 showed the lowest concentration (130 mg/L), while the maximum concentration was found in SP1 with a concentration of 483 mg/L. According to the mean hardness concentration (295.8 mg/L), the Culiacan River aquifer is classified as hard water but is close to being classified as very hard water (> 300 mg/L). The pH values in different seasons revealed that the pH value remains in neutral ranges (7–8), however, in the dry season it is slightly higher than in the rainy season, as reported by Yuan, et al. (2021). Regarding the redox potential it was very similar in all the sampling points with a mean value of 229.9 mV.
Regarding total nitrogen, the highest concentration was observed in SP1 with a maximum concentration of 9.145 mg/L. On the contrary, the lowest concentration of NT was found in SP2 (0.076 mg/L). The total nitrogen concentrations observed in the Culiacan River aquifer can be considered as high and can be related to agricultural activities in the study area. Within this aquifer, two reservoirs and three main rivers are found. These surface water bodies are also monitored, but low concentrations of TN are reported (0–2 mg/L). However, high nitrogen concentrations were confirmed in the Culiacan River aquifer according to nitrate concentrations. NO3− concentrations up to 9.097 mg/L were found in SP1. Han et al. (2020) report a mean NO3− concentration of 0.47 mg/L in an aquifer beneath a municipal solid waste landfill. Fang et al. (2020) mention that chemical fertilizers are probably responsible for nitrate pollution. Low phosphate concentrations were found at all sampling points (0–1 mg/L) in the Culiacan River aquifer.
Total organic carbon presented a low range concentration (0–1 mg/L). Fecal coliforms were detected in SP1 and SP6, with mean values of 260 and 320 MPN/100 mL, respectively, while in SP3 and SP5, fecal coliforms were lower than 4 MPN/100 mL. These results can be supported by the fact that SP1 and SP6 are located in urban areas and the rest of the sampling sites are located in agricultural areas. As a result of the ANOVA analysis, spatial significant differences between the means of NT, TDS, TH, sulfates, and HCO3− were found in the sampling sites with a confidence level of 95%. Based on the results obtained, the groundwater quality in the Culican River aquifer is mainly controlled by the agricultural activities that take place in the study area and by the wastewater that is infiltrated into this groundwater body.
3.2. Water quality index
Groundwater quality is very important because it is directly related to human health (Udeshani et al. 2020). The WQI was used to evaluate the status of the groundwater quality of the Culiacan River aquifer for use as drinking water according to the standards of the World Health Organization (WHO, 2008). For this evaluation, 10 parameters were used (pH, TDS, total hardness, calcium, magnesium, nitrates, chlorides, sulfates, fluorides, and total alkalinity). According to the classification presented in Table 1, the groundwater quality was classified as “Good water”. The values obtained from the water quality index in the different sampling sites were in SP1 = 72.4, SP2 = 58.6, SP3 = 64.2, SP4 = 63.8, and SP5 = 71.2. Sampling point SP6 is not located in an area of intense agricultural activity and showed a better groundwater quality, with a classification of “Excellent water”. The highest WQI was registered in SP1, where the influence of the urban area is evidenced.
Various researchers (Adimalla and Qian 2019; Acharya et al. 2018; Elsayed et al. 2020; Hamdi et al. 2018; Saikrishna et al. 2020; Verma et al. 2018) have used this WQI to assess groundwater quality for drinking purposes. For example, Adimalla and Qian (2019) evaluated the WQI of the groundwater of the Nanganur region and obtained an average value of 153 which was classified as poor drinking quality in most of the groundwater samples. Likewise, Verma et al. (2018) and Acharya et al. (2018) also used this WQI and determined that 24.44% and 20% of their samples, respectively, presented a very poor quality for drinking purposes. Moreover, Saikrishna et al. (2020) and Elsayed et al. (2020) discovered, through this WQI, that their groundwater has a low quality for drinking purposes. Therefore, this WQI has served as an alert for the responsible authorities to take the necessary measures.
