4.1. Geoelectrical resistivity
The geo-electric resistivity method (GRM), including vertical electrical sounding (VES) surveys, was applied using the schlumberger array. For study the subsurface formations, deposits, and salinization, longitudinal (green line) and transverse (pink line) geoelectric cross sections shown in Fig. 4. As shown in Fig. 4A, the apparent resistance (AR) of the subsurface materials varies from 1to 90 OA(m), along the longitudinal cross section (from the south to north of KPA (Fig. 3). Based on the results, layers with AR<20 OA(m) would have saline water or thin soil particles. Based on the mineralogy map, the bedrock of KPA consist of Qom formation, Upper and Lower Red Formation, and Miocene Marl, which in present of water, would produce saline groundwater. As shown in Fig. 4A, by moving toward the bedrock, AR decreases. Likewise, AR decrease by moving from west to east of KPA (Fig. 4B) which probably is due to mentioned formations and discharging saline groundwater from aquifer to the east of Kashan plain.
4.2. General Hydrogrochemical Characteristics
The geochemistry of groundwater samples in KPA and KPL provides visions into the source and mechanism of salinization, and KPA and KPL interaction in the study area. The following interpretations effort to determine the source and mechanism of salinization in KPA, and assessment the KPA/KPL interaction (Table 1).
Table 1. Physicochemical parameters, saturation indices, and hydrogeochemical parameters in KPA and KPL (chemical elements in mg/L and EC in µS/cm).
ID
|
depth (m)
|
pH
|
TDS
|
Ec (μS/cm)
|
Na
|
K
|
Ca
|
Mg
|
Cl
|
ALK (HCO3)
|
SO4
|
Br
|
KA1
|
35
|
7.16
|
7,391
|
13,000
|
1775
|
0.01
|
723
|
69
|
4047
|
130
|
770
|
3.32
|
KA2
|
35
|
7.12
|
8,751
|
4,330
|
2408
|
0.01
|
881
|
123
|
4473
|
300
|
848
|
4.73
|
KA3
|
120
|
7.04
|
5,061
|
2,100
|
729
|
0.01
|
846
|
189
|
1775
|
150
|
1,512
|
1.92
|
KA4
|
25
|
7.1
|
6,405
|
4,250
|
1523
|
0.01
|
523
|
96
|
2556
|
79
|
1,702
|
2.09
|
KA5
|
150
|
7.36
|
1,084
|
1,783
|
202
|
0.01
|
179
|
48
|
287
|
600
|
576
|
0.55
|
KA6
|
100
|
7.55
|
2,790
|
3,380
|
559
|
0.02
|
255
|
103
|
607
|
250
|
1,249
|
1.12
|
KA7
|
150
|
7.72
|
2,693
|
3,630
|
530
|
0.01
|
281
|
77
|
1180
|
87
|
619
|
0.71
|
KA8
|
90
|
7.87
|
1,509
|
2,210
|
368
|
0
|
167
|
59
|
177
|
720
|
726
|
0.52
|
KA9
|
38
|
7.85
|
896
|
1,462
|
250
|
0
|
84
|
45
|
71
|
660
|
433
|
0.39
|
KA10
|
0.5
|
8.11
|
3,937
|
4,420
|
1041
|
0
|
223
|
55
|
1034
|
170
|
1,573
|
0.81
|
KA11
|
50
|
7.44
|
3,030
|
4,000
|
768
|
0.01
|
204
|
89
|
1172
|
120
|
788
|
0.84
|
KA12
|
0.5
|
6.88
|
137,600
|
150,200
|
35472
|
0.17
|
10718
|
2,655
|
87935
|
14
|
890
|
29
|
KA13
|
100
|
7.38
|
6,778
|
10,100
|
1890
|
0.01
|
341
|
41
|
2689
|
42
|
1,813
|
1.93
|
KA14
|
1.5
|
7.68
|
4,002
|
6,620
|
984
|
0.07
|
207
|
61
|
1300
|
130
|
1,440
|
1.52
|
KA15
|
80
|
7.68
|
3,561
|
5,150
|
863
|
0.02
|
350
|
126
|
1053
|
190
|
1,155
|
1.66
|
KA16
|
35
|
7.25
|
4,881
|
7,500
|
1042
|
0.01
|
625
|
56
|
2026
|
31
|
1,129
|
2.37
|
KA17
|
300
|
7.58
|
1,604
|
2,300
|
371
|
