Adaptive measures sustainable groundwater management by identification of deep confined aquifers

There are different approaches to mitigate drought. The approaches can be preventive, as discussed above, by integrated management of surface and groundwater or using deep confined aquifers which should be identified before the calamity. The important consideration is that the potential aquifer identified is assessed to be resistant against droughts using geological, hydrogeological, geophysical and isotopic methods.

UNESCO (2003) has recently formed an Expert Group called Groundwater for Emergency Situations (GWES) to formulate guidelines for mitigating groundwater problems during emergency situations. The objective of this group has been to look at measures to be undertaken regarding groundwater supply and management during emergency situations such as floods, droughts, earthquakes, landslides etc. One of the recommendations of UNESCO (2003) is to identify aquifers which can yield good quantity and quality of groundwater during the extreme events. Worldwide attempts are in progress for such a task.

In the following, the Neyveli aquifer from southern India is discussed as a representative groundwater resource body that can be taken as an example to be used for mitigation of drought. The purpose of this example is to illustrate how and why this confined aquifer can be considered as useful for climate adaptation. The important criteria for such a selection or identification are: (i) hydrogeology; (ii) groundwater regime in time and space; (iii) characterization of recharge and discharge areas; and (iv) wide span of groundwater ages from Modern to >30 000 years BP. These points are discussed to bring home the usefulness of the Neyveli aquifer.

Regional hydrogeology of the study area

The Neyveli aquifer lies in Tamilnadu state, India, about 200 km south of Chennai (formerly Madras) on the eastern coast (11°15'-11°50'N, 79° 10'-79°50'E; Fig. 5). The average rainfall

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^—^ RECHARGE BOUNDARY (NLC) RECHARGE BOUNDARY (GSI) PERCHED AQUIFER AREA

Fig. 5. Geological map of Neyveli groundwater basin.

cc z

in the area is about 1200 mm. The geological cross-section A-A' in Figure 5 is shown in Figure 6. Thick Upper Miocene sediments, mainly Cuddalore sandstones, serve as an excellent aquifer under unconfined and confined conditions. Extensive clay layers with a thick lignite bed (8-16 m) are responsible for confining conditions (Fig. 6). The aquifer has been stressed with an average groundwater extraction rate of 7000 m3/h for facilitating open-cast lignite mining. This has resulted in development of a cone of depression around the mine areas. Groundwater occurs under phreatic, semi-confined and confined conditions. Measurements of 14C and 3H, as discussed later, led to identification of an area trending NE-SW as the recharge area. Under confined conditions two aquifers, upper and lower, are identified below the lignite bed. These two aquifers are confined and act as a single hydraulic unit although they are separated by clay beds. The level of potentio-metric surface of the lower and upper confined aquifers was initially approximately the same to justify them being considered as a single unit. Open-cast mining of lignite required lowering of the hydraulic pressure by pumping, which began in July 1961. The aquifer has been pumped continuously for the last 45 years.

LEGEND

• Sampled well location Lignite field boundary

^ Uncorrected 14C age isochrome / [variable interval]

Fig. 7. Systematic lateral distribution of radiocarbon isochrons (1985 sampling).

LEGEND

• Sampled well location Lignite field boundary

Fig. 8. Plot of S18O (%<> SMOW) versus uncorrected 14C ages of deep confined aquifer.

Fig. 8. Plot of S18O (%<> SMOW) versus uncorrected 14C ages of deep confined aquifer.

Demarcation of recharge area

One of the outstanding questions in understanding the groundwater system of the Neyveli aquifer has been identifying the recharge area for such a powerful artesian aquifer. This question, apart being addressed by the conventional hydrogeological approach, was also addressed through measurements of environmental isotopes of tritium, 14C,

Fig. 9. Plot of S13C versus uncorrected 14C age of Neyveli groundwater basin.

0 4 8 12 16 20 24 28 >32 Uncorrected t4C age(103yrbp) —►

Fig. 9. Plot of S13C versus uncorrected 14C age of Neyveli groundwater basin.

813C and S18O. For this purpose about 60 ground-water samples collected from wells deeper than 50 m were analysed during two campaigns in 1985 and 1991. The basic idea was to study not only spatial changes in isotopic composition but also temporal variation.

Fig. 10. (a) Cone of depression of groundwater in Neyveli Aquifer is mostly limited to the mine area. (b) Difference in the piezometric levels (in m) between 1986 and 1990 for Neyveli Aquifer.

