Origin of the fresh groundwater

Information on the origin of the fresh groundwater recharged by local precipitation or an influx of groundwater recharged far away can be obtained from the stable isotope compositions of oxygen, hydrogen and carbon. The S18O and 82H values may yield information on the altitude of the catchment area and the processes that occurred at the ground surface before groundwater recharge. We determined the isotope compositions of oxygen and hydrogen of 101 samples by means of two MICROMASS mass spectrometers (602C for the first two and 602E for hydrogen) and CO2 exchange with 20 ml of sample water. Hydrogen isotopes were determined on hydrogen produced by reduction of 20 ml water with zinc. The results are given in the common delta-notation referring to the Vienna-SMOW standard. The 813C values were determined on CO2 prepared from the DIC sample and referred to the marine PDB standard. The relationship between 818O and 82H values of surface water, shallow bank storage of the Sutlej River and fossil groundwater is shown in Fig. 6. The S18O and 82H values of the surface water and related recent groundwater form a narrow cluster on the global meteoric water line (GMWL) ranging from -6.3 to — 8.4%% for 818O and from -38 to — 56%o for 82H. This water originates from the Sutlej River with a catchment area at a mean altitude of 3700 m a.s.l. of the foothills of the Himalaya Mountains. The 818O and 82H values of rainwater taken in the Hindukush Mountains. (5300 m a.s.l.) and at the Khyber Pass fit a local meteoric water line of 82H = 8 x 818O + 15. This is in accordance with the observation that groundwater recharge in western Pakistan is predominantly derived from the winter monsoon but also through westerly storms from the eastern Mediterranean with higher deuterium excess of + 22% (Gat & Carmi 1970). Some rainwater samples that we took at Lahore and Marot/ Cholistan during the monsoon period delivered 818O and 82H values which also scatter around the GMWL.

The majority of 818O and 82H values obtained from 101 fossil fresh and saline groundwater

Fig. 6. The 818O/82H plot shows three pronounced point clusters. The surface and spring waters from the high Hindukush Mountains (5300 m a.s.l) fit the GMWL and are isotopically more depleted than the shallow groundwater and canal water derived from the Sutlej River with a mean altitude of the recharge area of 3700 m a.s.l. The isotope data of the latter also form a cluster on the MWL which intercepts with an assumed evaporation line through the cluster of isotope data of the explored fossil fresh groundwater body. This finding is a strong argument that this groundwater was recharged by seepage from the Old Hakra River. Water samples from the Khyber Pass provided evidence that Mediterranean storms reach this region.

Fig. 6. The 818O/82H plot shows three pronounced point clusters. The surface and spring waters from the high Hindukush Mountains (5300 m a.s.l) fit the GMWL and are isotopically more depleted than the shallow groundwater and canal water derived from the Sutlej River with a mean altitude of the recharge area of 3700 m a.s.l. The isotope data of the latter also form a cluster on the MWL which intercepts with an assumed evaporation line through the cluster of isotope data of the explored fossil fresh groundwater body. This finding is a strong argument that this groundwater was recharged by seepage from the Old Hakra River. Water samples from the Khyber Pass provided evidence that Mediterranean storms reach this region.

samples from test holes and tube wells form a cluster (S18O, —5.7 to — 4.2%; 82H, —44 to

— 37%) along an evaporation line: 82H = 4.1 S18O — 20. This evaporation line originates from the GMWL with an intercept at 8 O =

— 7.4% and 82H = — 49% within the cluster of the corresponding values of the young ground-water of Sutlej River. The deuterium excess (= 82H — 8 X 818O) between +8 and + 12% (Table 1) provides evidence that some of the river water might have seeped into the ground without evaporative enrichment of the heavy oxygen and hydrogen isotopes. As the stable oxygen and hydrogen isotope compositions of the fossil groundwater of Cholistan vary only slightly and are not correlated with the salinity of the water, it provides evidence that the fresh and brackish to saline groundwaters are of the same origin and were recharged under similar climatic conditions. The wide range of salinity is explained by the varying salt concentration in the sediments.