0.01
|
228
|
74
|
390
|
900
|
600
|
0.59
|
KA18
|
200
|
8
|
1,276
|
1,872
|
413
|
0.01
|
119
|
40
|
355
|
840
|
328
|
0.63
|
KA19
|
30
|
7.41
|
10,210
|
13,510
|
2383
|
0.03
|
849
|
337
|
4592
|
130
|
2,035
|
3.09
|
KA20
|
60
|
7.36
|
8,393
|
13,290
|
1400
|
0.02
|
1081
|
325
|
4810
|
64
|
773
|
1.64
|
KA21
|
80
|
7.33
|
8,970
|
11,720
|
2046
|
0.02
|
885
|
272
|
3510
|
150
|
2,247
|
2.77
|
KA22
|
80
|
7.57
|
10,636
|
14,250
|
2812
|
0.04
|
854
|
225
|
5259
|
140
|
1,477
|
4.7
|
KA23
|
170
|
7.43
|
2,686
|
3,290
|
465
|
0.01
|
327
|
88
|
720
|
230
|
1,071
|
0.99
|
KA24
|
120
|
7.48
|
1,667
|
2,270
|
482
|
0.01
|
173
|
57
|
377
|
210
|
563
|
0.63
|
KA25
|
150
|
7.35
|
3,603
|
3,930
|
842
|
0.01
|
440
|
79
|
1739
|
100
|
494
|
1.43
|
KA26
|
25
|
5.16
|
24,726
|
27,200
|
1941
|
0.03
|
867
|
730
|
11244
|
0.56
|
9,945
|
14.7
|
KA27
|
18
|
7.43
|
11,587
|
14,470
|
2897
|
0.03
|
1569
|
49
|
5680
|
63
|
1,390
|
2.49
|
KA28
|
50
|
7.58
|
4,935
|
6,560
|
1030
|
0.01
|
482
|
102
|
1474
|
62
|
1,841
|
2.17
|
KA29
|
80
|
7.78
|
2,003
|
1,619
|
538
|
0.01
|
94
|
86
|
745
|
150
|
528
|
0.66
|
KA30
|
40
|
7.19
|
6,635
|
10,420
|
1944
|
0.01
|
533
|
63
|
2867
|
100
|
1,220
|
1.56
|
KA31
|
140
|
7.99
|
2,298
|
1,923
|
765
|
0
|
99
|
10
|
816
|
190
|
594
|
0.57
|
KA32
|
15
|
7.66
|
3,099
|
2,730
|
796
|
0.02
|
204
|
111
|
994
|
230
|
978
|
1.19
|
KA33
|
52
|
7.52
|
4,182
|
6,350
|
1114
|
0.01
|
358
|
70
|
1680
|
85
|
954
|
1.47
|
KA34
|
14
|
6.95
|
4,581
|
6,720
|
1159
|
0.01
|
656
|
25
|
2733
|
24
|
530
|
1.95
|
KA35
|
0.5
|
5.76
|
292,500
|
204,000
|
74223
|
0.87
|
962
|
2438
|
204800
|
17
|
10,080
|
37
|
KA36
|
15
|
7.19
|
7,602
|
12,280
|
2200
|
0.01
|
137
|
9.49
|
3627
|
48
|
1,625
|
1.96
|
KA37
|
23
|
4.57
|
26,700
|
37,400
|
8284
|
0.09
|
1369
|
369
|
13710
|
0.5
|
2,976
|
12
|
KA38
|
14
|
4.17
|
20,315
|
28,800
|
3686
|
0.13
|
2848
|
677
|
11999
|
0.5
|
1,110
|
17
|
KA39
|
80
|
6.9
|
10,733
|
14,020
|
1818
|
0.03
|
1261
|
467
|
6009
|
1.6
|
1,169
|
2.58
|
KA40
|
60
|
7.37
|
6,650
|
7,660
|
2226
|
0.02
|
427
|
313
|
2291
|
1.3
|
1,385
|
1.57
|
PL1
|
0.5
|
6.66
|
235000
|
215000
|
87000
|
2600
|
440
|
19000
|
224000
|
104
|
12100
|
96
|
PL2
|
0.5
|
6.7
|
228000
|
203000
|
81300
|
3000
|
390
|
23800
|
202000
|
261
|
24000
|
113
|
PL3
|
0.5
|
6.66
|
245000
|
197000
|
83500
|
3200
|
240
|
26000
|
211000
|
240
|
29400
|
139
|
PL4
|
0.5
|
6.82
|
252000
|
219000
|
93700
|
1400
|
340
|
11900
|
193000
|
136
|
18500
|
87.4
|
PL5
|
0.5
|
6.57
|
251000
|
198000
|
79700
|
3200
|
230
|
24400
|
201000
|
200
|
26800
|
103
|
PL6
|
0.5
|
6.63
|
239000
|
215000
|
79900
|
2200
|
900
|
21600
|
209000
|
169
|
6600
|
93.7
|
PL7
|
0.5
|
6.48
|
229000
|
224000
|
86400
|
1800
|
1600
|
16200
|
207000
|
124
|
3900
|
81.4
|
PL8
|
0.5
|
6.71
|
248000
|
212000
|
85600
|
2700
|
380
|
19500
|
203000
|
188
|
15200
|
1.6
|
PL9
|
0.5
|
6.65
|
260000
|
207000
|
85700
|
2700
|
310
|
22800
|
200000
|
224
|
19600
|
113
|
PL10
|
0.5
|
6.63
|
208000
|
202000
|
86500
|
2600
|
250
|
22300
|
202000
|
324
|
27600
|
134
|
PL11
|
0.5
|
6.72
|
225000
|
220000
|
89600