3.3. Hydrogeochemical characteristics
Para analizar las características químicas del agua del acuífero en estudio, las propiedades estadísticas se muestran en la Tabla 4. This table shows that all the parameters are within international standards (FAO 1992; WHO 2008), except electrical conductivity, EC and TDS. The high values found in the Culiacan River aquifer are a consequence of the presence of Cl− and Na+ (Adimalla and Qian, 2019; Salgado-Mendez et al. 2019). This parameter is of vital interest in agricultural irrigation since the use of water under these conditions may cause problems in crops and reduce yields. TH concentrations were close to the allowed limit (WHO 2008) in some sampling sites. The lowest TH concentration (177.89 mg/L) was observed in SP1, while the highest TH concentration (350.29 mg/L) was seen in SP2. According to TH concentrations, the Culiacan River aquifer was classified as Hard and Very Hard (Table 3). Regarding bicarbonates, the maximum concentration (369 mg/L) was found in SP3 and the minimum concentration (11 mg/L) in SP2. Adimalla and Qian (2019) mention that the high concentration of bicarbonates in the aquifer water is due to the weathering of the carbonate and the dissolution of carbonic acid. Based on the ion concentrations reported in the Culiacan River aquifer, it could be attributed to marine intrusion.
Chemical constituents are key factors to understand the hydrogeochemistry of an aquifer (Cao et al. 2019; Elumalai et al. 2020; Gao et al. 2020; Mohanty and Rao 2019). The identification of the hydrochemical facies of the Culiacan River aquifer is presented in the Piper diagram shown in Fig. 5. This figure shows the mean values of cations and anions that were measured in the sampling wells from 2012 to 2019. The study shows that the water of the shallow aquifer is mainly of the Ca/Mg - HCO3−/Cl− type, which may indicate that it is a process of intrusion of saline water into the aquifer due to its proximity to the coastal zone (Bhadra et al. 2020; Hegazy et al. 2020). The three groups of cations (Ca2+; Mg2+; Na++K+) are shown in the left ternary plot, while the three groups of anions (Cl−; HCO3−+CO3−; SO42−) were plotted in the right ternary plot (Acharya et al. 2018; Badmus et al. 2020; Dragon et al. 2021). According to the cation plot, the Calcium type zone predominates in samples obtained at SP1, SP3, SP5, and SP6. The Sodium and Potassium type zone predominates in SP2 and SP4. Alkaline earth metals (Ca2++Mg2+) exceeded alkali metals (Na++K+) in hydrochemical facies. According to the anion plot, the Bicarbonate type zone predominates in all the sampling sites. Based on matrix transformation (diamond), the SP1, SP2, and SP4 sites could be considered as Mixed type zone and the Carbonate hardness zone predominates in SP3, SP5, and SP6 sites. These results are similar to those reported by Wisitthammasri et al. (2020), who suggest that the dominant control factor of groundwater chemistry is the interaction between rocks and filtered water.
Table 4
Descriptive statistics of groundwater physicochemical characteristics and geochemistry.