The recharge process of the fossil groundwater becomes understandable if we compare the evaporation line with that of the recent groundwater of the New Delhi area, India: 82H = 4.8 X 818O — 17.5 (Das et al. 1988). This shallow groundwater is recharged within alluvial sediments during which inundation of the Yamuna River, lasts many weeks. The phenomenon that the heavier stable oxygen and hydrogen isotopes of water become enriched under semi-arid and arid climate conditions before and during groundwater recharge has been known for a long time (Gonfiantini et al. 1974; Munnich et al. 1984; Miiller et al. 1984).

It can be ruled out that the 818O/82H cluster of the fossil groundwater samples joins a meteoric water line with a deuterium excess less than + 10% as this is typical for groundwater recharged during a cooler and wetter climate than that of the present day (Job et al. 1975). Such depleted stable isotope compositions with a MWL 82H = 8 X 818O + 4 were found for ground-waters in the Thar Desert of Western Rajasthan, India, where a conventional 14C age of 18 000 years BP is considered as evidence for recharge during a cooler and wetter climate (Chandrase-kharan et al. 1988).

The alternative explanation of the presence of groundwater with enrichment of heavy isotopes is diffuse recharge during relatively wet periods and partial evaporation through the unsaturated zone. This process would not be in conflict with the missing correlation between the deuterium excess and the salinity of the groundwater. However, this interpretation does not explain why the present fossil groundwater is restricted to the former course of the Old Hakra River.

The stable isotope composition of carbon of the DIC delivers complementary hints on the origin of the groundwater. The 813C value is controlled by the interaction of biogenic carbon dioxide with lime in the topsoil or other sources.

The 813C value of atmospheric carbon dioxide amounts to around — 7%, those of biogenic carbon range from —9 to — 35% depending on the type of photosynthesis pathways. Under different environmental conditions the 813C values of C3 plants (these are generally plants of the humid zone, e.g. wheat) range from —20 to — 35% ( — 25 to —28%«,). The 813C values of C4 plants (e.g. steppe grass, maize, sorghum, sugar cane) range from —9 to —16% (—12 to —14%). Characteristic values are given by Merwe (1982) and Hillaire-Marcel (1986).

The 813C values of DIC of groundwater recharged in sedimentary catchments range between —10 and —14%, while in crystalline areas 813C values up to — 20% are found. If marine carbonates were dissolved in the subsurface as the result of secondary chemical reactions, and / or if the recharge area is covered by C4 vegetation, or seepage from rivers or canals takes place, then 813C values in groundwater less negative than — 10% are found.

The 813C values of all groundwater samples taken in Cholistan vary from — 3.4 to —5.8%. The main reason for this relatively positive

813C

value might be that seepage from the former Old Hakra River was the predominant source of ground-water recharge. The 813C values of groundwater tend to become more negative downstream (Fig. 7). This trend might indicate that the influence of biogenic carbon dioxide becomes more dominant in the down-gradient direction. This interpretation fits with the concept that the Old Hakra River became shallower and wider towards the Derawar inland delta and therefore vegetation on river s! -3.0

70 60 50 40 30 20 10 0 km approx. distance

Fig. 7. Decline of the 813C values of DIC along the palaeochannel of the Old Hakra River. The x-axis shows the east-west distance between the sampling points.

Dingarh

Mo

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Mirgarh

Abbas

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70 60 50 40 30 20 10 0 km approx. distance banks was more abundant in the west. The 813C values of the surface water lie between — 8.7 and — 5.0%o while related recent groundwater attains 813C values ranging from —13.3 to — 11.6%c.

The groundwater study was carried out by the Hydrogeo-logy Directorate, Water and Power Development Authority (WAPDA), Lahore, and the Federal Institute for Geosciences and Natural Resources (BGR), Hannover, in the framework of a Pakistan-German Technical Cooperation Project. The project was funded by the Ministry of Water and Power, Government of Pakistan, and the Federal Ministry for Economic Cooperation and Development (BMZ), Government of the Federal Republic of Germany. Special gratitude is extended to the WAPDA drilling crews, field assistants and hydrogeologists who strongly supported groundwater sampling in the field. Thanks to the staff of the NLfB isotope laboratory in Hannover. We also thank Dr S. K. Gupta for reviewing our paper and valuable comments.

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