|
2300
|
630
|
17200
|
209000
|
100
|
7900
|
66.7
|
PL12
|
0.5
|
6.69
|
219000
|
200000
|
81000
|
3100
|
270
|
25200
|
204000
|
278
|
24500
|
97.7
|
PL13
|
0.5
|
6.64
|
249000
|
203000
|
84600
|
2700
|
260
|
22600
|
1970000
|
313
|
25900
|
124
|
PL14
|
0.5
|
6.63
|
273000
|
205000
|
88200
|
2300
|
250
|
19500
|
2110000
|
309
|
25900
|
107
|
PL15
|
0.5
|
6.7
|
248000
|
205000
|
89300
|
2400
|
260
|
19500
|
200000
|
359
|
27200
|
120
|
PL16
|
0.5
|
6.53
|
163000
|
220000
|
81600
|
2600
|
2000
|
19700
|
218000
|
64
|
2500
|
54.8
|
PL17
|
0.5
|
6.73
|
275000
|
203000
|
85200
|
2700
|
250
|
20700
|
208000
|
233
|
28700
|
108
|
PL18
|
0.5
|
6.47
|
219000
|
214000
|
72700
|
3000
|
2800
|
24400
|
213000
|
61
|
1700
|
56.3
|
PL19
|
0.5
|
6.81
|
247000
|
219000
|
94600
|
1600
|
380
|
14500
|
208000
|
258
|
16700
|
70.1
|
PL20
|
0.5
|
6.75
|
251000
|
216000
|
110000
|
2100
|
400
|
15500
|
204000
|
183
|
23700
|
61.4
|
PL21
|
0.5
|
6.63
|
236000
|
210000
|
74900
|
3000
|
710
|
25500
|
219000
|
202
|
7300
|
90.5
|
PL22
|
0.5
|
6.71
|
234000
|
211000
|
83200
|
2600
|
360
|
21800
|
209000
|
155
|
14800
|
76.5
|
PL23
|
0.5
|
6.98
|
250000
|
213000
|
83800
|
2600
|
570
|
21300
|
211000
|
250
|
11600
|
67.6
|
PL24
|
0.5
|
6.41
|
2130000
|
209000
|
66100
|
3000
|
4900
|
27200
|
207000
|
64
|
11100
|
68.3
|
PL25
|
0.5
|
6.3
|
181000
|
204000
|
61600
|
3700
|
4300
|
32300
|
219000
|
55
|
11100
|
67.9
|
PL26
|
0.5
|
6.63
|
164000
|
231000
|
92200
|
1500
|
1900
|
11200
|
198000
|
66
|
30000
|
47.9
|
Table 2. Saturation indices of Calcite, Dolomite, Gypsum, Anhydrate, Halite and Aragonite in water samples of KPA and KPL
ID
|
SICal
|
SIDol
|
SIGyp
|
SIAnh
|
SIHal
|
SIAra
|
ID
|
SICal
|
SIDol
|
SIGyp
|
SIAnh
|
SIHal
|
SIAra
|
KA1
|
0.05
|
-0.57
|
-0.51
|
-0.81
|
-4
|
-0.09
|
KA34
|
-0.66
|
-2.38
|
-0.61
|
-0.91
|
-4.33
|
-0.8
|
KA2
|
0.45
|
0.4
|
-0.47
|
-0.77
|
-3.85
|
0.31
|
KA35
|
-0.75
|
-0.52
|
-0.04
|
-0.17
|
-0.24
|
-0.89
|
KA3
|
0.19
|
0.06
|
-0.15
|
-0.45
|
-4.72
|
0.04
|
KA36
|
-1.12
|
-3.06
|
-0.85
|
-1.15
|
-3.94
|
-1.26
|
KA4
|
-0.32
|
-1.05
|
-0.29
|
-0.59
|
-4.25
|
-0.47
|
KA37
|
-2.41
|
-5.03
|
-0.13
|
-0.43
|
-2.99
|
-2.56
|
KA5
|
0.29
|
0.34
|
-0.87
|
-1.18
|
-6
|
0.15
|
KA38
|
-1.98
|
-4.2
|
-0.16
|
-0.46
|
-3.25
|
-2.12
|
KA6
|
-0.04
|
-0.15
|
-0.55
|
-0.85
|
-5.27
|
-0.18
|
KA39
|
-1.7
|
-3.46
|
-0.28
|
-0.58
|
-3.82
|
-1.84
|
KA7
|
-0.38
|
-0.99
|
-0.75
|
-1.05
|
-4.99
|
-0.53
|
KA40
|
-2.2
|
-4.2
|
-0.53
|
-0.83
|
-4.15
|
-2.35
|
KA8
|
0.3
|
0.48
|
-0.85
|
-1.15
|
-5.96
|
0.16
|
PL1
|
-1.11
|
0.03
|
-0.69
|
-0.83
|
-0.3
|
-1.25
|
KA9
|
0.04
|
0.14
|
-1.25
|
-1.55
|
-6.5
|
-0.1
|
PL2
|
-0.82
|
0.72
|
-0.49
|
-0.64
|
-0.41
|
-0.97
|
KA10
|
-0.31
|
-0.91
|
-0.56
|
-0.86
|
-4.78
|
-0.46
|
PL3
|
-1.1
|
0.41
|
-0.61
|
-0.75
|
-0.33
|
-1.25
|
KA11
|
-0.41
|
-0.84
|
-0.8
|
-1.11
|
-4.84
|
-0.55
|
PL4
|
-0.9
|
0.3
|
-0.59
|
-0.75
|
-0.44
|
-1.05
|
KA12
|
-0.23
|
-0.58
|
-0.2
|
-0.44
|
-1.38
|
-0.38
|
PL5
|
-1.18
|
0.25
|
-0.67
|
-0.83
|
-0.42
|
-1.32
|
KA13
|
-0.78
|
-2.16
|
-0.43
|
-0.73
|
-4.13
|
-0.93
|
PL6
|
-0.65
|
