Environmental isotope studies along an identified palaeochannel

Interpretation of satellite imagery of the western parts of the Jaisalmer district (western Rajasthan) 'revealed' the buried course of a river orientated in the NE-SW direction (Kar 1986; Bakliwal & Grover 1988) (Fig. 2a). In spite of the highly arid condition of the region, comparatively good quality groundwater is available along the course below 30 m depth. The aquifer consists of medium to fine sand with very little clay. A few dug wells do not dry up even in summer and the tube wells do not show reduction in water table, even after extensive utilization for human as well as livestock consumption. Groundwater away from this course is saline. The course is thought to belong to the legendary river Saraswati of Himalayan origin, mentioned in many early literary works and known to have existed 3000 years BP (Ghose et al. 1979; Valdiya 1996).

Quaternary alluvium composed of gravel, sand, silt and clay in varying proportions is the most widespread lithological unit. The thickness of this unit extends to a depth of 200 m. Much of the area occupied by this formation is either unproductive or contains saline groundwater. The central part of Jaisalmer district is occupied by Cenozoic and Mesozoic sandstone. Another widespread lithologi-cal unit encompassing about 52% area of the potential zone is Lathi sandstone (Department of Science and Technology 1999). It is white, yellow or reddish brown, poorly to moderately lithified medium-grained sandstone interspersed with silt, stone and shale. The upper part contains considerable fine-grained material. This unit attains maximum thickness (250 m) near Jaisalmer town.

To determine the origin and age of groundwater along the buried course of the river (palaeochannel), a study was carried out using environmental isotope. Samples were collected in the year 1998 from existing dug wells and tube wells in the area for analysis of 82H, 818O, 3H, 813C and 14C as well as for chemistry. Dug wells were quite deep with water levels below 30 m or more. Tube wells are cased with screens below 60 m depth. Figure 2b shows sample locations. Both dug wells and tube wells show similar characteristics, being evolved towards Na-Cl type (Fig. 3). A plot of 82H versus 818O for the samples is shown in Figure 4. It is seen that the stable isotopic values of a few groundwater samples from dug wells are enriched with respect to tube well samples. This could be due to the exposure of open water bodies to evaporation in an arid environment. The linear regression line of dug well samples has an equation 82H = 2.18 S18O - 28.7, r2 = 0.39, n = 12. The 82H-818O relationship (leaving aside the three dug well samples with enriched stable isotope values) would support the model of Allison et al. (1984), wherein direct infiltration during high rainfall event mixes with evaporated soil moisture and the mixed parcel eventually reaches the water table. The stable isotope (82H and 818O) values clearly show that the deep and shallow zones of the aquifer are generally not interconnected. Since no lithological separation exists between the deep and shallow zones, the deep aquifer receives a small fraction of vertical recharge from precipitation; this is spatially variable in accordance with the coarse grain content of the upper layer, following Allison et al. (1984) model. The regression line is parallel to the meteoric water line, which indicates that this recharge persisted over a long time. Both dug well and tube well samples are enriched compared to present-day Himalayan rivers (818O: -11% to -9%), which indicates

60 60

40 40

20 20

Ca Na HCO3 Cl

Fig. 3. Piper trilinear plot of Jaisalmer groundwater samples.

Fig. 4. Plot of 818O versus 82H for Jaisalmer samples.

that palaeochannel waters are not in dynamic contact with present-day Himalayan rivers. Figure 5 indicates that the tube well samples have negligible tritium, indicating the absence of modern recharge. However, a few dug wells do show small components of modern recharge.

Most of the dug well and tube well samples along the palaeochannel have electrical conductivity (EC) less than 4000 |mS/cm (Table 1), while groundwater away from this channel is highly saline, with conductivity more than 10 000 | S/cm. This indicates that salinity increases as one moves away from the channel.

The 14C age using an empirical model (Table 1) of the dug well samples is less than 5000 years BP (Fig. 6), which shows that they are old ground-waters, but no flow pattern is discernible from the results. The 14C age of tube well samples is more than 5000 years BP. The maximum age is from the saline pocket, Sadewala. There is a trend of an

■ Dug well samples o Tube well samples

Ca Na HCO3 Cl

0 2000 4000 6000 8000 10000 Conductivity (|S/cm)

Fig. 3. Piper trilinear plot of Jaisalmer groundwater samples.

Fig. 5. Plot of conductivity versus tritium content (TU) of Jaisalmer samples.

Table 1. Isotopic and conductivity data of palaeochannel samples

ID no.

