Recharge rate of the fresh and saline groundwater

The initial hydrogeological working hypothesis was that the fresh groundwater is still being replenished. However, at three different places in the Thar depth (m) 0

Recharge rate estimation: past:

14C (100-150 m): 3.4 mm/a (100-200 m): 6.8 mm/a present:

Fine to mediums and with thin clay interbeds, below 100 m depth partly coarse-sandy

Fig. 3. Layer-wise sampling during test hole drilling yielded the 14C age/depth gradient corresponding to a past recharge rate between 3.4 and 6.8 mm/a which exceeds the present-day maximum recharge rate of 0.4 mm/a by a factor of 8 -16. Groundwater recharge commenced with the last long-lasting pluvial period in this region at 13 000 year bp. The shallow groundwater sample was contaminated during sampling by isotope exchange with atmospheric CO2 shown in increased S13C and 14C values (Geyh & Ploethner 1995).

Desert, Western Rajasthan, India (Chandrasekharan et al. 1988; Sukhija et al. 1996), analysis of tritium and cobalt-60 tracer profiles revealed that the minimum groundwater recharge rate amounts to a minimum 1% of the annual rainfall of about 300 mm, except at one place where it is 13%. The potential evaporation rate is 2000 mm/a. In the Indian part of the Thar Desert Sharma & Gupta (1985, 1987) found higher groundwater recharge rates of between 5 and 12% of the annual rainfall of 200-400 mm, using the tritium-tagging method. However, south of the above-mentioned area, in Gujarat, 3 to 11% of the mean annual rainfall of 500 mm contributes to groundwater recharge, determined by the peak and total tritium methods (Sukhija & Rama 1973; Sukhija & Shah 1976).

We determined the recharge rate of our study area using the tritium value of water samples collected from dug wells. Bomb tritium is present in measurable quantities in the hydrological cycle since 1955 and is used to determine the present recharge rate of groundwater. It was measured without enrichment with a 6-l Oeschger-type counter filled with 3 bar of C2H6. The counting gas ethane was prepared from commercial fossil acetylene and hydrogen produced by reduction of a 20 ml water sample with hot magnesium. The background counting rate of the detector is 0.3 cpm and the 3H sensitivity 30 TU/cpm. The detection limit is about 1.2 TU for a counting time of 16 h.

Groundwater recharged after about 1960 contains tritium which can be detected with high reliability. Neither the fresh groundwater down to a depth of 130 m nor the brackish to saline ground-water down to 200 m contained tritium above the radiometric detection limit. Even the samples from dug wells were free of measurable tritium. From 1965 to 1990 rainwater contained 15-30 TU (IAEA 1981, 1983); we measured 16 to 48 TU in surface water and shallow bank storage from the Sutlej River in 1986. The decay-corrected total accumulated tritium (sum of the product of annual precipitation and its tritium value) cint was estimated to be about 330 (TU x m) for Pakistan for the period between 1950 and 1990.

These results allowed us to calculate the maximum recharge rate of groundwater. If the thickness of the water column in a dug well is h, the measured 3H value of groundwater is c, and the total porosity n is known, then the groundwater recharge rate r is given by:

c x h x n r —-x p cint with p as annual precipitation of 200 mm. For a water column of 1 m in the dug wells, a total porosity of 40% and a tritium value of <1.2 TU, a present recharge rate of <0.4 mm/a is obtained which makes up <0.2% of the mean annual rainfall of 200 mm. Consequently, according to these tritium measurements the present recharge of groundwater in Cholistan is negligible.

Chandrasekharan et al. (1988) found similar low recharge rates of around 1% of the annual precipitation in western Rajasthan. Edmunds (1998) applied the chloride method at many sites in the Sahara Desert and states that groundwater recharge ceases at an annual precipitation < 200 mm/a.

A methodically independent estimate of the recharge rate necessitated the 14C analysis of dissolved inorganic carbon (DIC) extracted from horizon-wise sampled fresh (depth < 155 m) and saline groundwater samples (depth 155-200 m) from the test hole T/H-26 (Fig. 3). The 14C age/depth gradient for the depth interval of 100 to 150 m (100-200 m) representing 5730 years (5920 years) of recharge is, however, only 115 a/m (59a/m). Adopting the total porosity of 40%, the inverse value equals the average recharge rate of 3.4 mm/a (6.8 mm/a). This rate is larger by a factor of 8 (17) than the present recharge rate of the study area derived from 3H measurements. This result provides evidence that the discovered fresh groundwater was not recharged under present climatic conditions. A change in the palaeohydrologic situation might have been responsible.

The recharge rates found in other studies are shown in Fig. 4. There is a trend of an increasing

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0 100 200 300 400 500 600 700 precipitation (mm/a)

Fig. 4. Isotopically determined present recharge rates from Gujarat and Rajasthan in India. Below 200 mm/a precipitation Edmunds (1998) assumes zero recharge (black line). Between 200 and 600 mm/a there is a clear trend of an increasing present recharge rate. The higher values determined by Sukhija & Rama (1973) and Sukhija & Shah (1976) cannot be explained. The recharge rate of fossil groundwater in the Thar Desert is shown as a black square. From the linear trend a past annual precipitation of about 550 mm may be derived, in agreement with the expectation by Enzel et al. (1999).

recharge rate of 0-5mm/a with annual precipitation of 200-600 mm/a. However, there are also values between 5 and 15 mm/a which were determined between 1973 and 1976 by Sukhija and coworkers. The reason for this discrepancy in results is not known. However, recharge rate estimates generated by the Water GAP Global Hydrological Model (Doll et al. 2003) and presented in the map Groundwater Resources of the World (BGR & UNESCO 2004) may yield an explanation. The model calculates groundwater recharge as a fraction of total runoff; however, for semi-arid and arid areas the model has been tuned against estimates of groundwater recharge derived from chloride and isotope data. The average annual diffuse groundwater recharge (1961 to 1990) of the Thar Desert of Pakistan is estimated to range from 0 to 5 mm/a, which increases towards India in the east from 5 to 20 mm/a.

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