Hydrochemical situation

A detailed overview on the spatial distribution of the fresh, brackish and saline groundwaters in the study area was obtained by an electrical resistivity and an air-borne electromagnetic survey. The 18-Vm line of the apparent resistivity of the

Fig. 1. Map of northern Cholistan. The former bed of the Old Hakra River stretched from Fort Abbas to the west of Yazman (light grey). The fresh groundwater body has been shifted to the south due to the southwesterly flow direction of the groundwater (dark grey with white dots).

water-saturated aquifer, related to 50 m depth below surface, traces a fresh groundwater body. It comprises a 14-km-wide and 98-km-long strip and is directed from east to SW a few kilometres south of the bed of the Old Hakra River (Fig. 1). This apparent resistivity corresponds to an electrical conductivity of groundwater alone of less than 1500 mS/cm corresponding to 900-1200 mg/l total dissolved solids (TDS). The average thickness and volume of the fresh groundwater aquifer is about 100 m and 10 km3, respectively. This resource is almost completely embedded in brackish and saline groundwater (Fig. 2). The electrical conductivity (EC) of the latter ranges from 3400 mS/cm at the top to 29000 mS/cm at a depth of 100 m, and 52 000 mS/cm in the east.

Sodium is the predominant cation. At EC < 5000 mS/cm bicarbonate is the dominant anion, while at EC > 5000 mS/cm chloride prevails. The fresh groundwater is a cation-exchange water of the Na-HCO3 or Na-HCO3-Cl water type. It changes to the Na-Cl-HCO3 and Na-Cl-SO4 type and finally to the Na-Cl water type with increasing depth and salinity.

The physical and chemical parameters of the fresh groundwater vary in the following ranges: pH = 7.0 to 8.8; T = 24.5 to 31.5°C; EC = 700 to 1200 mS/cm; TDS = 510 to 970 mg/l; sodium = 120 to 270 mg/l; calcium = 2 to 30 mg/l; magnesium = 2 to 40 mg/l; chloride = 30 to 220 mg/l; sulphate = 10 to 150 mg/l; bicarbonate = 200 to 500 mg/l. This fresh groundwater meets the chemical standards laid down for drinking water and stock watering. However, its suitability for irrigation purposes is limited due to the high proportion of sodium of up to 90 meq%.

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Fig. 2. Hydrogeological longitudinal section of the Old Hakra aquifer and the spatial distribution of the groundwater salinity. The white columns represent wells tapping the groundwater bodies of differing salinity. The salinity boundaries were spatially derived from the air-borne electromagnetic survey and calibrated by the hydrochemical analyses of single samples.

Fig. 2. Hydrogeological longitudinal section of the Old Hakra aquifer and the spatial distribution of the groundwater salinity. The white columns represent wells tapping the groundwater bodies of differing salinity. The salinity boundaries were spatially derived from the air-borne electromagnetic survey and calibrated by the hydrochemical analyses of single samples.

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The hydrochemical depth section shown in Fig. 2 may reflect past changes in precipitation: the higher the salinity of the groundwater the drier the climate. Conjunctive use through blending groundwater with surface water from the Sutlej River can reduce both the salt and the mining impacts. But these measures can only minimize the increasing problems with the fresh water supply, mainly used for irrigation, resulting in an acceleration of desertification (Scholz 1997). However, on the other hand the sustainable management of deep groundwater resources may accelerate the socioeconomic development of the Thar Desert and may even lead to de-desertification gradually with time (Zaigham 1999). This optimistic view becomes weakened dramatically as isotope analyses show that the studied groundwater resources are fossil and the gradients of the piezometric surface are relicts of former pluvial conditions.

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