Isotope investigation of dynamic changes due to longterm exploitation of groundwater

The Bhadka-Bheemda area in Barmer district is situated in the SW part of Rajasthan. At the request of the Groundwater Department, Rajasthan, environmental isotope investigations were undertaken in the 1980s to understand the recharge processes and dynamics of groundwater in these areas. A fresh look at the isotopic and chemical characteristics of groundwater was attempted subsequently in order to understand their response to large-scale exploitation of groundwater.

Figure 10 shows the geology of the area with sample locations. It receives a mean annual rainfall of 280 mm and the central portion is mainly underlain with Tertiary sediments. The northern part is covered by Lathi sandstone (Jurassic age). The other parts are covered by the Malani suite of igneous rocks (Precambrian-Lower Palaeozoic) (Groundwater Department Jodhpur 1983). Fresh groundwater forms a basin in the central portion. Here, the shallow aquifer is under phreatic conditions while the deeper aquifer is under

Fig. 10. Map of Bhadka-Bheemda area in Barmer district.

semi-confined or confined conditions. The shallow groundwater is generally brackish. These waters, along with the deep brackish groundwaters, are of Na-Cl type. The deep groundwater, which is relatively fresh, is Na-HCO3 type, which is being exploited at a rate of over 20 million cubic metres per year.

A set of samples, collected in 1987 as analysed for 2H, 18O, 3H, 14C and chemistry. It was observed that most of the deep (>100 m) fresh groundwater is isotopically depleted and plots along the meteoric water line on the 82H-818O plot (Fig. 11). This group also contains a brackish sample from Bhadka as well as a freshwater sample from Rajdhal from the northern dunal area. Negligible 3H and model ages (Pearson's model) ranging from 4000 to 9500 years BP (Navada et al. 1996) indicate that these are palaeowaters, probably recharged during a cooler and/or pluvial phase in the Holocene. The shallow and deep groundwaters (including samples from Lathi sandstone), which are brackish, fall along an evaporation line. Some samples in this group have measurable tritium indicating some components of modern recharge. Similar isotopic and chemical characteristics of shallow as well as deep samples at some locations indicate aquifer interconnection (e.g. Durgaram ki Dhani).

A second set of samples was collected from the area in 1996, and a third set in March 2000. They have been analysed for 82H, 818O, 813C, 3H and

14C as well as for chemistry. It was observed that most of the dug wells in the study area, sampled earlier, have been abandoned or have dried up. A few existing dug well samples were collected in 1996 for 14C and tritium measurement. In 2000, due to unavailability of dug wells, only tube well samples were collected. Many deep wells, which were sampled earlier, have been abandoned due to either failure of the wells or deterioration of water quality. In such cases, other nearby wells have been sampled.

Figure 12a shows electrical conductivities of tube well samples for all three sampling campaigns. It can be seen that the groundwater becomes more brackish with time except at Bhadka and Ratari where a reverse trend has been noticed. Deviation from the general trend may be due to sampling of a nearby well (in the case of failure of the previous well), which may not represent the groundwater of the failed well. Figure 12b clearly shows evaporative enrichment of deep groundwater with time. Exceptions are Ratari which is situated in the Lathi sandstone, and Rajdhal. Also, a slight increase in 14C values, from 58 pMC in 1996 to 61 pMC during 2000, is an indication of the contribution of younger water in the Ratari area. Figure 11 is the 82H-818O plot for all the samples collected during the three sampling campaigns in 1987, 1996 and 2000. Rainwater samples (monthly average of year 2000) show depleted values. The general trend of deep and shallow groundwater

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Fig. 11. Plot of 82H versus S18O for all samples collected from Barmer area.

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apparently remains the same. The distribution of stable isotopic data indicates the possibility of dispersed recharge through vertical infiltration with some evaporation prior to recharge during different climatic phases during the post 12 ka. However, from the S18O versus 14C plot (Fig. 13), variations in 14C values have been observed at Bheemda, Bhiyar, Nimla, Jogasar and Durgaram ki Dhani, and negligible variations at Bhadka, Rajdhal and Ratri. Although variations in the 14C values are seen at Bheemda, Bhiyar and Jogasar, a general trend is absent which indicates that different wells have been sampled at different times. Although there is a general tendency for samples to show lower 14C values, the variations are not significant at most locations. Small variations in their 14C content are indicative of lateral flow within an aquifer that has spatial variations in water quality and isotopic character inherited from vertical infiltration. Though mining of groundwater is apparent from the declining groundwater table, it is well reflected in the isotopic and chemical characteristics of the groundwater mixing with isotopically enriched saline waters. With the rate of abstraction increasing, the quality of water is bound to suffer further in the future, indicating a need for proper planning of groundwater resource management.

Fig. 12. Bar graphs showing electrical conductivity (a) and S18O values; (b) of samples collected during 1987, 1996 and 2000.

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