Well Number
|
Ca2+
mg/L
|
Mg2+
mg/L
|
Na+
mg/L
|
K+
mg/L
|
Cl-
mg/L
|
HCO3− mg/L
|
SO42+
mg/L
|
NO3−
mg/L
|
TDS
mg/L
|
EC
µS/cm
|
TH
mg/L
|
1
|
97.78
|
29.99
|
90.08
|
4.31
|
71.77
|
308.38
|
75.77
|
6.59
|
599.28
|
1001.57
|
177.89
|
2
|
81.53
|
22.75
|
52.04
|
10.10
|
124.73
|
164.19
|
18.95
|
0.03
|
557.57
|
868.14
|
350.29
|
3
|
85.05
|
23.18
|
72.58
|
4.93
|
23.99
|
353.34
|
53.01
|
3.72
|
530.23
|
831.33
|
316.39
|
4
|
77.29
|
19.77
|
78.52
|
5.70
|
38.07
|
309.12
|
68.35
|
3.90
|
538.12
|
861.43
|
314.69
|
5
|
74.92
|
32.29
|
99.32
|
4.94
|
83.03
|
299.51
|
93.34
|
1.93
|
632.01
|
1102.50
|
297.06
|
6
|
51.01
|
11.82
|
17.70
|
2.08
|
22.95
|
136.69
|
22.88
|
1.22
|
239.20
|
385.29
|
321.11
|
Mean
|
77.827
|
23.078
|
67.495
|
5.363
|
61.121
|
258.644
|
54.495
|
2.900
|
512.82
|
835.45
|
295.83
|
Min
|
41.9
|
9.448
|
9.32
|
0.952
|
14.782
|
11
|
15.930
|
0.0172
|
211.2
|
330
|
130.26
|
Max
|
111.4
|
40.88
|
117.2
|
42.82
|
140.609
|
373.36
|
99.172
|
9.097
|
688
|
1437
|
483
|
SD
|
16.6541
|
7.464
|
30.982
|
7.190
|
38.183
|
88.1068
|
29.112
|
2.33923
|
139.759
|
249.348
|
73.33
|
Coef. Var.
|
21.40%
|
32.34%
|
45.90%
|
134.07%
|
62.47%
|
34.06%
|
53.42%
|
80.65%
|
27.25%
|
29.85%
|
24.79%
|
FAO
|
0-400
|
0–60
|
0-920
|
0–2
|
0-1065
|
0-610
|
0-960
|
0–10
|
0-2000
|
0–3
|
-
|
WHO
|
75–200
|
50–150
|
200
|
12
|
200–600
|
-
|
200–400
|
50
|
500–1500
|
-
|
100–500
|
*FAO (1992); WHO (2008) |
The groundwater quality for irrigation purposes was evaluated using existing indices presented in Table 5. The sodium adsorption ratio (SAR) showed that the groundwater could be classified as excellent. Likewise, the results obtained for the soluble Residual Sodium Carbonate (CSR) classifies the water of the Culiacan River aquifer as good. However, water was classified as unsuitable for irrigation based on the soluble sodium percentage (SSP) index, in particular in SP1, SP3, SP4, and SP5 sites. Similar results were obtained when using the Kelly Ratio (KR). The percentage of Na (%Na) classified the water as Good only in SP6, but the rest of the sampling points could be classified as permissible. It has been reported that agricultural activities carried out with slightly alkaline groundwater can increase osmotic pressure due to sodium deposition in the soil, causing plants to be unable to absorb other nutrients from the soil-water medium (Ren et al. 2020). Magnesium Hazard (MH) was classified as suitable at all sampling points. Finally, the water of the Culiacan River aquifer could be considered adequate for irrigation purposes according to the permeability index (PI).
Table 5
Results of water quality indices of the Culiacan River aquifer.
SP
|
SAR
|
CSR
|
SSP
|
KR
|
Na%
|
MH
|
PI
|
Error
|
1
|
4.0885
|
-2.4368
|
51.3696
|
1.0563
|
51.5886
|
33.5662
|
47.1411
|
31.5022
|
2
|
2.6239
|
-3.2582
|
43.2030
|
0.7607
|
44.0724
|
31.4784
|
55.2746
|
40.3096
|
3
|
3.5959
|
-0.3687
|
50.6043
|
1.0245
|
50.9172
|
30.9703
|
53.6900
|
24.5878
|
4
|
4.1203
|
-0.4250
|
55.4221
|
1.2433
|
55.7525
|
29.6297
|
50.0145
|
24.9541
|
5
|
4.8266
|
-1.4946
|
57.4231
|
1.3487
|
57.6473
|
41.5030
|
39.9556
|
28.3630
|
6
|
1.1885
|
-1.3174
|
30.7338
|
0.4437
|
31.1348
|
27.7726
|
79.2404
|
24.6724
|
Figure 6 presents the ionic dominance pattern in meq/L and the Stiff diagram with the mean values of ions in water. The samples showed a mean ion balance error of 29%. All samples presented a positive value, which indicated the dominance of cations over anions (Hossain et al. 2020). In case of oral ingestion and/or dermal absorption of groundwater, the concentration of these ions would have little effect on the health of the region's inhabitants (Bodrud-Doza et al. 2020).