0.69
|
-0.72
|
-0.87
|
-0.41
|
-0.79
|
KA14
|
-0.45
|
-1.11
|
-0.62
|
-0.92
|
-4.71
|
-0.59
|
PL7
|
-0.42
|
0.76
|
-0.62
|
-0.77
|
-0.4
|
-0.57
|
KA15
|
-0.03
|
-0.18
|
-0.5
|
-0.81
|
-4.85
|
-0.18
|
PL8
|
-0.91
|
0.48
|
-0.67
|
-0.83
|
-0.4
|
-1.05
|
KA16
|
-0.6
|
-1.9
|
-0.32
|
-0.62
|
-4.5
|
-0.74
|
PL9
|
-0.98
|
0.49
|
-0.67
|
-0.83
|
-0.39
|
-1.13
|
KA17
|
0.53
|
0.91
|
-0.83
|
-1.14
|
-5.62
|
0.38
|
PL10
|
-0.89
|
0.75
|
-0.61
|
-0.76
|
-0.38
|
-1.03
|
KA18
|
0.28
|
0.42
|
-1.27
|
-1.57
|
-5.6
|
0.13
|
PL11
|
-0.94
|
0.17
|
-0.72
|
-0.87
|
-0.36
|
-1.08
|
KA19
|
0.03
|
0
|
-0.18
|
-0.48
|
-3.84
|
-0.12
|
PL12
|
-0.98
|
0.59
|
-0.64
|
-0.8
|
-0.39
|
-1.12
|
KA20
|
-0.12
|
-0.41
|
-0.45
|
-0.75
|
-4.04
|
-0.27
|
PL13
|
-0.89
|
0.72
|
-0.63
|
-0.78
|
-0.41
|
-1.04
|
KA21
|
0.11
|
0.05
|
-0.09
|
-0.39
|
-4.01
|
-0.03
|
PL14
|
-0.85
|
0.76
|
-0.61
|
-0.76
|
-0.35
|
-1
|
KA22
|
0.08
|
-0.07
|
-0.3
|
-0.6
|
-3.71
|
-0.07
|
PL15
|
-0.77
|
0.9
|
-0.58
|
-0.74
|
-0.39
|
-0.91
|
KA23
|
0.06
|
-0.13
|
-0.49
|
-0.8
|
-5.26
|
-0.09
|
PL16
|
-0.72
|
0.18
|
-0.73
|
-0.88
|
-0.36
|
-0.86
|
KA24
|
-0.17
|
-0.49
|
-0.9
|
-1.21
|
-5.5
|
-0.31
|
PL17
|
-1
|
0.49
|
-0.57
|
-0.72
|
-0.37
|
-1.14
|
KA25
|
-0.16
|
-0.72
|
-0.73
|
-1.03
|
-4.64
|
-0.3
|
PL18
|
-0.69
|
0.18
|
-0.78
|
-0.94
|
-0.42
|
-0.84
|
KA26
|
-2.56
|
-4.87
|
0.25
|
-0.04
|
-3.6
|
-2.7
|
PL19
|
-0.65
|
0.86
|
-0.59
|
-0.74
|
-0.35
|
-0.79
|
KA27
|
-0.02
|
-1.19
|
-0.1
|
-0.39
|
-3.66
|
-0.16
|
PL20
|
-0.8
|
0.58
|
-0.43
|
-0.57
|
-0.24
|
-0.94
|
KA28
|
-0.47
|
-1.29
|
-0.26
|
-0.57
|
-4.62
|
-0.61
|
PL21
|
-0.75
|
0.69
|
-0.75
|
-0.9
|
-0.38
|
-0.89
|
KA29
|
-0.59
|
-0.89
|
-1.21
|
-1.52
|
-5.18
|
-0.74
|
PL22
|
-1.07
|
0.24
|
-0.72
|
-0.87
|
-0.38
|
-1.21
|
KA30
|
-0.19
|
-0.96
|
-0.41
|
-0.71
|
-4.09
|
-0.33
|
PL23
|
-0.66
|
0.86
|
-0.62
|
-0.77
|
-0.37
|
-0.8
|
KA31
|
-0.49
|
-1.65
|
-1.14
|
-1.44
|
-5
|
-0.64
|
PL24
|
-0.49
|
0.39
|
-0.75
|
-0.91
|
-0.47
|
-0.64
|
KA32
|
-0.16
|
-0.26
|
-0.75
|
-1.06
|
-4.91
|
-0.31
|
PL25
|
-0.72
|
0.09
|
-0.82
|
-0.98
|
-0.43
|
-0.86
|
KA33
|
-0.37
|
-1.11
|
-0.57
|
-0.87
|
-4.54
|
-0.51
|
PL26
|
-0.49
|
0.37
|
-0.6
|
-0.76
|
-0.43
|
-0.64
|
The electric conductivity (EC) of the water samples varies from 1462 (freshwater end member (KA9) to 231000 (brine end member (SL26)) µS/cm. The mean value of EC for KPA and KPL is about 16,818 and 210,577 µS/cm, respectively. The total solid dissolve (TDS) ranges from 896 to 292500 mg/l (Fig. 5). The mean value of TDS for KPA and KPL is about 16,949 and 100,551 mg/l, respectively. The variation of TDS shows that 1, 45, 9, and 45% of the samples classify as freshwater, brackish water, saline, and brine, respectively. Pursuant to Fig. 5, the salinity increase by moving toward the groundwater flow direction (southwest to northeast). The pH ranges between 4.17 and 8.11 with an average of 6.93. The average temperature of the groundwater samples in KPA was 26.4°C. Na+ and Ca2+ are the dominant cations and Cl- and SO42– are the dominant anions in KPA. The Ca2+, Na+ and Cl– concentration varies from 84 to 10718, 202 to 110000 and 71 to 211000 mg/L, respectively. The concentration of SO42- varies from 328 to 29400 mg/L. The average concentration of HCO3- in KPA is 186 mg/L and the concentration decrease by moving from southeast of KPA toward the KPL. Also, Bromide concentration varies from 0.39 (KA9) to 139 (PL3) mg/l, respectively. Likewise, the mean value of Br for KPA and KPL is about 4.22 and 36.11 mg/l, respectively.