Location

Well type*

EC (ms/cm)

3H + 1ct (TU)

14C + 1a- (pMC)

13C (%<,)

18O (%o)

2H (%<,)

14C (model age) 91.9 pMC as A0

D1

Dharmikua

DW

2330

2.1 + 0.3

79.5 + 2.2

-9.6

-7.5

-47.1

1198

T1

Kishengarh

TW

3460

0.3 + 0.1

47.3 + 1.4

-5.7

- 5.6

-41.7

5492

D2

Kishengarh

DW

4180

1.1 + 0.2

91.9 + 1.7

-10.7

-6

- 40.9

0

D3

Kurriaberi

DW

2100

0.5 + 0.2

58.8 + 1.6

-8.3

- 5.7

- 42.6

3692

D4

Nathurakua

DW

3040

0.3 + 0.2

69.3 + 1.8

-7.9

- 6.3

-38.4

2334

T2

Ghantiyali

TW

3660

0.5 + 0.2

31.2 + 1.2

- 4.0

- 6.6

- 45.6

8932

D5

Ghantiyali

DW

2820

0.6 + 0.2

54.9 + 1.5

-7.1

- 6.0

- 41.2

4260

D6

Khairakua

DW

8900

- 4.8

- 41.5

D7

Gajesingh Ka Tar

DW

4620

2.1 + 0.3

64.9 + 1.9

- 7.7

- 4.7

- 35.2

2876

T3

Ranau

TW

1890

0.6 + 0.2

48.8 + 1.5

- 7.4

- 6.2

- 45.3

5233

D8

Ranau

DW

2060

1.7 + 0.3

- 6.0

- 46.1

T4

Sadewala

TW

7600

0.4 + 0.2

6.6 + 0.9

- 7.7

-3.4

- 26.3

21775

D9

Sadewala

DW

9120

0.8 + 0.2

- 6.3

-43.6

T5

Loungewala

TW

2740

0.4 + 0.2

10.4 + 0.9

- 5.6

- 6.2

- 44.0

18015

D10

Loungewala

DW

9370

1.0 + 0.2

- 5.9

-39.9

T6

Gumnewala

TW

4060

0.6 + 0.2

- 6.1

- 30.0

T7

Ghotaru

TW

2270

0.4 + 0.2

20.7 + 1.0

-7.3

- 6.9

- 48.7

12324

D12

Ghotaru

DW

3650

1.1 + 0.2

62.7 + 1.6

- 6.4

- 41.1

3161

T8

Asutar

TW

2560

0.4 + 0.2

36.1 + 1.3

-7.5

- 6.3

- 47.0

7726

D13

Asutar

DW

2390

0.3 + 0.1

D14

Langtala

HP

3400

0.3 + 0.1

68.6 + 2.0

- 6.2

- 5.0

-39.6

2418

D15

Langtala

DW

2380

1 + 0.2

64.8 + 1.7

- 6.9

- 6.0

- 46.1

2889

T9

Shahgarh

TW

10090

0.4 + 0.2

- 6.0

-38

D16

Ratnewala

DW

10330

- 4.7

D17

Dost Mohammad Ka Kuan

DW

1380

1

49.7 + 1.5

-1.2

5082

D18

Mituwala

DW

6780

0.6

57.9 + 1.7

-8.5

- 5.8

- 36.1

3820

25000

20000

CD

15000

CT

(0

C

10000

5000

■ Dug wells (shallow aquifer) Tube wells (deep aquifer)

■ Dug wells (shallow aquifer) Tube wells (deep aquifer)

Fig. 6. Plot of S13C versus 14C age of Jaisalmer samples.

increase in the apparent C age for groundwaters from Kishengarh to Loungewale, along the buried course. From the relative radiocarbon ages, a groundwater velocity of about 5m/a may be inferred, which is a normal value expected under similar desert conditions. The 813C content of shallow groundwater (dug wells) is depleted compared to other tube well samples.

Isotopic values indicate that this channel is not connected with the area across the border covering the dry Ghaggar River bed (Geyh & Ploethner 1995). The results of the isotope study indicate that direct headwater connection to the ground-waters in the study area from present-day Himalayan sources is remote. The stable isotope (2H, 18O), 3H and 14C values indicate that they are palaeowaters with negligible modern recharge component. Deep and shallow groundwaters are generally not interconnected, but some recharge to the deeper zones is possible. This indicates accumulation of groundwater over a 103-104 year time scale. The water quality difference may be related to small variations in the coarse grain fractions, which may be more along the paleochannel permitting slightly higher recharge and less evaporative enrichment prior to recharge.

The authors feel that the palaeochannel contains comparatively fresh water, which can be used for drinking and irrigation processes. Since the channel is presently not connected with any of the Himalayan rivers, over-exploitation may lead to depletion of groundwater resources.

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