3.4. Multivariate statistical analysis
Figure 7 shows the Pearson product-moment correlations between each pair of variables. These correlation coefficients range from − 1 to + 1 and measure the strength of a linear relationship between the variables (Abdelaziz et al. 2020; Elumalai et al. 2020; Goyal et al. 2021; Quevedo-Castro et al. 2019). In this study, significant correlations between water quality parameters were found. The results from the correlation matrix and PCA revealed a strong relationship of total fluorides (TF) with TC, HCO3−, SO42−, Na+, and TN. Furthermore, total chlorides are related to Ca2+, K+, Mg2+, TP, PO43−, TDS, and EC. Calcium is related to HCO3−, SO42−, Mg2+, Na+, TN, RP, TH, TDS, and EC. Bicarbonates are related to SO42−, Mg2+, Na+, TN, TP, PO43−, TH, TDS, and EC. Sulfates are related to Mg2+, Na+, TN, TH, TDS, and EC. Potassium is related to Na. Magnesium ions are correlated with Na+, TN, TH, TDS ions, and EC. Sodium ions are related to TN, TP, PO43−, TH, TDS, and EC. Total nitrogen is related to TP, PO43−, RP, TH, TDS, and EC. Total phosphorus is related to phosphates. Fecal coliforms correlate with pH. Redox potential is related to pH. Total hardness is related to TDS and EC. And the total dissolved solids parameter is related to EC. According to the Pearson correlation analysis, the vulnerability of the Culiacan River aquifer is obvious. This study showed that because many variables are correlated with each other, a small change in the concentration of a pollutant could lead to an important change in the chemical composition of the aquifer. This situation may be related to the water deficit reported in the present study.
PCA is based on 720 samples collected from 2012 to 2019. This multivariable analysis was supported on 18 water quality variables (TF, TC, Ca2+, HCO3−, SO42−, K, Mg, Na, TN, TP, FC, PO43−, RP, TOC, pH, TH, TDS, and EC) measured in the aquifer under study. The purpose of the analysis was to obtain a reduced number of linear combinations that better explain the water quality variability in the aquifer. In this case, 6 components were extracted since these components had Eigenvalues greater than or equal to 1.0. These components explained 86.26% of the original data variability.
Figure 8a shows the importance of the water quality parameters on the first two principal components (PC1 and PC2). They represented 54% of the total variation (PC1 = 36.38%; PC2 = 17.96%). This figure represents the weights (importance) of the variables that affect PC1 and PC2. Their corresponding spatial locations indicate similarities between them. The vectors represent the water quality parameters, and the length of the vectors is proportional to the influence of the parameters. The angle represents the linear correlation between the parameters and finally, the direction of the vector represents the direction in which this parameter varies the most.
Figure 8b shows the explained variances for each principal component. The first 10 principal components explain 96.7% of the accumulated variance of the data. Table 6 presents the principal components (PC) of the Culiacan River aquifer. Bold numbers show the importance of water quality parameters on the PC obtained. The first component (PC1) explains 36.3% of the total variance of the data. The ions Ca2+, HCO3−, SO42−, Mg2+ and Na+ represent the greatest variation of PC1. Wisitthammasri et al. (2020) mention that these ions are derived from anthropogenic contamination, such as wastewater, as well as geological factors, such as the impact of sodium sulfate bearing minerals. However, PC1 could also be related to the seawater intrusion process (Liu et al. 2021), which in turn can explain the high concentrations of these ions found in the Culiacan River aquifer. The PC2 showed strong positive importance and is comprised of TC, TP, and PO43−. This component is related to nutrients that come from agricultural areas. In PC3, the highest variance was represented by the FC and pH parameters. Some pollutants such as fecal coliforms are attributed to the infiltration process. This situation is evidenced by the presence of fecal coliforms (FC) in rivers and reservoirs located in the study area (Quevedo-Castro et al. 2019). PC4 is related to RP, TF, Ca2+, SO42−, which are also attributed to agricultural activity.