4.3. Hydrochemical Facies Evolution
Hydrochemical Facies Evolution Diagram (HFE-D) has been employed in order to provide information on salinization and freshening processes. The HFE-D (Fig. 6) shows the rate of Cl-, SO42-, HCO3-, Na+ and Ca2+ controlling the process of intrusion, while the aquifer is affected by two mechanisms; (1) direct cation exchange, and (2) reverse cation exchange. According to this figure, the mixing line (Saltwater intrusion phase) shows intrusion (brine endmember with Na-Cl face) by replacement of Sodium by Calcium that indicate Reverse Cation Exchange (RCE) and freshening (fresh endmember with Ca-HCO3 face) with exchange of Ca by Na that indicate direct cation exchange (Giménez-Forcada, 2010).
Fig. 6 also shows that hydrochemical facies in KPA to KPL is Ca-HCO3 (19%), Mix Ca-Cl (9%), Ca-Cl (17%), Mix Na-Cl, and Na-Cl (55%) and reverse cation exchange RCE is the main cause of salinization in KPA which lead to depletion in Na and enrichment in Ca concentration. In intrusion stage, bellow conservative mixing line (blue line) in HFE-D (red and black solid diamonds), groundwater slowly was being salinity from MixCa-MixHCO3 to Na-Cl.
4.4. Hydrogeochemical approach
Hydrochemical evaluations use Na/Cl, Br/Cl and I/Cl ion ratios as well as other major Ca2+, Mg2+, Na+, Cl-, SO42-, HCO3- ions to distinguish salinity and mixing of saline and fresh water (Fontes and Matray, 1993; Mirzavand et al., 2020a; Richter and Kreitler, 1993). To assess the influence of CaSO4 and NaCl dissolution on groundwater salinization, binary plots of (Na/Cl)/Cl, and (Ca2++Mg2+)/HCO3-+SO42- ratios are used (Richter and Kreitler, 1993). The seawater evaporation trend (SET) has been broadly applied to identify the source of salinization in sedimentary basins (Hanor et al., 1988; Kharaka and Hanor, 2005).
Waters influenced by halite dissolution and mixing with freshwater and brine fall above the SET (Mirzavand et al., 2020b), however, the saline groundwater originated from the evaporated sea waters distribute along the SET (Carpenter, 1978). Also, the Na/Cl molar ratio projected from the salt dissolution would be around 1 (Richter and Kreitler 1993). Likewise, the correlation of Ca2++Mg2+ versus HCO3-+SO42- could be useful for investigation of gypsum, calcite and dolomite dissolution effect on groundwater quality (Jahanshahi and Zare, 2017). The samples which signed by the line 1:1, show the gypsum, dolomite and calcite dissolution, nevertheless the samples distributed below and above the line 1:1, illustrate freshening, and intrusion processes, respectively (Ben Ammar et al., 2020).
