Recharge studies in Jalore Rajasthan

Jalore district (Fig. 7) is situated adjacent to Barmer in the SW part of Rajasthan. An environmental isotope investigation was undertaken to understand the groundwater recharge mechanism at the study area (Fig. 8). The region receives a mean annual rainfall of c. 380 mm and is drained by Sukri river, a tributary of the Luni river system, which is ephemeral in nature. The younger alluvium which is present along the river course is unconso-lidated to semi-consolidated coarse to fine sand and gravel. Older alluvium is of sub-Recent to Pleistocene age. Subsurface geology (Navada ei al. 1993) reveals the presence of a fault in a NE-SW direction along the Sukri river.

Younger and older Quaternary alluvium is the major hydrogeological unit. The younger alluvium comprises unconsolidated sediments - sand, silt and clays - in varying proportions and is found in floodplains. Older alluvium includes piedmont alluvial sediments, unsorted rock fragments, gravel, and sand with higher clay percentage; and higher terrace deposits - upper sandy loam and kankar layers and lower fine to medium sand, gravel and silt. In the central part along Sukri river in the east-west direction, there is a thick clay horizon on the northern bank and medium to coarse sand and gravel interbedded with clay lenses on the southern bank, where coarse sediments increases towards the east.

A number of samples from shallow (<50 m) and deep wells were collected and analysed for environmental isotope as well as chemical analyses.

Deep groundwater near the river course is generally fresh and Na-HCO3 type. Shallow and deep groundwater, away from the river course, are brackish and are Na-Cl type. Figure 9 shows the 82H versus 818O plot for the samples. The samples appear to fall into three groups. Shallow ground-waters along the river course are enriched in stable isotopes and fall into group C. They show a typical evaporation trend and have high 3H contents (5 to 20 TU) indicating a modern recharge component. This group also contains two deep well samples, including an artesian well, which are brackish, have negligible 3H and low 14C values. Shallow groundwaters, which are located away from the river course in the western and southwestern parts, have comparatively depleted stable isotope values and fall into group B. These samples have 3H concentrations ranging from 1 to 4 TU. A few fresh, deep well samples, from near the river courses, also have similar stable isotope (2H, 18O) values and measurable tritium contents, indicating that they have modern recharge components. Group A contains shallow well samples that are the most depleted in stable isotope values. A few samples from the deeper zone, which are brackish and have negligible 3H content, indicating absence of any recent recharge, are also included in this group.

It is observed that samples with depleted 18O values have low 3H contents and vice versa. It is also seen that old waters with low 3H values have

Fig. 7. Map of Jalore district.

high chloride contents (800 to 1000 ppm), whereas recent waters with high 3H values have low chloride content. This indicates that the groundwater near the river course is fresh water with enriched stable isotopic composition and high tritium value, showing the presence of modern recharge. The groundwater away from the river course is brackish, has depleted stable isotopes (2H, 18O) and low 3H contents and thus represents older water. Some interconnection between the shallow and deeper aquifer along the course is indicated by the stable isotopes (2H, 18O), 3H and 14C values (Navada et al. 1993).

The study indicates that the shallow aquifer receives recharge through vertical infiltration of local rainwater through the soil column, having spatially variable vertical permeability. Less permeable zones have less recharge, higher salinity

^ hills E3 older alluvium EH younger alluvium [H dry river bed E^l granites

Fig. 8. Sample location map and geological formation of the study area.

Dahiwa

Jeewana Bautra Valera

Ahore

Ahore

0 2.5 5 7.5 10 km of groundwater and low 3H. Stable isotope values reflect the evaporation signal overprinted on the primary isotopic signal of the rainfall at different times in the past.

-8.5-8.0-7.5-7.0-6.5-6.0-5.5-5.0-4.5-4.0-3.5-3.0-2.5 18O (%%) SMOW

Fig. 9. Plot of 82H versus 818O for dug well and tube well samples with label of high tritium content of dug well samples.

-8.5-8.0-7.5-7.0-6.5-6.0-5.5-5.0-4.5-4.0-3.5-3.0-2.5 18O (%%) SMOW

Fig. 9. Plot of 82H versus 818O for dug well and tube well samples with label of high tritium content of dug well samples.

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