Table 6
Principal components (PC) for groundwater chemistry of the Culiacan river aquifer.
Parameter
|
PC1
|
PC2
|
PC3
|
PC4
|
PC5
|
PC6
|
TF
|
0.13436
|
0.29535
|
-0.21115
|
-0.48059
|
-0.10206
|
0.01448
|
TC
|
0.08048
|
-0.52008
|
0.07951
|
0.04837
|
0.06252
|
0.01494
|
Ca2+
|
0.29903
|
-0.13205
|
-0.15608
|
0.32392
|
-0.04497
|
0.04234
|
HCO3−
|
0.31765
|
0.21928
|
0.03850
|
0.02459
|
-0.11931
|
-0.02172
|
SO42−
|
0.31923
|
0.10479
|
-0.06441
|
-0.31239
|
-0.10582
|
0.02381
|
K+
|
0.05979
|
-0.12263
|
0.35782
|
0.10433
|
0.43989
|
0.50115
|
Mg2+
|
0.31461
|
-0.23316
|
-0.10986
|
-0.04194
|
-0.18011
|
0.08934
|
Na+
|
0.35623
|
-0.03561
|
0.12101
|
-0.08407
|
-0.00632
|
0.19698
|
TN
|
0.26873
|
0.22231
|
-0.29097
|
0.02986
|
0.08342
|
0.04023
|
TP
|
0.15197
|
0.41424
|
0.27975
|
0.20985
|
0.11189
|
0.03253
|
FC
|
-0.04632
|
0.05674
|
-0.39159
|
-0.21559
|
0.64348
|
0.05895
|
PO43−
|
0.15107
|
0.41562
|
0.28183
|
0.20348
|
0.11654
|
0.03779
|
RP
|
0.07396
|
0.09607
|
-0.21566
|
0.54588
|
-0.11757
|
-0.27815
|
TOC
|
-0.08917
|
0.06978
|
-0.16462
|
0.08208
|
-0.38681
|
0.74827
|
pH
|
-0.02091
|
0.02617
|
0.53430
|
-0.30381
|
-0.14578
|
-0.17514
|
TH
|
0.30862
|
-0.05513
|
-0.02460
|
0.03854
|
0.31030
|
-0.07492
|
TDS
|
0.33697
|
-0.20413
|
0.08637
|
-0.00607
|
-0.01325
|
-0.12568
|
EC
|
0.33297
|
-0.19659
|
0.03882
|
-0.11190
|
-0.06291
|
-0.04318
|
Eigenvalue
|
6.54975
|
3.2335
|
2.05156
|
1.53662
|
1.11181
|
1.04424
|
Percentage of variance
|
36.387
|
17.964
|
11.398
|
8.537
|
6.177
|
5.801
|
Accumulated percentage
|
36.387
|
54.351
|
65.749
|
74.286
|
80.462
|
86.264
|
Figure 9 shows that separate groups of variables (water quality parameters) with similar characteristics correlate with each other (Adbelaziz et al. 2020; Bodrud-Doza et al. 2020; Gaikwad et al. 2020). After recalculating the distance between groups, the now closest groups are combined. The results of HCA are shown as a dendrogram, which can better explain the classification of the data. Two clusters were observed. Cluster 1 includes TF, TOC, FC, TC, SO4, K, TP, PO4, TN, RP, and PH. Cluster 2 includes Ca, Mg, HCO3, Na, TH, TDS, EC. Cluster 1 represents the chemical characteristics of groundwater regulated by both anthropogenic and natural processes. For example, TOC, FC, TP, PO4, TN and pH are mainly influenced by anthropogenic activities. Cluster 2 parameters are mainly controlled by natural processes, such as mineral dissolution and soil leaching. Group 2 indices are related to major ions, which are mainly controlled by natural processes, such as rock dissolution due to increasing rock-water interactions.