The brine from the dissolution of evaporite formations increases the molar ratio of Na+ and Cl- ions in water (Leonard and Ward, 1962). Besides, an increase in the concentration of SO42- and Ca2+ due to dissolution of gypsum and anhydrite minerals in these conditions is likely.
As shown in the geology map of the area (Fig.1), Miocene Marl, Qom, LR and UR formations contain significant evaporite minerals such as halite and gypsum. In order to determine halite dissolution in KPA and KPL causing salinization, the Na/Cl ratio was used (Fig. 7). The Na/Cl vs. Cl of the KPA samples varies close to Halite dissolution line 1:1 (Fig. 7) suggesting dissolution of halite is a probable source of salinity. High concentrations of Cl with Na/Cl vs. Cl ratios of more than 1 might have experienced ion exchange and KPL samples, shifting the results below the halite dissolution line, indicating reverse cation exchange process (Fig. 7). The Na/Cl vs. Cl diagram also indicates the enrichment in Cl ion from recharge zone of KPA toward the KPL due to high evaporation effect on discharge zone of KPA and KPL.
Also, for study the contribution of gypsum dissolution in salinization, correlation between Ca2++Mg2+and HCO3- + SO42-has been investigated (Fig. 8). Some of the samples lie on the line 1:1, representing the gypsum, calcite and dolomite dissolution effect on KPA and KPL samples, which indicates minor effect of this mineral on salinization. The samples that plotted above the equilibrium line revealed reverse cation exchange (salinization) and domination of gypsum dissolution while samples falling below the equilibrium line (dominant dolomite and calcite dissolution), confirm freshening process.
Plot of Ca2+ + Mg2+ vs. SO4- + HCO3- is a major indicator to identify ion exchange process (Fisher and Mullican, 1997). If ion exchange is the process, the points tend to shift to the right side of the plot due to an excess of SO4- + HCO3- . While the reverse ions exchange tend to shift to the left side due to excess of Ca2+ + Mg2+ over SO4- + HCO3-. Samples falling along the equilibrium line (1:1) showed the dissolution of calcite, dolomite and gypsum. Dissolution of carbonates contributed greatly to the regional groundwater evolution, especially with respect to the origins of Ca2+, Mg2+, and HCO3-, because most samples plotted along the 1:1 line. High SO42- in waters accounted for by weathering of soluble sedimentary minerals releasing Ca2+, Mg2+ and SO4 2- such as via dissolution of gypsum or anhydrite. The presence of some samples above the equilibrium line indicated the occurrence of inverse ion exchange in these samples. However, the dominant process in this graph was the reverse ion exchange. Fig. 8 also shows that some of the samples are clustered around the 1:1 line, indicating reverse-ion exchange. The imbalance of the ratio of sulfate and bicarbonate dissolution to calcium and magnesium may indicate that dissolution of carbonates and sulfates were not the only sources of calcium supply. The samples falling below the equilibrium line suggests significant contribution from non-carbonate sources and the excess would be balanced by alkalies (Na and K). As shown in Na vs. Cl diagram, contribution samples on Ca2++Mg2+ vs. SO42- diagram, reject intrusion from KPL to KPA.
Cl- vs. Br- concentration was presented in Fig. 9. Due to preferential partitioning of Cl over Br- into K+, Mg2+, and Na+ halogen salts during precipitation (Vengosh et al., 2002), brines produced by the halite dissolution, have low Br-/Cl- ratios (Hanshaw and Hill, 1969; Worden, 1996). Samples with high Cl- concentration, are depleted in Br- and unaffected by seawater evaporation as characterized by the SET (Carpenter, 1978; Kharaka and Hanor, 2005). Generally, samples affected by the dissolution of halite and mixing of fresh waters and brine shift to the upper part of the SET, while samples which effected by evaporated waters plotted on the SET. The analyzed KPA and KPL samples are plotted on the SET on a direct line and show a dilution or mixing trend. Also, the distribution of samples rejects intrusion from KPL to KPA and seawater sources for saline waters. Br- vs. Cl- plot shows that most of the KPA samples are aligned with seawater dilution line confirming that, while the KPL samples distributed along the SET, showing the dissolution of halite as major source of salinity.
4.5. Stable isotopes
Stable isotopic compositions of the water are useful methods for identify the source and mechanism of salinization. The relationship between δD and δ18O and δ18O/Br- has been used to investigate the groundwater salinization. Variations in δD and δ18O of waters are used to identify the source of water (Bouchaou et al., 2009; Farid et al., 2015; Isawi et al., 2016; Gil-Márquez et al., 2017; Nunes et al., 2017), while the initial isotopes concentration are typically modified by dilution during the mixing process (Clark and Fritz, 1997).