Fig. 10. (Continued)

Radiocarbon ages of groundwater

For 14C age calculation, we have assumed an initial activity of 100% modern carbon (pmc) in groundwater, and taken the half-life of 14C as 5730 + 40 years. The initial activity of 100 pmc was justified as the youngest groundwater with thermonuclear tritium had a 14C concentration > 100 pmc. The 14C ages were calculated on the piston flow model which has a distinct recharge area (Bath et al. 1979). Figure 7 shows the systematic lateral distribution of radiocarbon isochrons based on the 1985 sampling. The recharge area of the Neyveli confined aquifer was delineated on the basis of isotopic, geochemical and hydrogeolo-gical considerations. Figure 7 shows the delineated recharge area extending from NE to SW. The wells in the delineated recharge area have shown the presence of thermonuclear tritium, as well as radiocarbon indicating a modern age. Away from this area and towards the east, the wells tapping the confined aquifer lack tritium and the 14C ages show a gradual increase in flow direction. A systematic lateral increase in 14C ages from recharge area to discharge area varied from Modern to >30 000 years BP. In addition to these isotopes, the 813C and chloride show low values ( — 23 to —27% and 10 to 30 mg/l, respectively) in the identified recharge area. This investigation indicates clearly that the confined aquifer, though very large, has modern recharge feeding in its recharge area for sustenance during drought periods.

Palaeoclimate signatures

From the isotopic and geochemical observations (Sukhija et al. 1998) the palaeoclimatic signatures in the deep confined Neyveli aquifers during 20 000 to 12 000 years BP were deduced. Figure 8 shows the plot of S18O versus uncorrected 14C ages of the deep confined aquifer. Relatively enriched S18O values ( — 4.8 to —5.3%) corresponded to an age span of 20 000 to 12 000 years BP encompasing the last glacial period (18 000 + 2000 years BP) with aridity. The climate transition from an arid to a relatively humid period about 8000 to 12 000 years BP is marked by rather depleted values ( — 4.5 to — 6.3%o) implying that the groundwater recharge was related to precipitation events having varied isotopic ratios during this climate transition. The late Holocene (4000 years BP) groundwater has 818O values of

— 5.2 to — 6.3%c, indicating a rather humid but unstable climate.

Figure 9 shows the plot of 8 13C versus uncorrected 14C ages. Here again, relatively enriched 813C values (—10 to — 12%c) corresponding to 14C ages 20 000 to 12 000 years BP encompassing the last glacial period are attributed to growth and dominance of C4 plant species indicating an arid climate during this time span. In a similar fashion to 818O versus 14C during 12 000 to 8000 years BP, a rather depleting trend of 813C ( — 9.5 to

— 17%c) indicates growth of vegetation following the C3 pathway of carbon fixation. Further depletion of 813C from —13 to — 20%o is seen during 4000 years BP onwards indicating the abundance of C3 plant species. Thus in the Neyveli aquifer very clear signatures of changing climates are preserved.

Resilient nature of the Neyveli aquifer

The resilient nature of the Neyveli aquifer was tested by continuous withdrawal of the aquifer. Because of the characteristic recharge area of the aquifer, confined aquifers get a huge amount of natural recharge of c. 110 m3/year (Sukhija et al. 1996b; Rangarajan et al. 2005). The aquifer has also been studied for change in the pressure heads. Figure 10a and b show that in spite of very heavy continuous pumping, the cone of depression is mostly limited to the mine area. Maximum pressure head difference is about 10 to 12 m and at the top of the aquifer the pressure difference does not exceed a few metres. Similarly the quality variation of the aquifer is studied through chloride variation as chloride is a conservative tracer.

Figure 11a shows the isochlors based on chloride concentration of deep groundwater (1985). During 1985 chloride concentration was 10-30 mg/l in the recharge area, and there was a systematic increase towards the SE, indicating groundwater

Fig. 11. Isochlors based on chloride concentration (mg/l) of deep groundwater (1985) of Neyveli aquifer in: (a) 1985: (b) 1991.

Fig. 11. (Continued)

movement towards the SE. During 1991 (Fig. 11b) the general trend of isochlors remains similar except for local perturbation in the NE and SE. Thus there is some change in quality in certain parts of the aquifer, yet the overall quality of the aquifer is not altered significantly, indicating the resilient nature of the aquifer.

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