The δ2H and δ18O data of KPA and KPL with Global Meteoric Water Line (GMWL), Evaporation line (EL), and East Mediterranean Meteoric Water Line (EMMWL) are plotted in Fig. 10. The δ2H and δ18O varies from -10.03 to 7.03‰ with average of -6.95 ‰ for δ18O and -60.73 to 25.08 ‰ with mean value of -45.82 ‰ for δ2H (Table 3). Also, the δ34SSO4 varies from 5.95 to 22.55‰ in the study area (Table 3).
Table 3. δ2H, δ18O, and δ34SSO4 (‰) values in KPA and KPL
ID
|
δ34SSO4
|
δ2H
|
δ18O
|
d-excess
|
ID
|
δ34SSO4
|
δ2H
|
δ18O
|
d-excess
|
KA1
|
---
|
-55.26
|
-7.84
|
7.46
|
KA22
|
12.8
|
---
|
---
|
---
|
KA5
|
---
|
-54.19
|
-8.81
|
16.29
|
KA23
|
---
|
-57.23
|
-9.09
|
15.49
|
KA3
|
9.2
|
---
|
---
|
---
|
KA24
|
---
|
-57.51
|
-9.47
|
18.25
|
KA6
|
---
|
-49.91
|
-8.62
|
19.05
|
KA25
|
---
|
-57.79
|
-9.35
|
17.01
|
KA7
|
---
|
-59.06
|
-9.59
|
17.66
|
KA27
|
15.67
|
---
|
---
|
---
|
KA8
|
12.36
|
-55.71
|
-8.91
|
15.57
|
KA28
|
---
|
-54.54
|
-8.5
|
13.46
|
KA9
|
22.55
|
-60.73
|
-9.41
|
14.55
|
KA29
|
---
|
-58.04
|
-9.77
|
20.12
|
KA10
|
---
|
-50.44
|
-8.54
|
17.88
|
KA31
|
---
|
-56.19
|
-8.16
|
9.09
|
KA11
|
---
|
-56.88
|
-8.7
|
12.72
|
KA32
|
---
|
-59.65
|
-10.03
|
20.59
|
KA13
|
---
|
-56.18
|
-8.99
|
15.74
|
KA33
|
---
|
-58.89
|
-9.06
|
13.59
|
KA16
|
11.55
|
---
|
---
|
---
|
KA37
|
12.43
|
---
|
---
|
---
|
KA17
|
8.52
|
-56.11
|
-9.11
|
16.77
|
KA38
|
13.87
|
---
|
---
|
---
|
KA18
|
10.38
|
---
|
---
|
---
|
KA40
|
15.85
|
---
|
---
|
---
|
KA19
|
11.78
|
---
|
---
|
---
|
KPL3
|
5.95
|
21.78
|
6.03
|
-26.46
|
KA20
|
---
|
-56.36
|
-9.33
|
18.28
|
KPL12
|
9.99
|
25.08
|
7.03
|
-31.16
|
KA21
|
---
|
-53.74
|
-8.54
|
14.58
|
KPL26
|
8.84
|
22.68
|
6.48
|
-29.16
|
The fresh and saline groundwater samples plotted near the GMWL, with a lesser shift to the right due to membrane filtration, representing that the modern precipitations are the main source of groundwater. The brine samples collected from the wells in discharge zone of KPA, and KPL are plotted to the right side of the GMWL displaying enrichment in 18O and 2H due to evaporation effect. During the evaporation, solution preferentially depleted from lighter isotopes (Clark and Fritz, 1997) and enrichment in heavier isotope. This is compatible with the arid climatic condition in Kashan area situated in a desert region, where most of the precipitations evaporate prior to reaching the groundwater aquifer. In addition, over-pumping of the aquifer and groundwater is pumping from static storage.
The rocks forming the aquifer are mainly composed of marlstones, carbonates and evaporites which contain halite and gypsum. Thus, the source of salinity could be attributed to the deep brine waters that intruded into the aquifer during upconing and mixing process. Likewise, the saline groundwater is discharging to the northeast of the aquifer (discharge zone) and to the KPL which have been affected by intensive evaporation.
The relation between 18O and Br- (conservative parameter) could be used to investigate evaporation effect on water samples (Eissa, 2018). Fig. 11 shows increase in Br corresponds to the 18O enrichment, indicating evaporation process is one of the main causes of enhancing isotopic values in KPL.
4.6. Water-rock Interaction
The SI is a valuable measure to conclude whether the groundwater is under-saturated, saturated, or super-saturated with respect to halite, gypsum, calcite, dolomite, aragonite, and anhydrite. The saturation indexes (SI) quantify the deviation of water samples from balance state with respect to minerals. Six Saturation indexes are applied to determine the varies geochemical processes and reactions that change the chemistry of water (Clark, 2015).
Based on the results (Table. 2 and Fig. 12) saturation indices in KPA and KPL indicate different trend and pathways. In KPA, the SI regarding calcite and dolomite are supersaturated in the recharge zone of aquifer, while these minerals shown under-saturation conditions in the discharge zone. Gypsum and halite showed under-saturated state for all the groundwater samples.
According to Fig. 12, by moving toward the discharge zone, gypsum and halite, tends to be in saturation state, because of reverse ion exchange process. Also, the saturation of dolomite and calcite was typically related to carbonate dissolution through the groundwater flow path. As shown in Fig.12, all the KPL samples, display saturated state with regard to calcite and dolomite. Also, KPL tends to be in saturation state regarding to gypsum and halite. During the salinization process, due to reverse ion exchange, depletion in Na+ and Mg2+, and enrichment in Ca2+ occur leading to conditions favorable for further gypsum and halite dissolution. While, during the freshening, because of direct cation exchange, enrichment in Na+ and Mg2+, and depletion of Ca2+ occurs and lead to conditions suitable for more calcite and dolomite dissolution.
4.7. δ34S stable isotope as tracer for upconing and mixing Assessment
Fractionation of δ34S isotopes between sulphur compounds is a useful approach to understand the biological cycling and sulphate reduction in natural systems (Wallin and Ab, 2011). The δ34S value of SO42- in modern seawater is close to 21‰ VCDT (Robert and Seal, 2006), whereas ancient marine evaporites shows δ34SSO4 values from +10 to +36‰ VCDT (Han et al., 2011; Hosono et al., 2015; Massmann et al., 2003; Robert and Seal, 2006; Wallin and Ab, 2011). For Tertiary period, δ34SSO4 is greater than 20 ‰ VCDT (Clark and Fritz, 1997). Generally, S2- oxidation and SO42- reduction lead to depletion and enrichment in δ34SSO4, respectively (Clark and Fritz, 1997). The Sulphur compounds from different sources could have contributed to the geochemical reactions in groundwater. Hence, the δ34S isotopes can be a useful tracer for investigation of changes in the groundwater characteristics (Wallin and Ab, 2011) and upconing investigation. For determine the source of Sulphate and assessment upconing and mixing processes, 15 water samples were selected for δ34SSO4 analysis, including three KPL samples, and twelve groundwater samples, three (see Table 4).
Based on the results, except sample KA9 (fresh end-member) with δ34S of 22.6‰, other samples have δ34S range between 5.95‰ and 15.85‰ (see Fig. 13 and 14). Knowing the Tertiary age of geologic Formations in KPA, δ34SSO4 is greater than 20‰ (Fig. 14) which is compatible with freshwater end-member (δ34SSO4=22.55‰), while all the samples shown depletion in δ34SSO4.
Thus, sulphide minerals Galena (PbS), and Chalcopyrite (CuFeS2) in Dore mine in west of the aquifer (recharge zone) may have been the source of depleted δ34SSO4 produced by sulphide oxidation leached to the aquifer by seasonal runoff. Based on Fig.13, by moving to northeast of the aquifer pH decrease to 4.17, HCO3- decrease (Table 1). This is an indication that sulphide oxidation in KPA has occurred causing depletion of δ34SSO4. Sulphate and H2S formed through oxidation of sulphides or bacterial reduction, respectively, are isotopically significantly lighter at about +10‰. The most effective isotopic fractionation is caused by microbial reduction of dissolved sulphate to sulphide. During sulphate reduction, H2S and HCO3- are generated increasing the pH. Sulphate reduction (“rotten egg” odour) occurred in deep brine and mixing with sulphide oxidation in freshwater during upconing and moving by groundwater flow (southwest to northeast of KPA), δ34S shown values less than Tertiary values (δ34S<20‰) (Fig.14).
Table 4. δ34SSO4 (‰ CDT) values in KPA and KPL
KPL26
|
KPL12
|
KPL3
|
KA40
|
KA38
|
KA37
|
KA27
|
KA22
|
KA19
|
KA18
|
KA17
|
KA16
|
KA9
|
KA8
|
KA3
|
ID
|
8.84
|
9.99
|
8.95
|
15.85
|
13.87
|
12.43
|
15.67
|
12.8
|
11.78
|
10.38
|
8.52
|
11.55
|
22.55
|
12.36
|
9.2
|
δ34SSO4
|
Alternatively, in some systems, δ34S can contribute to the understanding of groundwater mixing, especially when δ34S plot against conservative tracer such as dissolved Cl- and δ18O (Wallin and Ab, 2011). Oxidation of sulphide yields sulphate with δ18O larger than that of the oxygen in water molecules by +4 to +20‰ (Taylor et al., 1984) observed here. The increase in δ34SSO4 values is commonly followed by an increase in δ18O and Cl values (Wallin and Ab, 2011). Based on the results, this pattern cannot be determined from plot between δ18O- δ34SSO4 and Cl- δ34S (Fig. 15a,b). So, there are other processes which changed this pattern described by Laaksoharju (1995). The sulphur in KPA and KPL should be changed due to sulphide oxidation and sulphate reduction in the aquifer.